Tesis previa a la obtención del Título de Ingeniero en Electrónica y ...
Tesis previa a la obtención del Título de Ingeniero en Electrónica y ...
Tesis previa a la obtención del Título de Ingeniero en Electrónica y ...
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ESCUELA POLITÉCNICA NACIONAL<br />
FACULTAD DE INGENIERÍA ELÉCTRICA<br />
ESTUDIO Y PLANIFICACIÓN DE UN MODELO DE RED<br />
PARA VIDEO-TELÉFONO<br />
<strong>Tesis</strong> <strong>previa</strong> a <strong>la</strong> <strong>obt<strong>en</strong>ción</strong> <strong><strong>de</strong>l</strong> <strong>Título</strong> <strong>de</strong> <strong>Ing<strong>en</strong>iero</strong> <strong>en</strong><br />
<strong>Electrónica</strong> y Telecomunicaciones<br />
Luis Santiago León Coronel<br />
Quito, Diciembre <strong>de</strong> 1998
ÍNDICE GENERAL<br />
Introducción ., 1<br />
CAPITULO1: PRINCIPIOS DE FUNCIONAMIENTO DEL VIDEO TELEFONO<br />
1.1 Descripción g<strong>en</strong>eral <strong>de</strong> los Sistemas <strong>de</strong> Vi<strong>de</strong>o-Telefonía Digital, 3<br />
1.1.1 Introducción 3<br />
1.1.2 Sistemas <strong>de</strong> Vi<strong>de</strong>o Teléfono 3<br />
1.1.2.1 Descripción <strong><strong>de</strong>l</strong> Servicio 4<br />
1.1.2.2 Facilida<strong>de</strong>s <strong><strong>de</strong>l</strong> Servicio 5<br />
1.1.2.3 Características <strong>de</strong> Calidad <strong><strong>de</strong>l</strong> Servicio Vi<strong>de</strong>o Telefónico 5<br />
1.1.2.4 Parámetros <strong><strong>de</strong>l</strong> Sistema Vi<strong>de</strong>o Telefónico 5<br />
1.2 Normas <strong>de</strong> Comunicación 7<br />
1.2.1 Norma H.320 7<br />
1.2.1.1 Diagrama <strong>de</strong> Bloques e I<strong>de</strong>ntificación <strong>de</strong> elem<strong>en</strong>tos 8<br />
1.2.1.2 Señales 9<br />
1.2.1.3 Velocidad Binaria-Opciones e Infraestructura 10<br />
1.2.1.4 Tipos <strong>de</strong> Terminal <strong>de</strong> un Vi<strong>de</strong>o Teléfono 11<br />
1.2.2 Establecimi<strong>en</strong>to <strong>de</strong> una l<strong>la</strong>mada <strong>de</strong> Vi<strong>de</strong>o Teléfono 12<br />
1.2.2.1 Normas y Procedimi<strong>en</strong>tos 12<br />
1.2.2.2 Transmisión y Despliegue <strong>de</strong> Pantal<strong>la</strong>s al iniciar una L<strong>la</strong>mada 16<br />
1.3 Algoritmos <strong>de</strong> Codificación y Decodificación 18<br />
1.3.1 JBIG (Codificación <strong>de</strong> Imág<strong>en</strong>es Progresivas <strong>de</strong> Doble Nivel) 19<br />
1.3.1.1 Codificación Progresiva 19<br />
1.3.1.2 Revisión <strong><strong>de</strong>l</strong> Algoritmo JBIG 21<br />
1.3.2 JPEG (Codificación <strong>de</strong> Imág<strong>en</strong>es Inmóviles a Color)......... 23<br />
1.3.2.1 Visión g<strong>en</strong>eral <strong>de</strong> los algoritmos JPEG 23<br />
1.3.3 Norma H.261 <strong>de</strong> codificación para vi<strong>de</strong>o confer<strong>en</strong>cia. 28<br />
1.3.3.1 Estructura y compon<strong>en</strong>tes <strong><strong>de</strong>l</strong> <strong>de</strong>codificador...... , 30<br />
1.3.3.2 Frecu<strong>en</strong>cia <strong>de</strong> muestreo. 31<br />
1.3.3.3 Codificación <strong><strong>de</strong>l</strong> canal <strong>de</strong> audio 31<br />
1.3.3.4 Codificación <strong><strong>de</strong>l</strong> canal <strong>de</strong> datos 31<br />
1.3.3.5 Codificación <strong>de</strong> fu<strong>en</strong>te <strong>de</strong> vi<strong>de</strong>o 31<br />
1.3.4 Estándar MPEG...... ..".... 32<br />
1.3.4.1 Requerimi<strong>en</strong>tos <strong>de</strong> <strong>la</strong> norma .....' 32<br />
1.3.4.2 Revisión <strong>de</strong> los algoritmos <strong>de</strong> compresión. 33<br />
1.3.4.3 Características <strong>de</strong> <strong>la</strong> sintaxis <strong><strong>de</strong>l</strong> sistema 36<br />
CAPITULO 2: LA RDSI EN VIDEO COMUNICACIONES<br />
2.1 Principios G<strong>en</strong>erales <strong>de</strong> <strong>la</strong> RDSI..... 38<br />
2.1.1 Establecimi<strong>en</strong>to <strong>de</strong> una RDSI 39<br />
2.1.2 P<strong>la</strong>n g<strong>en</strong>eral para el establecimi<strong>en</strong>to <strong>de</strong> una RSl. 39<br />
2.1.3 Interfuncionami<strong>en</strong>to <strong>en</strong>tre una RDSI y una RTPC 40<br />
2.1.4 Tratami<strong>en</strong>to <strong>de</strong> l<strong>la</strong>madas <strong>en</strong>tre RDSI y RTPC 41<br />
2.2 Vi<strong>de</strong>ocomunicaciones <strong>en</strong> <strong>la</strong> RDSI 44<br />
2.2.1 Servicios <strong>de</strong> Vi<strong>de</strong>o Comunicación <strong>en</strong> <strong>la</strong> RDSI... ,. 44<br />
Páginas
2.2.2 C<strong>la</strong>sificación <strong>de</strong> los servicios <strong>de</strong> Telecomunicaciones 45<br />
2.2.2.1 Servicios básicos 45<br />
2.2.2.2 Servicios complem<strong>en</strong>tarios............ 45<br />
2.3 RDSI <strong>de</strong> Banda Angosta y Banda Ancha 45<br />
2.3.1 RDSI <strong>de</strong> banda angosta.............. 46<br />
2.3.2 RDSI <strong>de</strong> banda ancha 48<br />
2.3.2.1 Modo <strong>de</strong> transfer<strong>en</strong>cia Asincrónico (ATM)....... 48<br />
2.4 Aplicaciones <strong>de</strong> <strong>la</strong> RDSI 54<br />
2.4.1 Consolidación <strong>de</strong> <strong>la</strong>s re<strong>de</strong>s............ 54<br />
2.4.2 Biblioteca electrónica digital.......... 54<br />
2.4.3 Media disponible <strong>en</strong> <strong>de</strong>manda....... 55<br />
2.4.4 Captura <strong>de</strong> información.............. 56<br />
2.4.5 Confer<strong>en</strong>cia. 56<br />
2.4.6 Simu<strong>la</strong>ción.... 57<br />
CAPITULO 3: TECNOLOGÍAS DEL VIDEO TELEFONO<br />
3.1 El Vi<strong>de</strong>o Teléfono <strong>de</strong> Imág<strong>en</strong>es Fijas 58<br />
3.1.1 Descripción y funcionami<strong>en</strong>to <strong>de</strong>! teléfono "PICASSO" <strong>de</strong> AT&T 58<br />
3.1.1.1 El sistema Picasso 58<br />
3.1.1.2 Características 58<br />
3.1.1.3 Modo <strong>de</strong> operación....... 60<br />
3.1.1.4 Funciones <strong><strong>de</strong>l</strong> teléfono Picasso...... 60<br />
3.1.2 Equipos y compon<strong>en</strong>tes <strong><strong>de</strong>l</strong> sistema 61<br />
3.1.2.1 Arquitectura <strong><strong>de</strong>l</strong> sistema Picasso 63<br />
3.1.2.2 Protocolos <strong>de</strong> comunicación 66<br />
3.1.3 Interactividad con oíros equipos 66<br />
3.1.3.1 Conectividad con PCs 67<br />
3.1.3.2 Conectividad con equipos <strong>de</strong> vi<strong>de</strong>o.................. 67<br />
3.1.3.3 Mouse para anotaciones... 67<br />
3.1.3.4 Mercado y aplicaciones.... 68<br />
3.2 El Vi<strong>de</strong>o Teléfono <strong>de</strong> Imág<strong>en</strong>es Móviles.... 70<br />
3.2.1 Introducción................ 70<br />
3.2.2 Descripción y funcionami<strong>en</strong>to <strong><strong>de</strong>l</strong> vi<strong>de</strong>o-teléfono 70<br />
3.2.2.1 Modo <strong>de</strong> utilización para los usuarios <strong>de</strong> un Vi<strong>de</strong>oteléfono...... 71<br />
3.2.2.2 Diseño físico... 72<br />
3.2.3 Descripción <strong><strong>de</strong>l</strong> Vi<strong>de</strong>o teléfono Alcatel...... 73<br />
3.2.3.1 Concepto <strong><strong>de</strong>l</strong> Vi<strong>de</strong>o teléfono Alcatel 73<br />
3.2.3.2 El compon<strong>en</strong>te es<strong>en</strong>cial CAP III...... 74<br />
3.2.4 Equipos y compon<strong>en</strong>etes <strong><strong>de</strong>l</strong> sistema 75<br />
3.2.4.1 Arquitectura <strong><strong>de</strong>l</strong> Vi<strong>de</strong>o teléfono 75<br />
3.3 La vi<strong>de</strong>o-confer<strong>en</strong>cia como aplicación particu<strong>la</strong>r 80<br />
3.3.1 Proshare Personal Confer<strong>en</strong>cing Vi<strong>de</strong>o System 200..... 81<br />
3.3.2 PictureTelLivePLS 500. 82<br />
3.3.3 TeleWork-5.. 83<br />
3.3.4 Otras aplicaciones <strong><strong>de</strong>l</strong> Vi<strong>de</strong>o teléfono 84
CAPITULO 4: ESTUDIO Y PLANIFICACIÓN DE UN MODELO DE RED PARA EL<br />
VIDEO- TELEFONO<br />
4.1 Estudio <strong>de</strong> <strong>la</strong>s re<strong>de</strong>s <strong>de</strong> vi<strong>de</strong>o telefonía porfibra óptica 89<br />
4.1.1 Fundam<strong>en</strong>tos <strong>de</strong> propagación <strong>de</strong> <strong>la</strong> luz y sistemas <strong>de</strong> fibra óptica..... ,. 89<br />
4..1.2 Tipos <strong>de</strong> fibra óptica... 90<br />
4.1.3 Estructura <strong><strong>de</strong>l</strong> cable <strong>de</strong> fibra óptica 91<br />
4.1.4 V<strong>en</strong>tajas al usar re<strong>de</strong>s <strong>de</strong> fibra óptica 94<br />
4.1.5 Pérdidas <strong>en</strong> <strong>la</strong>s fibras.... 95<br />
4.1.6 Topología <strong>de</strong> re<strong>de</strong>s 97<br />
4.1.7 Empalmes <strong>en</strong> <strong>la</strong>s fibras ópticas 98<br />
4.1.8 Apertura numérica 99<br />
4.2 P<strong>la</strong>nificación <strong>de</strong> un mo<strong><strong>de</strong>l</strong>o <strong>de</strong> red para esta tecnología <strong>en</strong> <strong>la</strong> ciudad<br />
<strong>de</strong> Quito para el sector que compr<strong>en</strong><strong>de</strong> <strong>la</strong> C<strong>en</strong>tral <strong>de</strong> Iñaquito 99<br />
4.2.1 Diseño <strong><strong>de</strong>l</strong> sistema 99<br />
4.2.1.1 Selección <strong><strong>de</strong>l</strong> número <strong>de</strong> fibras....... 99<br />
4.2.1.2 Cálculo <strong>de</strong> pérdidas..... 100<br />
4.2.1.3 At<strong>en</strong>uación <strong>de</strong> cable máxima permitida 102<br />
4.2.1.4 Cálculo <strong>de</strong> pérdidas <strong>en</strong> conectares. 102<br />
4.2.1.5 Cálculos requeridos según el tipo <strong>de</strong> fibra 103<br />
4.2.1.6 Selección <strong>de</strong> <strong>la</strong> fibra... 104<br />
4.2.1.7 Descripción y diseño para <strong>la</strong> red interna <strong>de</strong> fibra óptica 105<br />
4.3 Descripción <strong>de</strong> equipos y análisis económico 107<br />
4.3.1 Estimación <strong><strong>de</strong>l</strong> capital necesario... 110<br />
4.3.1.1 Costos <strong>de</strong> equipos y cable...... 110<br />
4.3.1.2 Costos por mano <strong>de</strong> obra y servicios 110<br />
4.3.1.3 Costos por servicios básicos..... 111<br />
4.3.1.4 Costos <strong>de</strong> operación 111<br />
4.3.1.5 Costos totales <strong>de</strong> operación...... 112<br />
4.3.2 Cobertura <strong>de</strong> costos <strong>de</strong> implem<strong>en</strong>tación y operación <strong><strong>de</strong>l</strong> sistema 112<br />
CONCLUSIONES Y RECOMENDACIONES 114<br />
ANEXOS<br />
Anexo 1: P<strong>la</strong>nos<br />
Red Principal<br />
Diagrama Vertical<br />
Equipos y compon<strong>en</strong>tes <strong><strong>de</strong>l</strong> sistema<br />
Mapa <strong>de</strong> ubicación<br />
Anexo 2: Normas y Recom<strong>en</strong>daciones<br />
Norma H: 221<br />
Norma H: 230<br />
Norma H: 242<br />
Norma H: 261<br />
Norma H: 320<br />
BIBLIOGRAFÍA
ESTUDIO Y PLANIFICACIÓN DE UN MODELO DE RED PARA<br />
VIDEO-TELEFONÍA<br />
INTRODUCCION.-<br />
La comunicación actual está caracterizada por hombres y máquinas que<br />
intercambian información <strong>de</strong> una manera s<strong>en</strong>cil<strong>la</strong>, fiable, segura y económica <strong>en</strong><br />
cualquier mom<strong>en</strong>to y lugar usando los medios a su elección, ya sean, voz, datos o<br />
imag<strong>en</strong>.<br />
Reci<strong>en</strong>tes estudios económicos cuantifican a <strong>la</strong>s telecomunicaciones como uno <strong>de</strong><br />
los principales factores con los que se mi<strong>de</strong> el <strong>de</strong>sarrollo <strong>de</strong> un país, por tanto<br />
mi<strong>en</strong>tras más completa sea <strong>la</strong> información que nos llegue, mucho más rápida será<br />
<strong>la</strong> respuesta que t<strong>en</strong>gamos, utilizando para el efecto m<strong>en</strong>os recursos, y por tanto<br />
optimizando tiempo y dinero <strong>en</strong> <strong>la</strong> realización <strong>de</strong> una tarea <strong>de</strong>terminada.<br />
Existe una c<strong>la</strong>ra t<strong>en</strong><strong>de</strong>ncia <strong>de</strong> llevar un creci<strong>en</strong>te número <strong>de</strong> servicios <strong>de</strong> vi<strong>de</strong>o<br />
tanto al lugar <strong>de</strong> trabajo como al hogar a través <strong>de</strong> <strong>la</strong>s telecomunicaciones, tales<br />
servicios serán necesarios para justificar <strong>la</strong> red <strong>de</strong> banda ancha a insta<strong>la</strong>rse <strong>en</strong> el<br />
futuro, al tiempo que serán los que t<strong>en</strong>drán que g<strong>en</strong>erar los necesarios b<strong>en</strong>eficios<br />
económicos. Esta nueva red t<strong>en</strong>drá <strong>la</strong> flexibilidad necesaria para ofrecer al<br />
usuario servicios <strong>de</strong> multimedia, si<strong>en</strong>do éste un instrum<strong>en</strong>to multifuncional que<br />
podrá soportar una combinación <strong>de</strong> comunicación <strong>de</strong> datos, voz y vi<strong>de</strong>o para un<br />
gran número <strong>de</strong> aplicaciones incluy<strong>en</strong>do teléfono, vi<strong>de</strong>oteléfono, y telefax.<br />
Un estudio realizado <strong>en</strong> 1991 por Arthur D. Little sugiere que <strong>la</strong>s re<strong>de</strong>s <strong>de</strong><br />
comunicación repres<strong>en</strong>tan el "carburante alternativo". El estudio concluye que si
<strong>la</strong>s teleconfer<strong>en</strong>cias, telecompras, teletrabajo, e intercambio electrónico <strong>de</strong><br />
docum<strong>en</strong>tos reemp<strong>la</strong>za <strong><strong>de</strong>l</strong> diez al veinte por ci<strong>en</strong>to <strong><strong>de</strong>l</strong> transporte <strong>en</strong> los<br />
Estados Unidos, se eliminarían: el transporte diario <strong>de</strong> seis millones <strong>de</strong> viajeros,<br />
aproximadam<strong>en</strong>te tresci<strong>en</strong>tos mil millones <strong>de</strong> peticiones <strong>de</strong> compra anuales, casi<br />
trece millones <strong>de</strong> viajes <strong>de</strong> negocios al año, mas <strong>de</strong> noveci<strong>en</strong>tos millones <strong>de</strong><br />
kilómetros <strong>de</strong> fletes (p.ej. correo) por tierra y por aire anuales. Por tanto el ahorro<br />
económico que ésto implica es una razón más que sufici<strong>en</strong>te como para realizar<br />
un análisis <strong>de</strong> <strong>la</strong>s posibilida<strong>de</strong>s y nuevas tecnologías <strong>de</strong> comunicación que exist<strong>en</strong><br />
actualm<strong>en</strong>te, y como se <strong>la</strong>s pue<strong>de</strong> adaptar a nuestro medio, utilizando los recursos<br />
exist<strong>en</strong>tes y adaptándolos a <strong>la</strong> tecnología <strong>de</strong> punta.<br />
Con este gran panorama, el pres<strong>en</strong>te trabajo ti<strong>en</strong>e por objeto estudiar una <strong>de</strong> <strong>la</strong>s<br />
aplicaciones <strong>de</strong> <strong>la</strong>s telecomunicaciones actuales como es <strong>la</strong> vi<strong>de</strong>o- telefonía, <strong>la</strong><br />
tecnología utilizada, <strong>la</strong>s normas y protocolos por los que esta regido y el campo<br />
<strong>de</strong> aplicación que se le pue<strong>de</strong> dar <strong>en</strong> nuestro medio.
CAPITULO I<br />
1.- PRINCIPIOS DE FUNCIONAMIENTO<br />
1.1 DESCRIPCIÓN GENERAL DE LOS SISTEMAS DE VIDEO TELÉFONO<br />
DIGITALES.-<br />
1.1.1 INTRODUCCIÓN<br />
Tomando <strong>en</strong> cu<strong>en</strong>ta que se han hecho gran<strong>de</strong>s esfuerzos a esca<strong>la</strong> mundial para<br />
e<strong>la</strong>borar equipos <strong>de</strong> ví<strong>de</strong>o telefonía basándose principalm<strong>en</strong>te <strong>en</strong> <strong>la</strong>s rápidas<br />
mejoras <strong>de</strong> <strong>la</strong> calidad <strong>de</strong> los algoritmos <strong><strong>de</strong>l</strong> codificador-<strong>de</strong>codificador (co<strong>de</strong>e) tanto<br />
<strong>de</strong> audio como <strong>de</strong> vi<strong>de</strong>o, tomando <strong>en</strong> cu<strong>en</strong>ta a<strong>de</strong>más que ya están a <strong>la</strong> v<strong>en</strong>ta<br />
ví<strong>de</strong>o teléfonos <strong>de</strong> algunas compañías y se han realizado <strong>la</strong>s primeras pruebas<br />
<strong>de</strong> ví<strong>de</strong>o teléfonos a esca<strong>la</strong> nacional e internacional; sabi<strong>en</strong>do a<strong>de</strong>más que <strong>la</strong> Red<br />
Digital <strong>de</strong> Servicios Integrados (RDSI) será apropiada para ofrecer éste servicio, y<br />
que <strong>en</strong> nuestro país ya se están realizando <strong>la</strong>s primeras pruebas <strong>de</strong> <strong>la</strong> RDSI, el<br />
pres<strong>en</strong>te capítulo t<strong>en</strong>drá por objeto realizar una <strong>de</strong>scripción g<strong>en</strong>eral <strong>de</strong> esta<br />
tecnología así como también <strong>de</strong> <strong>la</strong>s normas y protocolos por los que está regida.<br />
1.1.2 SISTEMAS VÍDEO TELEFÓNICOS .-<br />
El sistema ví<strong>de</strong>o telefónico es un sistema audiovisual, bidireccional que trabaja <strong>en</strong><br />
tiempo real, pudi<strong>en</strong>do transmitir voz e imág<strong>en</strong>es fijas o <strong>en</strong> movimi<strong>en</strong>to. La<br />
información visual transmitida es sufici<strong>en</strong>te como para repres<strong>en</strong>tar <strong>de</strong> manera<br />
a<strong>de</strong>cuada y con flui<strong>de</strong>z los movimi<strong>en</strong>tos <strong>de</strong> <strong>la</strong>s personas. Los sistemas<br />
unidireccionales <strong>de</strong> aplicación especial, como por ejemplo los sistemas <strong>de</strong><br />
vigi<strong>la</strong>ncia y algunos sistemas <strong>de</strong> recuperación <strong>de</strong> información, o el servicio <strong>de</strong><br />
ví<strong>de</strong>o confer<strong>en</strong>cia sin retorno, pue<strong>de</strong>n consi<strong>de</strong>rarse casos simplificados <strong><strong>de</strong>l</strong><br />
servicio ví<strong>de</strong>o telefónico.
1.1.2.1 Descripción <strong><strong>de</strong>l</strong> servicio.-<br />
El servicio ví<strong>de</strong>o telefónico es utilizado <strong>de</strong> manera muy simi<strong>la</strong>r al servicio telefónico<br />
vocal, con <strong>la</strong> v<strong>en</strong>taja <strong>de</strong> que los interlocutores pue<strong>de</strong>n verse, lo que supone varias<br />
nuevas aplicaciones. El servicio ví<strong>de</strong>o telefónico básico se caracteriza por <strong>la</strong><br />
transmisión continua <strong>de</strong> imág<strong>en</strong>es <strong>en</strong> movimi<strong>en</strong>to simultáneam<strong>en</strong>te con <strong>la</strong> voz <strong>de</strong> <strong>la</strong><br />
persona que participa <strong>en</strong> <strong>la</strong> l<strong>la</strong>mada mediante uno o dos canales a 64 Kbis/s. Se<br />
prevé que se pueda realizar dos tipos distintos <strong>de</strong> l<strong>la</strong>madas: l<strong>la</strong>madas <strong>de</strong> aparato a<br />
aparato (exig<strong>en</strong>cia básica) y l<strong>la</strong>madas multipunto, para ésta última opción se<br />
necesita una c<strong>en</strong>tral para mezc<strong>la</strong>r <strong>la</strong>s señales vocales y conmutar <strong>la</strong>s señales <strong>de</strong><br />
ví<strong>de</strong>o (esta opción todavía no se <strong>en</strong>cu<strong>en</strong>tra normalizada por el CCITT), <strong>la</strong><br />
comunicación ví<strong>de</strong>o telefónica multipunto se consi<strong>de</strong>ra un servicio suplem<strong>en</strong>tario.<br />
En este estudio es importante consi<strong>de</strong>rar el hecho <strong>de</strong> que <strong>en</strong> el servicio <strong>de</strong> vi<strong>de</strong>o<br />
teléfono, el número <strong>de</strong> abonados a este servicio, comparado con el número <strong>de</strong><br />
abonados <strong><strong>de</strong>l</strong> teléfono normal será evi<strong>de</strong>ntem<strong>en</strong>te inferior, por lo tanto estos<br />
aparatos <strong>de</strong>b<strong>en</strong> ser totalm<strong>en</strong>te compatibles con los teléfonos normales a fin <strong>de</strong><br />
evitar que los abonados al ví<strong>de</strong>o teléfono puedan comunicarse únicam<strong>en</strong>te <strong>de</strong>ntro<br />
<strong>de</strong> un grupo cerrado <strong>de</strong> usuarios, esta condición <strong>de</strong> compatibilidad <strong>de</strong>be cumplirse<br />
in<strong>de</strong>p<strong>en</strong>di<strong>en</strong>tem<strong>en</strong>te <strong>de</strong> <strong>la</strong> tecnología usada (análoga, digital, RDSI) <strong>en</strong> <strong>la</strong> c<strong>en</strong>tra!<br />
local a <strong>la</strong> que esté conectado el abonado.<br />
Para <strong>la</strong> intercomunicación <strong>en</strong>tre servicios ví<strong>de</strong>o telefónicos distintos, y con<br />
velocida<strong>de</strong>s <strong>de</strong> transmisión difer<strong>en</strong>tes, <strong>la</strong> comunicación básica se realizará con <strong>la</strong>s<br />
calida<strong>de</strong>s audiovisuales binarias más bajas.<br />
Al utilizar <strong>la</strong> RDSI como medio <strong>de</strong> comunicación, se pue<strong>de</strong> utilizar el protocolo<br />
2B+D (dos canales B más un canal D), <strong>en</strong> los cuales <strong>la</strong> comunicación <strong>de</strong> voz se <strong>la</strong><br />
transmite por el un canal B, <strong>la</strong> información <strong>de</strong> vi<strong>de</strong>o por el segundo canal B (<strong>de</strong><br />
ser necesario se pue<strong>de</strong> utilizar también parte <strong><strong>de</strong>l</strong> primer canal B para vi<strong>de</strong>o), y <strong>la</strong><br />
información <strong>de</strong> datos, señalización y sincronismo <strong>de</strong> señal, por ei canal D.
Para el proceso <strong>de</strong> liberación <strong>de</strong> l<strong>la</strong>madas, éste <strong>de</strong>be ser simi<strong>la</strong>r a <strong>la</strong> liberación <strong>de</strong><br />
una l<strong>la</strong>mada telefónica; <strong>la</strong> imag<strong>en</strong> y el sonido se liberan simultáneam<strong>en</strong>te.<br />
1.1.2.2 Facilida<strong>de</strong>s <strong>de</strong>servicio:<br />
Un servicio ví<strong>de</strong>o telefónico ofrece <strong>la</strong>s sigui<strong>en</strong>tes facilida<strong>de</strong>s:<br />
a) Transmisión <strong>de</strong> imág<strong>en</strong>es animadas tales como <strong>la</strong>s <strong><strong>de</strong>l</strong> busto <strong>de</strong> una<br />
persona o <strong>de</strong> un pequeño grupo <strong>de</strong> personas con una resolución<br />
mo<strong>de</strong>rada <strong>en</strong> cuanto a movimi<strong>en</strong>to.<br />
b) Transmisión <strong>de</strong> pa<strong>la</strong>bra asociada.<br />
c) Transmisión <strong>de</strong> información gráfica, tal como dibujos y docum<strong>en</strong>tos, con<br />
bu<strong>en</strong>a <strong>de</strong>finición.<br />
d) Servicio <strong>de</strong> ví<strong>de</strong>o confer<strong>en</strong>cia aplicando o no técnicas <strong>de</strong> división <strong>de</strong><br />
pantal<strong>la</strong>.<br />
1.1.2.3 Características <strong>de</strong> calidad <strong><strong>de</strong>l</strong> servicio vi<strong>de</strong>o telefónico.-<br />
- Se <strong>de</strong>be mant<strong>en</strong>er sincronismo <strong>de</strong> pa<strong>la</strong>bra y <strong>de</strong> movimi<strong>en</strong>to <strong>de</strong> los <strong>la</strong>bios<br />
(sincronismo <strong>la</strong>bial). La difer<strong>en</strong>cia no <strong>de</strong>be ser discernible subjetivam<strong>en</strong>te<br />
tanto <strong><strong>de</strong>l</strong> retardo <strong>de</strong> <strong>la</strong> pa<strong>la</strong>bra como <strong>de</strong> <strong>la</strong> señal <strong>de</strong> ví<strong>de</strong>o.<br />
- Debe contar con una calidad sonora lo sufici<strong>en</strong>tem<strong>en</strong>te aceptable como para<br />
\e se escuche con c<strong>la</strong>ridad <strong>la</strong> inte<br />
- La calidad vocal <strong>de</strong> este nuevo servicio <strong>de</strong>be ser por Ib m<strong>en</strong>os tan bu<strong>en</strong>a<br />
como <strong>la</strong> que se aplica al servicio telefónico RDSI <strong>de</strong> 64 Kt>¡t/s<br />
1.1.2.4 Parámetros <strong><strong>de</strong>l</strong> Sistema Vi<strong>de</strong>o Telefónico. -<br />
Normas <strong>de</strong> imag<strong>en</strong>:<br />
Las normas <strong>de</strong> ví<strong>de</strong>o <strong>de</strong> los aparatos <strong>de</strong> abonado serán compatibles con <strong>la</strong>s<br />
normas locales <strong>de</strong> radiodifusión y televisión, o fácilm<strong>en</strong>te convertibles a éstas.
Se recomi<strong>en</strong>dan <strong>la</strong>s dos c<strong>la</strong>ses <strong>de</strong> normas <strong>de</strong> imag<strong>en</strong> para el sistema ví<strong>de</strong>o<br />
telefónico que se muestran <strong>en</strong> el cuadro 1.1.<br />
Las normas <strong>de</strong> "c<strong>la</strong>se a" dan <strong>en</strong> <strong>la</strong> mayoría <strong>de</strong> los casos sufici<strong>en</strong>te <strong>de</strong>finición para<br />
<strong>la</strong> transmisión <strong>de</strong> imág<strong>en</strong>es <strong>en</strong> tiempo real <strong>de</strong> un grupo <strong>de</strong> personas (por ejemplo,<br />
<strong>en</strong> vi<strong>de</strong>o confer<strong>en</strong>cias) y <strong>de</strong> docum<strong>en</strong>tos gráficos.<br />
Las normas <strong>de</strong> "c<strong>la</strong>se b" dan sufici<strong>en</strong>te <strong>de</strong>finición para <strong>la</strong> transmisión <strong>en</strong> tiempo<br />
real <strong>de</strong> <strong>la</strong> imag<strong>en</strong> <strong><strong>de</strong>l</strong> busto <strong>de</strong> una persona o <strong>de</strong> un pequeño grupo. Para <strong>la</strong><br />
transmisión <strong>de</strong> información gráfica u otras imág<strong>en</strong>es fijas con bu<strong>en</strong>a <strong>de</strong>finición<br />
<strong>de</strong>berá aplicarse una técnica <strong>de</strong> exploración rápida, por ejemplo, un sistema <strong>de</strong><br />
625 ó 525 líneas horizontales <strong>de</strong> exploración y <strong>de</strong> 25 o 30 cuadros por segundo,<br />
que permita una <strong>de</strong>finición <strong>de</strong> c<strong>la</strong>se a <strong>en</strong> una anchura <strong>de</strong> banda <strong>de</strong> 1 MHz.<br />
C<strong>la</strong>se<br />
h<br />
Parámetros<br />
Numero <strong>de</strong> líneas horizontales <strong>de</strong><br />
explotación<br />
Imág<strong>en</strong>es por segundo<br />
Anchura <strong>de</strong> banda <strong>de</strong> vi<strong>de</strong>o<br />
Numero <strong>de</strong> líneas horizontales <strong>de</strong><br />
explotación<br />
Imág<strong>en</strong>es por segundo<br />
Anchura <strong>de</strong> banda <strong>de</strong> ví<strong>de</strong>o<br />
Cuadro 1.1 Normas <strong>de</strong> imag<strong>en</strong> para el sistema ví<strong>de</strong>o telefónico<br />
Región a <strong>la</strong> que se aplican los<br />
valores<br />
Regiones don<strong>de</strong> se aplican<br />
normas <strong>de</strong> difusión <strong>de</strong> <strong>la</strong><br />
televisión <strong>de</strong> 25 cuadros por<br />
segundo<br />
625<br />
25<br />
( <strong>en</strong>tre<strong>la</strong>zado 2:1 )<br />
5MHz<br />
313<br />
25<br />
( <strong>en</strong>tre<strong>la</strong>zado 2:1 )<br />
1 MHz<br />
Región a <strong>la</strong> que se aplican<br />
los valores<br />
Región don<strong>de</strong> se aplican<br />
normas <strong>de</strong> difusión <strong>de</strong> 30<br />
cuadros por segundo<br />
525<br />
30<br />
(<strong>en</strong>tre<strong>la</strong>zado 2:1 )<br />
4MHz<br />
263<br />
30<br />
(<strong>en</strong>tre<strong>la</strong>zado 2:1 )<br />
1 MHz
1.2 NORMAS DE COMUNICACION.-<br />
Los compon<strong>en</strong>tes <strong><strong>de</strong>l</strong> sistema <strong>en</strong> el ví<strong>de</strong>o teléfono están regidos por <strong>la</strong> norma<br />
H.320 <strong><strong>de</strong>l</strong> CCITT (Fig. 1.1) <strong>la</strong> misma que <strong>en</strong>globa una serie <strong>de</strong> normas adicionales<br />
que <strong>de</strong>scrib<strong>en</strong> y regu<strong>la</strong>n <strong>la</strong>s difer<strong>en</strong>tes partes <strong>de</strong> <strong>la</strong>s que está compuesto un ví<strong>de</strong>o<br />
teléfono.<br />
Así pues, se ti<strong>en</strong>e por ejemplo que <strong>la</strong> norma H.261 <strong><strong>de</strong>l</strong> 00ITT <strong>de</strong>scribe los<br />
algoritmos <strong>de</strong> codificación y <strong>de</strong>codificación <strong>de</strong> imág<strong>en</strong>es móviles, ello asegura que<br />
<strong>la</strong>s imág<strong>en</strong>es se puedan transmitir <strong>en</strong>tre dispositivos <strong>de</strong> difer<strong>en</strong>tes fabricantes.<br />
Una norma posterior que juega un importante papel <strong>en</strong> re<strong>la</strong>ción con <strong>la</strong><br />
comunicación es <strong>la</strong> H.321 <strong>la</strong> cual <strong>de</strong>fine <strong>la</strong> estructura <strong>de</strong> <strong>la</strong> trama <strong>de</strong> un protocolo<br />
para servicios audiovisuales con velocida<strong>de</strong>s <strong>de</strong> transmisión <strong>en</strong>tre 64 Kbits/s y<br />
1920 Kbits/s.<br />
La norma H.242 se usa <strong>en</strong> el ví<strong>de</strong>o teléfono para establecer y liberar <strong>la</strong> conexión<br />
<strong>en</strong>tre dos dispositivos terminales, transmitir datos durante <strong>la</strong> conexión y reaccionar<br />
fr<strong>en</strong>te a posibles errores, esta norma <strong>de</strong>scribe <strong>la</strong> señalización <strong>de</strong>ntro <strong>de</strong> banda.<br />
La norma H.230 <strong>de</strong>scribe el intercambio <strong>de</strong> tramas sincronizadas <strong>de</strong> <strong>la</strong> información<br />
<strong>de</strong> control usando <strong>la</strong> estructura <strong>de</strong> trama <strong>de</strong>finida <strong>en</strong> <strong>la</strong> norma H.221.1<br />
1.2.1.- NORMA H.320.-<br />
Como se m<strong>en</strong>cionó anteriorm<strong>en</strong>te, esta norma <strong>en</strong>globa y <strong>de</strong>scribe los<br />
compon<strong>en</strong>tes por los que está constituido un vi<strong>de</strong>o teléfono. A continuación se<br />
hace una <strong>de</strong>scripción g<strong>en</strong>eral <strong>de</strong> esta norma:<br />
1 En el capítulo <strong>de</strong> ANEXOS se pue<strong>de</strong> <strong>en</strong>contrar <strong>la</strong>s normas y recom<strong>en</strong>daciones mas importantes a <strong>la</strong>s que se<br />
hace refer<strong>en</strong>cia <strong>en</strong> el pres<strong>en</strong>te capítulo y <strong>en</strong> los posteriores.
MCU: Unidad <strong>de</strong> control Multipunto<br />
Fig. 1.1 Diagrama <strong>de</strong> bloques para ví<strong>de</strong>o teléfonos según <strong>la</strong> norma H:320.<br />
1.2.1.1 Diagrama <strong>de</strong> bloques e i<strong>de</strong>ntificación <strong>de</strong> elem<strong>en</strong>tos.-<br />
Una visión g<strong>en</strong>eral <strong><strong>de</strong>l</strong> ví<strong>de</strong>o teléfono se muestra <strong>en</strong> <strong>la</strong> figura 1.1; este consiste <strong>de</strong><br />
equipos terminales, re<strong>de</strong>s, unida<strong>de</strong>s <strong>de</strong> control multipunto (MCU), y otras <strong>en</strong>tida<strong>de</strong>s<br />
<strong><strong>de</strong>l</strong> sistema operativo. La configuración <strong>de</strong> este sistema se <strong>de</strong>tal<strong>la</strong> a continuación:<br />
• Equipo <strong>de</strong> <strong>en</strong>trada y salida <strong>de</strong> ví<strong>de</strong>o, que incluye cámaras, monitores y<br />
unida<strong>de</strong>s <strong>de</strong> procesami<strong>en</strong>to <strong>de</strong> ví<strong>de</strong>o.<br />
• Equipo <strong>de</strong> <strong>en</strong>trada y salida <strong>de</strong> audio, que incluye micrófonos, par<strong>la</strong>ntes, y<br />
unida<strong>de</strong>s <strong>de</strong> procesami<strong>en</strong>to <strong>de</strong> audio que prove<strong>en</strong> algunas funciones como<br />
eliminación <strong><strong>de</strong>l</strong> eco acústico.<br />
• Equipos telemáticos que son ayudas audiovisuales tales como pizarras<br />
electrónicas, que mejoran <strong>la</strong>s comunicaciones básicas <strong>en</strong> un ví<strong>de</strong>o teléfono.
• Control <strong>de</strong> Sistema: unidad que cumple algunas funciones como el acceso <strong>de</strong><br />
red a través <strong><strong>de</strong>l</strong> bloque <strong>de</strong> señalización terminal a red y control <strong>de</strong> terminal a<br />
terminal, para establecer un modo común <strong>de</strong> operación que <strong>de</strong> una apropiada<br />
función <strong><strong>de</strong>l</strong> terminal a través <strong><strong>de</strong>l</strong> bloque <strong>de</strong> señalización <strong>de</strong> terminal a terminal.<br />
• Co<strong>de</strong>e <strong>de</strong> Ví<strong>de</strong>o que lleva códigos <strong>de</strong> reducción <strong>de</strong> redundancia y<br />
<strong>de</strong>codificación para señales <strong>de</strong> ví<strong>de</strong>o, mi<strong>en</strong>tras que los co<strong>de</strong>e <strong>de</strong> audio hac<strong>en</strong> lo<br />
mismo para señales <strong>de</strong> audio. Retrasos <strong>en</strong> <strong>la</strong>s transmisiones <strong>de</strong> vi<strong>de</strong>o, son<br />
comp<strong>en</strong>sados con retrasos <strong>en</strong> los co<strong>de</strong>e <strong>de</strong> audio para mant<strong>en</strong>er los picos <strong>de</strong><br />
sincronización.<br />
• La unidad Mux/Dmux multiplexa señales <strong>de</strong> ví<strong>de</strong>o, audio, datos y señales <strong>de</strong><br />
control <strong>en</strong> un bit stream simple; y <strong>de</strong>multiplexa <strong>en</strong> un bit stream <strong>de</strong> recepción <strong>de</strong><br />
señales multimedia coher<strong>en</strong>tes. El ¡nterfaz <strong>de</strong> red hace adaptaciones<br />
necesarias <strong>en</strong>tre <strong>la</strong> red y e! terminal a<strong>de</strong>cuado<br />
1.2.1.2 Señales.-<br />
Las señales <strong>de</strong> un ví<strong>de</strong>o teléfono se pue<strong>de</strong>n c<strong>la</strong>sificar <strong>en</strong>: señales <strong>de</strong> ví<strong>de</strong>o, <strong>de</strong><br />
audio, <strong>de</strong> datos y control.<br />
- Las señales <strong>de</strong> audio son para tráfico continuo, y son requeridas para una<br />
transmisión <strong>en</strong> tiempo real.<br />
- Las señales <strong>de</strong> ví<strong>de</strong>o son también para tráfico continuo, el ritmo <strong>de</strong><br />
transmisión para estas señales <strong>de</strong>be ser tan alto como sea posible, para<br />
maximizar <strong>la</strong> calidad <strong>de</strong>ntro <strong>de</strong> <strong>la</strong> capacidad disponible <strong><strong>de</strong>l</strong> canal.<br />
- Las señales <strong>de</strong> datos (por ejemplo texto) por facilidad pue<strong>de</strong>n ocurrir solo<br />
ocasionalm<strong>en</strong>te cuando son estrictam<strong>en</strong>te requeridas y pue<strong>de</strong>n <strong>de</strong>sp<strong>la</strong>zarse<br />
temporalm<strong>en</strong>te por cualquier parte <strong>de</strong> <strong>la</strong> pantal<strong>la</strong>. Estas señales <strong>de</strong> datos<br />
están asociadas solo para mejoras opcionales <strong>en</strong> el sistema básico <strong><strong>de</strong>l</strong>
ví<strong>de</strong>o teléfono, por consigui<strong>en</strong>te, <strong>la</strong> apertura <strong>de</strong> una ruta para llevar tales<br />
señales, es precedida por gestiones <strong>en</strong>tre los terminales.<br />
- Señales <strong>de</strong> control.- La ruta para <strong>la</strong>s señales <strong>de</strong> control <strong>de</strong> terminal-a-red<br />
son suministradas <strong>en</strong> el canal-D, mi<strong>en</strong>tras que <strong>la</strong> ruta para <strong>la</strong>s señales <strong>de</strong><br />
control <strong>de</strong> terminal-a-terminal son suministradas <strong>en</strong> el BAS (señal <strong>de</strong><br />
asignación <strong>de</strong> velocidad binaria <strong>de</strong> 8 bits) o canal <strong>de</strong> servicio.<br />
1.2.1.3 Velocidad binaria - Opciones e infraestructura,-<br />
Los modos <strong>de</strong> comunicación <strong>de</strong> un ví<strong>de</strong>o teléfono se <strong>de</strong>fin<strong>en</strong> <strong>en</strong> <strong>la</strong> tab<strong>la</strong> #1.1 <strong>de</strong><br />
acuerdo con su configuración y código <strong>de</strong> canal.<br />
Modos para un<br />
Ví<strong>de</strong>o Teléfono<br />
A<br />
B<br />
Q<br />
(nota 3)<br />
Notas:<br />
9<br />
h<br />
i<br />
i<br />
k<br />
1<br />
m<br />
ao<br />
ai<br />
bi<br />
b2<br />
b3<br />
qi<br />
qa<br />
qs<br />
Tab<strong>la</strong> 1.1<br />
Modos <strong>de</strong> Comunicación para un Ví<strong>de</strong>o Teléfono<br />
Velocidad <strong>de</strong><br />
Canal<br />
(Kb¡t/s)<br />
64<br />
128<br />
nx64<br />
384<br />
768<br />
1152<br />
1536<br />
1536<br />
1920<br />
1920<br />
Canales<br />
RDSI<br />
(nota 2)<br />
B<br />
2B<br />
nB<br />
Ho<br />
2H0<br />
3H0<br />
4H0<br />
HH<br />
5H0<br />
H 12<br />
Básico<br />
No<br />
Aplicables<br />
InterfazRDSI<br />
Ritmo<br />
Primario<br />
Aplicable<br />
Audio<br />
Código<br />
Rec.G.711 (Npta4)<br />
Rea G.728<br />
Rea G.711<br />
Rea G.722<br />
Rea G.728<br />
Rea G.711<br />
Rea G.722<br />
Rea G.728<br />
Rea G.722<br />
(Nota 5)<br />
1. (Codificación <strong>de</strong> audio <strong>de</strong> modo b3) Adicionalm<strong>en</strong>te a lo indicado <strong>en</strong> G.728, una codificación<br />
<strong>de</strong> audio <strong>de</strong> alta calidad como <strong>la</strong> H.200/AV.253 pue<strong>de</strong> ser usada para este modo.<br />
10<br />
Ví<strong>de</strong>o<br />
No<br />
aplicable<br />
Rea H.261
2. Para múltiples canales <strong>de</strong> B/H0| todos los canales son sincronizados <strong>en</strong> el terminal <strong>de</strong> acuerdo<br />
con 2.7/H.221<br />
3. q = c/d/e/f correspondi<strong>en</strong>tem<strong>en</strong>te para n = 3/4/5/6, respectivam<strong>en</strong>te.<br />
4. Si un ví<strong>de</strong>o teléfono se interreiaciona con un terminal tipo G.725, un terminal G.722 <strong>de</strong> audio<br />
pue<strong>de</strong> ser usado <strong>en</strong> lugar <strong>de</strong> un terminal G.711 <strong>de</strong> audio.<br />
5. Otros modos (G.711 y G.728 audio) que han sido recom<strong>en</strong>dados pue<strong>de</strong>n ser incluidos por el<br />
procedimi<strong>en</strong>to <strong>de</strong>scrito <strong>en</strong> H.242.<br />
1.2.1.4 Tipos <strong>de</strong> terminal <strong>de</strong> un ví<strong>de</strong>o teléfono.-<br />
La lista indicada <strong>en</strong> <strong>la</strong> tab<strong>la</strong> 1.2 indica el tipo <strong>de</strong> terminal usado por un ví<strong>de</strong>o<br />
teléfono. La categoría <strong><strong>de</strong>l</strong> tipo <strong>de</strong> terminal va <strong>de</strong> acuerdo con los modos <strong>de</strong><br />
comunicación y <strong><strong>de</strong>l</strong> tipo <strong>de</strong> canales <strong>de</strong> comunicación con los cuales el terminal<br />
pue<strong>de</strong> comunicarse, así pues t<strong>en</strong>emos por ejemplo: mxB (tipo X con parámetros<br />
a-f), nxHo (tipo Y con parámetros 1-5; ver nota), Hn/Hi2 (tipo Z con parámetros<br />
a-(3) o sus combinaciones.<br />
Modo<br />
Ritmo <strong>de</strong><br />
Transfer<strong>en</strong>cia<br />
a0 B<br />
at B<br />
o, 2B<br />
b2 28<br />
b3 2B<br />
q! Nb<br />
q2 Nb<br />
q3 nB<br />
g HO<br />
n 2H0<br />
i 3H0<br />
] 4H0<br />
k Hn<br />
I 5H0<br />
m Hi7<br />
Codificación<br />
<strong>de</strong> Audio<br />
G.711<br />
G.728<br />
G.711<br />
G.722<br />
G.728<br />
G.711 (Nota 4)<br />
G.722 (Nota 4)<br />
G.728 (Nota 4)<br />
G.722<br />
G.722<br />
G.722<br />
G.722<br />
G.722<br />
G.722<br />
G.722<br />
Tab<strong>la</strong> 1.2<br />
Tipos <strong>de</strong> Terminal para un Ví<strong>de</strong>o Teléfono<br />
Tipo X (nota 2)<br />
A<br />
X<br />
X<br />
.<br />
.<br />
.<br />
.<br />
.<br />
.<br />
b1<br />
X<br />
X<br />
X<br />
.<br />
X<br />
.<br />
.<br />
-<br />
.<br />
b2/3<br />
X<br />
X<br />
X<br />
X<br />
X<br />
.<br />
.<br />
.<br />
.<br />
b4<br />
X<br />
.<br />
X<br />
.<br />
.<br />
.<br />
b5<br />
X<br />
.<br />
X<br />
X<br />
.<br />
.<br />
.<br />
.<br />
.<br />
.<br />
.<br />
q1<br />
X<br />
X<br />
X<br />
X<br />
X<br />
X<br />
.<br />
.<br />
.<br />
q2/3 q4<br />
X X<br />
X<br />
X<br />
X<br />
X<br />
X<br />
X<br />
X<br />
.<br />
.<br />
.<br />
X<br />
.<br />
X<br />
.<br />
.<br />
.<br />
q5<br />
•<br />
X<br />
X<br />
.<br />
X<br />
X<br />
.<br />
.<br />
1<br />
•<br />
.<br />
.<br />
.<br />
.<br />
X<br />
.<br />
.<br />
Tipo Y (Nota 3)<br />
2<br />
•<br />
.<br />
.<br />
.<br />
.<br />
.<br />
.<br />
X<br />
X<br />
.<br />
.<br />
3<br />
-<br />
.<br />
.<br />
.<br />
.<br />
.<br />
X<br />
X<br />
X<br />
.<br />
4<br />
.<br />
.<br />
-<br />
.<br />
.<br />
X<br />
X<br />
X<br />
X<br />
5<br />
•<br />
.<br />
.<br />
.<br />
.<br />
X<br />
X<br />
X<br />
X<br />
X<br />
11<br />
Tipo Z<br />
a<br />
•<br />
.<br />
.<br />
.<br />
.<br />
.<br />
.<br />
.<br />
.<br />
X<br />
.<br />
P<br />
•<br />
.<br />
.<br />
.<br />
.<br />
.<br />
.<br />
.<br />
X
1) Sin transmisión <strong>de</strong> ví<strong>de</strong>o: En <strong>la</strong> fase B1-2 y (si es pertin<strong>en</strong>te) <strong>en</strong> <strong>la</strong> fase<br />
CB1-2, el modo seleccionado incluye <strong>la</strong> posibilidad <strong>de</strong> apagado <strong>de</strong> ví<strong>de</strong>o.<br />
Durante <strong>la</strong> fase C cada usuario uni<strong>la</strong>teraim<strong>en</strong>te pue<strong>de</strong> seleccionar el modo<br />
<strong>de</strong> <strong>en</strong>c<strong>en</strong>dido <strong>de</strong> ví<strong>de</strong>o, alternativam<strong>en</strong>te, el terminal X pue<strong>de</strong> <strong>en</strong>viar el C&!<br />
(control e indicador.- señalización terminal a terminal que indica e informa<br />
cual es el funcionami<strong>en</strong>to <strong><strong>de</strong>l</strong> sistema), el BAS, el código VIR (indicador <strong>de</strong><br />
listo para activar vi<strong>de</strong>o), pero no se pue<strong>de</strong> seleccionar el modo <strong>de</strong><br />
<strong>en</strong>c<strong>en</strong>dido <strong>de</strong> ví<strong>de</strong>o hasta que el VIR se habilite <strong>de</strong>s<strong>de</strong> el terminal Y.<br />
Mi<strong>en</strong>tras permanezca el estado <strong>de</strong> ví<strong>de</strong>o <strong>en</strong> apagado, <strong>la</strong> pantal<strong>la</strong> <strong><strong>de</strong>l</strong> ví<strong>de</strong>o<br />
teléfono solo muestra símbolos o m<strong>en</strong>sajes que indican este estado (Ej.<br />
There is no fault).<br />
2) Configuración para <strong>la</strong> transmisión <strong>de</strong> ví<strong>de</strong>o.- Se pue<strong>de</strong> transmitir <strong>de</strong>s<strong>de</strong> una<br />
cámara normal <strong>de</strong> vi<strong>de</strong>o u otra configuración simi<strong>la</strong>r, <strong>en</strong> lugar <strong>de</strong> <strong>la</strong> señal <strong>de</strong><br />
un vi<strong>de</strong>o teléfono. Para esto, el C&! BAS código VIS (indicador <strong>de</strong><br />
susp<strong>en</strong>ción <strong>de</strong> ví<strong>de</strong>o) es utilizado para indicar <strong>la</strong> situación <strong>de</strong> remoto.<br />
3) Transmisión <strong>de</strong> ví<strong>de</strong>o pero sin imag<strong>en</strong> <strong>de</strong>spiegabfe.- Un procedimi<strong>en</strong>to<br />
simple <strong>en</strong> el terminal involucra una acción local para mostrar, no <strong>la</strong> señal<br />
<strong>en</strong>trante, pero si una explicación mediante m<strong>en</strong>sajes con símbolos, <strong>de</strong> que<br />
una imag<strong>en</strong> <strong>de</strong> ví<strong>de</strong>o pue<strong>de</strong> ser <strong>de</strong>splegada si el usuario así lo <strong>de</strong>sea. El<br />
usuario podría habilitar <strong>la</strong> señal <strong>en</strong>trante para que ésta pueda ser<br />
<strong>de</strong>splegada, pero esto <strong>de</strong>p<strong>en</strong><strong>de</strong>rá <strong>de</strong> una mutua acción <strong>de</strong> los usuarios,<br />
<strong>en</strong>tonces un nuevo código C&l BAS VDR (ví<strong>de</strong>o ready-to-disp<strong>la</strong>y) será<br />
<strong>de</strong>finido. Este punto esta tQcjaví^ so/netido a futuros estudios.<br />
17
1.3.- ALGORITMOS DE CODIFICACIÓN Y DECODIFICACION.-<br />
La mayoría <strong>de</strong> imág<strong>en</strong>es o aplicaciones <strong>de</strong> vi<strong>de</strong>o que están involucradas con <strong>la</strong><br />
transmisión o almac<strong>en</strong>ami<strong>en</strong>to requier<strong>en</strong> alguna forma <strong>de</strong> compresión <strong>de</strong> datos<br />
para reducir <strong>de</strong> cierta manera <strong>la</strong> <strong>de</strong>manda excesiva <strong>en</strong> el ancho <strong>de</strong> banda. El vi<strong>de</strong>o<br />
digital no comprimido requiere un ancho <strong>de</strong> banda <strong>de</strong> transmisión extremadam<strong>en</strong>te<br />
alto. La resolución <strong>de</strong> vi<strong>de</strong>o <strong>de</strong> <strong>la</strong> North American Televisión Standards Committee<br />
(NTSC), por ejemplo, ti<strong>en</strong>e un ritmo <strong>de</strong> transmisión promedio <strong>de</strong> aproximadam<strong>en</strong>te<br />
100 Mb/s. El grado <strong>de</strong> compresión requerido se lo consigue al explotar <strong>la</strong><br />
redundancia espacial y temporal pres<strong>en</strong>te <strong>en</strong> una señal <strong>de</strong> vi<strong>de</strong>o (estos tópicos se<br />
explican mas a<strong>de</strong><strong>la</strong>nte). Sin embargo <strong>la</strong>s pérdidas son inher<strong>en</strong>tes al proceso <strong>de</strong><br />
compresión, y por esto <strong>la</strong> señal reconstruida <strong><strong>de</strong>l</strong> flujo <strong>de</strong> bit comprimido no es<br />
idéntica a <strong>la</strong> señal <strong>de</strong> vi<strong>de</strong>o <strong>de</strong> ingreso. La compresión típicam<strong>en</strong>te introduce<br />
errores <strong>en</strong> <strong>la</strong> señal codificada.<br />
La compatibilidad <strong>en</strong>tre difer<strong>en</strong>tes aplicaciones y fabricantes es también muy<br />
<strong>de</strong>seable y a m<strong>en</strong>udo es<strong>en</strong>cial. A continuación se <strong>de</strong>scribe varios algoritmos <strong>de</strong><br />
compresión <strong>de</strong>sarrol<strong>la</strong>dos <strong>en</strong> años reci<strong>en</strong>tes:<br />
• La ISO (Organización Internacional <strong>de</strong> Normalización) - JBIG Joint Bilevel<br />
Image Group (El Consorcio <strong>de</strong> Imag<strong>en</strong> <strong>de</strong> Doble Nivel) ha perfeccionado un<br />
algoritmo <strong>de</strong> codificación progresiva que transmite estas imág<strong>en</strong>es <strong>en</strong> etapas<br />
<strong>de</strong> resolución más alta sucesivam<strong>en</strong>te. Las nuevas técnicas <strong>de</strong> codificación<br />
hac<strong>en</strong> posible suministrar esta capacidad progresiva, mi<strong>en</strong>tras que al mismo<br />
tiempo se consigue una mejor compresión <strong>de</strong> <strong>la</strong> que se obti<strong>en</strong>e por normas<br />
anteriores <strong>de</strong> codificación.<br />
• La ISO-JPEG Joint Photographic Expert Group (El Grupo <strong>de</strong> Expertos<br />
Fotográficos), ha <strong>de</strong>sarrol<strong>la</strong>do un algoritmo para <strong>la</strong> codificación <strong>de</strong> imág<strong>en</strong>es a<br />
color <strong>de</strong> recuadro simple. Este esta basado <strong>en</strong> <strong>la</strong> transformada discreta <strong><strong>de</strong>l</strong><br />
cos<strong>en</strong>o (DCT), pero también ti<strong>en</strong>e ext<strong>en</strong>siones para codificación progresiva.<br />
Empezando <strong>de</strong>s<strong>de</strong> un cuadro original rojo, ver<strong>de</strong>, azul (RGB) <strong>de</strong> 24 bits por
elem<strong>en</strong>to <strong>de</strong> cuadro (peí o pixel), los algoritmos JPEG dan una bu<strong>en</strong>a calidad<br />
<strong>de</strong> imag<strong>en</strong> con factores <strong>de</strong>'compresión <strong>de</strong> 10 a 20.<br />
• El (CCITT) - Study Group 15 (SG15) y su grupo <strong>de</strong> expertos <strong>en</strong> vi<strong>de</strong>o telefonía<br />
han <strong>de</strong>sarrol<strong>la</strong>do un conjunto <strong>de</strong> normas codificadas, conocida informalm<strong>en</strong>te<br />
como <strong>la</strong> norma Px64, para <strong>en</strong>viar cuadros <strong>de</strong> vi<strong>de</strong>o telefonía o vi<strong>de</strong>o<br />
confer<strong>en</strong>cia <strong>en</strong> una red con facilida<strong>de</strong>s <strong>de</strong> servicio digital integrado (RDSI), <strong>la</strong><br />
norma es aplicable a un rango <strong>de</strong> ancho <strong>de</strong> banda <strong>de</strong>s<strong>de</strong> los 56 Kb/s hasta los<br />
2 Mb/s, este cu<strong>en</strong>ta no solo con <strong>la</strong> DCT (transformada discreta <strong><strong>de</strong>l</strong> cos<strong>en</strong>o) sino<br />
también con <strong>la</strong> predicción comp<strong>en</strong>sada <strong>de</strong> movimi<strong>en</strong>to para <strong>la</strong> compresión <strong>de</strong><br />
datos g<strong>en</strong>erada por <strong>la</strong>s imág<strong>en</strong>es <strong>en</strong> movimi<strong>en</strong>to.<br />
• E! ISO - MPEG Motion Picture Experts Group (El Grupo <strong>de</strong> Expertos <strong>de</strong><br />
Figuras <strong>en</strong> Movimi<strong>en</strong>to) ha <strong>de</strong>sarrol<strong>la</strong>do algoritmos <strong>de</strong> compresión tanto <strong>de</strong><br />
vi<strong>de</strong>o como <strong>de</strong> audio que pue<strong>de</strong>n comprimir vi<strong>de</strong>o para transmisión y/o<br />
almac<strong>en</strong>ami<strong>en</strong>to <strong>en</strong> varios medios digitales incluy<strong>en</strong>do discos compactos,<br />
bases <strong>de</strong> datos <strong>de</strong> vi<strong>de</strong>os remotos, televisión por cable (CATV), fibras óptica,<br />
etc. El algoritmo MPEG utiliza toda <strong>la</strong> metodología Px64, así como también<br />
algunas nuevas técnicas.<br />
1.3.1 JBIG (CODIFICACIÓN DE IMAGEN PROGRESIVA DE DOBLE NIVEL)<br />
Esta sección pres<strong>en</strong>ta <strong>la</strong>s normas <strong>de</strong> codificación <strong>de</strong> imag<strong>en</strong> <strong>de</strong> doble nivel JBIG y<br />
como se re<strong>la</strong>ciona con <strong>la</strong>s otras normas, a<strong>de</strong>más se <strong>de</strong>scribe <strong>la</strong> codificación<br />
progresiva.<br />
1.3.1.1 Codificación Progresiva.-<br />
La codificación progresiva es codificación <strong>de</strong> multi resolución. Una imag<strong>en</strong> es<br />
capturada con un r<strong>en</strong>dimi<strong>en</strong>to <strong>de</strong> compresión <strong>de</strong> baja resolución más una<br />
secu<strong>en</strong>cia <strong>de</strong> (archivos que permit<strong>en</strong> duplicar <strong>la</strong> resolución <strong>de</strong> <strong>la</strong><br />
imag<strong>en</strong>). Cuando una imag<strong>en</strong> que ha sido codificada progresivam<strong>en</strong>te y se <strong>la</strong><br />
19
quiere <strong>de</strong>codificar, el r<strong>en</strong>dimi<strong>en</strong>to <strong>de</strong> baja resolución <strong>de</strong> <strong>la</strong> original llega a estar<br />
disponible primero, posteriorm<strong>en</strong>te se pue<strong>de</strong> duplicar <strong>la</strong> resolución seguido <strong>de</strong> una<br />
mayor cantidad <strong>de</strong> datos que son <strong>de</strong>codificados.<br />
La codificación progresiva ofrece <strong>la</strong>s sigui<strong>en</strong>tes v<strong>en</strong>tajas:<br />
• Almac<strong>en</strong>ami<strong>en</strong>to <strong>de</strong> imág<strong>en</strong>es <strong>en</strong> bases <strong>de</strong> datos que int<strong>en</strong>tan servir a pantal<strong>la</strong>s<br />
<strong>de</strong> difer<strong>en</strong>tes capacida<strong>de</strong>s <strong>de</strong> resolución.<br />
• Hojeo <strong>de</strong> imág<strong>en</strong>es <strong>de</strong> <strong>la</strong> base <strong>de</strong> datos <strong>en</strong> forma rápida y s<strong>en</strong>cil<strong>la</strong><br />
• Transmisión <strong>de</strong> imág<strong>en</strong>es sobre una red <strong>de</strong> paquetes<br />
AI almac<strong>en</strong>ar imág<strong>en</strong>es con codificación progresiva, una base <strong>de</strong> datos pue<strong>de</strong><br />
servir efici<strong>en</strong>tem<strong>en</strong>te a aparatos <strong>de</strong> salida que t<strong>en</strong>gan difer<strong>en</strong>tes capacida<strong>de</strong>s <strong>de</strong><br />
resolución. La base <strong>de</strong> datos <strong>en</strong>vía el código <strong>de</strong> r<strong>en</strong>dimi<strong>en</strong>to <strong>de</strong> baja resolución y<br />
so<strong>la</strong>m<strong>en</strong>te tantos archivos <strong><strong>de</strong>l</strong>ta como sean necesarios para recuperar <strong>la</strong> imag<strong>en</strong><br />
según <strong>la</strong> calidad que requiera <strong>la</strong> aplicación. Si un usuario primero mira una imag<strong>en</strong><br />
<strong>en</strong> una pantal<strong>la</strong> comparativam<strong>en</strong>te <strong>de</strong> baja resolución, tal como <strong>la</strong> <strong>de</strong> un tubo <strong>de</strong><br />
rayos catódicos (CRT), y más tar<strong>de</strong> requiere una copia impresa <strong>de</strong> más alta<br />
resolución, tal como <strong>en</strong> una impresora láser, se necesitará <strong>en</strong>viar solo unos pocos<br />
archivos <strong><strong>de</strong>l</strong>ta adicionales.<br />
Otra aplicación para <strong>la</strong> codificación progresiva son <strong>la</strong>s re<strong>de</strong>s paquete, don<strong>de</strong> los<br />
paquetes <strong>de</strong> información pue<strong>de</strong>n ser <strong>de</strong>scargados o no <strong>de</strong>scargados <strong>de</strong> acuerdo a<br />
como este congestionada <strong>la</strong> red. Los paquetes que llevan <strong>la</strong> información <strong>de</strong> <strong>la</strong><br />
resolución final duplicada serían <strong>en</strong>viados con baja prioridad; si ellos tuvieran que<br />
ser <strong>de</strong>scargados <strong>en</strong> mom<strong>en</strong>tos <strong>de</strong> congestión <strong>de</strong> <strong>la</strong> red, ninguna <strong>de</strong> <strong>la</strong>s regiones <strong>de</strong><br />
imág<strong>en</strong>es se per<strong>de</strong>rían o <strong>de</strong>struirían. La única <strong>de</strong>sv<strong>en</strong>taja sería que una imag<strong>en</strong><br />
sea ligeram<strong>en</strong>te m<strong>en</strong>os c<strong>la</strong>ra <strong>en</strong> algunas regiones.<br />
Una <strong>de</strong>sv<strong>en</strong>taja pot<strong>en</strong>cial <strong>de</strong> <strong>la</strong> codificación progresiva es <strong>la</strong> necesidad <strong>de</strong> uso <strong>de</strong><br />
un buffer (sección <strong>de</strong> almac<strong>en</strong>ami<strong>en</strong>to intermedia) lo sufici<strong>en</strong>tem<strong>en</strong>te gran<strong>de</strong> para<br />
mant<strong>en</strong>er <strong>la</strong> imag<strong>en</strong> <strong>en</strong> una <strong>de</strong>terminada resolución. El algoritmo JBIG ti<strong>en</strong>e una<br />
característica l<strong>la</strong>mada módulo o modo "secu<strong>en</strong>cial compatible", el cual pue<strong>de</strong><br />
20
obviar <strong>la</strong>s necesida<strong>de</strong>s <strong>de</strong> este buffer <strong>en</strong> el mom<strong>en</strong>to que una base <strong>de</strong> datos este<br />
almac<strong>en</strong>ando imág<strong>en</strong>es progresivam<strong>en</strong>te (para soportar un rango <strong>de</strong> resoluciones<br />
<strong>de</strong> pantal<strong>la</strong> efici<strong>en</strong>tem<strong>en</strong>te).<br />
1.3.1.2 REVISIÓN DE LOS ALGORITMOS JBIG<br />
Se <strong>de</strong>scribe a continuación algunos <strong>de</strong> los bloques funcionales principales <strong>de</strong> un<br />
codificador JBIG.<br />
Los <strong>de</strong>codifícadores son simi<strong>la</strong>res a los codificadores, y <strong>de</strong> alguna forma más<br />
simple porque <strong>la</strong> reducción <strong>de</strong> <strong>la</strong> resolución no es necesaria, por lo que <strong>la</strong><br />
<strong>de</strong>scripción <strong><strong>de</strong>l</strong> <strong>de</strong>codificador no se <strong>la</strong> realizará.<br />
Conceptualm<strong>en</strong>te un codificador JBIG pue<strong>de</strong> ser <strong>de</strong>scompuesto (ver figura 1.2) <strong>en</strong><br />
una ca<strong>de</strong>na <strong>de</strong> codificadores difer<strong>en</strong>ciales idénticos, seguidos por un codificador<br />
<strong>de</strong> <strong>en</strong>voltura <strong>de</strong> fondo (bottom-<strong>la</strong>yer <strong>en</strong>co<strong>de</strong>r). En ta figura 1.2(a), Id <strong>de</strong>nota <strong>la</strong><br />
imag<strong>en</strong> a ser codificada y Cd <strong>de</strong>nota su codificación.<br />
El corazón tanto <strong><strong>de</strong>l</strong> codificador difer<strong>en</strong>cial (figura 1.2.(b)) como <strong><strong>de</strong>l</strong> codificador <strong>de</strong><br />
<strong>en</strong>voltura <strong>de</strong> fondo (figura 1.2(c)) es un codificador aritmético adaptivo. Los<br />
codificadores aritméticos se los distingu<strong>en</strong> <strong>de</strong> los otros codificadores <strong>en</strong>trópicos<br />
tales como los codificadores Huffman <strong>en</strong> los cuales, al m<strong>en</strong>os conceptualm<strong>en</strong>te,<br />
mapean un conjunto <strong>de</strong> símbolos a ser codificados <strong>en</strong> un intervalo <strong>de</strong> unidad<br />
(0.0,1.0), lo que se transmite <strong>en</strong> vez <strong>de</strong> los símbolos es una repres<strong>en</strong>tación <strong>de</strong><br />
éste. La salida <strong>de</strong> un codificador aritmético es un número real simple, y a difer<strong>en</strong>cia<br />
•<strong><strong>de</strong>l</strong> codificador <strong>de</strong> Huffman, un codificador aritmético no requiere un número <strong>en</strong>tero<br />
<strong>de</strong> bits para repres<strong>en</strong>tar un símbolo <strong>de</strong> <strong>en</strong>trada. Como resultado, los codificadores<br />
aritméticos, son por lo g<strong>en</strong>eral más efici<strong>en</strong>tes que los codificadores Huffman.<br />
21
Codificador<br />
<strong>de</strong> Capa<br />
Difer<strong>en</strong>cial<br />
.1 Cd<br />
Típica<br />
Predicción<br />
(<strong>de</strong> londo)<br />
ld-1 '<br />
P<strong>la</strong>ntil<strong>la</strong><br />
adaptiva<br />
,<br />
Codrficador<br />
<strong>de</strong> Ca<br />
Drfer<strong>en</strong>c•tal<br />
(a)<br />
P<strong>la</strong>ntil<strong>la</strong><br />
Codificar<br />
Arrtmeti<br />
Fig.1J2 (a) Código JBIG que pue<strong>de</strong> ser <strong>de</strong>scompuesto <strong>en</strong> una ca<strong>de</strong>na <strong>de</strong> (b) codificadores difer<strong>en</strong>cial,<br />
seguidos por un (c) codificador <strong>de</strong> <strong>en</strong>voftura <strong>de</strong> fondo<br />
Una sub función algorítmica <strong>de</strong> los codificadores <strong>de</strong> capa difer<strong>en</strong>cial es <strong>la</strong><br />
reducción <strong>en</strong> <strong>la</strong> resolución, <strong>la</strong> cual mapea una imag<strong>en</strong> <strong>de</strong> resolución dada a una<br />
imag<strong>en</strong> <strong>de</strong> media resolución. Una forma para hacer esto sería simplem<strong>en</strong>te<br />
<strong>de</strong>scartar cada una <strong>de</strong> <strong>la</strong>s otras fi<strong>la</strong>s y columnas <strong>de</strong> <strong>la</strong> matriz <strong>de</strong> resolución, pero tal<br />
sub muestreo conduce a imág<strong>en</strong>es que son más pobres <strong>en</strong> calidad <strong><strong>de</strong>l</strong> mínimo<br />
requerido. El algoritmo <strong>de</strong> reducción <strong>de</strong> resolución basada <strong>en</strong> tab<strong>la</strong>s JBIG, crea<br />
una calidad excel<strong>en</strong>te con baja resolución para el texto, arte lineal, etc. La imag<strong>en</strong><br />
<strong>de</strong> baja resolución es creada pixel por pixel <strong>en</strong> el or<strong>de</strong>n usual <strong>de</strong> rastreo, es <strong>de</strong>cir,<br />
<strong>de</strong> arriba hacia abajo y <strong>de</strong> izquierda a <strong>de</strong>recha.<br />
Otras subfunciones algorítmicas <strong>de</strong> interés son los bloques <strong>de</strong> predicción típica, <strong>de</strong><br />
predicción <strong>de</strong>terminística y <strong>de</strong> <strong>la</strong>s p<strong>la</strong>ntil<strong>la</strong>s adaptativas. La predicción típica busca<br />
gran<strong>de</strong>s regiones continuas <strong>de</strong> color y cuando están pres<strong>en</strong>tes, pue<strong>de</strong>n<br />
sustancialm<strong>en</strong>te aligerar <strong>la</strong>s implem<strong>en</strong>taciones tanto <strong>de</strong> software como hardware.<br />
Adaptivi<br />
I<br />
Co<br />
22
La predicción <strong>de</strong>terminística explota <strong>la</strong> lógica <strong>de</strong> los algoritmos <strong>de</strong> reducción <strong>de</strong><br />
resolución para ganar una v<strong>en</strong>taja <strong>de</strong> alre<strong>de</strong>dor <strong>de</strong> un 5% <strong>de</strong> codificación. El<br />
algoritmo <strong>de</strong> p<strong>la</strong>ntil<strong>la</strong>s busca periodicida<strong>de</strong>s típicas <strong>de</strong> imág<strong>en</strong>es <strong>de</strong> medio tono y,<br />
cuando <strong>la</strong>s <strong>en</strong>cu<strong>en</strong>tra pue<strong>de</strong> explotar<strong>la</strong>s <strong>de</strong> gran manera, mejorando <strong>la</strong><br />
compresión.<br />
1.3.2 JPEG CODIFICACIÓN DE IMÁGENES INMÓVILES A COLOR<br />
Para evaluar <strong>la</strong> necesidad <strong>de</strong> <strong>la</strong> compresión <strong>de</strong> imag<strong>en</strong>, se consi<strong>de</strong>ra ios<br />
requerimi<strong>en</strong>tos <strong>de</strong> almac<strong>en</strong>ami<strong>en</strong>to/transmisión <strong>de</strong> una imag<strong>en</strong> no comprimida. Una<br />
imag<strong>en</strong> <strong>de</strong> color digital típica ti<strong>en</strong>e 512 x 480 pixeles. A 3 bytes por pixeles (1 para<br />
cada uno <strong>de</strong> los compon<strong>en</strong>tes rojo, ver<strong>de</strong> y azul), tal imag<strong>en</strong> requiere 737.280<br />
bytes <strong>de</strong> espacio <strong>de</strong> almac<strong>en</strong>ami<strong>en</strong>to. Transmitir <strong>la</strong> imag<strong>en</strong> no comprimida por un<br />
canal <strong>de</strong> 64 Kb/s toma alre<strong>de</strong>dor <strong>de</strong> 1.5 minutos. Los algoritmos JPEG ofrec<strong>en</strong><br />
"excel<strong>en</strong>te" calidad <strong>de</strong> compresión para <strong>la</strong> mayoría <strong>de</strong> imág<strong>en</strong>es hasta alre<strong>de</strong>dor <strong>de</strong><br />
1.0 bits/pixel. Este promedio <strong>de</strong> compresión <strong>de</strong> 24:1 reduce el almac<strong>en</strong>ami<strong>en</strong>to<br />
requerido <strong>de</strong> una imag<strong>en</strong> a color <strong>de</strong> 512 x 480 a 30.720 bytes, y su tiempo <strong>de</strong><br />
transmisión es alre<strong>de</strong>dor <strong>de</strong> 3.8 segundos. Las aplicaciones para compresión <strong>de</strong><br />
imág<strong>en</strong>es se <strong>la</strong>s pue<strong>de</strong> <strong>en</strong>contrar <strong>en</strong> publicidad, educación, T.V digital, vi<strong>de</strong>o<br />
comunicación, seguridad, <strong>en</strong>tre otras.<br />
1.3.2.1 Visión g<strong>en</strong>eral <strong>de</strong> los algoritmos JPEG<br />
El Comité JPEG no pudo satisfacer los requerimi<strong>en</strong>tos <strong>de</strong> todas <strong>la</strong>s aplicaciones <strong>de</strong><br />
compresión <strong>de</strong> imag<strong>en</strong> total con un algoritmo. Como resultado, el Comité propuso<br />
cuatro difer<strong>en</strong>tes modos <strong>de</strong> operación:<br />
• Modo Secu<strong>en</strong>cial basado <strong>en</strong> DCT( Transformada Discreta <strong><strong>de</strong>l</strong> Cos<strong>en</strong>o).- El<br />
modo secu<strong>en</strong>cia! DCT ofrece excel<strong>en</strong>tes tasas <strong>de</strong> compresión mant<strong>en</strong>i<strong>en</strong>do <strong>la</strong><br />
calidad <strong>de</strong> <strong>la</strong> imag<strong>en</strong>. La figura 1.3 pres<strong>en</strong>ta un diagrama simplificado <strong>de</strong> un<br />
co<strong>de</strong>e secu<strong>en</strong>cial DCT, <strong>en</strong> este modo, bloques1 <strong>de</strong> 8x8 <strong>de</strong> una imag<strong>en</strong> <strong>de</strong><br />
1 Un bloque consiste <strong>de</strong> 64 muestras que configuran <strong>la</strong> imag<strong>en</strong><br />
23
<strong>en</strong>trada son formateados para <strong>la</strong> compresión por medio <strong>de</strong> un rastreo <strong>de</strong><br />
imag<strong>en</strong> <strong>de</strong> izquierda a <strong>de</strong>recha y <strong>de</strong> arriba hacia abajo.<br />
Un subconjunto <strong><strong>de</strong>l</strong> modo secu<strong>en</strong>cial DCT ha sido i<strong>de</strong>ntificado por <strong>la</strong> JPEG para<br />
un "sistema <strong>de</strong> línea base". Este sistema <strong>de</strong>berá ayudar a asegurar <strong>la</strong><br />
interoperabilidad <strong>en</strong>tre los co<strong>de</strong>es <strong>de</strong> difer<strong>en</strong>tes marcas. Aun cuando un<br />
codificador DCT secu<strong>en</strong>cial completo pue<strong>de</strong> emplear ya sea <strong>la</strong> codificación<br />
<strong>en</strong>trópica aritmética o <strong>la</strong> <strong>de</strong> Huffman, un codificador <strong>de</strong> línea base pue<strong>de</strong><br />
so<strong>la</strong>m<strong>en</strong>te utilizar codificación Huffman.<br />
A continuación se <strong>de</strong>scribe los pasos <strong>de</strong> procesami<strong>en</strong>to para un codificador <strong>de</strong><br />
línea base. Un <strong>de</strong>codificador esta formado por los pasos inversos <strong><strong>de</strong>l</strong><br />
codificador.<br />
DCT y Cuantificación.- Todos los codificadores JPEG basados <strong>en</strong> DCT<br />
empiezan el proceso <strong>de</strong> codificación por <strong>la</strong> partición <strong>de</strong> <strong>la</strong> imag<strong>en</strong> <strong>de</strong> <strong>en</strong>trada <strong>en</strong><br />
un bloque <strong>de</strong> 8 x 8 recuadros <strong>de</strong> <strong>la</strong> muestra, éstas muestras son <strong>de</strong> 8 bits, a fin<br />
<strong>de</strong> que puedan mant<strong>en</strong>erse <strong>en</strong> el rango <strong>de</strong> -128 a + 127, los bloques se<br />
transforman al dominio <strong>de</strong> frecu<strong>en</strong>cia utilizando el FDCT (transformada directa).<br />
Las ecuaciones para <strong>la</strong> transformada directa e inversa <strong><strong>de</strong>l</strong> cos<strong>en</strong>o esta dadas<br />
por <strong>la</strong>s sigui<strong>en</strong>tes re<strong>la</strong>ciones:<br />
7 7<br />
FDCT: F(u,v) = % C(u)C(v) f(x,y) cos[7cu(2x-t-1)/16]cos[7iv(2y+1)/16] (1)<br />
1 1<br />
IDCT: f(x,y) = % V V C(t/)C(v) F(u,v) cos[7c(2x+1)f//16]cos[7c(2y+1)W16] (2)<br />
w=0 v=0<br />
don<strong>de</strong>: C(u) C(v) = 1/-\/2 para u,v = O ; C(¿/) C(v) = 1 <strong>en</strong> otro caso.<br />
24
salida <strong>de</strong> <strong>la</strong> imag<strong>en</strong>) por medio <strong><strong>de</strong>l</strong> uso <strong>de</strong> un factor q, el cual es por lo g<strong>en</strong>eral solo<br />
un factor aplicado a <strong>la</strong>s matrices <strong>de</strong> cuantificación.<br />
Luego <strong>de</strong> <strong>la</strong> cuantificación, los coefici<strong>en</strong>tes <strong>de</strong> los bloques por lo g<strong>en</strong>eral conti<strong>en</strong><strong>en</strong><br />
muchos coefici<strong>en</strong>tes AC <strong>de</strong> valor cero que <strong>de</strong>b<strong>en</strong> ser minimizados, ya que solo los<br />
coefici<strong>en</strong>tes AC no cero son codificados <strong>en</strong>tropicam<strong>en</strong>te, y para lo cual se utiliza un<br />
rastreo <strong>en</strong> zig zag para reor<strong>de</strong>nar estos coefici<strong>en</strong>tes (Fig.1.4)<br />
Fig.1,4 Rastreo <strong>en</strong> zig-zag <strong>de</strong> reor<strong>de</strong>nami<strong>en</strong>to <strong>de</strong> coefici<strong>en</strong>tes<br />
Modo Progresivo basado <strong>en</strong> DCT.- El modo progresivo DCT ha sido <strong>de</strong>finido<br />
por el JPEG para satisfacer <strong>la</strong> necesidad <strong>de</strong> una rápida <strong>de</strong>codificación <strong>de</strong><br />
imag<strong>en</strong>. Cuando se realiza una codificación <strong>de</strong> los coefici<strong>en</strong>tes cuantificados<br />
DCT <strong>en</strong> múltiples rastreos, <strong>la</strong> calidad <strong>de</strong> imag<strong>en</strong> <strong>de</strong>codificada se reconstruye<br />
progresivam<strong>en</strong>te <strong>de</strong>s<strong>de</strong> un nivel simple hasta una calidad acor<strong>de</strong> con <strong>la</strong>s<br />
matrices <strong>de</strong> cuantificación utilizadas, ya sea por selección espectral,<br />
aproximaciones sucesivas, o una combinación <strong>de</strong> <strong>la</strong>s dos.<br />
a) Selección Espectral.- En este método, los coefici<strong>en</strong>tes codificados DCT <strong>de</strong><br />
un bloque son primero divididos <strong>en</strong> bandas no tras<strong>la</strong>padas a lo <strong>la</strong>rgo <strong><strong>de</strong>l</strong><br />
26
mo<strong><strong>de</strong>l</strong>os toman v<strong>en</strong>taja <strong>de</strong> <strong>la</strong> baja s<strong>en</strong>sitividad <strong><strong>de</strong>l</strong> sistema visual humano hacia<br />
<strong>la</strong> información cromática-<strong>de</strong> alta frecu<strong>en</strong>cia al reducir <strong>la</strong> resolución cromática.<br />
Muchas imág<strong>en</strong>es (por lo g<strong>en</strong>eral RGB) son convertidas típicam<strong>en</strong>te a una<br />
repres<strong>en</strong>tación cromática <strong>de</strong> luminisc<strong>en</strong>cia antes <strong>de</strong> que este proceso tome<br />
lugar. Tanto <strong>la</strong> técnica <strong>de</strong> Huffman como <strong>la</strong>s técnicas aritméticas pue<strong>de</strong>n ser<br />
usadas para <strong>la</strong> codificación <strong>en</strong>trópica <strong>en</strong> cualquiera <strong>de</strong> los modos JPEG <strong>de</strong><br />
operación.<br />
1.3.3 NORMA H.261 DE CODIFICACIÓN PARA VIDEO CONFERENCIA<br />
Históricam<strong>en</strong>te, <strong>la</strong> norma H.261 fue <strong>de</strong>sarrol<strong>la</strong>da mucho antes que <strong>la</strong> JPEG. El<br />
<strong>de</strong>sarrollo <strong>de</strong> esta norma <strong>de</strong> transmisión <strong>de</strong> vi<strong>de</strong>o para los servicios <strong>de</strong> <strong>la</strong> RDSI <strong>de</strong><br />
bajo ritmo <strong>de</strong> transmisión ha pasado por varias etapas. Al comi<strong>en</strong>zo, <strong>la</strong> meta era<br />
diseñar un esquema <strong>de</strong> codificación para un promedio <strong>de</strong> transmisión <strong>de</strong> m x 384<br />
Kb/s, don<strong>de</strong> m estaba <strong>en</strong>tre 1 y 5. Más tar<strong>de</strong> se consi<strong>de</strong>ró un promedio <strong>de</strong><br />
transmisión <strong>de</strong> n x 64 Kb/s ( don<strong>de</strong> n estaba <strong>en</strong>tre 1 y 5). Sin embargo, a finales<br />
<strong>de</strong> 1989, <strong>la</strong> recom<strong>en</strong>dación H.261 <strong>de</strong> <strong>la</strong> CCITT fue hecha para un co<strong>de</strong>e <strong>de</strong> vi<strong>de</strong>o<br />
<strong>de</strong> p x 64 Kb/s, don<strong>de</strong> p va <strong>de</strong>s<strong>de</strong> 1 a 30.<br />
Tanto <strong>la</strong> línea base <strong><strong>de</strong>l</strong> JPEG como los co<strong>de</strong>e H.261 utilizan <strong>la</strong>s técnicas DCT y<br />
VLC (código <strong>de</strong> longitud variable). La mayor difer<strong>en</strong>cia <strong>en</strong>tre el esquema <strong>de</strong><br />
compresión JPEG y el H.261 es que el JPEG codifica cada recuadro<br />
individualm<strong>en</strong>te, mi<strong>en</strong>tras que el H.261 <strong>de</strong>sarrol<strong>la</strong> codificación interrecuadro. Aquí,<br />
los datos <strong><strong>de</strong>l</strong> cuadro <strong>en</strong> el recuadro previo pue<strong>de</strong>n ser usados para pre<strong>de</strong>cir los<br />
bloques <strong>de</strong> imág<strong>en</strong>es <strong>en</strong> el recuadro activo, como se lo muestra <strong>en</strong> <strong>la</strong> figura 1.5<br />
28
TRAMA PREVIA-<br />
TRAMA ACTIVA<br />
Fig.1.5 Bloque <strong>de</strong> comp<strong>en</strong>sación <strong>de</strong> movimi<strong>en</strong>to<br />
Como resultado, solo <strong>la</strong>s difer<strong>en</strong>cias, típicam<strong>en</strong>te <strong>de</strong> magnitud pequeña, <strong>en</strong>tre el<br />
bloque previo colocado y el bloque actual ti<strong>en</strong><strong>en</strong> que ser transmitidos. Hay varias<br />
consi<strong>de</strong>raciones <strong>de</strong> diseño <strong>en</strong> el H.261 que <strong>de</strong>b<strong>en</strong> ser consi<strong>de</strong>radas.<br />
Primero. La norma <strong>de</strong>fine es<strong>en</strong>cialm<strong>en</strong>te solo al <strong>de</strong>codificador. Sin embargo, el<br />
codificador, el cual no está especificado completa y explícitam<strong>en</strong>te por <strong>la</strong> norma,<br />
se espera que sea compatible con el <strong>de</strong>codificador.<br />
Segundo. Ya que <strong>la</strong> norma H.261 está diseñada para comunicaciones <strong>en</strong> tiempo<br />
real, utiliza solo el recuadro previo más cercano al pronosticado para reducir el<br />
retardo <strong>en</strong> <strong>la</strong> codificación.<br />
Tercero. Trata <strong>de</strong> ba<strong>la</strong>ncear <strong>la</strong>s complejida<strong>de</strong>s <strong><strong>de</strong>l</strong> hardware <strong><strong>de</strong>l</strong> codificador y el<br />
<strong>de</strong>codificador, ya que ambos son necesarios para <strong>la</strong> aplicación <strong><strong>de</strong>l</strong> vi<strong>de</strong>oteléfono<br />
<strong>de</strong> tiempo real.<br />
Cuarto. La norma H.261 establece un compromiso <strong>en</strong>tre el <strong>de</strong>sarrollo <strong>de</strong> <strong>la</strong><br />
codificación, los requerimi<strong>en</strong>tos <strong>de</strong> tiempo real, <strong>la</strong> complejidad <strong>de</strong> <strong>la</strong><br />
imptem<strong>en</strong>tación y <strong>la</strong> robustez dpi sistema.<br />
29
Quinto. La estructura y los parámetros <strong>de</strong> codificación final son <strong>en</strong>focados más<br />
hacia <strong>la</strong>s aplicaciones <strong>de</strong> bajo ritmo <strong>de</strong> transmisión. Esta alternativa es lógica,<br />
ya que <strong>la</strong> estructura y parámetros <strong>de</strong> codificación son más críticos al <strong>de</strong>sarrol<strong>la</strong>r<br />
co<strong>de</strong>es con promedios <strong>de</strong> bit muy bajos. En promedios <strong>de</strong> bit más altos, los<br />
valores <strong>de</strong> parámetros m<strong>en</strong>ores que el óptimo no afectan mucho al <strong>de</strong>sarrollo<br />
<strong><strong>de</strong>l</strong> co<strong>de</strong>e.<br />
1.3.3.1 Estructuras y compon<strong>en</strong>tes <strong><strong>de</strong>l</strong> <strong>de</strong>codificador.-<br />
La figura 1.6 muestra un diagrama funcional <strong>de</strong> un típico <strong>de</strong>codificador H.261. En<br />
términos g<strong>en</strong>erales, el flujo <strong>de</strong> bits recibidos se manti<strong>en</strong>e primero <strong>en</strong> el buffer<br />
receptor. El <strong>de</strong>codificador VLC <strong>de</strong>codifica el bit comprimido y distribuye <strong>la</strong><br />
información <strong>de</strong>codificada a los elem<strong>en</strong>tos que necesitan esa información.<br />
Fig. 1.6 Típico <strong>de</strong>codificador H.261<br />
Reproducción<br />
<strong>de</strong> Imág<strong>en</strong>es<br />
Comp<strong>en</strong>sador <strong>de</strong><br />
Movimi<strong>en</strong>to<br />
Predíctor/Frame<br />
Memoria<br />
El <strong>de</strong>codificador ti<strong>en</strong>e capacidad <strong>de</strong> comp<strong>en</strong>sación <strong>de</strong> movimi<strong>en</strong>to, permiti<strong>en</strong>do <strong>la</strong><br />
incorporación facultativa <strong>de</strong> ésta técnica <strong>en</strong> el codificador.<br />
30
Existe un segundo formato, quarter-CIF (QCIF) <strong>en</strong> el que ambas compon<strong>en</strong>tes (Y,<br />
y Cr, Cb) ti<strong>en</strong><strong>en</strong> <strong>la</strong> mitad <strong><strong>de</strong>l</strong> número <strong>de</strong> elem<strong>en</strong>tos <strong>de</strong> imag<strong>en</strong> y <strong>la</strong> mitad <strong><strong>de</strong>l</strong> número<br />
<strong>de</strong> líneas <strong>de</strong> estado.<br />
Todos los co<strong>de</strong>es <strong>de</strong>berían po<strong>de</strong>r operar usando QCIF. Sin embargo algunos<br />
co<strong>de</strong>es solo operan con CIF.<br />
X X X X X X<br />
X X X X X X<br />
O i O O<br />
X X X X X X<br />
x Muestra <strong>de</strong> luminancia<br />
O Muestra <strong>de</strong> crominancia<br />
— Bor<strong>de</strong> <strong>de</strong> bloque.<br />
Fig. 1.7 Posicionami<strong>en</strong>to <strong>de</strong> <strong>la</strong>s muestras <strong>de</strong> luminancia y crominancia.<br />
1.3.4.- ESTÁNDAR MPEG.-<br />
El MPEG es una norma internacional que fue creada para estandarizar una<br />
repres<strong>en</strong>tación <strong>de</strong> co<strong>de</strong>e <strong>de</strong> vi<strong>de</strong>o y audio que se ajuste a los medios <strong>de</strong><br />
almac<strong>en</strong>ami<strong>en</strong>to digital tales como los compact disc - memoria solo <strong>de</strong> lectura (CD<br />
ROM), cintas <strong>de</strong> audio digital (DAT), etc. La meta, sin embargo, ha sido <strong>de</strong>sarrol<strong>la</strong>r<br />
una nomra g<strong>en</strong>érica, que pueda ser usada <strong>en</strong> otras aplicaciones <strong>de</strong> vi<strong>de</strong>o digital,<br />
tales como <strong>la</strong>s telecomunicaciones.<br />
1.3.4.1 Requerimi<strong>en</strong>tos <strong>de</strong> <strong>la</strong> Norma<br />
• El requerimi<strong>en</strong>to primario <strong>de</strong> <strong>la</strong> norma <strong>de</strong> vi<strong>de</strong>o MPEG es que <strong>de</strong>be lograr <strong>la</strong> más<br />
alta calidad posible <strong><strong>de</strong>l</strong> vi<strong>de</strong>o <strong>de</strong>codificado con una velocidad <strong>de</strong> bit dado. La<br />
habilidad para <strong>de</strong>sarrol<strong>la</strong>r búsquedas rápidas hacia a<strong>de</strong><strong>la</strong>nte y hacia atrás es<br />
extremadam<strong>en</strong>te <strong>de</strong>seable si el medio <strong>de</strong> almac<strong>en</strong>ami<strong>en</strong>to ti<strong>en</strong>e <strong>la</strong>s capacida<strong>de</strong>s<br />
32
<strong>de</strong> búsqueda. La mayoría <strong>de</strong> aplicaciones requier<strong>en</strong> <strong>de</strong> una grado <strong>de</strong> e<strong>la</strong>sticidad<br />
para los errores <strong><strong>de</strong>l</strong> bit.<br />
1.3.4.2 Revisión <strong>de</strong> los Algoritmos <strong>de</strong> Compresión.-<br />
• Explotación <strong>de</strong> <strong>la</strong> Redundancia Espacial.- La compresión <strong><strong>de</strong>l</strong> vi<strong>de</strong>o MPEG<br />
utiliza una combinación <strong>de</strong> <strong>la</strong>s normas ISO JPEG (imag<strong>en</strong> estática) y <strong><strong>de</strong>l</strong> H.261<br />
<strong>de</strong> <strong>la</strong> CCITT (para vi<strong>de</strong>o telefonía y vi<strong>de</strong>oconfer<strong>en</strong>cia). Ya que el vi<strong>de</strong>o es una<br />
secu<strong>en</strong>cia <strong>de</strong> imág<strong>en</strong>es inmóviles, es posible comprimir o codificar una señal <strong>de</strong><br />
vi<strong>de</strong>o usando técnicas simi<strong>la</strong>res a <strong>la</strong> JPEG. Tales métodos <strong>de</strong> compresión son<br />
l<strong>la</strong>mados técnicas <strong>de</strong> codificación intra recuadro (intra-frame), don<strong>de</strong> cada<br />
recuadro <strong>de</strong> vi<strong>de</strong>o es comprimido individual e in<strong>de</strong>p<strong>en</strong>di<strong>en</strong>tem<strong>en</strong>te. La<br />
codificación intra recuadro explota <strong>la</strong> redundancia espacial que existe <strong>en</strong>tre los<br />
pixels adyac<strong>en</strong>tes <strong>de</strong> un recuadro.<br />
Como <strong>en</strong> JPEG y <strong>en</strong> H.261, el algoritmo <strong>de</strong> codificación <strong>de</strong> vi<strong>de</strong>o <strong>en</strong> el MPEG<br />
emplea un DCT bidim<strong>en</strong>sional basado <strong>en</strong> bloques. Un recuadro es primero<br />
dividido <strong>en</strong> bloques 8x8, y el DCT bidim<strong>en</strong>sional es luego aplicado,<br />
in<strong>de</strong>p<strong>en</strong>di<strong>en</strong>tem<strong>en</strong>te a cada bloque. Esta operación resulta <strong>de</strong> un bloque <strong>de</strong><br />
coefici<strong>en</strong>tes DCT <strong>de</strong> 8x8 <strong>en</strong> el cual <strong>la</strong> mayoría <strong>de</strong> <strong>la</strong> <strong>en</strong>ergía <strong>en</strong> el bloque<br />
original (pixel) está típicam<strong>en</strong>te conc<strong>en</strong>trado <strong>en</strong> unos pocos coefici<strong>en</strong>tes <strong>de</strong> baja<br />
frecu<strong>en</strong>cia. Se aplica un cuantificador a cada coefici<strong>en</strong>te DCT <strong>de</strong> los cuales<br />
muchos <strong>de</strong> ellos se setean a cero. Esta cuantificación es responsable <strong>de</strong> <strong>la</strong><br />
pérdida natural <strong>de</strong> información <strong>en</strong> los algoritmos <strong>de</strong> compresión <strong>de</strong> vi<strong>de</strong>o <strong>de</strong> <strong>la</strong><br />
JPEG, H.261 y MPEG. Se consigue <strong>la</strong> compresión por <strong>la</strong> transmisión<br />
únicam<strong>en</strong>te <strong>de</strong> los coefici<strong>en</strong>tes que sobreviv<strong>en</strong> a <strong>la</strong> operación <strong>de</strong> cuantificación<br />
y por <strong>la</strong> codificación <strong>en</strong>trópica <strong>de</strong> sus ubicaciones y amplitu<strong>de</strong>s. Esta norma<br />
permite <strong>la</strong> operación <strong>de</strong> cuantificación para conseguir un nivel más alto <strong>de</strong><br />
adaptación, un factor c<strong>la</strong>ve para lograr una bu<strong>en</strong>a calidad <strong>de</strong> imag<strong>en</strong>.<br />
• Explotación <strong>de</strong> <strong>la</strong> Redundancia Temporal.- Muchos <strong>de</strong> los requerimi<strong>en</strong>tos<br />
interactivos discutidos anteriorm<strong>en</strong>te pue<strong>de</strong>n ser satisfechos por <strong>la</strong> codificación<br />
33
intra recuadro (iníraframe). Sin embargo, como <strong>en</strong> <strong>la</strong> norma H.261, <strong>la</strong> calidad<br />
conseguida por <strong>la</strong> codificación intra recuadro por sí so<strong>la</strong> no es sufici<strong>en</strong>te para<br />
<strong>la</strong>s señales típicas <strong>de</strong> vi<strong>de</strong>o a un velocidad <strong>de</strong> bit <strong>de</strong> alre<strong>de</strong>dor <strong>de</strong> 1.5 Mb/s. La<br />
redundancia temporal resulta <strong>de</strong> un alto grado <strong>de</strong> corre<strong>la</strong>ción <strong>en</strong>tre los<br />
recuadros adyac<strong>en</strong>tes. El algoritmo H.261 explota esta redundancia al<br />
computar una señal difer<strong>en</strong>cial recuadro a recuadro l<strong>la</strong>mada el error <strong>de</strong><br />
predicción, <strong>la</strong> técnica <strong>de</strong> comp<strong>en</strong>sación <strong>de</strong> movimi<strong>en</strong>to se <strong>la</strong> emplea para<br />
corregir el movimi<strong>en</strong>to. Un acercami<strong>en</strong>to basado <strong>en</strong> el bloque se adopta para <strong>la</strong><br />
comp<strong>en</strong>sación <strong>de</strong> movimi<strong>en</strong>to, don<strong>de</strong> un bloque <strong>de</strong> pixeles, l<strong>la</strong>mado el bloque<br />
objeto, <strong>en</strong> el recuadro a ser codificado, concuerda con un conjunto <strong>de</strong> bloques<br />
<strong><strong>de</strong>l</strong> mismo tamaño <strong>en</strong> el recuadro previo l<strong>la</strong>mado recuadro <strong>de</strong> refer<strong>en</strong>cia. El<br />
bloque <strong>en</strong> el recuadro <strong>de</strong> refer<strong>en</strong>cia que "mejor concuerda" con el bloque objeto<br />
es usado como <strong>la</strong> predicción para el último, es <strong>de</strong>cir, el error <strong>de</strong> predicción es<br />
computado como ia difer<strong>en</strong>cia <strong>en</strong>tre el bloque objeto y el bloque que mejor<br />
concuer<strong>de</strong>. Este bloque <strong>de</strong> mejor concordancia esta asociado con un vector <strong>de</strong><br />
movimi<strong>en</strong>to que <strong>de</strong>scribe los <strong>de</strong>sp<strong>la</strong>zami<strong>en</strong>tos <strong>en</strong>tre él y el bloque objeto. La<br />
información <strong><strong>de</strong>l</strong> vector <strong>de</strong> movimi<strong>en</strong>to es también codificada y transmitida junto<br />
con el error <strong>de</strong> predicción. El error <strong>de</strong> predicción <strong>en</strong> sí es transmitido usando <strong>la</strong><br />
técnica <strong>de</strong> codificación intra recuadro basada <strong>en</strong> el DCT, resumida<br />
anteriorm<strong>en</strong>te. En el vi<strong>de</strong>o MPEG (como <strong>en</strong> el H.261), el tamaño <strong><strong>de</strong>l</strong> bloque<br />
para <strong>la</strong> comp<strong>en</strong>sación <strong>de</strong> movimi<strong>en</strong>to se lo escoge para que sea <strong>de</strong> 16 x 16,<br />
repres<strong>en</strong>tando un razonable intercambio <strong>en</strong>tre <strong>la</strong> compresión provista por <strong>la</strong><br />
comp<strong>en</strong>sación <strong>de</strong> movimi<strong>en</strong>to y el costo asociado con <strong>la</strong> transmisión <strong>de</strong> los<br />
vectores <strong>de</strong> movimi<strong>en</strong>to.<br />
Predicción Temporal Bidireccional.- La predicción temporal bidireccional,<br />
también l<strong>la</strong>mada interpo<strong>la</strong>ción comp<strong>en</strong>sada <strong>de</strong> movimi<strong>en</strong>tos, es una<br />
característica c<strong>la</strong>ve <strong><strong>de</strong>l</strong> vi<strong>de</strong>o MPEG. En ia predicción bidireccional, algunos <strong>de</strong><br />
los recuadros <strong><strong>de</strong>l</strong> vi<strong>de</strong>o son codificados, usando dos recuadros <strong>de</strong> refer<strong>en</strong>cia,<br />
uno previo y otro posterior. Un bloque <strong>en</strong> aquellos recuadros, pue<strong>de</strong> ser<br />
pronosticado por otro <strong><strong>de</strong>l</strong> recuadro <strong>de</strong> refer<strong>en</strong>cia previo (predicción <strong>en</strong><br />
34
avanzada) o <strong><strong>de</strong>l</strong> recuadro <strong>de</strong> refer<strong>en</strong>cia futura (predicción <strong>en</strong> retroceso), o por el<br />
promedio <strong>de</strong> los dos bloques, uno <strong>de</strong> cada recuadro <strong>de</strong> refer<strong>en</strong>cia<br />
(interpo<strong>la</strong>ción). En cada caso, el bloque <strong><strong>de</strong>l</strong> recuadro <strong>de</strong> refer<strong>en</strong>cia, está<br />
asociado con un vector <strong>de</strong> movimi<strong>en</strong>to, <strong>de</strong> modo que los vectores <strong>de</strong> ambos<br />
movimi<strong>en</strong>tos sean usados con <strong>la</strong> interpo<strong>la</strong>ción. La interpo<strong>la</strong>ción <strong>de</strong> movimi<strong>en</strong>to<br />
comp<strong>en</strong>sada para un bloque <strong>en</strong> un recuadro pronosticado bidireccionalm<strong>en</strong>te,<br />
es ilustrada <strong>en</strong> <strong>la</strong> figura 1.8. Los recuadros que son pronosticados<br />
bidireccionalm<strong>en</strong>te, nunca son por sí mismo utilizados como recuadros <strong>de</strong><br />
refer<strong>en</strong>cia. La predicción bidireccional proporciona varias v<strong>en</strong>tajas. La primera<br />
es que <strong>la</strong> compresión obt<strong>en</strong>ida es típicam<strong>en</strong>te más alta que <strong>la</strong> obt<strong>en</strong>ida con <strong>la</strong><br />
predicción <strong>en</strong> avance. Para obt<strong>en</strong>er <strong>la</strong> misma calidad <strong><strong>de</strong>l</strong> cuadro los recuadros<br />
pronosticados bidireccionalm<strong>en</strong>te se los pue<strong>de</strong> codificar con muchos m<strong>en</strong>os bit<br />
que los recuadros que utiliza so<strong>la</strong>m<strong>en</strong>te <strong>la</strong> predicción <strong>en</strong> avance. Sin embargo,<br />
<strong>la</strong> predicción bidireccional introduce un retardo extra <strong>en</strong> el proceso <strong>de</strong><br />
codificación, ya que los recuadros <strong>de</strong>b<strong>en</strong> ser codificados sin ninguna secu<strong>en</strong>cia.<br />
Más aún esta involucra una complejidad extra <strong>de</strong> codificación porque <strong>la</strong><br />
concordancia <strong>de</strong> bloque (el procedimi<strong>en</strong>to <strong>de</strong> codificación más int<strong>en</strong>sivo<br />
computacionalm<strong>en</strong>te), ti<strong>en</strong>e que ser <strong>de</strong>sarrol<strong>la</strong>do dos veces por cada bloque<br />
objeto, una vez con <strong>la</strong> refer<strong>en</strong>cia <strong>previa</strong> y otra vez con refer<strong>en</strong>cia posterior.<br />
1_- Bloque B = Bloque A<br />
2.- Bloque B = Bloque C<br />
3.- Bloque B = (Bloque A + Bloque C)/2<br />
Fig.1.8 Interpo<strong>la</strong>ción <strong>de</strong> comp<strong>en</strong>sación <strong>de</strong> movimi<strong>en</strong>to.<br />
35
1.3.4.3 Características <strong>de</strong> <strong>la</strong> Sintaxis <strong><strong>de</strong>l</strong> Bit-Stream.- La sintaxis <strong><strong>de</strong>l</strong> flujo<br />
<strong>de</strong> bit <strong>de</strong>be ser tan flexible como para soportar <strong>la</strong> variedad <strong>de</strong> aplicaciones<br />
creadas específicam<strong>en</strong>te para <strong>la</strong> norma <strong>de</strong> vi<strong>de</strong>o MPEG. Para este fin, <strong>la</strong><br />
sintaxis completa esta constituida <strong>de</strong> varias capas, cada una ejecutando una<br />
función lógica difer<strong>en</strong>te. La capa más externa es l<strong>la</strong>mada <strong>la</strong> capa <strong>de</strong> secu<strong>en</strong>cia<br />
<strong>de</strong> vi<strong>de</strong>o, <strong>la</strong> cual conti<strong>en</strong>e parámetros básicos tales como el tamaño <strong>de</strong> los<br />
recuadros <strong>de</strong> vi<strong>de</strong>o, <strong>la</strong> proporción <strong><strong>de</strong>l</strong> recuadro, velocidad <strong>de</strong> bit, y ciertos otros<br />
parámetros globales.<br />
Las figuras 1.9 (a) y (b) muestra un codificador y <strong>de</strong>codificador <strong>de</strong> vi<strong>de</strong>o MPEG.<br />
L 1<br />
«be<br />
lo<strong>en</strong><br />
_<br />
VeawOe<br />
Movirrin<br />
»•<br />
úeCotnp.<br />
* t<br />
Fig 1.9 (a) Codificador MPEG<br />
Esbmeóof<br />
Of Movim.<br />
; k<br />
-<br />
j A;<br />
tL<br />
Í TIpodt<br />
Imag<strong>en</strong><br />
Attwc<strong>en</strong>arn<br />
dfl kneQcfi<br />
Previa<br />
- Esoltira Piwa<br />
Almac<strong>en</strong>am.<br />
úe tmao<strong>en</strong><br />
FlXLTB<br />
36
LA RED DIGITAL DE SERVICIOS<br />
INTEGRADOS (RDSI) EN VIDEO<br />
COMUNICACIONES
CAPITULO II<br />
2. LA RED DIGITAL DE SERVICIO (INTEGRADOS (RDSI) EN<br />
VIDEO COMUNICACIONES<br />
2.1 PRINCIPIOS GENERALES DE LA RDSI<br />
Una Red Digital <strong>de</strong> Servicios Integrados (RDSI) es una red que se<br />
caracteriza por permitir una amplia gama <strong>de</strong> aplicaciones <strong>en</strong> <strong>la</strong> misma<br />
(audio, vi<strong>de</strong>o, datos). Esta es una red que ha evolucionado a partir <strong>de</strong><br />
una red digital integrada (RDI) para telefonía, y que ha incorporado<br />
progresivam<strong>en</strong>te funciones adicionales, características y propieda<strong>de</strong>s<br />
<strong>de</strong> red, incluy<strong>en</strong>do <strong>la</strong>s que son propias <strong>de</strong> otras re<strong>de</strong>s especializadas<br />
como son <strong>la</strong>s re<strong>de</strong>s <strong>de</strong> datos con conmutación <strong>de</strong> circuitos y<br />
paquetes, a fin <strong>de</strong> t<strong>en</strong>er <strong>en</strong> cu<strong>en</strong>ta los servicios actuales y los futuros.<br />
En esta evolución, <strong>la</strong> conectividad extremo a extremo se obti<strong>en</strong>e por<br />
medio <strong>de</strong> los recursos y equipos utilizados <strong>en</strong> <strong>la</strong>s re<strong>de</strong>s exist<strong>en</strong>tes,<br />
tales como transmisión digital, conmutación multiplex por división <strong>en</strong><br />
el tiempo y/o multiplex por división <strong>en</strong> el espacio.<br />
RDSI se sust<strong>en</strong>ta sobre tres aspectos fundam<strong>en</strong>tales:<br />
• La conectividad digital para <strong>la</strong> transfer<strong>en</strong>cia <strong>de</strong> señales <strong>de</strong><br />
información.<br />
• La conectividad y señalización <strong>en</strong> modo m<strong>en</strong>saje.<br />
• La polival<strong>en</strong>cia <strong>de</strong> acceso <strong><strong>de</strong>l</strong> usuario fr<strong>en</strong>te a los servicios <strong>de</strong><br />
telecomunicación.<br />
Tipos <strong>de</strong> RDSI:<br />
Exist<strong>en</strong> dos tipos <strong>de</strong> RDSI según el ancho <strong>de</strong> banda:<br />
1. RDSI <strong>de</strong> Banda Angosta: Hasta 2 MB/seg.<br />
2. RDS! <strong>de</strong> Banda Ancha: Mayores a 2 MB/seg.<br />
38
2.1.1 Establecimi<strong>en</strong>to <strong>de</strong> una RDSI.-<br />
Para que el establecimi<strong>en</strong>to <strong>de</strong> una RDSi t<strong>en</strong>ga s<strong>en</strong>tido, tanto <strong>de</strong>s<strong>de</strong> el punto <strong>de</strong><br />
vista <strong>de</strong> los servicios que presta, como <strong>de</strong> <strong>la</strong> r<strong>en</strong>tabilidad para <strong>la</strong>s empresas que <strong>la</strong><br />
ofrec<strong>en</strong>, se <strong>de</strong>be pres<strong>en</strong>tar al usuario <strong>de</strong>ntro <strong><strong>de</strong>l</strong> contexto <strong>de</strong> una Red Nacional. La<br />
RDSI como concepto <strong>de</strong> Red Nacional, <strong>de</strong>be brindar al usuario garantías <strong>de</strong>:<br />
• Cobertura total <strong><strong>de</strong>l</strong> servicio <strong>en</strong> cualquier lugar don<strong>de</strong> se<br />
disponga <strong>de</strong> esta red<br />
• Flexibilidad <strong>en</strong> el sistema<br />
• Confiabilidad <strong>de</strong> <strong>la</strong> red<br />
• Rapi<strong>de</strong>z <strong>de</strong> acceso<br />
• Calidad total tanto <strong>de</strong> fa red como <strong>de</strong> los servicios que presta<br />
Para cumplir estos objetivos, es importante realizar una normalización<br />
técnica, <strong>de</strong>sarrol<strong>la</strong>da con criterio amplio y futurista <strong>de</strong>ntro <strong>de</strong>!<br />
esc<strong>en</strong>ario internacional; por tanto es importante que cada país<br />
establezca sus normas nacionales RDSI antes <strong>de</strong> adquirir los<br />
sistemas <strong>de</strong> esta tecnología.<br />
A continuación se pres<strong>en</strong>ta, a breves rasgos, un p<strong>la</strong>n g<strong>en</strong>eral para el<br />
establecimi<strong>en</strong>to <strong>de</strong> una Red Digital <strong>de</strong> Servicios Integrados<br />
2.1.2 P<strong>la</strong>n G<strong>en</strong>eral para el Establecimi<strong>en</strong>to <strong>de</strong> RDSI.-<br />
(Se toma como ejemplo el p<strong>la</strong>n g<strong>en</strong>eral creado <strong>en</strong> Colombia para<br />
integración <strong>de</strong> servicios <strong>de</strong>bido a que sigue normas y lineami<strong>en</strong>tos<br />
internacionales, y por tanto pue<strong>de</strong>n ser aplicados a nuestro medio).<br />
1) Se <strong>de</strong>be realizar una <strong>de</strong>scripción completa <strong>de</strong> <strong>la</strong> infraestructura y<br />
situación interna <strong>de</strong> <strong>la</strong> red y <strong><strong>de</strong>l</strong> <strong>en</strong>torno, tanto actual, como <strong>de</strong><br />
acuerdo con p<strong>la</strong>nes y programas <strong>de</strong> <strong>de</strong>sarrollo establecidos.<br />
2) Se <strong>de</strong>be realizar un diagnóstico <strong>de</strong> esta infraestructura y <strong><strong>de</strong>l</strong><br />
estado <strong>de</strong> <strong>la</strong> red exist<strong>en</strong>te, para según esto, po<strong>de</strong>r realizar <strong>la</strong><br />
imp<strong>la</strong>ntación <strong>de</strong> nuevos servicios.<br />
39
3) Se <strong>de</strong>be realizar un estudio <strong>de</strong> causalidad para <strong>la</strong> imp<strong>la</strong>ntación e<br />
implem<strong>en</strong>tación <strong>de</strong> "Integración <strong>de</strong> Servicios"<br />
4) Un estudio selectivo para <strong>la</strong> imp<strong>la</strong>ntación <strong>de</strong> "Integración <strong>de</strong><br />
Servicios"<br />
5) La e<strong>la</strong>boración <strong>de</strong> un p<strong>la</strong>n coher<strong>en</strong>te con <strong>la</strong> realidad <strong><strong>de</strong>l</strong>.medio,<br />
para <strong>la</strong> implem<strong>en</strong>tación <strong>de</strong> "Sistemas <strong>de</strong> Integración <strong>de</strong> Servicios"<br />
6) Gestiones, y ejecución <strong><strong>de</strong>l</strong> pian.<br />
7) Diseño <strong><strong>de</strong>l</strong> Sistema.<br />
8) E<strong>la</strong>boración <strong>de</strong> P<strong>la</strong>nos y trámites correspondi<strong>en</strong>tes.<br />
Una vez que se cu<strong>en</strong>ta con un p<strong>la</strong>n g<strong>en</strong>eral <strong>de</strong> acción, que esté <strong>de</strong><br />
acuerdo con <strong>la</strong>s necesida<strong>de</strong>s y realidad <strong>de</strong> cada país, el sigui<strong>en</strong>te<br />
paso será buscar una inter-re<strong>la</strong>ción <strong>en</strong>tre <strong>la</strong> red exist<strong>en</strong>te y <strong>la</strong> nueva<br />
red, hasta que <strong>la</strong> transición <strong>en</strong>tre <strong>la</strong> una y <strong>la</strong> otra se haya completado.<br />
2.1.3 Interfuncionami<strong>en</strong>to <strong>en</strong>tre una RDSI y una Red Telefónica<br />
Pública Conmutada (RTPC).<br />
Dado que <strong>la</strong> transición <strong>en</strong>tre una RTPC a una RDSI pue<strong>de</strong> abarcar un<br />
di<strong>la</strong>tado período, se necesitará durante mucho tiempo <strong>la</strong> interrealción<br />
<strong>en</strong>tre una RDSI y <strong>la</strong> RTPC, En esta situación, es probable que se<br />
necesit<strong>en</strong> funciones <strong>de</strong> intercomunicación <strong>en</strong> varios lugares, y no <strong>en</strong><br />
un solo punto. Durante <strong>la</strong> transición hacia <strong>la</strong> RDSI, aparecerán puntos<br />
<strong>de</strong> i nterf unción a mi<strong>en</strong>to que quizás no sean necesarios más tar<strong>de</strong>.<br />
(Fig. 2.1)<br />
40
SoT<br />
a) Esc<strong>en</strong>ario Irrtercerrtrales<br />
b) Esc<strong>en</strong>ario Irrtra c<strong>en</strong>tra les<br />
Fig.2.1 Interre<strong>la</strong>ción <strong>en</strong>tre RTPC y RDSI<br />
Los puntos más comunes don<strong>de</strong> pue<strong>de</strong> haber interfuncionami<strong>en</strong>to son;<br />
- D<strong>en</strong>tro <strong>de</strong> <strong>la</strong> c<strong>en</strong>tral local,<br />
- En <strong>la</strong>s c<strong>en</strong>trales <strong>de</strong> tránsito,<br />
- En <strong>la</strong>s c<strong>en</strong>trales internacionales <strong>de</strong> cabecera<br />
2.1.4 Tratami<strong>en</strong>to <strong>de</strong> l<strong>la</strong>madas <strong>en</strong>tre RDSI - RTPC.-<br />
En g<strong>en</strong>eral, pue<strong>de</strong> necesitarse una interpe<strong>la</strong>ción <strong>de</strong> sistemas <strong>de</strong><br />
señalización, <strong>en</strong> especial para l<strong>la</strong>madas ¡nterc<strong>en</strong>trales, <strong>en</strong>tre el<br />
sistema <strong>de</strong> señalización <strong>de</strong> <strong>la</strong> RTPC (que pue<strong>de</strong> ser <strong>de</strong>ntro <strong>de</strong> banda)<br />
y el sistema <strong>de</strong> señalización No 7 (PU RDSI) <strong>de</strong> una RDSI. Los<br />
procesos <strong>de</strong> interfuncionami<strong>en</strong>to se especifican <strong>en</strong> <strong>la</strong> Recom<strong>en</strong>dación<br />
Q.699.<br />
Para l<strong>la</strong>madas vocales se necesita una indicación especial para que<br />
<strong>la</strong> c<strong>en</strong>tral local <strong>de</strong> <strong>la</strong> RDS! sepa que se ha producido<br />
interfuncionami<strong>en</strong>to <strong>en</strong>tre RDSI y RTPC, y son los protocolos<br />
respectivos (Q.761, Q764 <strong>de</strong> <strong>la</strong> RDSI y protocolos <strong>de</strong> <strong>la</strong> Rec 1.451) los<br />
Usuario<br />
RTPC<br />
4]
que dispon<strong>en</strong> <strong>de</strong> <strong>la</strong> capacidad para indicar esta situación <strong>de</strong><br />
interpe<strong>la</strong>ción. En todo caso se <strong>de</strong>b<strong>en</strong> cumplir los sigui<strong>en</strong>tes requisitos<br />
para una intercomunicación <strong>en</strong>tre RDSI y RTPC:<br />
- Indicar al terminal que conecte el canal B, <strong>de</strong> modo que<br />
pueda recibir tonos y anuncios <strong>de</strong>ntro <strong>de</strong> banda cuando se<br />
originan l<strong>la</strong>madas <strong>de</strong> RDSI a RTPC,<br />
- Indicar al terminal RDSI que pue<strong>de</strong> no disponerse <strong>de</strong> <strong>la</strong><br />
información <strong>de</strong> selección <strong>de</strong> servicio y <strong>de</strong> dirección o <strong>de</strong> una parte<br />
<strong>de</strong> <strong>la</strong> misma; se pue<strong>de</strong> <strong>en</strong>tonces pedir al terminal que acepte <strong>la</strong><br />
l<strong>la</strong>mada sin verificar <strong>la</strong> compatibilidad fuera <strong>de</strong> banda,<br />
- Indicar al equipo terminal <strong>de</strong> datos que provea por anticipado <strong>la</strong>s<br />
señales <strong>de</strong> <strong>en</strong>trada <strong>en</strong> contacto <strong>de</strong>ntro <strong>de</strong> <strong>la</strong> banda <strong>en</strong> <strong>la</strong>s l<strong>la</strong>madas<br />
RDSI - RTPC.<br />
Para l<strong>la</strong>madas no vocales <strong>en</strong>tre abonados RDSI-RTPC, pue<strong>de</strong> ser<br />
necesario una ¡nterrel ación con respecto a <strong>la</strong> capacidad <strong>de</strong><br />
inte reo néctar terminales <strong>de</strong> <strong>la</strong> RTPC equipados con mo<strong>de</strong>ms y<br />
terminales compatibles con un acceso RDSI. En el futuro, esta<br />
situación pue<strong>de</strong> incluir un medio para verificar <strong>la</strong> compatibilidad y el<br />
suministro <strong>de</strong> un grupo común <strong>de</strong> mo<strong>de</strong>ms para realizar <strong>la</strong> conversión<br />
A/D y <strong>la</strong> adaptación <strong>de</strong> velocidad.<br />
En principio exist<strong>en</strong> dos maneras posibles <strong>de</strong> ofrecer <strong>la</strong> conmutación<br />
<strong>de</strong> datos <strong>en</strong>tre un cli<strong>en</strong>te <strong>de</strong> RDSI y uno <strong>de</strong> RTPC:<br />
1, El terminal <strong>de</strong> datos <strong>de</strong> cli<strong>en</strong>te <strong>de</strong> RDSI se conecta a un<br />
mo<strong>de</strong>m que a su vez está conectado a un dispositivo <strong>de</strong><br />
conversión A/D, por tanto, el trato que se le dará a <strong>la</strong> l<strong>la</strong>mada<br />
será como <strong>en</strong> telefonía normal. Se requerirá <strong>de</strong> ulteriores<br />
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2.2 VÍDEO COMUNICACIONES EN LA RDSI<br />
2.2.1 Servicios <strong>de</strong> Ví<strong>de</strong>o Comunicación <strong>en</strong> <strong>la</strong> RDSI:<br />
En <strong>la</strong>s re<strong>de</strong>s conv<strong>en</strong>cionales solo se pue<strong>de</strong> utilizar un servicio a <strong>la</strong><br />
vez, por ejemplo, teléfono o fax. Esto ha llevado a una situación <strong>en</strong><br />
<strong>la</strong> que <strong>la</strong>s líneas están <strong>de</strong>dicadas a un uso específico que es, como<br />
teléfono o para transmisión <strong>de</strong> datos, tanto <strong>en</strong> comunicaciones<br />
privadas como <strong>de</strong> negocios. Con ta RDSI esta limitación ya no existe.<br />
Las sigui<strong>en</strong>tes facilida<strong>de</strong>s es<strong>en</strong>ciales son características <strong>de</strong> <strong>la</strong>s<br />
comunicaciones RDSI:<br />
a) Conexiones digitales extremo a extremo y por tanto<br />
transfer<strong>en</strong>cia <strong>de</strong> toda <strong>la</strong> información con una velocidad <strong>de</strong><br />
64 Kbits/s.<br />
b) Capacidad <strong>de</strong> transmisión <strong>de</strong> dos canales, por ejemplo<br />
voz y fax.<br />
c) Servicios nuevos y mejorados <strong>en</strong> una red para voz,<br />
texto, datos e imag<strong>en</strong>, que se pue<strong>de</strong>n usar <strong>de</strong>s<strong>de</strong> una<br />
línea <strong>de</strong> abonado.<br />
d) Facilida<strong>de</strong>s nuevas y mejoradas, como i<strong>de</strong>ntificación<br />
<strong><strong>de</strong>l</strong> abonado l<strong>la</strong>mante, oferta <strong>de</strong> l<strong>la</strong>mada y <strong>de</strong>svió <strong>de</strong><br />
l<strong>la</strong>mada con <strong>la</strong> oportunidad <strong>de</strong> muchas y nuevas<br />
aplicaciones.<br />
La explotación <strong>de</strong> <strong>la</strong>s anteriores facilida<strong>de</strong>s <strong>de</strong>p<strong>en</strong><strong>de</strong> <strong><strong>de</strong>l</strong> uso <strong>de</strong> los<br />
dispositivos terminales. Los dispositivos terminales actualm<strong>en</strong>te<br />
disponibles, como el teléfono RDSI, el vi<strong>de</strong>o teléfono RDSI, el<br />
teléfono RDSi mas PC y funciones <strong>de</strong> servidor proporcionan una<br />
44
nueva calidad <strong>de</strong> comunicaciones y una mayor eficacia <strong>en</strong> el puesto<br />
<strong>de</strong> trabajo.<br />
2.2,2 CLASIFICACIÓN DE LOS SERVICIOS DE TELECOMUNICACIONES<br />
2.2.2.1 Servicios Básicos.-<br />
Estos servicios pue<strong>de</strong>n ser prestados por <strong>la</strong> red, in<strong>de</strong>p<strong>en</strong>di<strong>en</strong>tem<strong>en</strong>te<br />
<strong>de</strong> otros servicios. Así por ejemplo se ti<strong>en</strong>e:<br />
- Servicios vocales<br />
- Vi<strong>de</strong>otelefonia<br />
- Vid eoconf er<strong>en</strong> cía<br />
- Transmisión <strong>de</strong> datos<br />
- Facsímil<br />
2.2.2.2 Servicios Su píeme ntarios.-<br />
Complem<strong>en</strong>tan a un servicio básico para mejorar o modificar su<br />
pres<strong>en</strong>tación. Van siempre asociados a un servicio básico. Por<br />
ejemplo se ti<strong>en</strong>e:<br />
- Servicios <strong>de</strong> M<strong>en</strong>sajería:<br />
- Correo electrónico<br />
- Servicio <strong>de</strong> Consulta:<br />
- Vi<strong>de</strong>otex<br />
- Vi<strong>de</strong>oteca<br />
- Audioteca<br />
- Base <strong>de</strong> datos<br />
2.3 RDSI DE BANDA ANGOSTA Y BANDA ANCHA.-<br />
Como se m<strong>en</strong>cionó anteriorm<strong>en</strong>te, exist<strong>en</strong> dos tipos <strong>de</strong> RDSI según el<br />
ancho <strong>de</strong> banda:<br />
1. RDSI <strong>de</strong> Banda Angosta: Con velocida<strong>de</strong>s <strong>de</strong> hasta 2 MB/seg.<br />
2. RDSI <strong>de</strong> Banda Ancha: Con velocida<strong>de</strong>s mayores a 2 MB/seg.<br />
45
A continuación se <strong>de</strong>scribe el tipo <strong>de</strong> canales RDSI que son mas<br />
comúnm<strong>en</strong>te utilizados para comunicación <strong>de</strong> audio, vi<strong>de</strong>o y datos <strong>de</strong><br />
acuerdo al ancho <strong>de</strong> banda que se utilice <strong>en</strong> <strong>la</strong> transmisión <strong>de</strong><br />
información, y <strong>la</strong> velocidad <strong>de</strong> canal <strong>de</strong> los mismos<br />
Tipos <strong>de</strong> Canales:<br />
Exist<strong>en</strong> difer<strong>en</strong>tes tipos <strong>de</strong> canales según el tipo <strong>de</strong> banda (angosta ó<br />
ancha) que se esté utilizando y.esto influye <strong>en</strong> ia velocidad <strong><strong>de</strong>l</strong> canal,<br />
así pues se ti<strong>en</strong>e.<br />
- Banda Angosta:<br />
CANAL RDSI VELOCIDAD DE CANAL<br />
H<br />
- Banda Ancha:<br />
B 64 Kb/s<br />
D 16 Kb/s<br />
Ho 384 Kb/s<br />
Hn 1536 Kb/s<br />
Hi2 _ 192.0 Kb/s<br />
CANAL RDSI VELOCIDAD DE CANAL<br />
H21: 32768 Kb/s<br />
H22: Del or<strong>de</strong>n <strong>de</strong> 43 a 45 Mb/s<br />
2.3.1 RDSI <strong>de</strong> Banda Angosta.- (RDSlba).-<br />
La RDSI <strong>de</strong> banda angosta se caracteriza por un limitado ancho <strong>de</strong><br />
banda (<strong>de</strong> hasta 2Mb/s), y constituye una primera etapa <strong>en</strong> el<br />
<strong>de</strong>sarrollo <strong>de</strong> <strong>la</strong> RDSI, <strong>en</strong> <strong>la</strong> cual se ofrece <strong>la</strong> posibilidad <strong>de</strong> t<strong>en</strong>er una<br />
gama <strong>de</strong> aplicaciones (vocales y no vocales) <strong>en</strong> <strong>la</strong> misma red pero <strong>de</strong><br />
46
un modo limitado, principalm<strong>en</strong>te <strong>en</strong> lo que ti<strong>en</strong>e que ver a<br />
accesibilidad <strong>de</strong> servicios y velocidad <strong>de</strong> transmisión<br />
Así pues se ti<strong>en</strong>e dos tipos <strong>de</strong> accesos<br />
• Acceso Básico: 2B + D (2 hilos) don<strong>de</strong> D = 16 Kb/s<br />
B = 64 Kb/s<br />
Por el canal D se pue<strong>de</strong> transmitir datos y señalización<br />
alternadam<strong>en</strong>te, realizando lo que se <strong>de</strong>nomina conmutación <strong>de</strong><br />
paquetes1 (SAPI = 16) (SAPI= I<strong>de</strong>ntificador <strong><strong>de</strong>l</strong> Punto <strong>de</strong> Acceso a!<br />
Servicio).<br />
Por el canal B se realiza <strong>la</strong> transmisión <strong>de</strong> datos <strong>en</strong> conmutación <strong>de</strong><br />
circuitos2 (SAPI = 0).<br />
• Acceso Primario<br />
a) 23B + D (U.S.A. y Japón) — 1544 Kbps<br />
b) 30B + D (Europa) — 2048 Kbps<br />
Con <strong>la</strong>s sigui<strong>en</strong>tes variantes: 4Ho, 3Ho, 5Ho+ D, HH, H12 + D<br />
Este tipo <strong>de</strong> accesos se los conoce como <strong>de</strong> segunda fase <strong>en</strong> <strong>la</strong><br />
evolución <strong>de</strong> <strong>la</strong> RDSI.<br />
La RDSI <strong>de</strong> banda angosta soporta <strong>la</strong>s actuales aplicaciones que se<br />
ti<strong>en</strong>e <strong>en</strong> el campo <strong>de</strong> <strong>la</strong> multimedia, siempre y cuando se utilic<strong>en</strong><br />
algoritmos <strong>de</strong> compresión.<br />
Conmutación <strong>en</strong> Modo Paquete.- Es aquel <strong>en</strong> el que el cont<strong>en</strong>ido <strong>de</strong> <strong>la</strong> información es corto, con m<strong>en</strong>sajes seccionados, no necesita<br />
memoria pasiva <strong>en</strong> los nodos, es mas fácil para <strong>de</strong>tectar y corregir errores.<br />
2 • «<br />
Conmutación <strong>en</strong> Circuitos.- Es aquel <strong>en</strong> el que perman<strong>en</strong>tem<strong>en</strong>te hay un circuito <strong>de</strong> <strong>en</strong><strong>la</strong>ce (transpar<strong>en</strong>te, sin injer<strong>en</strong>cia intelig<strong>en</strong>te <strong>en</strong> <strong>la</strong><br />
señal)<br />
47
2.3.2 RDSI <strong>de</strong> Banda Ancha (RDSIBA).-<br />
En años reci<strong>en</strong>tes, <strong>la</strong> evolución <strong>de</strong> <strong>la</strong> infraestructura <strong>de</strong> <strong>la</strong>s<br />
telecomunicaciones ha estado <strong>de</strong>terminada por el viraje que se ha<br />
dado <strong>de</strong> to análogo a lo digital, y últimam<strong>en</strong>te por <strong>la</strong> llegada <strong>de</strong> <strong>la</strong>s<br />
re<strong>de</strong>s <strong>de</strong> banda ancha. Este avance tecnológico ha ocasionado un<br />
cambio <strong>en</strong> <strong>la</strong> prestación <strong>de</strong> servicios, <strong>en</strong>tre realizarlos con líneas<br />
utilizando TDM (Múltiplex por División <strong>de</strong> Tiempo), a realizarlos<br />
utilizando re<strong>de</strong>s <strong>de</strong> banda ancha con Modo <strong>de</strong> Transfer<strong>en</strong>cia<br />
Asincrónica (ATM), utilizando <strong>la</strong>s tecnologías <strong>de</strong> <strong>la</strong> Red Óptica<br />
Sincrónica (SONET)/(SDH)Jerarquía Digital Sincrónica.<br />
RDSIBA es una ext<strong>en</strong>sión <strong>de</strong> <strong>la</strong> RDSI <strong>de</strong> banda angosta, <strong>la</strong> cual<br />
provee mayor ancho <strong>de</strong> banda, para manejar voz, información, y<br />
servicios <strong>de</strong> ví<strong>de</strong>o <strong>de</strong> alta velocidad. Se <strong>de</strong>fine <strong>la</strong> arquitectura <strong>de</strong> <strong>la</strong><br />
RDSIBA, estableci<strong>en</strong>do tres capas:<br />
- La capa Física<br />
- La capa ATM<br />
- La capa <strong>de</strong> adaptación <strong>de</strong> ATM.<br />
La capa física <strong><strong>de</strong>l</strong> mo<strong><strong>de</strong>l</strong>o RDSIBA, <strong>de</strong>fine los requisitos <strong>de</strong> <strong>la</strong><br />
interface eléctrica u óptica, así como también <strong>la</strong> velocidad <strong>de</strong> <strong>la</strong> línea,<br />
<strong>la</strong> cual <strong>de</strong>termina <strong>la</strong>s tasas <strong>de</strong> transmisión con <strong>la</strong> que van a ser<br />
manejadas.<br />
La capa ATM <strong>de</strong>fine el formato <strong>de</strong> <strong>la</strong> célu<strong>la</strong>, mi<strong>en</strong>tras que <strong>la</strong> capa <strong>de</strong><br />
adaptación <strong><strong>de</strong>l</strong> ATM, <strong>de</strong>fine el proceso <strong>de</strong> conversión <strong>de</strong> tráfico <strong>de</strong><br />
<strong>en</strong>trada a <strong>la</strong> célu<strong>la</strong> que ti<strong>en</strong>e una ext<strong>en</strong>sión fija <strong>de</strong> 53 bytes.<br />
2.3.2.1 Modo <strong>de</strong> Transfer<strong>en</strong>cia Asincrónica (ATM).-<br />
Para <strong>en</strong>t<strong>en</strong><strong>de</strong>r que es ATM, es apropiado dar una breve introducción<br />
<strong>de</strong> STM (Modo Sincrónico <strong>de</strong> Transfer<strong>en</strong>cia). STM es usado por <strong>la</strong>s<br />
re<strong>de</strong>s troncales <strong>de</strong> telecomunicaciones para transferir voz<br />
48
empaquetada y datos cuando se ti<strong>en</strong>e que cubrir distancias <strong>la</strong>rgas. Es<br />
un mecanismo <strong>de</strong> re<strong>de</strong>s con conmutación <strong>de</strong> circuitos, don<strong>de</strong> se<br />
establece una conexión <strong>en</strong>tre dos puntos antes <strong>de</strong> que comi<strong>en</strong>ce <strong>la</strong><br />
transfer<strong>en</strong>cia <strong>de</strong> datos, y se termina cuando los dos puntos extremos<br />
han concluido <strong>la</strong> comunicación. Así pues, los puntos extremos asignan<br />
y reservan el ancho <strong>de</strong> banda <strong>de</strong> <strong>la</strong> conexión durante todo el tiempo<br />
que dure <strong>la</strong> comunicación, aun cuando no se esté transmiti<strong>en</strong>do datos.<br />
La forma <strong>en</strong> que los datos son transportados a través <strong>de</strong> una red STM<br />
es dividi<strong>en</strong>do el ancho <strong>de</strong> banda <strong>de</strong> los <strong>en</strong><strong>la</strong>ces STM (<strong>en</strong><strong>la</strong>ces T-1 y T-<br />
3) <strong>en</strong> una unidad fundam<strong>en</strong>tal <strong>de</strong> transmisión l<strong>la</strong>mada segm<strong>en</strong>to <strong>de</strong><br />
tiempo. Estos segm<strong>en</strong>tos están organizados <strong>en</strong> un tr<strong>en</strong> que conti<strong>en</strong>e<br />
un número fijo <strong>de</strong> segm<strong>en</strong>tos y están rotu<strong>la</strong>dos <strong>de</strong> 1 a N. El tr<strong>en</strong> se<br />
repite periódicam<strong>en</strong>te cada T períodos <strong>de</strong> tiempo, con los segm<strong>en</strong>tos<br />
<strong>en</strong> el tr<strong>en</strong> siempre <strong>en</strong> <strong>la</strong> misma posición y con el mismo rótulo. Pue<strong>de</strong><br />
haber hasta M tr<strong>en</strong>es difer<strong>en</strong>tes rotu<strong>la</strong>dos <strong>de</strong> 1 a M, todos repiti<strong>en</strong>do<br />
con el período <strong>de</strong> tiempo T y todos llegando <strong>de</strong>ntro <strong><strong>de</strong>l</strong> mismo período<br />
<strong>de</strong> tiempo. (Los parámetros N.T.M están <strong>de</strong>terminados por comités <strong>de</strong><br />
estándares y son difer<strong>en</strong>tes para Europa y Estados Unidos).<br />
Como se pue<strong>de</strong> observar <strong>de</strong> lo anteriorm<strong>en</strong>te dicho, <strong>en</strong> el Modo <strong>de</strong><br />
Transfer<strong>en</strong>cia Sincrónico, una vez que et segm<strong>en</strong>to <strong>de</strong> tiempo es<br />
asignado a una conexión, g<strong>en</strong>eralm<strong>en</strong>te permanece asignado para el<br />
uso exclusivo <strong>de</strong> esa conexión a través <strong><strong>de</strong>l</strong> tiempo <strong>de</strong> vida <strong>de</strong> ésta; <strong>en</strong><br />
otras pa<strong>la</strong>bras, si una conexión ti<strong>en</strong>e datos para transmitir, pone sus<br />
datos <strong>de</strong>ntro <strong><strong>de</strong>l</strong> segm<strong>en</strong>to asignado (time-slot), pero si <strong>la</strong> conexión no<br />
ti<strong>en</strong>e ningún dato para transmitir, este segm<strong>en</strong>to se va vacío, y por lo<br />
g<strong>en</strong>eral suce<strong>de</strong> que muchas conexiones se van vacías, por lo que<br />
constituye un <strong>de</strong>sperdicio significativo <strong>de</strong> ancho <strong>de</strong> banda y limita el<br />
número <strong>de</strong> conexiones que pue<strong>de</strong>n ser soportadas simultáneam<strong>en</strong>te.<br />
Por lo tanto <strong>la</strong>s compañías <strong>de</strong> telecomunicaciones están investigando<br />
el uso y <strong>la</strong> tecnología a<strong>de</strong>cuada <strong>de</strong> re<strong>de</strong>s <strong>de</strong> fibra óptica para po<strong>de</strong>r<br />
49
ealizar <strong>en</strong><strong>la</strong>ces <strong>de</strong> comunicación <strong>en</strong>tre varios países y <strong>en</strong><strong>la</strong>ces<br />
interoceánicos con velocida<strong>de</strong>s <strong>de</strong> Gigabits por segundo Se <strong>de</strong>sea<br />
llevar <strong>en</strong> forma integrada, tanto tráfico <strong>en</strong> tiempo real tal como <strong>la</strong> voz<br />
y el vi<strong>de</strong>o <strong>de</strong> alta resolución que pue<strong>de</strong> tolerar alguna pérdida pero no<br />
<strong>de</strong>mora, como también el tráfico que no es <strong>en</strong> tiempo real tal como los<br />
datos y <strong>la</strong> transfer<strong>en</strong>cia <strong>de</strong> archivos que pue<strong>de</strong>n tolerar alguna<br />
<strong>de</strong>mora pero no pérdida.<br />
El problema <strong>de</strong> llevar tráfico <strong>de</strong> difer<strong>en</strong>tes características sobre el<br />
mismo medio y <strong>de</strong> una forma integrada, es que el requerimi<strong>en</strong>to <strong>de</strong><br />
ancho <strong>de</strong> banda pico <strong>de</strong> estas fu<strong>en</strong>tes <strong>de</strong> tráfico pue<strong>de</strong> ser bastante<br />
alto, como <strong>en</strong> el vi<strong>de</strong>o <strong>de</strong> alta resolución, pero <strong>la</strong> duración <strong>de</strong><br />
transmisión <strong>de</strong> datos pue<strong>de</strong> ser bastante pequeña.<br />
En otras pa<strong>la</strong>bras, los datos vi<strong>en</strong><strong>en</strong> <strong>en</strong> ráfagas y <strong>de</strong>b<strong>en</strong> ser<br />
transmitidos a <strong>la</strong> velocidad pico <strong>de</strong> <strong>la</strong> ráfaga, pero el promedio <strong>de</strong><br />
tiempo <strong>de</strong> llegada <strong>en</strong>tre <strong>la</strong>s ráfagas pue<strong>de</strong> ser bastante <strong>la</strong>rgo y<br />
aleatoriam<strong>en</strong>te distribuido. Para tales conexiones <strong>en</strong> ráfaga,<br />
reservarse un segm<strong>en</strong>to a velocidad pico <strong>en</strong> todo mom<strong>en</strong>to sería un<br />
<strong>de</strong>sperdicio consi<strong>de</strong>rable <strong>de</strong> ancho <strong>de</strong> banda, cuando <strong>en</strong> promedio<br />
solo 1 <strong>de</strong> cada 10 segm<strong>en</strong>tos lleva datos. Así-, usar el modo <strong>de</strong><br />
transfer<strong>en</strong>cia STM se vuelve inefici<strong>en</strong>te a medida que el ancho <strong>de</strong><br />
banda <strong>de</strong> <strong>en</strong><strong>la</strong>ce, <strong>la</strong> tasa pico <strong>de</strong> transfer<strong>en</strong>cia <strong>de</strong> tráfico y <strong>la</strong><br />
característica- g<strong>en</strong>eral <strong>de</strong> ráfagas <strong><strong>de</strong>l</strong> tráfico expresada como <strong>la</strong><br />
re<strong>la</strong>ción pico/promedio, sub<strong>en</strong>.<br />
Por esta razón se concibió ATM, <strong>en</strong> <strong>la</strong> cual <strong>la</strong> i<strong>de</strong>a principal era, <strong>en</strong><br />
lugar <strong>de</strong> siempre i<strong>de</strong>ntificar una conexión por el número <strong>de</strong><br />
segm<strong>en</strong>tos, simplem<strong>en</strong>te llevar el i<strong>de</strong>ntificador <strong>de</strong> <strong>la</strong> conexión junto<br />
con los datos <strong>en</strong> cualquier segm<strong>en</strong>to, y mant<strong>en</strong>er pequeño el tamaño<br />
<strong><strong>de</strong>l</strong> segm<strong>en</strong>to, <strong>de</strong> manera tal que si cualquier segm<strong>en</strong>to que se<br />
perdiera <strong>en</strong> ruta <strong>de</strong>bido a congestión, no se pierdan muchos datos, y<br />
50
<strong>en</strong> algunos caso incluso que se puedan recuperar. Estos conceptos<br />
se podían re<strong>la</strong>cionar mucho con los <strong>de</strong> conmutación <strong>en</strong> paquetes, y es<br />
por esta razón que lo l<strong>la</strong>maron "Conmutación rápida <strong>de</strong> paquetes con<br />
paquetes <strong>de</strong> longitud fija y corta".<br />
Así, dos puntos extremos <strong>en</strong> una red ATM están asociados <strong>en</strong>tre ellos<br />
por medio <strong>de</strong> un ¡<strong>de</strong>ntificador l<strong>la</strong>mado "I<strong>de</strong>ntificador <strong>de</strong> Circuito<br />
Virtual" (VCI) <strong>en</strong> vez <strong>de</strong> estar asociados por un segm<strong>en</strong>to <strong>de</strong> tiempo o<br />
un número <strong>de</strong> segm<strong>en</strong>tos como <strong>en</strong> <strong>la</strong> red STM. El VCI es Nevado <strong>en</strong><br />
<strong>la</strong> sección <strong><strong>de</strong>l</strong> <strong>en</strong>cabezado <strong><strong>de</strong>l</strong> paquete rápido. El paquete rápido <strong>en</strong> sí<br />
mismo es transportado <strong>en</strong> el mismo tipo <strong>de</strong> segm<strong>en</strong>to anterior, pero ya<br />
no es necesario un rótulo o <strong>de</strong>signación <strong>de</strong> segm<strong>en</strong>to. Los términos<br />
paquete rápido (fast packet), celda (cell) y segm<strong>en</strong>to (bucket) se usan<br />
<strong>en</strong> forma intercambiable <strong>en</strong> <strong>la</strong> literatura ATM y se refier<strong>en</strong> a <strong>la</strong> misma<br />
cosa.<br />
Multiplexación estadística.-<br />
La conmutación rápida <strong>de</strong> paquetes int<strong>en</strong>ta resolver el problema <strong><strong>de</strong>l</strong><br />
segm<strong>en</strong>to no usado <strong>de</strong> STM, multiplexando estadísticam<strong>en</strong>te varias<br />
conexiones sobre el mismo <strong>en</strong><strong>la</strong>ce, basado <strong>en</strong> sus características <strong>de</strong><br />
tráfico. En otras pa<strong>la</strong>bras, si <strong>en</strong> un número gran<strong>de</strong> <strong>de</strong> conexiones,<br />
éstas constan <strong>de</strong> muchas ráfagas (p.ej. si su tasa pico/promedio es<br />
<strong>de</strong> 10:1 o mas alta), <strong>en</strong>tonces todas el<strong>la</strong>s pue<strong>de</strong>n ser asignados al<br />
mismo <strong>en</strong><strong>la</strong>ce con <strong>la</strong> esperanza <strong>de</strong> que estadísticam<strong>en</strong>te el<strong>la</strong>s no<br />
dispararán <strong>la</strong>s ráfagas simultáneam<strong>en</strong>te. Y si algunas <strong>de</strong> el<strong>la</strong>s<br />
dispararan <strong>la</strong>s ráfagas simultáneam<strong>en</strong>te, <strong>de</strong>berá haber sufici<strong>en</strong>te<br />
e<strong>la</strong>sticidad <strong>de</strong> manera que los segm<strong>en</strong>tos puedan ser almac<strong>en</strong>ados<br />
temporalm<strong>en</strong>te y puestos <strong>en</strong> segm<strong>en</strong>tos libres posteriorm<strong>en</strong>te<br />
disponibles. Esto se l<strong>la</strong>ma multiplexación estadística, y permite que <strong>la</strong><br />
suma <strong>de</strong> los requerimi<strong>en</strong>tos <strong>de</strong> ancho <strong>de</strong> banda pico <strong>de</strong> todas <strong>la</strong>s<br />
conexiones sobre un <strong>en</strong><strong>la</strong>ce, puedan exce<strong>de</strong>r el ancho <strong>de</strong> banda<br />
agregado disponible <strong><strong>de</strong>l</strong> <strong>en</strong><strong>la</strong>ce, bajo ciertas condiciones <strong>de</strong> disciplina.<br />
51
Esto era imposible para una red STM, y es <strong>la</strong> principal difer<strong>en</strong>cia con<br />
una red ATM.<br />
Características principales <strong>de</strong> una Red ATM.-<br />
El intercambio <strong>de</strong> célu<strong>la</strong> <strong>de</strong> relevo, y específicam<strong>en</strong>te el Modo <strong>de</strong><br />
Transfer<strong>en</strong>cia Asincrónica (ATM), se ha diseñado para superar una <strong>de</strong><br />
<strong>la</strong>s principales <strong>de</strong>sv<strong>en</strong>tajas <strong>de</strong> TDM, esto es, <strong>la</strong> <strong>de</strong>p<strong>en</strong><strong>de</strong>ncia <strong>de</strong> TDM<br />
<strong>en</strong> <strong>la</strong> asignación <strong>de</strong> amplitud <strong>de</strong> banda fija y su utilización <strong>en</strong> niveles<br />
por <strong>de</strong>bajo <strong>de</strong> lo óptimo <strong>de</strong> <strong>la</strong> capacidad <strong>de</strong> red disponible. Con <strong>la</strong><br />
célu<strong>la</strong> <strong>de</strong> relevo, <strong>la</strong> información <strong>de</strong> cualquier tipo, ya sea voz, ví<strong>de</strong>o,<br />
información (datos) o trafico <strong>de</strong> imág<strong>en</strong>es, se pue<strong>de</strong> dividir, <strong>de</strong><br />
acuerdo con el mo<strong><strong>de</strong>l</strong>o ATM, <strong>en</strong> célu<strong>la</strong>s <strong>de</strong> ext<strong>en</strong>sión fija,<br />
específicam<strong>en</strong>te <strong>de</strong> 53 bytes. A cada célu<strong>la</strong> se le asigna un<br />
<strong>en</strong>cabezado que <strong>la</strong> asocia con una aplicación <strong>de</strong>terminada y éstas son<br />
intercambiadas a lo <strong>la</strong>rgo <strong>de</strong> cualquier combinación <strong>de</strong> trayectorias y<br />
<strong>de</strong> módulos, que sean necesarios para transmitir<strong>la</strong>s a su <strong>de</strong>stino. A<br />
difer<strong>en</strong>cia <strong>de</strong> TDM, con ATM, una aplicación pue<strong>de</strong> apropiarse, <strong>en</strong><br />
cualquier mom<strong>en</strong>to, <strong>de</strong> toda <strong>la</strong> amplitud <strong>de</strong> banda ociosa, a través <strong>de</strong><br />
un acceso <strong>de</strong>terminado <strong>de</strong> <strong>la</strong> insta<strong>la</strong>ción, permiti<strong>en</strong>do así, una<br />
utilización mucho mas efici<strong>en</strong>te <strong>de</strong> ia capacidad <strong>de</strong> <strong>la</strong> red.<br />
Una célu<strong>la</strong> ATM, según lo especificado por el comité T1S1, sub<br />
comité <strong>de</strong> <strong>la</strong> ANSÍ consiste <strong>de</strong> 53 bytes <strong>de</strong> los cuales 48 bytes son <strong>de</strong><br />
carga útil transportando información (datos) <strong><strong>de</strong>l</strong> usuario, y 5 bytes son<br />
<strong>de</strong> <strong>en</strong>cabezado, que conti<strong>en</strong><strong>en</strong> información <strong><strong>de</strong>l</strong> canal y <strong>de</strong> <strong>la</strong><br />
trayectoria, así como también control <strong>de</strong> flujo, tipo <strong>de</strong> carga útil, e<br />
información sobre <strong>la</strong> <strong>de</strong>tección <strong>de</strong> errores. El tamaño <strong>de</strong> <strong>la</strong> célu<strong>la</strong> fija<br />
<strong>de</strong> ATM <strong>de</strong> 53 bytes, simplifica el proceso que se requiere <strong><strong>de</strong>l</strong><br />
hardware, y así mismo produce una conducta mas fácil <strong>de</strong> pre<strong>de</strong>cir, a<br />
lo <strong>la</strong>rgo <strong>de</strong> toda <strong>la</strong> red.<br />
52
<strong>la</strong> red SONET/SDH, servirá como el medio <strong>de</strong> transporte sobre el cual<br />
tos paquetes ATM serán <strong>en</strong>viados. La jerarquía SONET/SDH, ofrec<strong>en</strong><br />
mejoras significativas al multiplexor asincrónico, <strong>en</strong> lo re<strong>la</strong>cionado a<br />
flexibilidad, tasa <strong>de</strong> transmisión <strong>en</strong> manejo y garantías <strong>en</strong> e!<br />
r<strong>en</strong>dimi<strong>en</strong>to.<br />
2.4 APLICACIONES DE LA RDSL-<br />
Las sigui<strong>en</strong>tes aplicaciones son ejemplos <strong>de</strong> usos futuros <strong>de</strong> <strong>la</strong> red<br />
RDSI que podrán ser utilizados por difer<strong>en</strong>tes tipos <strong>de</strong> empresas,<br />
especialm<strong>en</strong>te aquel<strong>la</strong>s que están consi<strong>de</strong>rando <strong>la</strong> posibilidad <strong>de</strong><br />
utilizar tecnología avanzada.<br />
2.4.1 Consolidación <strong>de</strong> <strong>la</strong>s Re<strong>de</strong>s.-<br />
Actualm<strong>en</strong>te los usuarios utilizan un número variado <strong>de</strong> re<strong>de</strong>s para<br />
transmitir su información datos y/o voz e imág<strong>en</strong>es. La <strong>de</strong>manda<br />
busca que estas re<strong>de</strong>s se consoli<strong>de</strong>n <strong>en</strong> una so<strong>la</strong> columna vertebral,<br />
reduci<strong>en</strong>do los costos <strong>de</strong> <strong>la</strong>s mismas, al utilizar una so<strong>la</strong> estructura<br />
administrativa, que permita dar mejores servicios. Se requerirá <strong>de</strong><br />
esta red que provea interfaces para el Frame Re<strong>la</strong>y, HDLC/SDLC,<br />
X.25, LAN (Tok<strong>en</strong> Ring y Ethernet), apoyo <strong>de</strong> ATM, así como también<br />
PBX <strong>de</strong> voz y fax. Adicionalm<strong>en</strong>te se requiere <strong>de</strong> este apoyo para el<br />
manejo <strong>de</strong> varias velocida<strong>de</strong>s bajas, así como también <strong>de</strong> interfaces<br />
<strong>de</strong> alta velocidad, para nuevas aplicaciones.<br />
2.4.2 Biblioteca <strong>Electrónica</strong> Digital.-<br />
Una <strong>de</strong> <strong>la</strong>s aplicaciones que impondrá una cantidad significativa <strong>de</strong><br />
requisitos a <strong>la</strong> red, es <strong>la</strong> biblioteca electrónica digital. La habilidad<br />
para localizar y <strong>en</strong>tregar información <strong>de</strong> multimedia (voz, ví<strong>de</strong>o,<br />
datos) <strong>de</strong> varias bibliotecas (servidores), <strong>en</strong> cualquier mom<strong>en</strong>to, es -<strong>la</strong><br />
54
ase <strong>de</strong> <strong>la</strong> estructura <strong>de</strong> esta aplicación. La banda ancha y el tiempo<br />
real <strong>de</strong> utilización para este tipo <strong>de</strong> aplicación, llevará un número<br />
significativo <strong>de</strong> requisitos <strong>de</strong> <strong>la</strong> red.<br />
La localización <strong>de</strong> <strong>la</strong> información <strong>de</strong>be ser transpar<strong>en</strong>te a <strong>la</strong> <strong>de</strong>manda<br />
<strong><strong>de</strong>l</strong> usuario, se <strong>de</strong>be pres<strong>en</strong>tar con el mínimo <strong>de</strong> <strong>de</strong>mora <strong>de</strong> <strong>en</strong>trega<br />
<strong>de</strong> información. La red o re<strong>de</strong>s a través <strong>de</strong> (as cuales <strong>la</strong> <strong>de</strong>manda <strong>de</strong><br />
servicio es satisfecha <strong>de</strong>b<strong>en</strong> prestar un servicio <strong>de</strong> calidad <strong>en</strong> tiempo<br />
real. El tiempo <strong>de</strong> conexión a un servidor, es usualm<strong>en</strong>te corto. El<br />
usuario pue<strong>de</strong> pasar <strong>de</strong> una biblioteca a otra rápidam<strong>en</strong>te, <strong>de</strong> manera<br />
que durante una sesión particu<strong>la</strong>r, éste pueda requerir conexión<br />
simultánea a varias bibliotecas difer<strong>en</strong>tes. El tráfico es asimétrico ( <strong>la</strong><br />
cantidad <strong>de</strong> información transmitida a una estación <strong>de</strong> trabajo es<br />
significativam<strong>en</strong>te mas gran<strong>de</strong> que el requerimi<strong>en</strong>to hecho por <strong>la</strong><br />
estación <strong>de</strong> trabajo <strong><strong>de</strong>l</strong> usuario).<br />
Este tipo <strong>de</strong> aplicación, g<strong>en</strong>era una información única <strong>de</strong><br />
administración <strong>de</strong> re<strong>de</strong>s para aplicaciones <strong>de</strong> contabilidad y cu<strong>en</strong>tas,<br />
así como también el manejo <strong>de</strong> fal<strong>la</strong>s y el <strong>de</strong>sempeño <strong>de</strong> asist<strong>en</strong>cia,<br />
cuando ocurr<strong>en</strong> problemas.<br />
2.4.3 Media Disponible <strong>en</strong> <strong>de</strong>manda.-<br />
Esta aplicación existirá <strong>en</strong> dos ambi<strong>en</strong>tes, el <strong><strong>de</strong>l</strong> consumidor normal<br />
(hogar), y el empresarial. En <strong>la</strong> actualidad, muchas pruebas <strong>de</strong><br />
consumidores se <strong>de</strong>sarrol<strong>la</strong>n <strong>en</strong> todo el mundo buscando <strong>en</strong>t<strong>en</strong><strong>de</strong>r<br />
tanto <strong>la</strong> parte <strong><strong>de</strong>l</strong> área <strong>de</strong> los negocios como los aspectos técnicos <strong>de</strong><br />
<strong>la</strong> medida disponible <strong>en</strong> <strong>de</strong>manda. En el ambi<strong>en</strong>te <strong>de</strong> hogar,<br />
adicionalm<strong>en</strong>te a <strong>la</strong>s pelícu<strong>la</strong>s <strong>en</strong> <strong>de</strong>manda, f<strong>en</strong>óm<strong>en</strong>os como <strong>la</strong><br />
compra <strong>de</strong>s<strong>de</strong> el hogar, los juegos, e incluso <strong>de</strong>sempeñar activida<strong>de</strong>s<br />
llórales, <strong>de</strong>sempeñan una función pot<strong>en</strong>cial, a medida que esta<br />
aplicación se <strong>de</strong>sarrol<strong>la</strong>. Los requisitos <strong>de</strong> esta aplicación son<br />
-Tjpl<br />
gjmi<strong>la</strong>res a los <strong>de</strong> <strong>la</strong> biblioteca electrónica digital.<br />
55
2.4.4 Captura <strong>de</strong> información.-<br />
La distribución <strong>de</strong> información a ios distintos servidores, haci<strong>en</strong>do eí<br />
seguimi<strong>en</strong>to <strong>de</strong> cu<strong>en</strong>tas y efectuando tareas como e! archivo <strong>de</strong><br />
información, <strong>en</strong> <strong>la</strong> medida <strong>en</strong> que su utilización aum<strong>en</strong>ta g<strong>en</strong>era<br />
requisitos <strong>en</strong> <strong>la</strong> red <strong><strong>de</strong>l</strong> usuario.<br />
La <strong>de</strong>manda <strong>de</strong> <strong>la</strong> red para <strong>la</strong> aplicación <strong>de</strong> captura <strong>de</strong> información,<br />
está ori<strong>en</strong>tada hacia <strong>la</strong> distribución y mant<strong>en</strong>imi<strong>en</strong>to <strong>de</strong> <strong>la</strong> información<br />
<strong>de</strong> multimedia, a través <strong>de</strong> <strong>la</strong> red para <strong>la</strong> biblioteca electrónica digital<br />
y para <strong>la</strong> aplicación <strong>de</strong> media <strong>en</strong> <strong>de</strong>manda. Por ejemplo, para<br />
distribuir y tomar <strong>en</strong> cu<strong>en</strong>ta <strong>la</strong> información <strong>de</strong> varios servidores a los<br />
cuales <strong>la</strong> información pue<strong>de</strong> ser <strong>en</strong>viada, y archivada, son dos<br />
problemas importantes, que <strong>de</strong>b<strong>en</strong> ser resueltos. La información será<br />
asimétrica, con muy poca información fluy<strong>en</strong>do hacia el orig<strong>en</strong>. Una<br />
transmisión libre <strong>de</strong> errores es un requisito importante, y si ocurre un<br />
error <strong>en</strong> <strong>la</strong> transmisión, se requiere arreg<strong>la</strong>rlo; sin embargo un error<br />
no pue<strong>de</strong> ser captado por todas <strong>la</strong>s personas, <strong>de</strong> manera que el<br />
proceso <strong>de</strong> arreglo se hace mas compiejo y <strong>la</strong> transmisión <strong>de</strong><br />
información pue<strong>de</strong> necesitar <strong>de</strong> una susp<strong>en</strong>sión temporal.<br />
2.4.5 Confer<strong>en</strong>cia.-<br />
La confer<strong>en</strong>cia <strong>de</strong> "habitación" y <strong>la</strong> confer<strong>en</strong>cia con co<strong>la</strong>boración <strong>de</strong><br />
un <strong>de</strong>sktop, están consi<strong>de</strong>radas como <strong>la</strong>s aplicaciones que se<br />
convertirán <strong>en</strong> impulsoras <strong>de</strong> servicios <strong>de</strong> banda ancha <strong>en</strong> los<br />
ambi<strong>en</strong>tes empresariales <strong><strong>de</strong>l</strong> futuro. La facilidad <strong>de</strong> los usuarios <strong>en</strong><br />
sus estaciones <strong>de</strong> trabajo, no so<strong>la</strong>m<strong>en</strong>te para ver a <strong>la</strong> persona al otro<br />
extremo, sino también para po<strong>de</strong>r mirar <strong>la</strong>s mismas imág<strong>en</strong>es <strong>en</strong> <strong>la</strong><br />
pantal<strong>la</strong> <strong>de</strong> su estación <strong>de</strong> trabajo, dará un valor significativo a <strong>la</strong><br />
^fuerza <strong>de</strong> trabajo <strong>de</strong> <strong>la</strong> estación empresarial mo<strong>de</strong>rna.<br />
56
Esta aplicación g<strong>en</strong>era muchos y muy distintos tipos <strong>de</strong> requisitos<br />
sobre <strong>la</strong> red, A difer<strong>en</strong>cia <strong>de</strong> <strong>la</strong>s que se discutieron anteriorm<strong>en</strong>te,<br />
aquí <strong>la</strong> transmisión es mas simétrica que <strong>en</strong> <strong>la</strong>s aplicaciones<br />
anteriores. En el caso <strong>de</strong> un error <strong>en</strong> <strong>la</strong> transmisión, el recobrarlo<br />
pue<strong>de</strong> ser simi<strong>la</strong>r a que el error no hubiera t<strong>en</strong>ido lugar.<br />
2.4.6 Simu<strong>la</strong>ción.-<br />
La capacidad <strong>de</strong> simu<strong>la</strong>r fórmu<strong>la</strong>s ci<strong>en</strong>tíficas y <strong>de</strong> recrear ev<strong>en</strong>tos<br />
reales será aprovechada por muchas empresas <strong>en</strong> el futuro. Por<br />
ejemplo, <strong>la</strong> recreación <strong>de</strong> un acci<strong>de</strong>nte, para <strong>la</strong> revisión <strong>de</strong> expertos<br />
<strong>de</strong> <strong>la</strong> industria <strong>de</strong> los seguros y <strong>la</strong> visualización ci<strong>en</strong>tífica, se ti<strong>en</strong><strong>en</strong><br />
como impulsores pot<strong>en</strong>ciales <strong>de</strong> este tipo <strong>de</strong> aplicación. Estas serán<br />
aplicaciones <strong>de</strong> tiempo real, que requerirán cantida<strong>de</strong>s variables <strong>de</strong><br />
amplitud <strong>de</strong> banda asimétrica.<br />
57
EL VIDEO TELEFONO DE<br />
IMÁGENES FIJAS Y MÓVILES
CAPITULO III<br />
3.1. EL VIDEO TELÉFONO DE IMÁGENES FIJAS.-<br />
3.1.1.- DESCRIPCIÓN Y FUNCIONAMIENTO DEL TELÉFONO "PICASSO" DE AT&T<br />
La vi<strong>de</strong>o telefonía <strong>en</strong> g<strong>en</strong>eral se divi<strong>de</strong> <strong>en</strong> dos etapas: una es <strong>la</strong> transmisión, a más<br />
<strong>de</strong> <strong>la</strong> voz y datos, <strong>de</strong> imág<strong>en</strong>es inmóviles, y <strong>la</strong> segunda compr<strong>en</strong><strong>de</strong> <strong>la</strong> transmisión<br />
<strong>de</strong> imág<strong>en</strong>es móviles <strong>en</strong> tiempo real. La primera parte <strong>de</strong> este capítulo ti<strong>en</strong>e por<br />
objeto el estudio <strong>de</strong> ví<strong>de</strong>o teléfonos con transmisión <strong>de</strong> imág<strong>en</strong>es fijas, y para el<br />
efecto se realiza un estudio <strong><strong>de</strong>l</strong> ví<strong>de</strong>o teléfono "Picasso" <strong>de</strong> AT&T, ya que este<br />
mo<strong><strong>de</strong>l</strong>o, a más <strong>de</strong> que se lo <strong>en</strong>cu<strong>en</strong>tra <strong>en</strong> nuestro medio, a nivel tecnológico está<br />
muy <strong>de</strong>sarrol<strong>la</strong>do.<br />
3.1.1.1 El Sistema Picasso.-<br />
Introducido <strong>en</strong> 1993, el Picasso AT&T es un teléfono <strong>de</strong> imág<strong>en</strong>es inmóviles con<br />
transmisiones simultáneas <strong>de</strong> voz e imág<strong>en</strong>es a color <strong>de</strong> alta resolución sobre una<br />
línea telefónica. Los dos usuario par<strong>la</strong>ntes pue<strong>de</strong>n observar y discutir <strong>la</strong>s<br />
imág<strong>en</strong>es transmitidas mi<strong>en</strong>tras usan un mouse para apuntar y hacer anotaciones<br />
sobre <strong>la</strong>s mismas. Las imág<strong>en</strong>es pue<strong>de</strong>n ser grabadas y almac<strong>en</strong>adas por un PC a<br />
través <strong>de</strong> una interfaz apropiado.<br />
3.1.1.2 Características.-<br />
El sistema Picasso ti<strong>en</strong>e un amplio set <strong>de</strong> características que prove<strong>en</strong> al usuario <strong>la</strong><br />
habilidad <strong>de</strong> transmitir, observar y manipu<strong>la</strong>r repres<strong>en</strong>taciones <strong>de</strong> objetos e<br />
imág<strong>en</strong>es capturadas.<br />
a) Anotaciones.- La aplicación conocida como "anotación" permite a los usuarios,<br />
tanto al que l<strong>la</strong>ma como al que recibe, realizar anotaciones y dibujos sobre <strong>la</strong><br />
imag<strong>en</strong> que está si<strong>en</strong>do mostrada <strong>en</strong> <strong>la</strong> pantal<strong>la</strong>, <strong>de</strong> esta manera se da <strong>la</strong><br />
oportunidad <strong>de</strong> discutir <strong>de</strong>talles, y <strong>de</strong>stacar áreas <strong>de</strong> particu<strong>la</strong>r interés. El usuario<br />
pue<strong>de</strong> realizar <strong>la</strong>s anotaciones <strong>en</strong> <strong>la</strong> pantal<strong>la</strong> con <strong>la</strong> ayuda <strong>de</strong> un mouse conectado<br />
58
<strong>en</strong> uno <strong>de</strong> los pórticos RS-232 <strong><strong>de</strong>l</strong> teléfono. Las anotaciones podrán ser<br />
almac<strong>en</strong>adas como parte <strong>de</strong> <strong>la</strong> imag<strong>en</strong>, o borradas haci<strong>en</strong>do un simple doble clic<br />
con el botón <strong><strong>de</strong>l</strong> mouse.<br />
b) Perfeccionami<strong>en</strong>to <strong>de</strong> Imag<strong>en</strong>.- Se provee al usuario un gran grupo <strong>de</strong><br />
utilitarios <strong><strong>de</strong>l</strong> sistema Picasso. Este grupo <strong>de</strong> utilitarios mejora <strong>la</strong> apari<strong>en</strong>cia visual<br />
<strong>de</strong> <strong>la</strong> imag<strong>en</strong> y <strong>la</strong> velocidad <strong>de</strong> transmisión, Estos incluy<strong>en</strong>:<br />
• Perfeccionami<strong>en</strong>to <strong>de</strong> fotografía. Imág<strong>en</strong>es oscuras pue<strong>de</strong>n ser ac<strong>la</strong>radas<br />
significativam<strong>en</strong>te aplicando <strong>la</strong> técnica conocida como histograma <strong>de</strong><br />
ecualización,<br />
• Perfeccionami<strong>en</strong>to <strong>de</strong> texto. Las imág<strong>en</strong>es que se combinan con el texto<br />
pue<strong>de</strong>n ser mejoradas por afinami<strong>en</strong>to <strong><strong>de</strong>l</strong> texto usando una técnica conocida<br />
como <strong>de</strong>tección <strong>de</strong> marg<strong>en</strong>.<br />
• Operación libre <strong>de</strong> fluctuación. Una trama simple que captura movimi<strong>en</strong>to <strong>de</strong><br />
objetos pue<strong>de</strong> g<strong>en</strong>erar un efecto <strong>de</strong> fluctuación <strong>de</strong>bido al modo <strong>de</strong> interfer<strong>en</strong>cia<br />
<strong>de</strong> operación <strong>de</strong> <strong>la</strong> mayoría <strong>de</strong> los estándares <strong>de</strong> vi<strong>de</strong>o <strong>de</strong> <strong>la</strong>s cámaras. Si un<br />
objeto esta <strong>en</strong> movimi<strong>en</strong>to durante <strong>la</strong> captura, cada mitad <strong>de</strong> <strong>la</strong> imag<strong>en</strong><br />
capturada pue<strong>de</strong> ser ligeram<strong>en</strong>te difer<strong>en</strong>te <strong>de</strong>bido al retardo <strong>en</strong>tre los campos<br />
<strong>de</strong> vi<strong>de</strong>o. Cuando <strong>la</strong> imag<strong>en</strong> es <strong>de</strong>splegada, aparec<strong>en</strong> <strong>la</strong>s fluctuaciones. Este<br />
efecto pue<strong>de</strong> ser eliminado tanto por visualización <strong>de</strong> uno solo <strong>de</strong> los campos o<br />
por construcción <strong>de</strong> un nuevo cuadro por medio <strong>de</strong> interpo<strong>la</strong>ción <strong>de</strong> los<br />
campos. Ambos métodos afectan <strong>la</strong> resolución vertical <strong>de</strong> <strong>la</strong> imag<strong>en</strong>. La técnica<br />
<strong>de</strong> interpo<strong>la</strong>ción, sin embargo, usualm<strong>en</strong>te produce mejor calidad <strong>de</strong> imag<strong>en</strong>.<br />
c) Calidad <strong>de</strong> Imag<strong>en</strong> versus Tiempo <strong>de</strong> transmisión.- Los algoritmos <strong>de</strong><br />
compresión JPEG usados <strong>en</strong> el sistema Picasso produc<strong>en</strong> pérdida durante el<br />
proceso, esto es, alguna información se pier<strong>de</strong> irremediablem<strong>en</strong>te cuando se<br />
realiza compresión <strong>de</strong> imag<strong>en</strong>. Una alta compresión da como resultado mayor<br />
información que se pier<strong>de</strong>, y una gran pérdida afecta a <strong>la</strong> calidad <strong>de</strong> <strong>la</strong> imag<strong>en</strong>.<br />
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3.1.1.3 Modo <strong>de</strong> Operación.-<br />
Para efectuar <strong>la</strong> transmisión se necesita t<strong>en</strong>er insta<strong>la</strong>do un teléfono Picasso <strong>en</strong><br />
cada extremo <strong>de</strong> <strong>la</strong> línea, y un monitor o PC <strong>de</strong> recepción. El sistema <strong>de</strong> ví<strong>de</strong>o<br />
teléfono Picasso ofrece <strong>la</strong>s sigui<strong>en</strong>tes opciones durante una l<strong>la</strong>mada:<br />
- Al inicio <strong>de</strong> una l<strong>la</strong>mada telefónica el usuario pue<strong>de</strong> capturar imág<strong>en</strong>es<br />
propias para <strong>en</strong>viar<strong>la</strong>s como parte introductoria <strong>de</strong> <strong>la</strong> conversación.<br />
- Si se va a <strong>en</strong>viar muchas imág<strong>en</strong>es, este ví<strong>de</strong>o teléfono pue<strong>de</strong> acelerar <strong>la</strong><br />
transmisión usando el modo Sil<strong>en</strong>t Ví<strong>de</strong>o (activado por medio <strong>de</strong> m<strong>en</strong>ú) que<br />
permite hab<strong>la</strong>r sin vi<strong>de</strong>o durante <strong>la</strong> transmisión usando este modo, y <strong>la</strong>s<br />
imág<strong>en</strong>es luego se <strong>en</strong>viarán mucho más rápido.<br />
Pue<strong>de</strong> realizarse anotaciones y com<strong>en</strong>tarios <strong>de</strong> los puntos más<br />
sobresali<strong>en</strong>tes <strong>de</strong> una imag<strong>en</strong>, los mismos que pue<strong>de</strong>n ser archivado para<br />
revisarlos posteriorm<strong>en</strong>te,<br />
- Se pue<strong>de</strong> a<strong>de</strong>más organizar y archivar <strong>la</strong>s imág<strong>en</strong>es que se <strong>de</strong>sea <strong>en</strong>viar<br />
durante <strong>la</strong> conversación <strong>en</strong> <strong>la</strong> memoria interna <strong><strong>de</strong>l</strong> teléfono Picasso (memoria<br />
para 32 cuadros) o <strong>en</strong> algún dispositivo externo <strong>de</strong> memoria.<br />
Provee a<strong>de</strong>más <strong>de</strong> iluminación apropiada para <strong>la</strong> captura <strong>de</strong> bu<strong>en</strong>as<br />
imág<strong>en</strong>es. Si se <strong>en</strong>vía imág<strong>en</strong>es que conti<strong>en</strong><strong>en</strong> texto, éste t<strong>en</strong>drá el estilo y<br />
tamaño apropiado para que sea legible, especialm<strong>en</strong>te cuando se hace uso <strong>de</strong><br />
pantal<strong>la</strong>s pequeñas.<br />
De esta manera, <strong>la</strong>s personas <strong>en</strong>tre <strong>la</strong>s que se realiza <strong>la</strong> comunicación, pue<strong>de</strong>n<br />
discutir sobre <strong>la</strong>s imág<strong>en</strong>es que están vi<strong>en</strong>do, modificar<strong>la</strong>s, sombrear áreas <strong>de</strong><br />
interés, realizar com<strong>en</strong>tarios escritos, etc. consigui<strong>en</strong>do <strong>de</strong> esta forma, elevar <strong>la</strong><br />
calidad y utilidad <strong>de</strong> <strong>la</strong> comunicación. A<strong>de</strong>más, <strong>en</strong> cualquiera <strong>de</strong> los dos terminales<br />
se pue<strong>de</strong> almac<strong>en</strong>ar imág<strong>en</strong>es para revisar<strong>la</strong>s <strong>en</strong> un futuro.<br />
3.1.1.4 Funciones <strong><strong>de</strong>l</strong> teléfono Picasso.-<br />
Entre <strong>la</strong>s principales funciones que nos ofrece el ví<strong>de</strong>o teléfono Picasso se pue<strong>de</strong><br />
m<strong>en</strong>cionar <strong>la</strong>s sigui<strong>en</strong>tes:<br />
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Comunicaciones simultáneas <strong>de</strong> voz e imag<strong>en</strong>.<br />
Capacidad para almac<strong>en</strong>ar internam<strong>en</strong>te hasta 32 imág<strong>en</strong>es.<br />
Permite escoger <strong>en</strong>tre resolución <strong>de</strong> imág<strong>en</strong>es y <strong>la</strong> velocidad <strong>de</strong><br />
transmisión<br />
Permite hacer com<strong>en</strong>tarios.<br />
Muestra m<strong>en</strong>ús <strong>en</strong> pantal<strong>la</strong> para manejo <strong><strong>de</strong>l</strong> equipo<br />
Permite <strong>la</strong> operación a control remoto<br />
Selección para funcionami<strong>en</strong>to como teléfono <strong>de</strong> tonos o pulsos<br />
Opciones flexibles <strong>de</strong> interfaz:<br />
* Soporta equipos compatibles con formatos NTSC para<br />
captura, muestreo y almac<strong>en</strong>ami<strong>en</strong>to.<br />
+ Posee un puerto serial <strong>de</strong> 9 pines para com<strong>en</strong>tarios.<br />
* Posee un puerto paralelo por conectar el PC.<br />
* Interfaz <strong>de</strong> ví<strong>de</strong>o <strong>en</strong>trada / salida.<br />
* Inerfaz <strong>de</strong> S-Vi<strong>de</strong>o <strong>en</strong>trada / salida.<br />
Velocidad estándar <strong>de</strong> 14.4 Kbps, con mó<strong>de</strong>m V.32<br />
Interfaz para P C (opcional)<br />
Permite a<strong>de</strong>más todas <strong>la</strong>s facilida<strong>de</strong>s <strong>de</strong> un teléfono estándar <strong>de</strong> voz<br />
como: facilidad <strong>de</strong> espera, remarcado <strong><strong>de</strong>l</strong> último número, 5 números <strong>de</strong><br />
memoria para marcado rápido, control <strong>de</strong> volum<strong>en</strong> para par<strong>la</strong>nte y<br />
timbrado etc.<br />
3,1.2 EQUIPOS Y COMPONENTES DEL SISTEMA<br />
Los compon<strong>en</strong>tes típicos <strong>de</strong> un sistema con ví<strong>de</strong>o teléfono Picasso se muestra <strong>en</strong><br />
<strong>la</strong> Figura 3.1. El equipo básico necesario para realizar una comunicación por vi<strong>de</strong>o<br />
teléfono Picasso es:<br />
- Un vi<strong>de</strong>o teléfono Picasso (uno a cada <strong>la</strong>do <strong>de</strong> <strong>la</strong> línea)<br />
- Una cámara <strong>de</strong> vi<strong>de</strong>o (una a cada <strong>la</strong>do <strong>de</strong> <strong>la</strong> línea)<br />
- Un monitor (uno a cada <strong>la</strong>do <strong>de</strong> <strong>la</strong> línea)<br />
- Una línea telefónica<br />
El tipo <strong>de</strong> cables y adaptadores que se necesita, <strong>de</strong>p<strong>en</strong><strong>de</strong> <strong><strong>de</strong>l</strong> tipo <strong>de</strong> señal <strong>de</strong><br />
vi<strong>de</strong>o usada y <strong><strong>de</strong>l</strong> tipo <strong>de</strong> aparato <strong>de</strong> vi<strong>de</strong>o <strong><strong>de</strong>l</strong> que se disponga.<br />
Exist<strong>en</strong> varios tipos <strong>de</strong> cables y adaptadores; el cable RCA que provee el teléfono<br />
Picasso es usado por <strong>la</strong> mayoría <strong>de</strong> aparatos <strong>de</strong> vi<strong>de</strong>o.<br />
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El vi<strong>de</strong>o teléfono Picasso esta compuesto por <strong>la</strong>s sigui<strong>en</strong>tes partes:<br />
• Unidad base <strong><strong>de</strong>l</strong> teléfono Picasso<br />
• cordón espiral <strong>de</strong> teléfono<br />
• Adaptador <strong>de</strong> AC<br />
• Unidad <strong>de</strong> control remoto<br />
• dispositivo <strong>de</strong> anotación (mouse serial)<br />
• batería <strong>de</strong> 9 voltios<br />
• 2 baterías 7WV (para el control remoto)<br />
• Adaptador BNC-RCA<br />
La fig. 3.2 muestra <strong>la</strong> localización <strong>de</strong> los jacks <strong>de</strong> vi<strong>de</strong>o, <strong>de</strong> los pórticos <strong>de</strong><br />
<strong>en</strong>trada/salida para conectar los difer<strong>en</strong>tes aparatos <strong>de</strong> captura y disp<strong>la</strong>y, <strong>de</strong><br />
anotación e interfaz para PC <strong><strong>de</strong>l</strong> teléfono Picasso:<br />
CONFIGURACIÓN DEL VIDEO TELEFONO<br />
PICASSO<br />
VIDEO TELEFONO<br />
PICASSO<br />
Fig 3,1 Compon<strong>en</strong>tes <strong>de</strong> un sistema con Vi<strong>de</strong>o Teléfono "Picasso"<br />
MONITOR<br />
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Fig. 3.2 LocaÜzación <strong>de</strong> jacks <strong>de</strong> vi<strong>de</strong>o y pórticos <strong>de</strong> <strong>en</strong>trada/salida<br />
3.1.2.1 Arquitectura <strong><strong>de</strong>l</strong> Sistema Picasso.-<br />
La Fig. 3,3 muestra un diagrama <strong>de</strong> bloques simplificado <strong>de</strong> los subsistemas que<br />
son ¡mplem<strong>en</strong>tados <strong>en</strong> el sistema Picasso.<br />
a) Procesador Principal. Todo el control <strong>de</strong> cada elem<strong>en</strong>to esta dado por el<br />
procesador principal (host processor), el cual se interconecta con todos los <strong>de</strong>más<br />
subsistemas. La función principal <strong>de</strong> los procesadores host es juntar <strong>la</strong> compresión<br />
<strong>de</strong> datos <strong>de</strong> vi<strong>de</strong>o, imag<strong>en</strong>, y audio <strong>de</strong>s<strong>de</strong> los respectivos procesadores y realizar<br />
una transmisión fiable <strong>de</strong> esos datos y otros sistemas <strong>de</strong> información a través <strong>de</strong><br />
un mó<strong>de</strong>m.<br />
En el <strong>la</strong>do <strong>de</strong> recepción, el procesador principal toma datos <strong>de</strong>s<strong>de</strong> el terminal<br />
lejano vía mó<strong>de</strong>m, chequea <strong>la</strong> integridad <strong>de</strong> éstos, para luego <strong>de</strong>sempaquetar esta<br />
información ya sea <strong>en</strong> vi<strong>de</strong>o, imag<strong>en</strong>, audio u otro tipo <strong>de</strong> información.<br />
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Pórtico<br />
Paralelo<br />
Procesador Host<br />
Micro procesador.<br />
:..GomercÍat ;.,.-.<br />
Fig.3.3 Diagrama <strong>de</strong> bloques <strong>de</strong> los subsistemas <strong><strong>de</strong>l</strong> Vi<strong>de</strong>o Teléfono Picasso<br />
Entrada<br />
<strong>de</strong> Vi<strong>de</strong>o<br />
El procesador host esta basado <strong>en</strong> el microcontro<strong>la</strong>dor MC68302 <strong>de</strong> Motoro<strong>la</strong>. Este<br />
dispositivo usa un chip Motoro<strong>la</strong> 68000 como su Unidad C<strong>en</strong>tral <strong>de</strong> Procesami<strong>en</strong>to<br />
(CPU). El MC68302 ti<strong>en</strong>e también varias funciones periféricas <strong>de</strong> procesami<strong>en</strong>to<br />
que manejan tanto <strong>la</strong> <strong>en</strong>trada como <strong>la</strong> salida, <strong>de</strong> este modo se <strong>de</strong>scargan estas<br />
operaciones <strong>de</strong>s<strong>de</strong> el CPU.<br />
b) El Mó<strong>de</strong>m.- El sistema Picasso usa un estándar industrial para mó<strong>de</strong>m V.32bis<br />
a 2.400 baudios, (mó<strong>de</strong>m con cance<strong>la</strong>ción <strong>de</strong> eco). Este mó<strong>de</strong>m es capaz <strong>de</strong><br />
transmitir datos con un ritmo <strong>de</strong> transmisión <strong>de</strong> hasta 14.4 Kb/s. También pue<strong>de</strong><br />
soportar bajos ritmos <strong>de</strong> transmisión (12, 9.6, 7.2, y 4.8 Kb/s) si el <strong>de</strong> 14.4 Kb/s no<br />
es exitoso.<br />
El sistema <strong>de</strong> mó<strong>de</strong>ms está basado <strong>en</strong> un grupo <strong>de</strong> chips para mó<strong>de</strong>ms <strong>de</strong> alta<br />
velocidad <strong>de</strong> AT&T Microelectronics. Este grupo <strong>de</strong> chips incluye un procesador<br />
para señal digital ROM-co<strong>de</strong>d AT&T Microelectrionics DSP16A, el chip <strong>de</strong> interfaz<br />
lógico CAMIL2, y el co<strong>de</strong>e lineal AT&T T7525 para conversión análogo/digital y<br />
64
digital/análogo, facilitan una conexión con <strong>la</strong> línea telefónica.<br />
c) Procesador <strong>de</strong> Audio.- La compresión y <strong>de</strong>scompresión <strong>de</strong> los datos <strong>de</strong> audio<br />
son ejecutadas por el procesador <strong>de</strong> audio usando un algoritmo <strong>de</strong> predicción lineal<br />
(CELP). Tanto el procesador <strong>de</strong> audio como el grupo <strong>de</strong> chips <strong><strong>de</strong>l</strong> mó<strong>de</strong>m, se<br />
basan <strong>en</strong> un procesador digital <strong>de</strong> señal ROM-co<strong>de</strong>d DSP16A.<br />
c) Subsistema <strong>de</strong> vi<strong>de</strong>o para Picasso.- Todo lo re<strong>la</strong>cionado con <strong>la</strong>s funciones <strong>de</strong><br />
vi<strong>de</strong>o se lleva a cabo por el subsistema <strong>de</strong> vi<strong>de</strong>o Picasso, Estas funciones<br />
incluy<strong>en</strong>:<br />
• Vi<strong>de</strong>o I/O<br />
• Conversiones A/D y D/A<br />
• Captura <strong>de</strong> tramas<br />
• Compresión y <strong>de</strong>scompresión <strong>de</strong> imag<strong>en</strong><br />
• Almac<strong>en</strong>ami<strong>en</strong>to y recuperación <strong>de</strong> imag<strong>en</strong><br />
• Texto sobrepuesto<br />
• Interfaz con PC, y<br />
• Anotaciones.<br />
El sistema Picasso usa los sigui<strong>en</strong>tes formatos <strong>de</strong> vi<strong>de</strong>o:<br />
• NTSC (National Televisión Standards Commitee), y<br />
• S-VHS (Super-VHS)<br />
La señal <strong>de</strong> <strong>en</strong>trada esta separada <strong>en</strong> luminancia (Y), crominancia roja (Cr), y<br />
crominancia azul (Cb). Cada una <strong>de</strong> estas compon<strong>en</strong>tes, por turnos, son<br />
digitalizadas y almac<strong>en</strong>adas <strong>en</strong> <strong>la</strong> memoria <strong>de</strong> acceso aleatorio <strong>de</strong> vi<strong>de</strong>o (VRAM)<br />
para cada trama <strong>de</strong> vi<strong>de</strong>o capturada. La máxima resolución espacial es <strong>de</strong> 512 x<br />
480 pixels. Para compresión y <strong>de</strong>scompresión <strong>de</strong> imág<strong>en</strong>es, el algoritmo que se<br />
usa, se basa <strong>en</strong> el estándar ISO-JPEG.<br />
El subsistema <strong>de</strong> vi<strong>de</strong>o esta basado <strong>en</strong> el procesador AT&T Microelectronics<br />
DSP3210, este chip soporta una resolución <strong>de</strong> 32 bits, y ti<strong>en</strong>e acceso para 2<br />
65
Mbytes <strong>de</strong> memoria para compresión <strong>de</strong> imag<strong>en</strong>, almac<strong>en</strong>ami<strong>en</strong>to y procesami<strong>en</strong>to<br />
por software <strong>de</strong> <strong>la</strong> imag<strong>en</strong>. Adicionalm<strong>en</strong>te, este subsistema conti<strong>en</strong>e 512 Kilobytes<br />
(KB) <strong>de</strong> VRAM para captura <strong>de</strong> imag<strong>en</strong>, y 128 KB <strong>de</strong> VRAM para datos <strong>de</strong><br />
anotaciones.<br />
d) POTS/Tec<strong>la</strong>do.- Los POTS (Servicio Telefónico <strong>de</strong> Configuración Antigua) y <strong>la</strong><br />
circuitería asociada a los par<strong>la</strong>ntes <strong>de</strong> teléfono provee el interfaz análogo <strong>de</strong> audio.<br />
Estos circuitos están basados <strong>en</strong> el Vi<strong>de</strong>o Teléfono 2500 (que se lo estudia<br />
posteriorm<strong>en</strong>te). Simi<strong>la</strong>rm<strong>en</strong>te, el sistema <strong>de</strong> tec<strong>la</strong>do <strong><strong>de</strong>l</strong> Picasso es una versión<br />
modificada <strong><strong>de</strong>l</strong> tec<strong>la</strong>do <strong><strong>de</strong>l</strong> Vi<strong>de</strong>o Teléfono 2500. También conti<strong>en</strong>e <strong>la</strong> circuitería<br />
necesaria para control remoto.<br />
3.1.2.2 Protocolos <strong>de</strong> Comunicación.- El Sistema Picasso multiplexa cuatro<br />
difer<strong>en</strong>tes tipos <strong>de</strong> datos sobre una línea física. El tipo <strong>de</strong> datos incluye<br />
vi<strong>de</strong>o/imag<strong>en</strong>, anotaciones, audio, y control. Se usa el protocolo X.25 para ejecutar<br />
estas tareas, X25 proporciona un sistema <strong>de</strong> <strong>de</strong>tección <strong>de</strong> errores y retransmisión<br />
para obt<strong>en</strong>er una fiable transmisión <strong><strong>de</strong>l</strong> flujo <strong>de</strong> datos. El canal <strong>de</strong> control se lo usa<br />
para realizar el control <strong>de</strong> terminal a terminal, y para contro<strong>la</strong>r <strong>la</strong> calidad <strong>de</strong><br />
información, <strong>en</strong>tre otras funciones.<br />
3.1.3 INTERCONECTIVIDAD CON OTROS EQUIPOS.-<br />
El sistema Picasso es compatible con cualquier dispositivo <strong>de</strong> vi<strong>de</strong>o <strong>de</strong> interfaz<br />
NTSC o S-VHS. Los ejemplos incluy<strong>en</strong> <strong>la</strong>s vi<strong>de</strong>o cámaras, camcor<strong>de</strong>rs, cámaras<br />
<strong>de</strong> imag<strong>en</strong> fija, cámaras <strong>de</strong> docum<strong>en</strong>tos, VCRs, monitores <strong>de</strong> TV, Cds fotos, e<br />
impresoras <strong>de</strong> vi<strong>de</strong>o. La figura 3.4 muestra los distintos dispositivos <strong>de</strong> captura,<br />
almac<strong>en</strong>ami<strong>en</strong>to, y disp<strong>la</strong>y que se pue<strong>de</strong>n conectar al vi<strong>de</strong>o teléfono Picasso.<br />
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3.1.3.1 Conectividad con PC'S:<br />
El Picasso posee un puerto paralelo para conectarse directam<strong>en</strong>te con un PC<br />
cualquiera lo cual permite <strong>en</strong>viar imág<strong>en</strong>es almac<strong>en</strong>adas <strong>en</strong> cualquier programa o<br />
viceversa, almac<strong>en</strong>ar imág<strong>en</strong>es <strong>en</strong>viadas <strong>de</strong>s<strong>de</strong> un punto remoto.<br />
3.1.3.2 Conectividad con Equipos <strong>de</strong> Vi<strong>de</strong>o:<br />
Picasso permite <strong>en</strong>viar imág<strong>en</strong>es <strong>de</strong> cualquier equipo <strong>de</strong> ví<strong>de</strong>o, ya sea VCR,<br />
camcor<strong>de</strong>r, V-8, super V-8, etc. El sistema incluye puertos para ví<strong>de</strong>o normal y<br />
para super ví<strong>de</strong>o.<br />
CÁMARA<br />
FOTOGRÁFICA<br />
CÁMARA<br />
CAMCORDER<br />
Dispositivos <strong>de</strong><br />
Captura<br />
Dispositivos <strong>de</strong><br />
Almac<strong>en</strong>ami<strong>en</strong>to<br />
TELEFONO PICASSO<br />
CAMCORDER<br />
Fig. 3.4 Dispositivos <strong>de</strong> captura, almac<strong>en</strong>ami<strong>en</strong>to y disp<strong>la</strong>y <strong><strong>de</strong>l</strong> teléfono Picasso<br />
3.1.3.3 Mouse para anotaciones:<br />
VCR<br />
Dispositivos <strong>de</strong><br />
Disp<strong>la</strong>y<br />
MONITOR<br />
VIDEO<br />
PRINTER<br />
Para que una comunicación sea completa no es únicam<strong>en</strong>te necesario transmitir<br />
una imag<strong>en</strong>, sino también el que ambas partes posean mecanismos que les<br />
permitan hacer anotaciones sobre <strong>la</strong>s mismas. Por tal motivo, se incorpora un<br />
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mouse que permite realizar este tipo <strong>de</strong> anotaciones. El Picasso dispone para el<br />
efecto <strong>de</strong> un puerto serial al que va conectado un mouse común y corri<strong>en</strong>te, el cual<br />
pue<strong>de</strong> seña<strong>la</strong>r regiones especificas <strong>de</strong> una imag<strong>en</strong> para que pueda ser modificada<br />
<strong>de</strong> acuerdo al criterio <strong><strong>de</strong>l</strong> usuario, e incluso almac<strong>en</strong>ar estas imág<strong>en</strong>es<br />
modificadas.<br />
A<strong>de</strong>más posee también un Control Remoto para <strong>en</strong>viar imág<strong>en</strong>es a lugares<br />
remotos <strong>en</strong> el cual se esta llevando a cabo una pres<strong>en</strong>tación, mediante un control<br />
remoto, el confer<strong>en</strong>cista pue<strong>de</strong> manejar a su gusto <strong>la</strong>s imág<strong>en</strong>es a mostrarse <strong>en</strong><br />
pantal<strong>la</strong>.<br />
3.1.3.4 Mercado y Aplicaciones.-<br />
Des<strong>de</strong> que se introdujo al mercado <strong>en</strong> 1993, el sistema Picasso ha sido usado <strong>en</strong><br />
una amplia variedad <strong>de</strong> aplicaciones y para muchos difer<strong>en</strong>tes mercados<br />
alre<strong>de</strong>dor <strong><strong>de</strong>l</strong> mundo. Estos mercados incluy<strong>en</strong> servicios creativos, publicidad,<br />
ing<strong>en</strong>iería industrial, etc. El sistema Picasso es particu<strong>la</strong>rm<strong>en</strong>te bu<strong>en</strong>o para<br />
satisfacer aplicaciones <strong>en</strong> el campo <strong>de</strong> <strong>la</strong> medicina e ing<strong>en</strong>iería industrial.<br />
Cuidados Médicos. La comunidad médica fue <strong>la</strong> que primero empezó a<br />
interesarse por el sistema Picasso para el propósito <strong>de</strong> intercambio <strong>de</strong> información<br />
y consulta <strong>en</strong> forma remota. Estas aplicaciones se han visto ahora expandidas al<br />
incluir el adiestrami<strong>en</strong>to remoto, <strong>la</strong> información <strong>de</strong> emerg<strong>en</strong>cia se transmite por<br />
ejemplo <strong>de</strong>s<strong>de</strong> <strong>la</strong>s ambu<strong>la</strong>ncias hacia los c<strong>en</strong>tros hospita<strong>la</strong>rios, o se pue<strong>de</strong><br />
interactuar con algún <strong>la</strong>boratorio requerido.<br />
El sistema Picasso permite hacer consultas <strong>de</strong> una forma interactiva y fácil<br />
<strong>en</strong>tre médicos <strong>de</strong> difer<strong>en</strong>tes especialida<strong>de</strong>s y <strong>en</strong> cualquier parte <strong><strong>de</strong>l</strong> mundo para<br />
ayudar <strong>en</strong> un diagnóstico o tratami<strong>en</strong>to. Otras disciplinas médicas <strong>en</strong> <strong>la</strong>s cuales el<br />
sistema Picasso ha empezado a introducirse incluye <strong>la</strong> <strong>de</strong>rmatología, oftalmología,<br />
y cuidados <strong>de</strong> emerg<strong>en</strong>cia.<br />
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3.2 EL VIDEO TELEFONO DE IMÁGENES MÓVILES<br />
3.2.1 INTRODUCCIÓN<br />
En el pres<strong>en</strong>te capítulo se estudiará el servicio <strong>de</strong> Vi<strong>de</strong>otelefonía que esta <strong>de</strong>finida<br />
para re<strong>de</strong>s que ofrec<strong>en</strong> canales a 64 Kbit/s y concebida para <strong>la</strong> RDSI, a <strong>la</strong> que se<br />
le consi<strong>de</strong>ra actualm<strong>en</strong>te como idónea para ofrecer este servicio.<br />
Al mom<strong>en</strong>to se está estudiando servicios vi<strong>de</strong>o telefónicos que necesitan uno o<br />
dos canales a 64 Kbits/s. En caso <strong>de</strong> normalizarse dos velocida<strong>de</strong>s binarias, será<br />
necesario prever una intercomunicación básica, con <strong>la</strong>s calida<strong>de</strong>s audiovisuales<br />
<strong>de</strong> <strong>la</strong> velocidad más baja. Esa intercomunicación básica <strong>de</strong>berán ofrecer<strong>la</strong> los<br />
terminales. A continuación se estudiará dos <strong>de</strong> <strong>la</strong>s tecnologías y marcas que<br />
actualm<strong>en</strong>te están <strong>de</strong>sarrol<strong>la</strong>ndo vi<strong>de</strong>o teléfonos como son: AT&T (con e! vi<strong>de</strong>o<br />
teléfono 2500) y ALCATEL (con el vi<strong>de</strong>o teléfono 2838 ó 4040)<br />
3.2.2 DESCRIPCIÓN DEL VIDEO TELEFONO 2500 DE AT&T.-<br />
Este tipo <strong>de</strong> ví<strong>de</strong>o teléfono a difer<strong>en</strong>cia <strong><strong>de</strong>l</strong> ví<strong>de</strong>o teléfono Picasso permite a los<br />
usuarios verse mutuam<strong>en</strong>te, con imág<strong>en</strong>es móviles, <strong>en</strong> tiempo real, a todo color y<br />
<strong>en</strong> tonos naturales durante <strong>la</strong>s l<strong>la</strong>madas que se efectúan.<br />
La pantal<strong>la</strong> <strong>de</strong> ví<strong>de</strong>o <strong><strong>de</strong>l</strong> teléfono se monta justo <strong>en</strong>cima <strong>de</strong>! tec<strong>la</strong>do numérico, esta<br />
pantal<strong>la</strong> se inclina y gira para obt<strong>en</strong>er el mejor ángulo <strong>de</strong> visión, ti<strong>en</strong>e controles <strong>de</strong><br />
contraste y regu<strong>la</strong>ción <strong>de</strong> brillo <strong>de</strong> <strong>la</strong> imag<strong>en</strong>, y un botón para verse uno mismo<br />
para así mostrar al usuario exactam<strong>en</strong>te como lo ve <strong>la</strong> otra persona al otro <strong>la</strong>do <strong><strong>de</strong>l</strong><br />
hilo durante <strong>la</strong> l<strong>la</strong>mada.<br />
Este tipo <strong>de</strong> ví<strong>de</strong>o teléfonos ti<strong>en</strong>e también un altavoz incorporado, con un<br />
micrófono direccional especialm<strong>en</strong>te diseñado para reducir los ruidos <strong>de</strong> fondo y el<br />
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eco <strong>de</strong> <strong>la</strong> habitación, permiti<strong>en</strong>do así que más <strong>de</strong> una persona se sume a <strong>la</strong><br />
conversación.<br />
La tecnología <strong>de</strong> compresión <strong>de</strong> audio, los procesadores <strong>de</strong> señales digitales y los<br />
mó<strong>de</strong>ms insta<strong>la</strong>dos <strong>de</strong>ntro <strong>de</strong> éstos vi<strong>de</strong>oteléfonos fueron <strong>de</strong>sarrol<strong>la</strong>dos por<br />
AT&T.<br />
El mó<strong>de</strong>m transmite a una velocidad <strong>de</strong> 19,2 kilobits por segundo. La pantal<strong>la</strong> es<br />
<strong>de</strong> 3,3 pulgadas <strong>en</strong> diagonal, a color, <strong>de</strong> cristal liquido (LCD). La cámara<br />
electrónica a color, <strong>de</strong> 1/3 <strong>de</strong> pulgada esta incorporada al teléfono justo sobre <strong>la</strong><br />
pantal<strong>la</strong>, y ti<strong>en</strong>e un alcance <strong>de</strong> foco <strong>de</strong> 1 a 9 pies con igual calidad. El teléfono se<br />
pue<strong>de</strong> insta<strong>la</strong>r fácilm<strong>en</strong>te <strong>en</strong> un jack telefónico modu<strong>la</strong>r (RJ11) y <strong>en</strong> un<br />
tomacorri<strong>en</strong>te <strong>de</strong> 110 voltios con 60 hertz <strong>de</strong> frecu<strong>en</strong>cia.<br />
En <strong>la</strong> actualidad se están haci<strong>en</strong>do estudios y <strong>de</strong>sarrol<strong>la</strong>ndo <strong>la</strong> tecnología<br />
a<strong>de</strong>cuada para que este ví<strong>de</strong>o teléfono pueda funcionar <strong>en</strong> otras configuraciones<br />
<strong>de</strong> red, y por líneas que aceptan velocida<strong>de</strong>s <strong>de</strong> transmisión difer<strong>en</strong>tes <strong>de</strong> 19,2<br />
kilobits, para que sea un producto que pueda ser utilizado <strong>en</strong> cualquier parte <strong><strong>de</strong>l</strong><br />
mundo adaptándose así a <strong>la</strong>s difer<strong>en</strong>tes normas y tecnologías.<br />
La <strong>en</strong>trada <strong>de</strong> luminancia es <strong>de</strong> 128 pixels por 112 líneas. Los compon<strong>en</strong>tes <strong>de</strong><br />
cada producto incluy<strong>en</strong> un mó<strong>de</strong>m, un co<strong>de</strong>e, una cámara, y una pantal<strong>la</strong> <strong>de</strong> ví<strong>de</strong>o,<br />
un procesador <strong>de</strong> audio, y un software específico para este tipo <strong>de</strong> teléfonos.<br />
3.2.2.1 Modo <strong>de</strong> utilización para los usuarios <strong>de</strong> un vi<strong>de</strong>oteléfono.-<br />
Para <strong>la</strong> utilización <strong>de</strong> este tipo <strong>de</strong> tecnología, se requiere que cada usuario t<strong>en</strong>ga<br />
uno <strong>de</strong> estos aparatos a cada <strong>la</strong>do <strong>de</strong> <strong>la</strong> línea; una vez que se establece <strong>la</strong><br />
comunicación como <strong>la</strong> <strong>de</strong> un teléfono normal, una o ambas partes <strong>de</strong>ci<strong>de</strong>n<br />
cambiar <strong>de</strong> una l<strong>la</strong>mada <strong>de</strong> voz, a una l<strong>la</strong>mada <strong>de</strong> audio y vi<strong>de</strong>o. Se oprime el<br />
botón <strong>de</strong> ví<strong>de</strong>o <strong>en</strong> cualquiera <strong>de</strong> los dos ví<strong>de</strong>o teléfonos, activándose así el<br />
mó<strong>de</strong>m que conecta los dos teléfonos, permiti<strong>en</strong>do el intercambio <strong>de</strong> datos <strong>de</strong><br />
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audio y <strong>de</strong> ví<strong>de</strong>o codificados digitalm<strong>en</strong>te. So<strong>la</strong>m<strong>en</strong>te una <strong>de</strong> <strong>la</strong>s partes ti<strong>en</strong>e que<br />
oprimir el botón <strong>de</strong> ví<strong>de</strong>o para que los aparatos pas<strong>en</strong> a <strong>la</strong> conexión <strong>de</strong> mó<strong>de</strong>m.<br />
Sin embargo, para mant<strong>en</strong>er <strong>la</strong> privacidad <strong>de</strong> <strong>la</strong>s partes, no se emite ningún dato<br />
<strong>de</strong> ví<strong>de</strong>o hasta que se oprima el botón correspondi<strong>en</strong>te, aun cuando se <strong>en</strong>víe una<br />
señal digital <strong>de</strong> audio y se reciba señales digitales <strong>de</strong> audio y ví<strong>de</strong>o. Por lo tanto, si<br />
<strong>la</strong> persona a qui<strong>en</strong> se ha l<strong>la</strong>mado oprime el botón <strong>de</strong> ví<strong>de</strong>o, usted podrá ver a esa<br />
persona, pero esa persona no podrá verlo, hasta que usted oprima el botón <strong>de</strong><br />
ví<strong>de</strong>o <strong>en</strong> su ví<strong>de</strong>o teléfono. Otra característica <strong>de</strong> privacidad <strong><strong>de</strong>l</strong> ví<strong>de</strong>o teléfono es<br />
el obturador manual que permite al usuario cubrir <strong>la</strong> l<strong>en</strong>te <strong>de</strong> <strong>la</strong> cámara <strong><strong>de</strong>l</strong> ví<strong>de</strong>o<br />
teléfono para impedir que se transmita imág<strong>en</strong>es, aun habi<strong>en</strong>do oprimido el botón<br />
<strong>de</strong> ví<strong>de</strong>o.<br />
En este tipo <strong>de</strong> ví<strong>de</strong>o teléfono, cuando se oprime el botón <strong>de</strong> ví<strong>de</strong>o, se interrumpe<br />
<strong>la</strong> conversación por unos segundos mi<strong>en</strong>tras los mó<strong>de</strong>ms inician el <strong>en</strong><strong>la</strong>ce. Ningún<br />
otro aparato telefónico o mó<strong>de</strong>m pue<strong>de</strong> utilizar <strong>la</strong>s líneas telefónicas<br />
simultáneam<strong>en</strong>te con el Ví<strong>de</strong>o Teléfono 2500, <strong>de</strong>bido a que estos son mó<strong>de</strong>ms <strong>de</strong><br />
alta velocidad que utilizan datos modu<strong>la</strong>dos. Esto significa que <strong>la</strong> conexión <strong>de</strong><br />
ví<strong>de</strong>o se corta si otra persona levanta una ext<strong>en</strong>sión telefónica <strong>en</strong> <strong>la</strong> misma línea<br />
para unirse a <strong>la</strong> conversación. Sin embargo, este ví<strong>de</strong>o teléfono ti<strong>en</strong>e un altavoz<br />
interno que permite utilizarlo sin usar <strong>la</strong>s manos y <strong>en</strong>tab<strong>la</strong>r una conversación <strong>en</strong>tre<br />
varias personas.<br />
3.2.2.2 Diseño Físico.-<br />
Debido a <strong>la</strong> electrónica <strong>de</strong> alta velocidad, el Vi<strong>de</strong>o Teléfono 2500 podría haber<br />
t<strong>en</strong>ido insuperables problemas con interfer<strong>en</strong>cias electromagnéticas (EMI),<br />
interfer<strong>en</strong>cias <strong>de</strong> radiofrecu<strong>en</strong>cia (RFI), <strong>de</strong>scargas electrostáticas (ESD), y<br />
cal<strong>en</strong>tami<strong>en</strong>to, si no se hubiese tomado precauciones <strong>en</strong> el diseño original. Los<br />
diseñadores <strong>de</strong> <strong>la</strong> parte física, trabajaron juntam<strong>en</strong>te con los diseñadores<br />
industriales, asesores <strong>de</strong> EMI y calor, y diseñadores <strong>de</strong> hardware eléctrico para<br />
<strong>de</strong>sarrol<strong>la</strong>r un diseño arquitectónico que esté dirigido a todo lo concerni<strong>en</strong>te para<br />
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obt<strong>en</strong>er una bu<strong>en</strong>a imag<strong>en</strong>, un correcto tamaño <strong><strong>de</strong>l</strong> teléfono, y un costo efectivo <strong><strong>de</strong>l</strong><br />
paquete.<br />
Para mitigar el electromagnetismo y <strong>la</strong> interfer<strong>en</strong>cia <strong>de</strong> radiofrecu<strong>en</strong>cia, se conecta<br />
un p<strong>la</strong>no <strong>de</strong> tierra al host <strong>de</strong> alta velocidad y a <strong>la</strong>s tarjetas <strong>de</strong> circuitos <strong><strong>de</strong>l</strong> co<strong>de</strong>e <strong>de</strong><br />
vi<strong>de</strong>o. A<strong>de</strong>más, los circuitos <strong>de</strong> alta velocidad son agrupados tan estrecham<strong>en</strong>te<br />
juntos como sea posible, <strong>de</strong>ntro <strong>de</strong> cubiertas <strong>de</strong> metal para minimizar <strong>la</strong>s<br />
interconexiones.<br />
El gran número <strong>de</strong> salidas, orificios y <strong>la</strong> proximidad <strong><strong>de</strong>l</strong> grupo <strong>de</strong> tarjetas <strong>de</strong><br />
circuitos pusieron <strong>en</strong> relieve el problema <strong>la</strong>s <strong>de</strong>scargas electrostáticas (ESD). Para<br />
prev<strong>en</strong>ir los daños <strong>de</strong> los compon<strong>en</strong>tes por <strong>la</strong> ESD, cualquier <strong>de</strong>scarga <strong>de</strong>bería<br />
estar ligada directam<strong>en</strong>te a tierra antes <strong>de</strong> que llegue <strong>de</strong>masiado lejos <strong>de</strong>ntro <strong>de</strong> <strong>la</strong><br />
tarjeta <strong>de</strong> circuitos, así mismo, el diseño provee <strong>de</strong> un camino a tierra para <strong>la</strong> ESD<br />
que g<strong>en</strong>eralm<strong>en</strong>te esta <strong>en</strong> el filo superior o inferior <strong>de</strong> <strong>la</strong> tarjeta <strong>de</strong> circuitos. Ese<br />
camino se empata con un cable ver<strong>de</strong> <strong>de</strong> tierra (estandarizado) o <strong>en</strong> el extremo <strong>de</strong><br />
<strong>la</strong> línea telefónica.<br />
3.2.3 DESCRIPCIÓN DEL VIDEO TELEFONO ALCATEL.<br />
(Mo<strong><strong>de</strong>l</strong>os 2838/4040)<br />
3.2.3.1 Concepto <strong><strong>de</strong>l</strong> ví<strong>de</strong>o teléfono Alcatel.<br />
El ví<strong>de</strong>o teléfono Alcatel ti<strong>en</strong>e una alta calidad <strong>de</strong> imag<strong>en</strong> y permite el contacto<br />
visual directo con el interlocutor gracias a una solución que emplea un espejo semi<br />
transpar<strong>en</strong>te. Va <strong>en</strong> el interior <strong>de</strong> una caja s<strong>en</strong>cil<strong>la</strong> y ti<strong>en</strong>e un interfaz <strong>de</strong> usuario<br />
simple y amigable.<br />
Una conexión RDSI con un interfaz <strong>de</strong> frecu<strong>en</strong>cia básica (BRl) <strong>de</strong> 64 Kb/s es un<br />
excel<strong>en</strong>te vehículo para servicios <strong>de</strong> vi<strong>de</strong>o telefonía. La repres<strong>en</strong>tación <strong>de</strong> <strong>la</strong><br />
imag<strong>en</strong> pue<strong>de</strong> ser llevada a 56 kb/s y <strong>la</strong> repres<strong>en</strong>tación <strong>de</strong> sonido a 8 kb/s. Si se<br />
usa una conexión full RDSI BRl para el servicio <strong>de</strong> vi<strong>de</strong>o telefonía, <strong>en</strong>tonces el<br />
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itmo <strong>de</strong> transmisión total disponible es <strong>de</strong> 128 kb/s con dos canales <strong>de</strong> soporte (ej.<br />
canales B) que pue<strong>de</strong>n ser divididos para llevar <strong>la</strong> repres<strong>en</strong>tación digital tanto <strong><strong>de</strong>l</strong><br />
audio como <strong><strong>de</strong>l</strong> vi<strong>de</strong>o. Por ejemplo, <strong>la</strong> señal <strong>de</strong> <strong>la</strong> imag<strong>en</strong> pue<strong>de</strong> ser llevada a 112<br />
kb/s usando el estándar px64, mi<strong>en</strong>tras que <strong>la</strong> voz acompañante pue<strong>de</strong> ser llevada<br />
a 16 kb/s, <strong>de</strong> esta manera, se provee <strong>de</strong> imag<strong>en</strong> y sonido <strong>de</strong> alta calidad.<br />
El ví<strong>de</strong>o teléfono RDSI fabricado por Alcatel SEL se pue<strong>de</strong> conectar tanto<br />
directam<strong>en</strong>te a un abonado <strong>de</strong> <strong>la</strong> red pública RDSI como también a una c<strong>en</strong>tral<br />
privada. La imp<strong>la</strong>ntación <strong>de</strong> futuros protocolos <strong>de</strong> canal-D es muy s<strong>en</strong>cil<strong>la</strong>, solo<br />
por medio <strong>de</strong> modificaciones al software. La codificación y <strong>de</strong>codificación <strong>de</strong> ví<strong>de</strong>o,<br />
constituye el corazón <strong><strong>de</strong>l</strong> ví<strong>de</strong>o teléfono RDSI. El diseño consta <strong>de</strong> unida<strong>de</strong>s <strong>de</strong><br />
procesami<strong>en</strong>to especiales que proporcionan velocida<strong>de</strong>s <strong>de</strong> proceso muy<br />
elevadas, y también <strong>de</strong> una memoria c<strong>en</strong>tral.<br />
Se necesita una eficacia máxima <strong><strong>de</strong>l</strong> procesador <strong>de</strong> hasta 1,2 x 109 operaciones<br />
por segundo para el proceso <strong>de</strong> los algoritmos <strong>de</strong> compresión y <strong>de</strong>scompresión <strong>de</strong><br />
datos. La función <strong><strong>de</strong>l</strong> co<strong>de</strong>e se integra <strong>en</strong> el mismo hardware usado para g<strong>en</strong>erar <strong>la</strong><br />
señalización <strong>de</strong>ntro <strong>de</strong> <strong>la</strong> banda y formar <strong>la</strong> trama <strong>de</strong> transmisión.<br />
Se ha <strong>de</strong>sarrol<strong>la</strong>do una p<strong>la</strong>ca <strong>de</strong> procesami<strong>en</strong>to con cuatro circuitos integrados (IC)<br />
procesadores para imp<strong>la</strong>ntar el co<strong>de</strong>e.<br />
3.2.3.2 El compon<strong>en</strong>te es<strong>en</strong>cial "CAP ///"(Celu<strong>la</strong>r Array Processor).<br />
Se ha <strong>de</strong>sarrol<strong>la</strong>do para el vi<strong>de</strong>oteléfono un circuito específico que realiza<br />
aproximadam<strong>en</strong>te 750.000 funciones <strong>de</strong> transistor. El "CAP ¡II" (Celu<strong>la</strong>r Array<br />
Processor) es un procesador integrado con un total <strong>de</strong> seis unida<strong>de</strong>s <strong>de</strong> proceso<br />
idénticas, cada una <strong>de</strong> estas unida<strong>de</strong>s <strong>de</strong> procesami<strong>en</strong>to ti<strong>en</strong>e una memoria<br />
interna <strong>de</strong> 1024 bytes. Este ví<strong>de</strong>o teléfono ti<strong>en</strong>e cuatro unida<strong>de</strong>s <strong>de</strong> "CAP ¡II" con<br />
un total <strong>de</strong> 24 elem<strong>en</strong>tos <strong>de</strong> proceso. Esto proporciona un total <strong>de</strong> 24 Kbytes <strong>de</strong><br />
memoria <strong>de</strong> chip. El IC procesador está especialm<strong>en</strong>te diseñado para funciones<br />
<strong>de</strong> señal y pue<strong>de</strong> procesar campos <strong>de</strong> datos unidim<strong>en</strong>sionales o bidim<strong>en</strong>sionales.<br />
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El circuito <strong>de</strong> procesador lo diseñó LSl-Logic, EE.UU. y su estructura permite una<br />
alta integración conforme a <strong>la</strong> tecnología VLSI disponible sin necesidad <strong>de</strong><br />
modificaciones substanciales <strong>de</strong> software.<br />
3.2.4 EQUIPOS Y COMPONENTES DEL SISTEMA.-<br />
3.2.4.1 Arquitectura <strong><strong>de</strong>l</strong> Vi<strong>de</strong>o Teléfono<br />
En términos g<strong>en</strong>erales un Vi<strong>de</strong>o teléfono ti<strong>en</strong>e un procesador que actúa como un<br />
policía <strong>de</strong> tráfico <strong>en</strong> una intersección ocupada. En esta intersección, los datos<br />
tomados <strong>de</strong>s<strong>de</strong> el co<strong>de</strong>e <strong>de</strong> ví<strong>de</strong>o y el procesador <strong>de</strong> audio <strong>de</strong>b<strong>en</strong> ser empaquetados<br />
y <strong>en</strong>viados a través <strong><strong>de</strong>l</strong> mó<strong>de</strong>m para <strong>la</strong> transmisión. A<strong>de</strong>más, los datos <strong><strong>de</strong>l</strong> equipo <strong><strong>de</strong>l</strong><br />
terminal a distancia vi<strong>en</strong><strong>en</strong> a través <strong><strong>de</strong>l</strong> mó<strong>de</strong>m y <strong>de</strong>b<strong>en</strong> ser chequeados para verificar<br />
su integridad, luego <strong>de</strong>sempaquetados y separados <strong>en</strong> datos <strong>de</strong> ví<strong>de</strong>o y audio,<br />
posteriorm<strong>en</strong>te son <strong>en</strong>viados ai co<strong>de</strong>e <strong>de</strong> ví<strong>de</strong>o y al procesador <strong>de</strong> audio<br />
respectivam<strong>en</strong>te. En suma, mi<strong>en</strong>tras el tráfico <strong><strong>de</strong>l</strong> mó<strong>de</strong>m esta si<strong>en</strong>do contro<strong>la</strong>do, el<br />
usuario esta haci<strong>en</strong>do solicitu<strong>de</strong>s a través <strong>de</strong> <strong>la</strong>s tec<strong>la</strong>s presionadas que son<br />
s<strong>en</strong>sadas por el procesador POTS (servicio telefónico <strong>de</strong> configuración antigua), para<br />
posteriorm<strong>en</strong>te ir al procesador c<strong>en</strong>tral (host) para su ejecución.<br />
La figura 3.5 conti<strong>en</strong>e un diagrama <strong>de</strong> bloques simplificado <strong><strong>de</strong>l</strong> Vi<strong>de</strong>o teléfono 2500.<br />
A continuación se realiza una explicación <strong>de</strong> cada bloque.<br />
a) PROCESADOR PRINCIPAL: El procesador principal es un Motoro<strong>la</strong> MC68302 el cual<br />
ti<strong>en</strong>e un microprocesador Motoro<strong>la</strong> 68000 <strong>en</strong> calidad <strong>de</strong> unidad <strong>de</strong> proceso c<strong>en</strong>tral<br />
(CPU). En esta arquitectura con tecnología RISC (Conjunto reducido <strong>de</strong><br />
comunicaciones para el computador), los procesadores periféricos manejan el<br />
protocolo serial <strong>de</strong> comunicaciones, reduci<strong>en</strong>do <strong>la</strong> cargada <strong><strong>de</strong>l</strong> CPU.<br />
"75
it/s. El algoritmo ampliado, CELP+, fue <strong>de</strong>sarrol<strong>la</strong>do <strong>en</strong> los <strong>la</strong>boratorios AT&T <strong>en</strong><br />
1991.<br />
d) Co<strong>de</strong>e <strong>de</strong> vi<strong>de</strong>o: El co<strong>de</strong>e <strong>de</strong> vi<strong>de</strong>o codifica y <strong>de</strong>codifica imág<strong>en</strong>es <strong>de</strong> vi<strong>de</strong>o y<br />
maneja tres ¡nterfaces:<br />
1. Interfaz <strong>de</strong> datos y comando principal.- Esta interfaz permite un intercambio <strong>de</strong><br />
datos robusto1, una secu<strong>en</strong>cia <strong>de</strong> arranque, g<strong>en</strong>eración <strong>de</strong> caracteres para <strong>la</strong><br />
pantal<strong>la</strong>, ajuste <strong>de</strong> parámetros <strong>de</strong> codificación <strong><strong>de</strong>l</strong> ví<strong>de</strong>o, ajuste <strong><strong>de</strong>l</strong> color,<br />
diagnóstico, y otras características.<br />
2. Interfaz analógica <strong>de</strong> ingreso <strong>de</strong> ví<strong>de</strong>o.<br />
3. Interfaz analógica <strong>de</strong> salida <strong>de</strong> ví<strong>de</strong>o.<br />
e) Pantal<strong>la</strong>: La pantal<strong>la</strong> <strong>de</strong> cristal líquido <strong>de</strong> 3.3 pulgadas (LCD) muestra imág<strong>en</strong>es <strong>de</strong><br />
vi<strong>de</strong>o <strong>en</strong> color usando superposición <strong>de</strong> campo <strong>en</strong> vez <strong>de</strong> un cuadro completo <strong>de</strong><br />
ví<strong>de</strong>o el cual requiere un <strong>en</strong>tre<strong>la</strong>zado por cuadro. En suma, el ingreso <strong>de</strong> ví<strong>de</strong>o es una<br />
variante <strong><strong>de</strong>l</strong> sistema NTSC. Por ejemplo, utiliza el temporizador <strong><strong>de</strong>l</strong> NTSC, pero no el<br />
voltaje ni <strong>la</strong> sincronización, aún cuando el término también se refiere a <strong>la</strong>s normas <strong>de</strong><br />
televisión que este Comité configuró para los Estados Unidos. En ese contexto, NTSC<br />
significa 525 líneas <strong>de</strong> resolución transmitidas a 60 semicuadros (<strong>en</strong>tre<strong>la</strong>zados) por<br />
segundo.<br />
f) Cámaras y l<strong>en</strong>tes: La cámara provee una variante <strong><strong>de</strong>l</strong> NTSC <strong>en</strong> señal <strong>de</strong> salida <strong><strong>de</strong>l</strong><br />
ví<strong>de</strong>o a color y una función <strong>de</strong> ba<strong>la</strong>nce <strong>de</strong> b<strong>la</strong>nco automático. (El ba<strong>la</strong>nce <strong>de</strong> b<strong>la</strong>nco<br />
automático significa que el color es ba<strong>la</strong>nceado automáticam<strong>en</strong>te, usando <strong>la</strong>s áreas<br />
b<strong>la</strong>ncas <strong>de</strong> <strong>la</strong> imag<strong>en</strong> como refer<strong>en</strong>cia). Sony Corporation diseñó <strong>la</strong> cámara<br />
específicam<strong>en</strong>te para esta aplicación y ahora <strong>la</strong> fabrica para <strong>la</strong> AT&T.<br />
1 Robusto implica que <strong>de</strong>tecta su propios problemas durante el intercambio <strong>de</strong> datos<br />
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Tx<br />
Fkj. 3.5 Diagrama <strong>de</strong> bloques <strong><strong>de</strong>l</strong> VIDEO TELEFONO AT&T 2500. La mo<strong>de</strong>rna tecnología incluye compresión <strong>de</strong> audio y<br />
ví<strong>de</strong>o y un mó<strong>de</strong>m <strong>de</strong> afta velocidad<br />
g) Interfaz <strong>de</strong> Usuario: La ¡nterfaz <strong>de</strong> usuario es lo que e! cli<strong>en</strong>te contro<strong>la</strong> y recibe.<br />
Esta ¡nterfaz incluye <strong>la</strong> visión g<strong>en</strong>eral <strong><strong>de</strong>l</strong> equipo, el sonido y <strong>la</strong> facilidad <strong>de</strong> uso <strong><strong>de</strong>l</strong><br />
equipo. El vi<strong>de</strong>o teléfono (Fig. 3.6) ti<strong>en</strong>e que ser tan fácil <strong>de</strong> usar como un teléfono<br />
vocal normal, para esto, se tuvo que construir con atributos físicos que<br />
proporcion<strong>en</strong> al cli<strong>en</strong>te seguridad <strong>de</strong> privacidad <strong>en</strong> su operación. Se estimó, por lo<br />
tanto, que se <strong>de</strong>berá t<strong>en</strong>er obligatoriam<strong>en</strong>te un obturador mecánico visible fr<strong>en</strong>te a<br />
<strong>la</strong> cámara con lo que el usuario sabe que <strong>la</strong> imag<strong>en</strong> suya no será transmitida.<br />
Este tipo <strong>de</strong> vi<strong>de</strong>o comunicaciones pue<strong>de</strong> hacer al inicio que el cli<strong>en</strong>te se si<strong>en</strong>ta un<br />
poco incómodo, por lo tanto, <strong>la</strong>s acciones <strong>de</strong>b<strong>en</strong> ser hechas tan simples como<br />
sean posibles. El set ti<strong>en</strong>e un gran botón azul etiquetado con VIDEO <strong>en</strong> <strong>la</strong> esquina<br />
<strong>de</strong>recha <strong><strong>de</strong>l</strong> área <strong><strong>de</strong>l</strong> campo <strong>de</strong> botones. Después <strong>de</strong> que una l<strong>la</strong>mada se ha<br />
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El botón <strong>de</strong> FOCUS sirve para <strong>en</strong>focar <strong>la</strong> imag<strong>en</strong> que se recibe, provee <strong>de</strong> una<br />
forma con <strong>la</strong> que el usuario pueda componer sus imág<strong>en</strong>es y chequear su<br />
iluminación y esc<strong>en</strong>ario.<br />
Durante una l<strong>la</strong>mada el usuario pue<strong>de</strong> activar SELF VIEW si quiere saber si su<br />
imag<strong>en</strong> y voz han sido transmitidas. Ya que <strong>la</strong> transmisión <strong>de</strong> vi<strong>de</strong>o continúa<br />
cuando SELF VIEW está activado, <strong>la</strong> pantal<strong>la</strong> informa al usuario qui<strong>en</strong> esta <strong>en</strong><br />
modo SELF VIEW y que esta todavía <strong>en</strong>viándose voz e imag<strong>en</strong>.<br />
3.3 LA VIDEO CONFERENCIA COMO APLICACIÓN PARTICULAR.-<br />
El servicio <strong>de</strong> vi<strong>de</strong>oconfer<strong>en</strong>cia ha l<strong>la</strong>mado <strong>la</strong> at<strong>en</strong>ción <strong><strong>de</strong>l</strong> público como un servicio<br />
no telefónico (<strong>en</strong> e! estricto s<strong>en</strong>tido). El rápido a<strong>de</strong><strong>la</strong>nto <strong>de</strong> <strong>la</strong> tecnología digital ha<br />
culminado con <strong>la</strong> Red Digital <strong>de</strong> Servicios Integrados que es especialm<strong>en</strong>te<br />
a<strong>de</strong>cuada para proporcionar el servicio <strong>de</strong> vi<strong>de</strong>o confer<strong>en</strong>cia. Se espera que <strong>la</strong><br />
<strong>de</strong>manda <strong>de</strong> este servicio aum<strong>en</strong>te rápidam<strong>en</strong>te.<br />
Una configuración básica <strong>de</strong> este sistema se muestra <strong>en</strong> <strong>la</strong> Figura 3.7 <strong>en</strong> <strong>la</strong> cual a<br />
más <strong>de</strong> los equipos <strong>de</strong> vi<strong>de</strong>oconfer<strong>en</strong>cia <strong>de</strong> escritorio (cámara, monitor, micrófono,<br />
software, etc.) se necesita <strong>de</strong> un servicio RDSI (más cualquier hardware adicional<br />
que se requiera) y dos pórticos <strong>de</strong> <strong>la</strong> PC. Es importante cumplir con <strong>la</strong>s normas<br />
nacionales e internacionales <strong>en</strong> cuanto al sistema <strong>de</strong> vi<strong>de</strong>o, que compr<strong>en</strong><strong>de</strong><br />
cámara, monitores, esquema <strong>de</strong> codificación para co<strong>de</strong>es, y los sistemas <strong>de</strong><br />
transmisión <strong>de</strong> <strong>la</strong> red. A! seleccionar el equipo terminal y <strong>la</strong> red para una<br />
vi<strong>de</strong>oconfer<strong>en</strong>cia se <strong>de</strong>be t<strong>en</strong>er <strong>en</strong> cu<strong>en</strong>ta <strong>la</strong> finalidad, <strong>la</strong> frecu<strong>en</strong>cia <strong>de</strong> utilización,<br />
<strong>la</strong> calidad y el aspecto económico.<br />
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VIDEOCONFERENC1AS PUNTO A PUNTO A TRAVEZ DE ISDN<br />
Flujo AA/ digital<br />
comprimido<br />
Fig, 3.7 Configuración básica <strong>de</strong> un sistema <strong>de</strong> vi<strong>de</strong>oconfer<strong>en</strong>cia.<br />
Tarjeta ví<strong>de</strong>o/sonido<br />
y co<strong>de</strong>e<br />
En esta parte <strong><strong>de</strong>l</strong> capítulo se estudia, principalm<strong>en</strong>te tres sistemas: el Picture Tel<br />
Uve PCS 50, el ProShare Vi<strong>de</strong>o System 200 <strong>de</strong> Intel y el TeleWork-5 <strong>de</strong> Vivo;<br />
todos estos se pue<strong>de</strong>n conectar sobre líneas RDSI. El POTS (servicio telefónico<br />
antiguo simple) no pue<strong>de</strong> proporcionar <strong>la</strong> sufici<strong>en</strong>te velocidad, y los <strong>en</strong><strong>la</strong>ces <strong>de</strong> alta<br />
velocidad usualm<strong>en</strong>te ti<strong>en</strong><strong>en</strong> un precio alto.<br />
3.3.1 ProShare Personal Confer<strong>en</strong>cing Vi<strong>de</strong>o System 200.<br />
ProShare Personal Confer<strong>en</strong>cing Vi<strong>de</strong>o System <strong>de</strong> Intel es muy fácil <strong>de</strong> usar y se<br />
ofrece a un precio razonable. Actualm<strong>en</strong>te esta realizando <strong>la</strong> transición <strong>de</strong>s<strong>de</strong> sus<br />
oríg<strong>en</strong>es PCs hacia estar integrado completam<strong>en</strong>te al estándar H.320. Su precio<br />
<strong>de</strong> lista es <strong>de</strong> $1,999 USD, sin embargo <strong>la</strong> mayor parte <strong>de</strong> sus unida<strong>de</strong>s se v<strong>en</strong><strong>de</strong>n<br />
por $999 USD a través <strong>de</strong> un acuerdo <strong>de</strong> v<strong>en</strong>ta conjunta que asocia cada sistema<br />
con una or<strong>de</strong>n para un nuevo servicio RDSI.<br />
NT-1<br />
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El paquete incluye: una tarjeta <strong>de</strong> vi<strong>de</strong>o Smart Vi<strong>de</strong>o Recor<strong>de</strong>r, un<br />
micrófono/audífono, una interfaz RDSI y tres paquetes <strong>de</strong> software; el audio es full<br />
dúplex. La calidad <strong>de</strong> vi<strong>de</strong>o <strong>de</strong> ProShare es fluctuante, usa una tarjeta <strong>de</strong> captura<br />
<strong>de</strong> vi<strong>de</strong>o Intel Smart Vi<strong>de</strong>o Recor<strong>de</strong>r integrada al sistema, que produce, sin<br />
embargo, una imag<strong>en</strong> granulosa y velocida<strong>de</strong>s <strong>de</strong> cuadro que rara vez exce<strong>de</strong>n <strong>de</strong><br />
10 fps (cuadros por segundo). Intel está trabajando para mejorar <strong>la</strong> calidad <strong>de</strong><br />
vi<strong>de</strong>o.<br />
Los tres paquetes <strong>de</strong> software que vi<strong>en</strong><strong>en</strong> con el equipo son:<br />
• El Intel ProShare Personal Confer<strong>en</strong>cing Vi<strong>de</strong>o System que es <strong>la</strong> versión más<br />
reci<strong>en</strong>te <strong>de</strong> <strong>la</strong> aplicación PCs original <strong>de</strong> Intel. Ti<strong>en</strong>e una interfaz parecida al<br />
vi<strong>de</strong>oteléfono que es fácil <strong>de</strong> usar y con una resolución <strong>de</strong> vi<strong>de</strong>o <strong>de</strong> 325 por<br />
264.<br />
• El ProShare Vi<strong>de</strong>o System 200 Room Vi<strong>de</strong>o Confer<strong>en</strong>cing <strong>de</strong> Intel es una<br />
nueva versión apegada al estándar H.320. Su interfaz es simi<strong>la</strong>r, pero algunas<br />
funciones, incluso <strong>la</strong> <strong>de</strong> marcado <strong>de</strong> teléfono rápido, no son tan fáciles <strong>de</strong> usar.<br />
• El Intel ProShare Persona Confer<strong>en</strong>cing Premier Edition proporciona facilidad<br />
<strong>en</strong> el uso <strong><strong>de</strong>l</strong> sistema y <strong>en</strong> g<strong>en</strong>eral un bu<strong>en</strong> <strong>de</strong>sempeño. Su pizarrón <strong>de</strong> carpeta<br />
con l<strong>en</strong>güetas es intuitivo y almac<strong>en</strong>a una cantidad muy gran<strong>de</strong> <strong>de</strong> imág<strong>en</strong>es.<br />
3.3.2 Picture Tel Live PCS 500.-<br />
Es el mejor <strong>de</strong> los sistemas <strong>de</strong> vi<strong>de</strong>o confer<strong>en</strong>cias personales; proporciona una<br />
calidad <strong>de</strong> audio y vi<strong>de</strong>o sobresali<strong>en</strong>tes, soporta H.320, es robusto para una<br />
interoperabilidad, superior a una amplia selección <strong>de</strong> características docum<strong>en</strong>to-<br />
confer<strong>en</strong>cias. Asimismo, el precio es el más elevado, pero no hay duda que se<br />
obti<strong>en</strong>e un producto <strong>de</strong> alta calidad. Su precio está alre<strong>de</strong>dor <strong>de</strong> los $2,495 USD.<br />
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vi<strong>de</strong>o <strong>en</strong> software se apoyan por lo g<strong>en</strong>eral <strong>en</strong> el procesador <strong>de</strong> <strong>la</strong> computadora<br />
para comprimir y <strong>de</strong>scomprimir el vi<strong>de</strong>o.) Su calidad <strong>de</strong> imag<strong>en</strong> no esta a <strong>la</strong> par con<br />
<strong>la</strong> <strong>de</strong> los sistemas basados <strong>en</strong> hardware.<br />
Este sistema transmite vi<strong>de</strong>o a resolución QCIF y pue<strong>de</strong> recibir QCIF o CIF. La<br />
calidad <strong>de</strong> imag<strong>en</strong> es aceptable. "Vivo" produce su propio software <strong>de</strong> vi<strong>de</strong>o H.320<br />
y lo junta con una interfaz RDSI IBM WaveRunner Digital Mó<strong>de</strong>m; soporta los<br />
co<strong>de</strong>e <strong>de</strong> audio G.711 y G.722 <strong>de</strong> H.320. Manejadores <strong>de</strong> pórtico permit<strong>en</strong> que <strong>la</strong><br />
¡nterfaz integrada RDSI WaveRunner trabaje con paquetes <strong>de</strong> comunicaciones que<br />
por lo común requier<strong>en</strong> mó<strong>de</strong>ms y líneas telefónicas análogas.<br />
Los audífonos proporcionan audio full-duplex, aunque <strong>en</strong> <strong>la</strong> actualidad no son aun<br />
muy confortables. Work-5 trabaja con una variedad <strong>de</strong> paquetes <strong>de</strong> docum<strong>en</strong>to-<br />
confer<strong>en</strong>cia.<br />
3.3.4 Otras Aplicaciones <strong><strong>de</strong>l</strong> Vi<strong>de</strong>oteléfono.-<br />
La tecnología, estándares y p<strong>la</strong>taformas <strong>de</strong>sarrol<strong>la</strong>das por <strong>la</strong> vi<strong>de</strong>otelefonía hace<br />
que sea posible que se provea <strong>de</strong> un amplio rango <strong>de</strong> aplicaciones para el usuario.<br />
Estas aplicaciones incluy<strong>en</strong> confer<strong>en</strong>cias con vi<strong>de</strong>o y multimedia, educación a<br />
distancia, aplicaciones <strong>de</strong> vi<strong>de</strong>o <strong>en</strong> tiempo real (como vi<strong>de</strong>o sobre <strong>de</strong>manda) y<br />
servicios informativos multimedia, <strong>en</strong>tre otros.<br />
• Vi<strong>de</strong>o m<strong>en</strong>sajería.- Un servicio <strong>de</strong> vi<strong>de</strong>o comunicación básica es <strong>la</strong> vi<strong>de</strong>o<br />
m<strong>en</strong>sajería. Este servicio incluye <strong>la</strong> grabación <strong>de</strong> vi<strong>de</strong>o m<strong>en</strong>sajes <strong>en</strong> un<br />
mailbox, simi<strong>la</strong>r al que se utiliza con los m<strong>en</strong>sajes <strong>de</strong> voz <strong>de</strong> los sistemas<br />
actuales. También m<strong>en</strong>sajes <strong>de</strong> correo electrónico pue<strong>de</strong>n ser mejorados para<br />
incluir compon<strong>en</strong>tes <strong>de</strong> vi<strong>de</strong>o, y suplem<strong>en</strong>tariam<strong>en</strong>te imág<strong>en</strong>es fijas o <strong>en</strong><br />
movimi<strong>en</strong>to con voz y sonido que pue<strong>de</strong>n ser ligados con m<strong>en</strong>sajes <strong>de</strong> texto.<br />
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Educación a Distancia.- Otra importante aplicación <strong>de</strong> <strong>la</strong> vi<strong>de</strong>otelefonía es <strong>la</strong><br />
educación a distancia. En una aplicación para <strong>la</strong> educación a distancia, el<br />
instructor pue<strong>de</strong> <strong>en</strong>señar a sus estudiantes <strong>de</strong>s<strong>de</strong> una localización remota. Los<br />
estudiantes pue<strong>de</strong>n ver y oír a su instructor remoto con una gran calidad y<br />
variedad <strong>de</strong> información; esta información incluye imág<strong>en</strong>es <strong>de</strong> fotografía, notas<br />
<strong>de</strong> lectura que el instructor pue<strong>de</strong> escribir <strong>en</strong> tiempo real, y segm<strong>en</strong>tos cortos<br />
<strong>de</strong> material <strong>de</strong> vi<strong>de</strong>o.<br />
Vi<strong>de</strong>o sobre <strong>de</strong>manda.- Otro importante servicio <strong>en</strong> esta área es el vi<strong>de</strong>o<br />
sobre <strong>de</strong>manda, el cual da a los usuarios <strong>la</strong> oportunidad <strong>de</strong> seleccionar un<br />
vi<strong>de</strong>o para verlo cuando ellos lo especifiqu<strong>en</strong>. El servicio <strong>de</strong> vi<strong>de</strong>o sobre<br />
<strong>de</strong>manda pue<strong>de</strong> ser llevado por un servicio <strong>de</strong> cable y por <strong>la</strong>s compañías<br />
telefónicas.<br />
Servicio <strong>de</strong> información multimedia.- Los terminales <strong>de</strong> vi<strong>de</strong>o y <strong>la</strong><br />
infraestructura <strong>de</strong> re<strong>de</strong>s <strong>de</strong> vi<strong>de</strong>o acelerarán <strong>la</strong> introducción <strong>de</strong> otros nuevos<br />
servicios. Así por ejemplo, los cli<strong>en</strong>tes podrán acce<strong>de</strong>r al vi<strong>de</strong>o interactivo y<br />
re<strong>la</strong>cionar datos <strong>de</strong>s<strong>de</strong> difer<strong>en</strong>tes fu<strong>en</strong>tes. Como ejemplo, <strong>la</strong> búsqueda <strong><strong>de</strong>l</strong><br />
prototipo <strong>de</strong> un docum<strong>en</strong>to alerta y revisa el sistema, l<strong>la</strong>ma a una base <strong>de</strong> datos<br />
<strong>de</strong> librería <strong>de</strong> imág<strong>en</strong>es, <strong>de</strong>mostrando <strong>la</strong> forma como <strong>en</strong> el futuro se podrá<br />
acce<strong>de</strong>r y hojear varios vi<strong>de</strong>o-periódicos y vi<strong>de</strong>o-publicaciones, hacer compras<br />
y pedidos <strong>de</strong>s<strong>de</strong> el hogar a través <strong>de</strong> vi<strong>de</strong>o catálogos, etc.<br />
El Ví<strong>de</strong>o Teléfono <strong>en</strong> combinación con una Cámara <strong>de</strong> Docum<strong>en</strong>tos.- La<br />
posibilidad <strong>de</strong> expandir el ví<strong>de</strong>o teléfono <strong>de</strong> forma modu<strong>la</strong>r, por ejemplo,<br />
conectando una cámara <strong>de</strong> docum<strong>en</strong>tos como <strong>la</strong> <strong>de</strong>sarrol<strong>la</strong>da por Alcatel Sel<br />
ti<strong>en</strong>e una consi<strong>de</strong>rable importancia.<br />
Esta opción da lugar a importantes y nuevas aplicaciones que podrían contribuir<br />
substancialm<strong>en</strong>te a un increm<strong>en</strong>to <strong>de</strong> <strong>la</strong> eficacia <strong>en</strong> el trabajo. Así, por<br />
85
ejemplo, una empresa <strong>de</strong> diseño podría explicar <strong>la</strong> apari<strong>en</strong>cia <strong>de</strong> un producto<br />
nuevo a sus cli<strong>en</strong>tes usando simplem<strong>en</strong>te el ví<strong>de</strong>o teléfono y <strong>la</strong> cámara <strong>de</strong><br />
objetos.<br />
El cli<strong>en</strong>te podría así consi<strong>de</strong>rar <strong>de</strong> inmediato si el producto cubre o no sus<br />
expectativas, y pue<strong>de</strong>, si es necesario, expresar sin ambigüedad y c<strong>la</strong>ram<strong>en</strong>te<br />
cualquier modificación que consi<strong>de</strong>re necesaria. Este ejemplo <strong>de</strong> aplicación<br />
pue<strong>de</strong> tras<strong>la</strong>darse a muchas áreas por ejemplo <strong>de</strong> <strong>la</strong> economía, administración,<br />
publicidad, marketing etc., como <strong>la</strong> cooperación espontanea <strong>en</strong>tre cli<strong>en</strong>tes,<br />
arquitectos, ing<strong>en</strong>ieros <strong>de</strong> estructuras <strong>en</strong> <strong>la</strong> construcción o <strong>de</strong> <strong>la</strong> industria textil<br />
permiti<strong>en</strong>do llegar a un acuerdo <strong>en</strong>tre diseñador y taller.<br />
El uso <strong><strong>de</strong>l</strong> ví<strong>de</strong>o teléfono con cámara <strong>de</strong> objetos es b<strong>en</strong>eficioso siempre que <strong>la</strong><br />
g<strong>en</strong>te situada <strong>en</strong> distintos lugares quiera c<strong>la</strong>rificar <strong>de</strong>talles re<strong>la</strong>tivos a un<br />
<strong>de</strong>terminado objeto.<br />
El ví<strong>de</strong>o teléfono RDS1 <strong>de</strong> Alcatel Sel ti<strong>en</strong>e una serie <strong>de</strong> características<br />
especiales que permit<strong>en</strong> nuevas aplicaciones como <strong>la</strong>s sigui<strong>en</strong>tes:<br />
Imag<strong>en</strong> <strong>en</strong> Imag<strong>en</strong>:<br />
La facilidad <strong>de</strong> imag<strong>en</strong> <strong>en</strong> imag<strong>en</strong> muestra <strong>la</strong> imag<strong>en</strong> que se transmite <strong>en</strong> <strong>la</strong><br />
esquina superior <strong>de</strong>recha <strong>de</strong> <strong>la</strong> imag<strong>en</strong> que se recibe. Esto asegura una<br />
verificación continua <strong>de</strong> <strong>la</strong> imag<strong>en</strong> que se esta transmiti<strong>en</strong>do <strong>en</strong> su composición y<br />
cont<strong>en</strong>ido, tanto como si <strong>la</strong> imag<strong>en</strong> sea <strong>la</strong> <strong><strong>de</strong>l</strong> usuario o <strong>la</strong> <strong>de</strong> un objeto<br />
seleccionado.<br />
El usuario pue<strong>de</strong> también si esta <strong>en</strong> "transmisión", saber si su propia imag<strong>en</strong> se<br />
esta <strong>en</strong>viando. Esta posibilidad se pue<strong>de</strong> fácilm<strong>en</strong>te activar y eliminar <strong>de</strong> <strong>la</strong><br />
pantaUa por medio <strong>de</strong> una tec<strong>la</strong> <strong>de</strong> teléfono.<br />
86
(Fig.3.8). Se pue<strong>de</strong> observar el <strong>de</strong>partam<strong>en</strong>to a través <strong><strong>de</strong>l</strong> <strong>en</strong><strong>la</strong>ce RDSI así creado<br />
y <strong>de</strong>tectar cualquier cambio, tomándose <strong>la</strong>s a<strong>de</strong>cuadas medidas si fuera necesario.<br />
RED<br />
PUBLICA<br />
E LE HE O»BOUR<br />
WAVORCOtirEHIDO DE<br />
WEHOS<br />
Fig. 3.8 Sistema TeleNeighbour para protección y supervisión <strong>de</strong> ev<strong>en</strong>tos<br />
Otro esc<strong>en</strong>ario es aquel <strong>en</strong> el que el propietario l<strong>la</strong>ma a <strong>la</strong> cámara <strong>de</strong> supervisión.<br />
De esta manera, por ejemplo, pue<strong>de</strong> conocer <strong>de</strong>s<strong>de</strong> su lugar <strong>de</strong> vacaciones<br />
situaciones no <strong>de</strong>seables que afect<strong>en</strong> a su resi<strong>de</strong>ncia. Esto es posible<br />
actualmerite-<strong>en</strong> Europa al haberse ya introducido el proíocoip
CAPITULO IV<br />
ESTUDIO Y PLANIFICACIÓN<br />
UN MODELO DE RED PARA<br />
VIDEOTELÉFONO
CAPITULO IV<br />
ESTUDIO Y PLANIFICACIÓN DE UN MODELO DE RED PARA<br />
VIDEO TELEFONO.-<br />
4.1 ESTUDIO DE LAS REDES DE VIDEO TELEFONÍA POR FIBRA ÓPTICA<br />
En los últimos años se ha dado un gran interés por el estudio y <strong>de</strong>sarrollo <strong>de</strong> re<strong>de</strong>s<br />
<strong>de</strong> vi<strong>de</strong>ocomunicaciones a gran esca<strong>la</strong>, que permitan establecer una rápida<br />
comunicación <strong>en</strong>tre usuarios y el intercambio <strong>de</strong> gran cantidad <strong>de</strong> información<br />
(audio, datos vi<strong>de</strong>o) con el mínimo <strong>de</strong> errores y con alta calidad <strong>de</strong> <strong>la</strong> misma. Se ha<br />
comprobado que un excel<strong>en</strong>te medio <strong>de</strong> transmisión es <strong>la</strong> red <strong>de</strong> fibra óptica, por lo<br />
tanto, <strong>la</strong> insta<strong>la</strong>ción <strong>de</strong> una amplia red <strong>de</strong> cables <strong>de</strong> fibra óptica y puntos <strong>de</strong><br />
distribución <strong>de</strong> abonados requier<strong>en</strong> una eficaz p<strong>la</strong>nificación para el diseño <strong>de</strong> <strong>la</strong><br />
red.<br />
4.1.1 FUNDAMENTOS DE PROPAGACIÓN DE LA LUZ EN SISTEMAS DE<br />
FIBRAS ÓPTICAS.<br />
La luz se pue<strong>de</strong> propagar a través <strong>de</strong> un cable <strong>de</strong> fibra óptica por reflexión o<br />
refracción. La forma <strong>de</strong> propagación <strong>de</strong> <strong>la</strong> luz <strong>de</strong>p<strong>en</strong><strong>de</strong>rá <strong><strong>de</strong>l</strong> modo <strong>de</strong> propagación<br />
y <strong><strong>de</strong>l</strong> índice <strong>de</strong> contorno <strong>de</strong> <strong>la</strong> fibra. En terminología <strong>de</strong> fibra óptica, <strong>la</strong> pa<strong>la</strong>bra<br />
modo simple significa una trayectoria, esto es que <strong>la</strong> luz pue<strong>de</strong> tomar un solo<br />
camino a lo <strong>la</strong>rgo <strong><strong>de</strong>l</strong> cable <strong>de</strong> fibra óptica, se lo conoce como monomodo; si <strong>la</strong> luz<br />
pue<strong>de</strong> seguir mas <strong>de</strong> una trayectoria <strong>de</strong>ntro <strong><strong>de</strong>l</strong> cable, se lo conocerá como<br />
multimodo<br />
Por el índice <strong>de</strong> refracción <strong>la</strong>s fibras se <strong>la</strong>s pue<strong>de</strong> c<strong>la</strong>sificar <strong>en</strong> dos tipos: Fibra<br />
óptica <strong>de</strong> índice gradual y fibra óptica <strong>de</strong> índice escalonado<br />
En <strong>la</strong>s fibras <strong>de</strong> índice escalonado se produce un cambio abrupto <strong>en</strong> el índice <strong>de</strong><br />
refracción <strong>de</strong> <strong>la</strong> luz, mi<strong>en</strong>tras que <strong>en</strong> <strong>la</strong>s fibras <strong>de</strong> índice gradual el índice <strong>de</strong><br />
refracción cambia gradualm<strong>en</strong>te.<br />
89
Para realizar <strong>la</strong> transmisión <strong>de</strong>'información a través <strong>de</strong> fibras ópticas, es necesario<br />
utilizar conversores eléctrico/ópticos y óptico/eléctricos los cuales conviert<strong>en</strong> <strong>la</strong><br />
señal eléctrica <strong>en</strong> óptica y viceversa. Hay dos dispositivos que se usan<br />
comúnm<strong>en</strong>te como conversores eléctrico/ópticos para g<strong>en</strong>erar luz <strong>en</strong> un sistema<br />
<strong>de</strong> comunicaciones por fibra óptica: Diodos Emisores <strong>de</strong> Luz (LEDs) y Diodos<br />
Láser <strong>de</strong> Inyección (ILDs). Aun cuando los LED prove<strong>en</strong> m<strong>en</strong>os <strong>en</strong>ergía y operan<br />
a m<strong>en</strong>or velocidad, satisfac<strong>en</strong> ampliam<strong>en</strong>te <strong>la</strong>s aplicaciones <strong>en</strong> <strong>la</strong>s que se requiere<br />
velocida<strong>de</strong>s <strong>de</strong> varios ci<strong>en</strong>tos <strong>de</strong> megabits y distancias <strong>de</strong> transmisión <strong>de</strong> varios<br />
kilómetros. Para altas velocida<strong>de</strong>s y <strong>la</strong>rgas distancias se <strong>de</strong>berá consi<strong>de</strong>rar los<br />
diodos láser. En el otro extremo <strong>de</strong> <strong>la</strong> fibra, un fotodiodo realiza <strong>la</strong> función <strong>de</strong><br />
conversor óptico/eléctrico.<br />
4.1.2 TIPOS DE FIBRA OPTICA.-<br />
Hay tres tipos <strong>de</strong> fibras ópticas disponibles hoy <strong>en</strong> día. Estos están construidos ya<br />
sean <strong>en</strong> vidrio, <strong>en</strong> plástico o una combinación <strong>de</strong> <strong>la</strong>s dos. Las tres varieda<strong>de</strong>s son:<br />
• Con núcleo y <strong>en</strong>voltura <strong>de</strong> plástico<br />
• Con núcleo <strong>de</strong> vidrio y <strong>en</strong>voltura <strong>de</strong> plástico (l<strong>la</strong>mada fibra PCS)<br />
• Con núcleo y <strong>en</strong>voltura <strong>de</strong> vidrio (l<strong>la</strong>mada fibra SCS).<br />
Las fibras plásticas ti<strong>en</strong><strong>en</strong> gran<strong>de</strong>s v<strong>en</strong>tajas sobre <strong>la</strong>s fibras <strong>de</strong> vidrio, primero<br />
porque <strong>la</strong>s fibras plásticas son más flexibles que <strong>la</strong>s <strong>de</strong> vidrio, son fáciles <strong>de</strong><br />
insta<strong>la</strong>r, pue<strong>de</strong>n resistir mejor <strong>la</strong> presión, son m<strong>en</strong>os costosas y su peso es<br />
aproximadam<strong>en</strong>te un 60% m<strong>en</strong>or que <strong>la</strong>s <strong>de</strong> vidrio.<br />
Las <strong>de</strong>sv<strong>en</strong>tajas <strong>de</strong> <strong>la</strong>s fibras plásticas' están <strong>en</strong> que ti<strong>en</strong><strong>en</strong> una at<strong>en</strong>uación<br />
re<strong>la</strong>tivam<strong>en</strong>te alta, no pue<strong>de</strong>n propagar <strong>la</strong> luz con tanta efici<strong>en</strong>cia como <strong>la</strong>s <strong>de</strong><br />
vidrio, <strong>la</strong>s fibras plásticas están limitadas para trayectorias cortas como <strong>la</strong>s que se<br />
utilizan <strong>de</strong>ntro <strong>de</strong> un edificio.<br />
En cuanto al tipo <strong>de</strong> cables <strong>de</strong> fibra óptica, hay básicam<strong>en</strong>te dos tipos: los loóse<br />
(suelto, flojo) buffer y los tight (ajustado) buffer.<br />
90
Los cables loóse buffer están diseñados para ser usados <strong>en</strong> exteriores y <strong>en</strong><br />
ambi<strong>en</strong>tes don<strong>de</strong> ios cambios <strong>de</strong> temperatura son probables <strong>de</strong> ocurrir, estos son<br />
i<strong>de</strong>ales para <strong>la</strong>rgas distancias <strong>en</strong> líneas <strong>de</strong> telecomunicación.<br />
Los cables tight buffer están diseñados para cableado horizontal, <strong>en</strong> aplicaciones<br />
don<strong>de</strong> <strong>la</strong>s variaciones <strong>de</strong> temperatura son mínimas y con recorridos cortos <strong>de</strong><br />
cable<br />
4.1.3 ESTRUCTURA DEL CABLE DE FIBRA ÓPTICA<br />
Los cables se compon<strong>en</strong> <strong>de</strong> un núcleo <strong>de</strong> polietil<strong>en</strong>o eximido <strong>de</strong> 4,5 mm <strong>de</strong><br />
diámetro con un elem<strong>en</strong>to c<strong>en</strong>tral resist<strong>en</strong>te <strong>de</strong> acero galvanizado <strong>de</strong> 1,8 mm <strong>de</strong><br />
diámetro. El núcleo incorpora varios surcos helicoidales para alojar <strong>la</strong>s fibras<br />
ópticas. Varios <strong>de</strong> estos núcleos se agrupan y proteg<strong>en</strong> con una capa <strong>de</strong> aluminio<br />
<strong>la</strong>minado/polietil<strong>en</strong>o <strong>de</strong> alta <strong>de</strong>nsidad para obt<strong>en</strong>er un cable que cont<strong>en</strong>ga 10, 20,<br />
30, 50, 70, 90,120, 150 o 210 fibras. (Fig. 4.1)<br />
SECCIONES TRANSVERSALES DE CABLES ÓPTICOS<br />
10<br />
20<br />
50 70<br />
Fig.4.1 Secciones transversales <strong>de</strong> cables ópticos <strong>de</strong> diversos tamaños que se utilizan <strong>en</strong><br />
<strong>la</strong> red <strong>de</strong> vi<strong>de</strong>ocomunícaciones, mostrando su construcción a partir <strong><strong>de</strong>l</strong> mismo<br />
núcleo básico estriado.<br />
La tab<strong>la</strong> 4.1 resume <strong>la</strong>s características <strong>de</strong> estos cables.<br />
30<br />
91
Características <strong>de</strong> <strong>la</strong>s Fibras<br />
Máximo diámetro exterior (mm)<br />
Peso Aproximado<br />
(Kg/Km)<br />
Mínimo radio <strong>de</strong> curvatura<br />
Estático<br />
Dinámico<br />
Número <strong>de</strong> Fibras<br />
20<br />
16<br />
210<br />
200<br />
200<br />
30<br />
18<br />
230<br />
200<br />
200<br />
TABLA 4.1 Principales características <strong>de</strong> los cables <strong>de</strong> fibra óptica<br />
Para los puntos <strong>de</strong> distribución se utiliza un cable especial <strong>de</strong> fibra óptica<br />
construido a base <strong>de</strong> un núcleo cilindrico <strong>de</strong> 4,5 mm <strong>de</strong> diámetro con cinco surcos,<br />
cada uno <strong>de</strong> ellos capaz <strong>de</strong> alojar una fibra óptica. Exist<strong>en</strong> tres versiones para <strong>la</strong><br />
insta<strong>la</strong>ción <strong>en</strong> distintos <strong>en</strong>tornos:<br />
1.- Versión para uso <strong>de</strong>ntro <strong>de</strong> edificios: pres<strong>en</strong>ta una cubierta <strong>de</strong> aluminio y<br />
50<br />
20<br />
320<br />
230<br />
250<br />
70<br />
21<br />
450<br />
230<br />
250<br />
90<br />
27<br />
500<br />
230<br />
250<br />
120<br />
30<br />
720<br />
300<br />
300<br />
polietil<strong>en</strong>o <strong>de</strong> alta <strong>de</strong>nsidad (Fig.4.2) Con un diámetro exterior <strong>de</strong> 7.5 mm<br />
F8fU CfTCA 1WLT1MOOO C€ MXE GíUDIW.<br />
Híuemo peseiEKTE CENTÍW, MEW.CG<br />
cuero DE PUSTCD con<br />
SURCOS B(V<br />
CW« EWCUJUMS<br />
ENHEUCE<br />
OttTA DE ALUMfW LCW3TUWAL<br />
CUSEUIA EXTEflCfl OE PCLETUIO<br />
NEGRO DE AOA DENSJWD<br />
Cable do fibra óptica<br />
con núcleo estriado<br />
para Insta<strong>la</strong>ciones <strong>en</strong><br />
edificios o conduceío-<br />
150<br />
33<br />
720<br />
300<br />
300<br />
92<br />
210<br />
35<br />
900<br />
300<br />
300<br />
Fig, 4.2 Estructura <strong>de</strong> un cable<br />
<strong>de</strong> fibra óptica.
2.- Versión para insta<strong>la</strong>ción <strong>de</strong> muros exteriores: cable obt<strong>en</strong>ido mediante<br />
incorporación <strong>de</strong> un material <strong>de</strong> rell<strong>en</strong>o <strong>en</strong> el núcleo y con una cubierta <strong>de</strong><br />
polietil<strong>en</strong>o <strong>de</strong> alta <strong>de</strong>nsidad.<br />
3.- Versión para insta<strong>la</strong>ción aérea: este cable autosoportado ti<strong>en</strong>e una sección<br />
<strong>en</strong> forma <strong>de</strong> 8, con un elem<strong>en</strong>to resist<strong>en</strong>te <strong>de</strong> acero galvanizado (fig.4.3).<br />
Utiliza <strong>la</strong> misma estructura impermeable que el cable anterior.<br />
El cable <strong>de</strong> fibra <strong>de</strong> abonado es <strong><strong>de</strong>l</strong> tipo U, provisto <strong>de</strong> tres refuerzos, como se<br />
aprecia <strong>en</strong> <strong>la</strong> Fig. 4.4. se fabrica <strong>en</strong> dos versiones. Una <strong>de</strong>stinada a insta<strong>la</strong>ciones<br />
aéreas y exteriores, incorpora un rell<strong>en</strong>o <strong>de</strong> ge<strong>la</strong>tina <strong>de</strong> petróleo impermeable y un<br />
refuerzo externo <strong>de</strong> kev<strong>la</strong>r; su diámetro exterior es <strong>de</strong> 7,5 mm. La otra, para uso<br />
interno, ti<strong>en</strong>e una cubierta <strong>de</strong> polietil<strong>en</strong>o <strong>de</strong> 5 mm <strong>de</strong> diámetro.<br />
Fig. 4.3 Cable <strong>de</strong> fibra óptica con sección <strong>en</strong> forma <strong>de</strong> 8<br />
Cable Coaxial: Este cable es <strong><strong>de</strong>l</strong> tipo 3,9/17,3 mm, con un conductor c<strong>en</strong>tral <strong>de</strong><br />
cobre, ais<strong>la</strong>mi<strong>en</strong>to <strong>de</strong> polietil<strong>en</strong>o celu<strong>la</strong>r y conductor externo <strong>de</strong> aluminio soldado<br />
longitudinalm<strong>en</strong>te. Ti<strong>en</strong>e una impedancia <strong>de</strong> 75 ± 2 Q y una at<strong>en</strong>uación <strong>de</strong> 3,8 dB<br />
por 100m a 420 Mhz.<br />
93
RELLENO DE GRASA<br />
FIBRA DE V1DEOCOM<br />
KEVLAR TRENZADO<br />
CINTA TERFAN<br />
PORTADOR PVC<br />
HELICOIDAL<br />
ELEMENTOS RESIS-<br />
TENTES DE FIBRA<br />
DE VIDRIO<br />
CUBIERTA DE POUET1LENO DE ALTA DENSIDAD<br />
F¡g.4.4 Cable óptico monofibra <strong>de</strong> usuario. Tipo U<br />
4.1.4 VENTAJAS AL USAR REDES DE FIBRA OPTICA.-<br />
Las fibras ópticas ofrec<strong>en</strong> un número significativo <strong>de</strong> v<strong>en</strong>tajas, <strong>en</strong> comparación con<br />
otras re<strong>de</strong>s, <strong>en</strong>tre <strong>la</strong>s que po<strong>de</strong>mos m<strong>en</strong>cionar:<br />
• Aum<strong>en</strong>to <strong>en</strong> <strong>la</strong> capacidad <strong><strong>de</strong>l</strong> ancho <strong>de</strong> banda; un solo par <strong>de</strong> fibras ópticas<br />
pue<strong>de</strong>n proveer 3 Giga bps <strong>de</strong> amplitud <strong>de</strong> banda. Los progresos <strong>en</strong> los<br />
sistemas láser utilizados para g<strong>en</strong>erar señales ópticas y los foto diodos<br />
utilizados para captar<strong>la</strong>s, ya han llevado a límites superiores <strong>la</strong>s tasas <strong>de</strong><br />
transmisión, mas allá <strong>de</strong> los billones <strong>de</strong> bits por segundo.<br />
• Baja at<strong>en</strong>uación <strong>de</strong> señal y altas tasas <strong>de</strong> transmisión. La at<strong>en</strong>uación <strong>de</strong> <strong>la</strong><br />
señal ti<strong>en</strong>e <strong>en</strong> cu<strong>en</strong>ta <strong>la</strong> pérdida que ocurre durante <strong>la</strong> transmisión, <strong>la</strong> cual es<br />
medida <strong>en</strong> <strong>de</strong>cibeles (dB). Cuando <strong>la</strong> señal recibida es m<strong>en</strong>or que <strong>la</strong> señal<br />
transmitida, <strong>la</strong> perdida se atribuye, <strong>en</strong>tre otras causas, a <strong>la</strong> ¡mpedancia<br />
inher<strong>en</strong>te <strong><strong>de</strong>l</strong> medio <strong>de</strong> transmisión. Con el cable metálico, <strong>la</strong>s pérdidas <strong>en</strong> <strong>la</strong><br />
transmisión, ocasionadas por <strong>la</strong> at<strong>en</strong>uación <strong>de</strong> <strong>la</strong> señal, aum<strong>en</strong>tan<br />
proporcionalm<strong>en</strong>te con <strong>la</strong> frecu<strong>en</strong>cia <strong>de</strong> <strong>la</strong> señal y <strong>la</strong> distancia que <strong>la</strong> misma<br />
<strong>de</strong>be recorrer. En contraste, <strong>la</strong> fibra óptica, exhibe una impedancia inher<strong>en</strong>te<br />
94
aja, lo que significa que el medio pue<strong>de</strong> manejar tasas mas altas <strong>de</strong><br />
transmisión para distancias mayores, sin necesidad <strong>de</strong> repetidores para<br />
reg<strong>en</strong>erar <strong>la</strong> señal.<br />
• Alta integridad <strong>en</strong> <strong>la</strong> información; los <strong>en</strong><strong>la</strong>ces <strong>de</strong> fibra óptica, aum<strong>en</strong>tan <strong>de</strong> una<br />
manera significativa el nivel <strong>de</strong> r<strong>en</strong>dimi<strong>en</strong>to cuando se los compara con el cable<br />
metálico/cobre, como resultado, los sistemas <strong>de</strong> fibra óptica, no requier<strong>en</strong> el<br />
uso excesivo <strong>de</strong> protocolos <strong>de</strong> prueba <strong>de</strong> errores.<br />
• Inmunidad a <strong>la</strong> infer<strong>en</strong>cia electromagnética (HE) y a <strong>la</strong> interfer<strong>en</strong>cia <strong>de</strong> <strong>la</strong>s<br />
frecu<strong>en</strong>cias <strong>de</strong> radio (IFR), ME e IFR, son <strong>la</strong>s principales causas <strong>de</strong> errores <strong>en</strong><br />
<strong>la</strong> información (datos), <strong>en</strong> <strong>la</strong> transmisión a través <strong>de</strong> re<strong>de</strong>s <strong>de</strong> cobre/metálicas.<br />
La fibra óptica, es inmune a este tipo <strong>de</strong> alteraciones, y es también inmune a<br />
los problemas ocasionados por condiciones <strong>de</strong> tiempo adversas, una <strong>de</strong> <strong>la</strong>s<br />
mayores <strong>de</strong>sv<strong>en</strong>tajas <strong>de</strong> <strong>la</strong>s microondas como medio <strong>de</strong> transmisión.<br />
• Durabilidad.- Un corte <strong>de</strong> sección <strong>de</strong> una fibra óptica, muestra cuatro<br />
segm<strong>en</strong>tos: el Núcleo, el cual transmite <strong>la</strong> señal <strong>de</strong> luz, el Revestimi<strong>en</strong>to<br />
Metálico, el cual manti<strong>en</strong>e <strong>la</strong> señal <strong>de</strong> luz <strong>de</strong>ntro <strong><strong>de</strong>l</strong> núcíeo, <strong>la</strong> Cobertura, y el<br />
Reforzami<strong>en</strong>to, el cual le da consist<strong>en</strong>cia al cable. Como resultado, <strong>la</strong> fibra<br />
ti<strong>en</strong>e <strong>la</strong> resist<strong>en</strong>cia, que le permite ser conducida a través <strong>de</strong> pare<strong>de</strong>s, pisos y<br />
ductos subterráneos, sin que se dañe.<br />
4.1.5 PÉRDIDAS EN LAS FIBRAS<br />
a) Pérdidas <strong>en</strong> Transmisión.-<br />
Las perdidas <strong>en</strong> <strong>la</strong> transmisión por cables <strong>de</strong> fibras ópticas son una <strong>de</strong> <strong>la</strong>s<br />
características más importantes <strong>de</strong> <strong>la</strong>s fibras. Estas pue<strong>de</strong>n darse por una<br />
reducción <strong>en</strong> <strong>la</strong> fu<strong>en</strong>te <strong>de</strong> luz lo que motivará <strong>la</strong> reducción <strong><strong>de</strong>l</strong> ancho <strong>de</strong> banda <strong><strong>de</strong>l</strong><br />
sistema, el ritmo <strong>de</strong> transmisión <strong>de</strong> <strong>la</strong> información, <strong>la</strong> efici<strong>en</strong>cia, y sobretodo <strong>la</strong><br />
capacidad <strong><strong>de</strong>l</strong> sistema. Las causas predominantes <strong>de</strong> éstas pérdidas <strong>en</strong> <strong>la</strong>s fibras<br />
son; pérdidas por microtorceduras <strong>de</strong> <strong>la</strong> fibra, perdidas por conexión <strong>de</strong> fibras,<br />
perdidas por malos empalmes, perdidas por malos acoples<br />
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) Pérdidas por Absorción<br />
Las pérdidas por absorción <strong>en</strong> fibras ópticas son análogas a <strong>la</strong>s perdidas por<br />
disipación <strong>de</strong> calor <strong>en</strong> los cables <strong>de</strong> cobre; <strong>la</strong>s impurezas <strong>en</strong> <strong>la</strong>s fibras absorb<strong>en</strong> <strong>la</strong><br />
luz y <strong>la</strong> conviert<strong>en</strong> <strong>en</strong> calor, <strong>la</strong> pureza <strong>de</strong> los vidrios <strong>en</strong> <strong>la</strong> fabricación <strong>de</strong> fibras<br />
ópticas <strong>de</strong>be ser <strong>de</strong> aproximadam<strong>en</strong>te <strong><strong>de</strong>l</strong> 99,9999 por ci<strong>en</strong>to. Exist<strong>en</strong> tres<br />
factores que contribuy<strong>en</strong> a <strong>la</strong>s perdidas por absorción <strong>en</strong> una fibra óptica<br />
1. La absorción ultravioleta<br />
2. La absorción infrarroja<br />
3. La absorción por resonancia iónica<br />
1.- Absorción Ultravioleta. La absorción ultravioleta es causada por los<br />
electrones <strong>de</strong> val<strong>en</strong>cia <strong><strong>de</strong>l</strong> material <strong>de</strong> silicio con el que se fabrica <strong>la</strong> fibra. La luz<br />
ioniza los electrones <strong>de</strong> val<strong>en</strong>cia <strong>de</strong>ntro <strong><strong>de</strong>l</strong> conductor. La ionización es<br />
equival<strong>en</strong>te a <strong>la</strong>s pérdidas <strong>en</strong> el total <strong><strong>de</strong>l</strong> campo <strong>de</strong> luz y contribuye a <strong>la</strong>s pérdidas<br />
<strong>de</strong> transmisión <strong>de</strong> <strong>la</strong> fibra.<br />
2.- Absorción Infrarroja La absorción infrarroja es el resultado <strong>de</strong> fotones <strong>de</strong> luz<br />
que son absorbidos por los átomos <strong>de</strong> <strong>la</strong>s molécu<strong>la</strong>s <strong><strong>de</strong>l</strong> vidrio <strong>en</strong> el núcleo. Los<br />
fotones absorbidos se conviert<strong>en</strong> <strong>en</strong> vibraciones mecánicas <strong>de</strong> tipo aleatorio<br />
típicas <strong><strong>de</strong>l</strong> calor<br />
3.- Absorción <strong>de</strong> resonancia Iónica Esta es causada por los iones OH <strong>en</strong> el<br />
material. La fu<strong>en</strong>te <strong>de</strong> los iones OH son <strong>la</strong>s molécu<strong>la</strong>s <strong>de</strong> agua <strong>la</strong>s mismas que son<br />
atrapadas <strong>en</strong> el vidrio <strong>en</strong> el proceso <strong>de</strong> fabricación. La absorción iónica también es<br />
causada por <strong>la</strong>s molécu<strong>la</strong>s <strong>de</strong> hierro, cobre y cromo que pue<strong>de</strong>n estar pres<strong>en</strong>tes.<br />
c) Pérdidas por Difracción.<br />
Durante el proceso <strong>de</strong> fabricación el vidrio es extruido (estirado <strong>en</strong> <strong>la</strong>rgas fibras <strong>de</strong><br />
diámetro pequeño), y <strong>la</strong> t<strong>en</strong>sión aplicada causa el <strong>en</strong>friami<strong>en</strong>to <strong><strong>de</strong>l</strong> vidrio<br />
<strong>de</strong>sarrol<strong>la</strong>ndo irregu<strong>la</strong>rida<strong>de</strong>s submicroscópicas perman<strong>en</strong>tes <strong>en</strong> <strong>la</strong> fibra. Cuando<br />
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los rayos <strong>de</strong> luz se propagan a lo <strong>la</strong>rgo <strong>de</strong> <strong>la</strong> fibra, chocan con estas impurezas y<br />
son difractados; <strong>la</strong> difracción causa que <strong>la</strong> luz se disperse o se propague hacia<br />
afuera <strong>en</strong> muchas direcciones y se escape hacia el manto <strong>de</strong> <strong>la</strong> fibra<br />
repres<strong>en</strong>tando perdidas <strong>en</strong> <strong>la</strong> fu<strong>en</strong>te <strong>de</strong> luz<br />
4.1.6 TOPOLOGÍA DE REDES,<br />
Red Punto a Punto. La topología punto a punto es común hoy <strong>en</strong> día, esta<br />
requiere dos nodos comunicados directam<strong>en</strong>te, <strong>en</strong><strong>la</strong>zados normalm<strong>en</strong>te por un<br />
par <strong>de</strong> hebras <strong>de</strong> fibras ópticas, una para transmisión y otra para recepción. Entre<br />
ias aplicaciones punto a punto se incluy<strong>en</strong>:<br />
• Canales para computación<br />
• Terminales multiplexíng<br />
• Transmisión <strong>de</strong> Vi<strong>de</strong>o<br />
La Fig. 4.5 muestra el montaje para una red <strong>de</strong> fibra óptica punto a punto.<br />
COMPUTADOR<br />
A<br />
m<br />
RS232<br />
T E<br />
90<br />
ÓPTICA<br />
BIT DRIVER<br />
FIBRA ÓPTICA DÚPLEX CABLE<br />
RS232<br />
TERMINAL<br />
97<br />
Fig. 4.5 Montaje para una<br />
red <strong>de</strong> Fibra Óptica punto a<br />
punto
Red <strong>en</strong> Estrel<strong>la</strong>. Las re<strong>de</strong>s <strong>en</strong> estrel<strong>la</strong> son arreglos <strong>en</strong> torno a un único hub que<br />
pue<strong>de</strong> actuar como un controtador c<strong>en</strong>tral para <strong>la</strong> red. La transmisión <strong>en</strong>viada<br />
<strong>de</strong>s<strong>de</strong> un nodo o terminal <strong>de</strong>be primero pasar a través <strong><strong>de</strong>l</strong> hub. Las aplicaciones<br />
comunes <strong>de</strong> una red estrel<strong>la</strong>r incluy<strong>en</strong> PBX y Mainframe<br />
Topología Anillo. En una red tipo Anillo todos los terminales son <strong>en</strong><strong>la</strong>zados <strong>en</strong><br />
series punto a punto. Si una parte fal<strong>la</strong>, el sistema se cae, a m<strong>en</strong>os que se use<br />
compon<strong>en</strong>tes <strong>de</strong> bypass.<br />
Para evitar conflictos, cada sistema usa un bit patrón (patern), l<strong>la</strong>mado tok<strong>en</strong>. El<br />
tok<strong>en</strong> circu<strong>la</strong> hacia cada nodo, autorizando al nodo a capturar el tok<strong>en</strong> y el ring a<br />
transmitir datos.<br />
Topología Bus. Las re<strong>de</strong>s basadas <strong>en</strong> topología bus usan también un diseño<br />
tok<strong>en</strong> o un diseño <strong>de</strong> acceso conocido como carrier-s<strong>en</strong>se múltiple access con<br />
<strong>de</strong>tección <strong>de</strong> colisiones (CSMA/CD) o con anu<strong>la</strong>ción <strong>de</strong> colisiones (CSMA/CA)<br />
Como un anillo, los m<strong>en</strong>sajes <strong>en</strong> el bus son transmitidos a todos los terminales.<br />
(Fig. 4.6) Los sistemas más popu<strong>la</strong>res que usan topología bus son Ethernet y<br />
MAP (Manufacturing Automation Protoco!).<br />
4.1.7 EMPALMES EN FIBRAS OPTICAS.-<br />
Para realizar empalmes <strong>en</strong> <strong>la</strong>s fibras se emplea un bloque e<strong>la</strong>stómero mol<strong>de</strong>ado<br />
con un surco <strong>de</strong> sección rectangu<strong>la</strong>r, ligeram<strong>en</strong>te m<strong>en</strong>or que <strong>la</strong> sección <strong>de</strong> <strong>la</strong> fibra.<br />
Este surco se ll<strong>en</strong>a con un adhesivo transpar<strong>en</strong>te polimerizable y se cubre el<br />
bloque con una <strong>la</strong>mina <strong>de</strong> vidrio. Una vez <strong>de</strong>spojadas <strong><strong>de</strong>l</strong> recubrimi<strong>en</strong>to primario y<br />
tal<strong>la</strong>das <strong>en</strong> su extremo, <strong>la</strong>s dos fibras a conectarse se empujan <strong>de</strong>s<strong>de</strong> <strong>la</strong>dos<br />
opuestos, a través <strong><strong>de</strong>l</strong> adhesivo, y son automáticam<strong>en</strong>te alineadas por <strong>la</strong>s fuerzas<br />
que produce <strong>la</strong> <strong>de</strong>formación <strong><strong>de</strong>l</strong> e<strong>la</strong>stómero. El empalme se protege luego <strong>en</strong> un<br />
receptáculo rell<strong>en</strong>o con <strong>la</strong> misma resina polímera. Tras comprobar el empalme con<br />
un microscopio, el adhesivo se polimeriza mediante <strong>la</strong> exposición a <strong>la</strong> luz<br />
ultravioleta, y <strong>la</strong> conexión se protege <strong>en</strong> un receptáculo metálico. Los diez<br />
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empalmes que requiere cada estructura <strong>de</strong> núcleo con fibras embutidas <strong>en</strong> surcos<br />
(o dos estructuras <strong>de</strong> 5 fibras) 'se insta<strong>la</strong>n <strong>en</strong> un estuche <strong>de</strong> empalmes circu<strong>la</strong>r que<br />
protege <strong>la</strong>s fibras <strong>de</strong>scubiertas.<br />
4.1.8 Apertura Numérica (An)<br />
Es un parámetro importante <strong>en</strong> fibras ópticas, y es igual al s<strong>en</strong>o <strong><strong>de</strong>l</strong> semiángulo<br />
máximo <strong><strong>de</strong>l</strong> cono <strong>en</strong> el cual están cont<strong>en</strong>idas todas <strong>la</strong>s direcciones <strong>de</strong> los rayos<br />
que inci<strong>de</strong>n <strong>en</strong> <strong>la</strong> sección transversal <strong>de</strong> <strong>la</strong> fibra y produc<strong>en</strong> reflexión total.<br />
4.2 PLANIFICACIÓN DE UN MODELO DE RED PARA ESTA<br />
TECNOLOGÍA (CON FIBRA ÓPTICA)<br />
4.2.1 Diseño <strong><strong>de</strong>l</strong> Sistema<br />
Como un primer paso, se realiza un p<strong>la</strong>no o diagrama <strong>de</strong> <strong>de</strong>talles <strong>en</strong> el que están<br />
especificados <strong>la</strong> distancia, el recorrido <strong>de</strong>! cable y el <strong>en</strong>torno <strong>de</strong> operación. Cada<br />
sistema simple punto a punto, requiere <strong>de</strong> un diagrama esquemático <strong><strong>de</strong>l</strong> sistema<br />
(En el anexo 1 se adjuntan los p<strong>la</strong>nos y diagramas m<strong>en</strong>cionados).<br />
Como topología <strong>de</strong> red se utilizará <strong>en</strong> el pres<strong>en</strong>te trabajo una tipo estrel<strong>la</strong>, cuyas<br />
características se <strong>de</strong>scribió anteriorm<strong>en</strong>te:<br />
4.2.1.1 Selección <strong><strong>de</strong>l</strong> Número <strong>de</strong> Fibras<br />
La selección <strong>de</strong> <strong>la</strong> cantidad <strong>de</strong> fibras <strong>en</strong> el cable matriz, es un dato importante que<br />
influye <strong>en</strong> <strong>la</strong> futura capacidad <strong>de</strong> <strong>la</strong> red <strong>de</strong> comunicación. Muchos <strong>de</strong> los <strong>la</strong>zos <strong>de</strong><br />
cables <strong>de</strong> fibra óptica individuales insta<strong>la</strong>dos ahora se integrarán a <strong>la</strong> red <strong>de</strong><br />
comunicaciones futura. Puesto que este sistema proveerá <strong>de</strong> servicio para un<br />
difer<strong>en</strong>te número <strong>de</strong> aplicaciones, el número <strong>de</strong> fibras insta<strong>la</strong>do <strong>en</strong> el backbone y <strong>la</strong><br />
distribución <strong><strong>de</strong>l</strong> cable <strong>de</strong>be ser cuidadosam<strong>en</strong>te consi<strong>de</strong>rado.<br />
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La <strong>de</strong>cisión con respecto al número <strong>de</strong> fibras ópticas a insta<strong>la</strong>r es muy <strong>de</strong>p<strong>en</strong>di<strong>en</strong>te<br />
<strong>de</strong> <strong>la</strong> aplicación, así pues se ti<strong>en</strong>e:<br />
• Aplicaciones <strong>de</strong> vi<strong>de</strong>o. Estas usualm<strong>en</strong>te emplean soluciones <strong>de</strong> una o dos<br />
fibras insta<strong>la</strong>das <strong>en</strong> topologías punto a punto o <strong>en</strong> estrel<strong>la</strong>. Vi<strong>de</strong>o con<br />
aplicaciones <strong>de</strong> seguridad son <strong>en</strong> un solo s<strong>en</strong>tido, portante, se requiere <strong>de</strong> una<br />
fibra, mi<strong>en</strong>tras que el vi<strong>de</strong>o interactivo requiere dos fibras.<br />
• Telemetría.- Involucra <strong>la</strong> transmisión <strong>de</strong> datos a distancia monitoreados <strong>de</strong>s<strong>de</strong><br />
<strong>la</strong> estación. Esta aplicación es g<strong>en</strong>eralm<strong>en</strong>te <strong>en</strong> un solo s<strong>en</strong>tido <strong>de</strong> transmisión<br />
y requiere <strong>de</strong> una so<strong>la</strong> fibra. Ciertos tipos <strong>de</strong> telemetría, sin embargo, pue<strong>de</strong>n<br />
ser interactivos. Correcciones <strong>de</strong> señal, basados <strong>en</strong> receptores <strong>de</strong> datos, son<br />
<strong>en</strong>viadas a los mismos aparatos remotos. Esta aplicación típicam<strong>en</strong>te usa dos<br />
fibras, una para transmisión <strong>de</strong> datos y <strong>la</strong> otra para recepción <strong>de</strong> corrección <strong>de</strong><br />
señal.<br />
• Comunicación <strong>de</strong> Voz y Datos.- Hoy <strong>en</strong> día <strong>la</strong>s comunicaciones <strong>de</strong> voz y<br />
datos g<strong>en</strong>eralm<strong>en</strong>te emplean dos fibras por nodo. Para muchos <strong>de</strong> los usuarios<br />
finales se prevé <strong>la</strong> insta<strong>la</strong>ción <strong>de</strong> un par <strong>de</strong> fibras adicionales <strong>de</strong> protección.<br />
Esta filosofía es consist<strong>en</strong>te con <strong>la</strong> actual topología <strong>de</strong> red<br />
Para el pres<strong>en</strong>te proyecto se utilizaran 4 fibras por nodo, 2 para comunicación y 2<br />
<strong>de</strong> reserva.<br />
4.2.1.2 Cálculos <strong>de</strong> Pérdidas.<br />
Entre <strong>la</strong>s principales causas <strong>de</strong> pérdidas se ti<strong>en</strong>e <strong>la</strong>s <strong>de</strong> los conectores, pérdidas<br />
por <strong>en</strong>vejecimi<strong>en</strong>to (transmisor, receptor y conectores), variaciones <strong>de</strong> temperatura<br />
y el marg<strong>en</strong> <strong>de</strong> error que se <strong>de</strong>be asignar al sistema.<br />
Para el sigui<strong>en</strong>te cálculo, se usará <strong>la</strong> información <strong>de</strong> acuerdo a lo que se ha<br />
obt<strong>en</strong>ido <strong>de</strong> <strong>la</strong>s hojas <strong>de</strong> especificaciones <strong>de</strong> los fabricantes.<br />
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Especificaciones <strong><strong>de</strong>l</strong> fabricante,<br />
Fu<strong>en</strong>te <strong>de</strong> luz " LED<br />
Tipo <strong>de</strong> fibra 62.5/125 índice Gradual<br />
Longitud <strong>de</strong> Onda <strong>de</strong> Operación 850 nm<br />
R<strong>en</strong>dimi<strong>en</strong>to promedio <strong>de</strong> transmisión -14 dBm<br />
S<strong>en</strong>sibilidad <strong>de</strong> Recepción (10~9 BER) -52 dBm<br />
Rango Dinámico 2.0 dB<br />
Cálculo <strong><strong>de</strong>l</strong> Promedio <strong>de</strong> Pérdidas <strong>en</strong> el En<strong>la</strong>ce (l_LB)(l_ink Loss Budget)<br />
A. Determinación <strong>de</strong> <strong>la</strong> ganancia <strong><strong>de</strong>l</strong> sistema .<br />
R<strong>en</strong>dimi<strong>en</strong>to promedio <strong>de</strong> transmisión -14 dBm<br />
- S<strong>en</strong>sibilidad <strong>de</strong> Recepción -(-52 dBm)<br />
Ganancia <strong><strong>de</strong>l</strong> Sistema: 38 dBm<br />
B. Cálculo <strong>de</strong> Pérdidas por Márg<strong>en</strong>es <strong>de</strong> Error<br />
1. Márg<strong>en</strong>es <strong>de</strong> Operación. Estas pérdidas se dan por variaciones <strong>de</strong> <strong>la</strong><br />
longitud <strong>de</strong> onda <strong>en</strong> <strong>la</strong> fu<strong>en</strong>te <strong>de</strong> transmisión, estos cambios <strong>en</strong> <strong>la</strong> fu<strong>en</strong>te <strong>de</strong><br />
transmisión y <strong>en</strong> <strong>la</strong> s<strong>en</strong>sibilidad <strong>de</strong> recepción se <strong>de</strong>b<strong>en</strong> a <strong>la</strong> edad<br />
(<strong>en</strong>vejecimi<strong>en</strong>to), y a variaciones <strong>de</strong> temperatura <strong>en</strong> los compon<strong>en</strong>tes <strong>de</strong>ntro<br />
<strong>de</strong> los rangos <strong>de</strong> operación <strong><strong>de</strong>l</strong> sistema. Si no exist<strong>en</strong> especificaciones <strong><strong>de</strong>l</strong><br />
fabricante <strong><strong>de</strong>l</strong> sistema, se pue<strong>de</strong>n consi<strong>de</strong>rar los valores <strong>de</strong> 2 dB para LED y<br />
3 dB para láser.<br />
2. Oíros Márg<strong>en</strong>es <strong>de</strong> Error: Algunos fabricantes pue<strong>de</strong>n especificar otros<br />
márg<strong>en</strong>es <strong>de</strong> error que disminuy<strong>en</strong> <strong>la</strong> ganancia <strong><strong>de</strong>l</strong> sistema tales como <strong>la</strong><br />
dispersión, el jitter, e! ancho <strong>de</strong> banda etc., y si se proporciona esta<br />
información, se <strong>de</strong>bería también restar <strong><strong>de</strong>l</strong> promedio <strong>de</strong> <strong>la</strong> ganancia <strong><strong>de</strong>l</strong><br />
sistema.<br />
Determinación <strong><strong>de</strong>l</strong> Marg<strong>en</strong> <strong>de</strong> Error<br />
Por Marg<strong>en</strong> <strong>de</strong> Operación Se usa 2.0 dB<br />
Por otros Márg<strong>en</strong>es no formu<strong>la</strong>do<br />
Por Marg<strong>en</strong> <strong>de</strong> Reparaciones: 1.0 dB<br />
Total: = 2.0 dB + 1.0 dB = 3.0 dB<br />
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C. Determinación <strong><strong>de</strong>l</strong> Promedio <strong>de</strong> Pérdidas <strong>en</strong> el En<strong>la</strong>ce (LLB)<br />
Restando los valores para cada uno <strong>de</strong> los márg<strong>en</strong>es <strong>de</strong> error (dB) <strong>de</strong> <strong>la</strong><br />
ganancia <strong><strong>de</strong>l</strong> sistema, nos dará <strong>la</strong> máxima pérdida permitida para el sistema<br />
<strong>de</strong> cable <strong>de</strong> terminal a terminal. Esta pérdida máxima a m<strong>en</strong>udo se refiere al<br />
Link Loss Budget (LLB). En algunos casos, el fabricante <strong>de</strong> hardware ti<strong>en</strong>e<br />
ya calcu<strong>la</strong>dos los valores <strong><strong>de</strong>l</strong> LLB. Los márg<strong>en</strong>es por reparación, a m<strong>en</strong>os<br />
que se diga lo contrario, usualm<strong>en</strong>te no se incluye <strong>en</strong> los cálculos <strong><strong>de</strong>l</strong> LLB<br />
realizados por el fabricante, <strong>en</strong>tonces se <strong>de</strong>be aún restar ésto <strong>en</strong> el dis<strong>en</strong>o<br />
<strong><strong>de</strong>l</strong> sistema;<br />
Cálculo <strong><strong>de</strong>l</strong> LLB:<br />
Ganancia <strong><strong>de</strong>l</strong> Sistema<br />
m<strong>en</strong>os el Marg<strong>en</strong> <strong>de</strong> Error<br />
Promedio <strong>de</strong> Pérdidas<br />
4.2.1.3 At<strong>en</strong>uación <strong>de</strong> Cable Máxima Permitida<br />
38.0 dB<br />
- 3.0 dB<br />
35.0 dB<br />
Cada difer<strong>en</strong>te ruta <strong>de</strong> cable <strong>en</strong> el sistema <strong>de</strong>bería ser analizada para <strong>de</strong>terminar<br />
<strong>la</strong> at<strong>en</strong>uación máxima permitida <strong>de</strong> <strong>la</strong> fibra. Usualm<strong>en</strong>te, <strong>la</strong> calidad <strong>de</strong> <strong>la</strong> fibra que<br />
se escoja <strong>de</strong>berá satisfacer los requerimi<strong>en</strong>tos <strong><strong>de</strong>l</strong> <strong>en</strong><strong>la</strong>ce consi<strong>de</strong>rando el valor<br />
mas alto <strong>de</strong> pérdidas. Para gran<strong>de</strong>s sistemas <strong>en</strong> los que se ti<strong>en</strong>e varias<br />
ext<strong>en</strong>siones <strong>de</strong> <strong>en</strong><strong>la</strong>ces, pue<strong>de</strong> ser más económico usar algunas calida<strong>de</strong>s <strong>de</strong> fibra<br />
que satisfagan los requerimi<strong>en</strong>tos <strong>de</strong> cada <strong>en</strong><strong>la</strong>ce, <strong>en</strong> lugar <strong>de</strong> seleccionar una so<strong>la</strong><br />
calidad.<br />
4,2.1.4. Cálculo <strong>de</strong> Pérdidas <strong>en</strong> Conectores.<br />
Se <strong>de</strong>be sumar los valores <strong>de</strong> at<strong>en</strong>uación local individual (<strong>en</strong> dB) para cada par <strong>de</strong><br />
conectores a lo <strong>la</strong>rgo <strong>de</strong> <strong>la</strong> fibra, <strong>de</strong>s<strong>de</strong> el transmisor hacia el receptor, excluy<strong>en</strong>do<br />
los conectores propios <strong><strong>de</strong>l</strong> transmisor y receptor. (Tab<strong>la</strong> 4.3)<br />
Multimodo<br />
Tipo <strong>de</strong> Conect<strong>en</strong><br />
ST<br />
SMA 906<br />
Biconic<br />
Monomodo<br />
Tab<strong>la</strong> 4.3 Pérdidas <strong>en</strong> conectores<br />
Pérdidas <strong>en</strong> dB<br />
Típica Máxima<br />
0.5<br />
1.2<br />
0.8<br />
1.8<br />
0.7<br />
1.5<br />
0.7<br />
1.3<br />
102
Para <strong>en</strong><strong>la</strong>ces con 10o más conectores, es razonable usar los valores típicos para<br />
el p<strong>la</strong>neami<strong>en</strong>to <strong><strong>de</strong>l</strong> sistema. Para m<strong>en</strong>os <strong>de</strong> 10 conectores es recom<strong>en</strong>dable usar<br />
los valores máximos.<br />
Cálculo <strong>de</strong> pérdidas <strong>en</strong> los Conectores <strong><strong>de</strong>l</strong> Sistema. De acuerdo al recorrido y<br />
distancias <strong><strong>de</strong>l</strong> <strong>en</strong><strong>la</strong>ce, se ti<strong>en</strong><strong>en</strong> 7 conectores utilizados para el <strong>en</strong><strong>la</strong>ce, los mismos<br />
que se indican <strong>en</strong> los p<strong>la</strong>nos respectivos. (Excluy<strong>en</strong>do los conectores <strong><strong>de</strong>l</strong><br />
Transmisor y Receptor)1<br />
Siete Conectores ( c/u 1.8 dB) 12.6 dB<br />
Cálculo <strong>de</strong> Otras Pérdidas<br />
Se <strong>de</strong>b<strong>en</strong> sumar <strong>la</strong>s pérdidas <strong>de</strong> cualquier otro compon<strong>en</strong>te que contribuya a <strong>la</strong>s<br />
pérdidas <strong>en</strong> el recorrido <strong>de</strong> <strong>la</strong> fibra, <strong>de</strong>s<strong>de</strong> el transmisor hacia el receptor<br />
Cálculo <strong>de</strong> otros compon<strong>en</strong>tes con pérdida:<br />
Para el pres<strong>en</strong>te proyecto: Ninguno 0.0 dB<br />
Restando <strong>la</strong>s perdidas <strong><strong>de</strong>l</strong> Link Loss Budget nos da <strong>la</strong> máxima perdida permitida <strong>en</strong><br />
el cable a una longitud <strong>de</strong> onda específica.<br />
Por lo tanto, <strong>en</strong> el pres<strong>en</strong>te caso, <strong>la</strong> máxima pérdida permitida para el cable a<br />
850 nm 22.4 dB<br />
4.2.1.5 Cálculos Requeridos Según el Tipo <strong>de</strong> Fibra<br />
Dividi<strong>en</strong>do el máximo valor permitido para pérdida <strong>en</strong> cables por el total <strong>de</strong> longitud<br />
<strong><strong>de</strong>l</strong> cable <strong>en</strong> el <strong>en</strong><strong>la</strong>ce, se ti<strong>en</strong>e <strong>la</strong> máxima at<strong>en</strong>uación permisible para fibras<br />
normalizadas <strong>en</strong> dB/km<br />
Nótese que este valor final repres<strong>en</strong>ta <strong>la</strong> at<strong>en</strong>uación máxima absoluta permisible<br />
bajo <strong>la</strong>s especificaciones <strong><strong>de</strong>l</strong> transmisor/receptor dados por el fabricante<br />
103
La at<strong>en</strong>uación máxima normalizada para el cable se <strong>de</strong>termina dividi<strong>en</strong>do el<br />
máximo valor permisible <strong>de</strong> pérdidas <strong>en</strong> el cable por el total <strong>de</strong> <strong>la</strong> longitud <strong>de</strong> cable<br />
<strong>en</strong> el <strong>en</strong><strong>la</strong>ce.<br />
El total <strong>de</strong> <strong>la</strong> longitud <strong><strong>de</strong>l</strong> cable2 , L, para este diseño es:<br />
L = 30m + 15m +270m + 15m + 50m<br />
L = 380m ó 0.38 km<br />
Por tanto <strong>la</strong> máxima at<strong>en</strong>uación normalizada para este tipo <strong>de</strong> cable es:<br />
22.4 dB ó 58.94 dB/Km para una fibra <strong>de</strong> 850 nm con una longitud <strong>de</strong> 0.38 Km<br />
4.2.1.6 Selección <strong>de</strong> <strong>la</strong> Fibra.<br />
Varias propieda<strong>de</strong>s <strong>de</strong> <strong>la</strong>s fibras como <strong>la</strong> at<strong>en</strong>uación, <strong>la</strong> apertura numérica (NA), y<br />
el diámetro <strong><strong>de</strong>l</strong> núcleo se <strong>de</strong>b<strong>en</strong> tomar <strong>en</strong> cu<strong>en</strong>ta. Todas <strong>la</strong>s fibras pue<strong>de</strong>n ser<br />
comparadas sobre un kilómetro <strong>de</strong> longitud. Ciertam<strong>en</strong>te los tipos <strong>de</strong> fibras ti<strong>en</strong><strong>en</strong><br />
que probarse satisfactoriam<strong>en</strong>te para aplicaciones especiales.<br />
Cuatro son los tamaños que se consi<strong>de</strong>ran mas a m<strong>en</strong>udo:<br />
Núcleo<br />
(miaron)<br />
50<br />
62.5<br />
85<br />
100<br />
Envoltura<br />
(micron)<br />
125<br />
125<br />
125<br />
140<br />
Ancho <strong>de</strong><br />
850nm<br />
400<br />
100<br />
200<br />
150<br />
Banda (MHz)<br />
1300nm<br />
Todas son fibras mulíimodo, <strong>de</strong> índice gradual para asegurar un a<strong>de</strong>cuado ancho<br />
<strong>de</strong> banda y pérdidas bastante bajas.<br />
Para nuestro caso seleccionaremos fibras <strong>de</strong> 62.5/125 y <strong>de</strong> 850 nm por t<strong>en</strong>er esta<br />
un sufici<strong>en</strong>te ancho <strong>de</strong> banda tanto para aplicaciones <strong>de</strong> vi<strong>de</strong>o telefonía como para<br />
otros servicios.<br />
1 Datos obt<strong>en</strong>idos <strong><strong>de</strong>l</strong> diseño que se muestra <strong>en</strong> el anexo 1<br />
2 Datos obt<strong>en</strong>idos <strong><strong>de</strong>l</strong> diseño que se muestra <strong>en</strong> el anexo 1<br />
400<br />
400<br />
600<br />
500<br />
104
4.2.1.7 Descripción y Diseño para <strong>la</strong> Red Interna <strong>de</strong> Fibra Óptica<br />
Según el pres<strong>en</strong>te diseño, se parte <strong>de</strong>s<strong>de</strong> un Panel Principal (Main Cross-<br />
Connect (MC)) que estará ubicado <strong>en</strong> <strong>la</strong> C<strong>en</strong>tral <strong>de</strong> Iñaquito. (Se <strong>de</strong>be tomar <strong>en</strong><br />
cu<strong>en</strong>ta que <strong>la</strong> red <strong>en</strong>tre c<strong>en</strong>trales es <strong>de</strong> fibra óptica). Este Panel Principal vi<strong>en</strong>e<br />
si<strong>en</strong>do el c<strong>en</strong>tro administrativo <strong>de</strong> <strong>la</strong> red, provee interconectividad <strong>en</strong>tre varios<br />
Sub Paneles <strong>de</strong> Distribución (IC) que pue<strong>de</strong>n estar ubicados <strong>en</strong> el mismo<br />
edificio o <strong>en</strong> otros edificios <strong><strong>de</strong>l</strong> sector. El (MC) se interconecta a los difer<strong>en</strong>tes<br />
(IC) mediante un Backbone <strong>de</strong> fibra óptica. El MC funcionará como un hub<br />
físico principal para una configuración <strong>en</strong> estrel<strong>la</strong>. Fig. 4.7<br />
Quct Astdtítono Cstbte<br />
Fig. 4.7 Diseño g<strong>en</strong>eral <strong>de</strong> <strong>la</strong> red <strong>de</strong> fibra óptica,<br />
Para el diseño <strong>de</strong> <strong>la</strong> red interna que va <strong>en</strong> un edificio, se <strong>de</strong>be consi<strong>de</strong>rar <strong>la</strong><br />
utilización <strong>de</strong> un Sub Panel <strong>de</strong> Distribución (Intermedíate Cross-Connect (IC))<br />
que sirve <strong>de</strong> <strong>en</strong><strong>la</strong>ce <strong>en</strong>tre el panel principal y los paneles <strong>de</strong> telecomunicación<br />
<strong>de</strong> <strong>la</strong> red horizontal <strong>de</strong> cada piso.<br />
Por último, <strong>en</strong> cada piso irá ubicado un Panel <strong>de</strong> Telecomunicación (TC), que<br />
sirve <strong>de</strong> <strong>en</strong><strong>la</strong>ce y distribución para <strong>la</strong> red horizontal y terminales <strong>de</strong> usuario.<br />
Un <strong>la</strong>zo <strong>de</strong> 72 fibras irá <strong>de</strong>s<strong>de</strong> el MC hasta el IC#1 ubicado, para nuestro caso,<br />
<strong>en</strong> <strong>la</strong> Av. Gaspar <strong>de</strong> Vil<strong>la</strong>rruel 1477 y calle Japón edificio La Nacional, se<br />
105
utilizará <strong>en</strong><strong>la</strong>ce aéreo a través <strong>de</strong> postes <strong>de</strong> hormigón (postes <strong>de</strong> luz<br />
exist<strong>en</strong>tes). Entre el MC y el ÍC#1 existe una distancia aproximada <strong>de</strong> 380m.<br />
Se <strong>de</strong>ja previsto un segundo <strong>en</strong><strong>la</strong>ce <strong>de</strong> 72 fibras que pue<strong>de</strong> unir el MC con un<br />
IC#2, que podrá estar ubicado <strong>en</strong> cualquier otro edificio don<strong>de</strong> se rerquiera <strong>de</strong><br />
este servicio Por lo tanto, dos <strong>en</strong><strong>la</strong>ces típicos e in<strong>de</strong>p<strong>en</strong>di<strong>en</strong>tes pue<strong>de</strong>n utilizar<br />
un cable <strong>de</strong> 144 fibras que sal<strong>en</strong> <strong>de</strong>s<strong>de</strong> el MC hacia un punto <strong>de</strong> empalme<br />
(Manhole) y <strong>de</strong> allí se reparte hacia cada IC (72 fibras para cada uno) Ver<br />
(Anexo 1)<br />
Diseño Interconexión Intrabuilding (D<strong>en</strong>tro <strong><strong>de</strong>l</strong> Edificio).<br />
El diseño esta consi<strong>de</strong>rado para un edificio <strong>de</strong> 7 pisos interconectado con fibra<br />
óptica. Se consi<strong>de</strong>ra <strong>la</strong> sigui<strong>en</strong>te estructura <strong>de</strong> cableado:<br />
Uno <strong>de</strong> los pisos se <strong>de</strong>be diseñar como el Hub principal para una configuración<br />
<strong>en</strong> estrel<strong>la</strong>, <strong>de</strong>s<strong>de</strong> aquí funcionara el IC, para nuestro caso estará ubicado <strong>en</strong> el<br />
cuarto <strong>de</strong> equipos ubicado <strong>en</strong> P<strong>la</strong>nta Baja. En los otros pisos se ubicarán los<br />
armarios <strong>de</strong> telecomunicaciones (TC) (anexo 1), cada TC estará <strong>en</strong><strong>la</strong>zado con<br />
el IC por medio <strong>de</strong> un cable <strong>de</strong> 8 fibras.<br />
J&rízonteJ Catite<br />
Fig. 4.8 Diseño g<strong>en</strong>eral <strong>de</strong> <strong>la</strong> red horizontal<br />
tetacommumcatúms<br />
Ctosst<br />
106
Diseño Horizontal. Se consi<strong>de</strong>ran 5 áreas <strong>de</strong> trabajo por cada piso, que<br />
necesitarán ser cableados con" fibra óptica, <strong>en</strong> cada área <strong>de</strong> trabajo se proveerá <strong>de</strong><br />
tomas (outlets). El cableado tradicional para <strong>la</strong> red horizontal interconectará cada<br />
área <strong>de</strong> trabajo con el armario <strong>de</strong> telecomunicaciones ubicado <strong>en</strong> cada piso.<br />
El TC estará diseñado como el Hub <strong>de</strong> <strong>la</strong> configuración física <strong>en</strong> estrel<strong>la</strong> con<br />
<strong>en</strong><strong>la</strong>ces <strong>de</strong> cables para cada área <strong>de</strong> trabajo. (Fig. 4.8)<br />
4.3 DESCRIPCIÓN DE EQUIPOS Y ANÁLISIS ECONÓMICO.-<br />
Para <strong>la</strong> insta<strong>la</strong>ción <strong><strong>de</strong>l</strong> sistema <strong>de</strong>scrito anteriorm<strong>en</strong>te se requiere <strong><strong>de</strong>l</strong> sigui<strong>en</strong>te<br />
equipo que se <strong>de</strong>tal<strong>la</strong> a continuación.<br />
Equipos y Cableado <strong><strong>de</strong>l</strong> Backbone (Red Vertical)<br />
MC (Main Cross-Connect) .^___________<br />
Cantidad Descripción P, Unitario<br />
{U.S.D.)<br />
Hardware<br />
107<br />
P. Total<br />
(U.S.D)<br />
Gabinetes metálicos para 72 puertos (56 habilitados) <strong>de</strong> Fibra óptica tipo SC 392.10 784.20<br />
24 6 Fiber Connector panel wrth two dúplex multimo<strong>de</strong> SC adapt<strong>en</strong> 91.50 2,196.00<br />
12 Fiber Buffertube Fan-Out Krt for direct termínation of loóse tube cable 57.19 686.28<br />
Unicam SC muftimo<strong>de</strong> Field-lnstal<strong>la</strong>ble connector<br />
Patch Corete and Cable Assemblies<br />
72 iDuplex SC to two ST compatible mutómo<strong>de</strong> cable 84.97 6,117.84<br />
MC hacia el Manhole<br />
Cable<br />
156 m 144 fiber muHimo<strong>de</strong> MIC Rise-Rated cable; 18.60 2,901.6
Manhote<br />
Hardware<br />
1 Cajetín <strong>de</strong> empalmes 92.10 92.10<br />
Manga <strong>de</strong> empalme <strong>de</strong> 144 fibras<br />
12 Ban<strong>de</strong>jas <strong>de</strong> empalme para 12 fibras c/u<br />
21.19 254.28<br />
108<br />
12.50 12.50<br />
Manhole hacia el IC<br />
Cable<br />
224 m Cable <strong>de</strong> 72 fibras muftimodo MIC Rise-Rated 14.50 3,248.00<br />
(P<strong>la</strong>nta Baja)<br />
IC (Intermedíate Cross-Connect)<br />
Cantidad Descripción P. Unitario<br />
(U.S.D)<br />
Hardware<br />
Connectors<br />
P. Total<br />
(U.S.D)<br />
1 Gabinete metálico para 72 puertos (56 habilitados) <strong>de</strong> Fibra óptica tipo SC 392.10 392.10<br />
10 Panel <strong>de</strong> conexión <strong>de</strong> 6 fibras con adaptadores dobles muftimodo SC 91.50 915.00<br />
6 Fiber Buffer tube Fan-Out Ktt for direct termination of loóse tube cable 57.19<br />
400.33<br />
56 Unicam SC muttimo<strong>de</strong> Field-lnstal<strong>la</strong>ble connector 19.61 1,098.16<br />
Patch Cords and Cable Assemblies<br />
28 [Dúplex SC to two ST compatible multimo<strong>de</strong> cable 84.97 2,379.16<br />
IC toTC<br />
Cable<br />
135 m 8 fiber muttimo<strong>de</strong> MIC Rise-Rated cable; 1,093.50
TC#1 (Primer Piso)<br />
Hardware<br />
Con e ctores<br />
1 12 Fiber Rack-Mount Connector Module Housing (8 habilitados) 326.80 326.80<br />
6 Fiber Connector panel with two dúplex muttimo<strong>de</strong> of loóse tube cable 91.50 183.00<br />
6 Fiber Buffer tube Fan-Out Kft for direct termínation of loóse tube cable<br />
Unicam SC muttimo<strong>de</strong> Field-lnstal<strong>la</strong>ble connector<br />
Patch Cords and Cable Assembfíes<br />
109<br />
57.19 57.19<br />
4 (Dúplex SC to two ST compatible mufómo<strong>de</strong> cable; l<strong>en</strong>gth: 84.97 339.88<br />
TC#2 - #6 (Típico para cada piso) (5 pisos)<br />
Hardware<br />
1(5) 12 Fiber Wall-Mouned premises interconnect c<strong>en</strong>ter with a dúplex SC adapter 196.08 980.40<br />
2(10) 6 Fiber Connector panel with two dúplex muttimo<strong>de</strong> of loóse tube cable 91.50 915.00<br />
1(5) 6 Fiber Buffer tube Fan-Out Kft for direct termination of loóse tube cable 57.19 285.95<br />
Connectors<br />
8(40) Unicam SC multimo<strong>de</strong> Field-lnstal<strong>la</strong>ble connector 19.61 784.40<br />
Patch Cords and Cable Assemblies<br />
4 (20) Dúplex SC to two ST compatible multimo<strong>de</strong> cable; l<strong>en</strong>gth: 84.97 1,699.4<br />
SUBTOTAL1 31,123.79<br />
Equipos y Cableado Red Horizontal<br />
Cantidad Descripción P. Unitario<br />
(U.S.D)<br />
19.61<br />
P. Total<br />
(U.S.D)<br />
35 Costo por punto <strong>de</strong> insta<strong>la</strong>ción 150.00 5250.00<br />
SUBTOTAL 2 5,250.00<br />
TOTAL 36,37^79<br />
El costo para implem<strong>en</strong>tar <strong>la</strong> red horizontal se <strong>la</strong> consi<strong>de</strong>ra por punto <strong>de</strong> insta<strong>la</strong>ción<br />
(el punto compr<strong>en</strong><strong>de</strong> el cable, <strong>la</strong> toma (outlet) y los accesorios necesarios <strong>de</strong><br />
insta<strong>la</strong>ción).
En el pres<strong>en</strong>te diseño se p<strong>la</strong>ntea t<strong>en</strong>er <strong>en</strong> total 35 puntos <strong>de</strong> insta<strong>la</strong>ción, por tanto<br />
et costo <strong>de</strong> <strong>la</strong> implem<strong>en</strong>tación <strong>de</strong> <strong>la</strong> parte horizontal es <strong>de</strong> $5,250 U.S.D.<br />
Cosío total <strong>de</strong> equipos y cable: $36,373.79 U.S.D.<br />
Una vez que se ha <strong>de</strong>finido <strong>la</strong> infraestructura básica necesaria para implem<strong>en</strong>tar<br />
esta red <strong>de</strong> vi<strong>de</strong>o telefonía que se ha <strong>de</strong>scrito anteriorm<strong>en</strong>te, es necesario<br />
<strong>de</strong>terminar los costos estimados <strong>de</strong> su implem<strong>en</strong>tación<br />
4,3.1 ESTIMACIÓN DEL CAPITAL NECESARIO.-<br />
De acuerdo a los requisitos <strong>de</strong> infraestructura <strong><strong>de</strong>l</strong> sistema <strong>de</strong>scrito <strong>en</strong> el punto<br />
4.3.1 se pres<strong>en</strong>ta a continuación el <strong>de</strong>talle <strong>de</strong> los costos <strong><strong>de</strong>l</strong> sistema, tanto <strong>en</strong><br />
moneda nacional como <strong>en</strong> dó<strong>la</strong>res (moneda refer<strong>en</strong>cial)<br />
Cotización <strong><strong>de</strong>l</strong> dó<strong>la</strong>r estimado al 21 <strong>de</strong> diciembre <strong>de</strong> 1998: 6.500 sucres<br />
4.3.1.1 COSTO DE EQUIPOS Y CABLE<br />
Según el análisis anterior, el monto total <strong>de</strong> equipo y cable necesario para<br />
implem<strong>en</strong>tar <strong>la</strong> red diseñada asci<strong>en</strong><strong>de</strong> a: $ 36,373.79 U.S.D.<br />
4.3.1.2 COSTOS POR MANO DE OBRA Y SERVICIOS<br />
Para el análisis <strong>de</strong> este punto se consi<strong>de</strong>ran los sigui<strong>en</strong>tes aspectos:<br />
Tiempo estimado para <strong>la</strong> realización <strong>de</strong> <strong>la</strong> obra: 4 semanas<br />
• Pago m<strong>en</strong>sual <strong>de</strong> personal<br />
• Costos por servicios básicos<br />
• Costos <strong>de</strong> operación<br />
• Imprevistos<br />
110
Pago m<strong>en</strong>sual <strong>de</strong> personal<br />
a) Personal Técnico - Administrativo<br />
Descripción Sueldo (sucres)<br />
<strong>Ing<strong>en</strong>iero</strong> <strong>de</strong> re<strong>de</strong>s<br />
Técnico Resi<strong>de</strong>nte<br />
Secretaria<br />
Total Pagos<br />
b) Personal <strong>de</strong> Operación<br />
Descripción Sueldo (sucres)<br />
Maestro electricista (catg. 4)<br />
electricista (catg. 3)<br />
Ayudante 1 (Catg 2)<br />
Bo<strong>de</strong>guero<br />
Total Pagos<br />
6,000.000<br />
4,000.000<br />
1,500.000<br />
1,300.000<br />
950.000<br />
600.000<br />
600.000<br />
Cantidad<br />
1<br />
1<br />
1<br />
Cantidad<br />
1<br />
2<br />
4<br />
1<br />
Total<br />
6,000.000<br />
4,000.000<br />
1,500.000<br />
9,500.000<br />
Total<br />
1,300.000<br />
1,900.000<br />
2,400.000<br />
600.000<br />
6,200.000<br />
Total <strong>de</strong> Pago m<strong>en</strong>sual a personal: 17,700.000<br />
4.3.1.3 COSTOS POR SERVICIOS BÁSICOS<br />
Por servicios básicos se <strong>en</strong>ti<strong>en</strong><strong>de</strong> los costos por consumo <strong>de</strong> <strong>en</strong>ergía eléctrica,<br />
agua potable y teléfono, e! cual y para fines <strong>de</strong> estimación <strong>de</strong> cálculos se<br />
consi<strong>de</strong>rará como el 3% <strong><strong>de</strong>l</strong> costo <strong>de</strong> mano <strong>de</strong> obra (pagos a personal)<br />
Costo total (3% <strong>de</strong> M.O) 531.000 (sucres)<br />
4.3.1.4 COSTOS DE OPERACIÓN<br />
En este punto se consi<strong>de</strong>ran los gastos por movilización, (transporte, combustible,<br />
mant<strong>en</strong>imi<strong>en</strong>to) como <strong><strong>de</strong>l</strong> 10% <strong><strong>de</strong>l</strong> costo <strong>de</strong> Mano <strong>de</strong> Obra (M.O.)(pagos personal)<br />
111
4.3.1.5 COSTOS TOTALES DE OPERACIÓN<br />
Pagos a personal 17,700.000<br />
Servicios básicos (3% <strong>de</strong> M.O) 531.000<br />
Costos <strong>de</strong> operación (10% <strong>de</strong> M.O) 1,700.000<br />
Imprevistos (10% <strong>de</strong> M.O) 1,700.000<br />
Total 21,631.000 sucres<br />
3,330.00 U.S.D<br />
4.3,2 Cobertura <strong>de</strong> costos <strong>de</strong> implem<strong>en</strong>tación y operación <strong><strong>de</strong>l</strong> sistema<br />
Para cubrir los costos <strong>de</strong> operación y mant<strong>en</strong>imi<strong>en</strong>to es necesario cobrar<br />
m<strong>en</strong>sualm<strong>en</strong>te a los usuarios <strong>de</strong> este servicio una tarifa que se <strong>de</strong>termina <strong>de</strong> <strong>la</strong><br />
sigui<strong>en</strong>te manera:<br />
Costo total <strong>de</strong> impiem<strong>en</strong>tación <strong><strong>de</strong>l</strong> sistema: 36,373.79 + 3,330 = 39,703.79 U.S.D<br />
Número <strong>de</strong> usuarios 35<br />
Cuota por usuario = 39.703.79 = 1,134.39 U.S.D<br />
35<br />
Se estima un período <strong>de</strong> pago (n) igual a diez años (120 mese)<br />
La tasa <strong>de</strong> interés / para evaluación <strong><strong>de</strong>l</strong> proyecto (<strong>en</strong> dó<strong>la</strong>res ) se consi<strong>de</strong>ra <strong><strong>de</strong>l</strong><br />
12% anual (1% m<strong>en</strong>sual)<br />
Fórmu<strong>la</strong> <strong>de</strong> cálculo <strong>de</strong> m<strong>en</strong>sualida<strong>de</strong>s (M): M = CE<br />
Por lo tanto se ti<strong>en</strong>e:<br />
' 0.01(1 +0.01)120<br />
M = 1,134.39<br />
(1 + 0.01)120-1<br />
i(I + i)<br />
= 16.27 dó<strong>la</strong>res/mes durante 10 años<br />
112
CONCLUSIONES Y RECOMENDACIONES.-<br />
Muchos <strong>de</strong> los servicios no vocales requier<strong>en</strong> <strong>de</strong> un nuevo equipo terminal<br />
ubicado <strong>en</strong> <strong>la</strong>s insta<strong>la</strong>ciones <strong><strong>de</strong>l</strong> abonado, como <strong>en</strong> el caso <strong><strong>de</strong>l</strong> ví<strong>de</strong>o teléfono,<br />
que requiere <strong>de</strong> un aparato especial para <strong>la</strong> comunicación ví<strong>de</strong>o telefónica. Asi<br />
pues cada aparato <strong>de</strong> vi<strong>de</strong>o telefonía osci<strong>la</strong> <strong>en</strong>tre los $700 a $1200 U.S.D.<br />
Para fom<strong>en</strong>tar <strong>la</strong> utilización <strong><strong>de</strong>l</strong> nuevo servicio, pue<strong>de</strong> ser necesario suministrar<br />
el equipo <strong>de</strong> insta<strong>la</strong>ción <strong>de</strong> abonado con una formu<strong>la</strong> conv<strong>en</strong>i<strong>en</strong>te y económica.<br />
Se pue<strong>de</strong>n dar varias alternativas para <strong>la</strong> adquisición <strong>de</strong> este tipo <strong>de</strong> aparatos<br />
tales como:<br />
1) ofrecer el equipo a precio <strong><strong>de</strong>l</strong> mercado;<br />
2) arr<strong>en</strong>dar el equipo;<br />
3) v<strong>en</strong><strong>de</strong>r e! equipo al precio <strong>de</strong> promoción para estimu<strong>la</strong>r <strong>la</strong> <strong>de</strong>manda; u<br />
4) ofrecer el equipo gratuitam<strong>en</strong>te.<br />
Los costos <strong>de</strong> <strong>la</strong> dos últimas opciones pue<strong>de</strong>n cubrirse con los ingresos que<br />
g<strong>en</strong>er<strong>en</strong> los costos por prestación <strong><strong>de</strong>l</strong> servicio o ser comp<strong>en</strong>sados por otros<br />
inversionistas <strong>en</strong> los servicios que t<strong>en</strong>gan productos conexos o que se<br />
b<strong>en</strong>efici<strong>en</strong> <strong>de</strong> los nuevos servicios no vocales.<br />
En este estudio es importante consi<strong>de</strong>rar el hecho <strong>de</strong> que <strong>en</strong> el servicio <strong>de</strong><br />
vi<strong>de</strong>o teléfono, el número <strong>de</strong> abonados comparado con el número <strong>de</strong> abonados<br />
<strong><strong>de</strong>l</strong> servicio telefónico normal será evi<strong>de</strong>ntem<strong>en</strong>te inferior, por lo tanto estos<br />
aparatos <strong>de</strong>b<strong>en</strong> ser totalm<strong>en</strong>te compatibles con los teléfonos normales a fin <strong>de</strong><br />
evitar que los abonados al ví<strong>de</strong>o teléfono puedan comunicarse únicam<strong>en</strong>te<br />
<strong>de</strong>ntro <strong>de</strong> un grupo cerrado <strong>de</strong> usuarios, esta condición <strong>de</strong> compatibilidad <strong>de</strong>be<br />
cumplirse in<strong>de</strong>p<strong>en</strong>di<strong>en</strong>tem<strong>en</strong>te <strong>de</strong> <strong>la</strong> tecnología usada (análoga, digital, RDSI)<br />
<strong>en</strong> <strong>la</strong> c<strong>en</strong>tral local a <strong>la</strong> que esté conectado el abonado.<br />
114
Las normas <strong>de</strong> codificación <strong>de</strong> imág<strong>en</strong>es son cruciales para el crecimi<strong>en</strong>to <strong>de</strong><br />
los servicios visuales y sistemas <strong>de</strong> comunicación, sin ellos, <strong>la</strong> comunicación<br />
<strong>en</strong>tre los terminales y los sistemas llegan a ser extremadam<strong>en</strong>te l<strong>en</strong>ta, y<br />
costosa. Las normas JBIG, JPEG, Px64 y MPEG suministran algoritmos <strong>de</strong><br />
compresión para todos los tipos <strong>de</strong> imág<strong>en</strong>es que pue<strong>de</strong>n ser llevadas <strong>en</strong><br />
servicios multimedia<br />
El mercado <strong>de</strong> los productos multimedia <strong>en</strong> especial <strong>en</strong> lo que ti<strong>en</strong>e que ver con<br />
comunicación audio-visual, esta recién empezando a emerger. Sin embargo, los<br />
estudios <strong>de</strong> mercado realizados a nivel mundial, indican que estos servicios<br />
ti<strong>en</strong><strong>en</strong> una <strong>de</strong>manda cada vez mayor, y si a esto le añadimos <strong>la</strong> reducción <strong>de</strong><br />
costos <strong>de</strong> los equipos, y <strong>la</strong> ampliación <strong>de</strong> <strong>la</strong>s características <strong>de</strong> los productos, el<br />
futuro <strong>de</strong> éste tipo <strong>de</strong> servicios parece estar asegurado. Adicionalm<strong>en</strong>te el<br />
increm<strong>en</strong>to <strong>en</strong> el <strong>de</strong>sarrollo tecnológico tanto <strong>en</strong> software como <strong>en</strong> hardware<br />
con flexibilidad <strong>en</strong> el diseño, con po<strong>de</strong>r <strong>de</strong> modificación y reutilización, y una<br />
rápida introducción <strong>de</strong> estos productos <strong>en</strong> el mercado hac<strong>en</strong> prever un futuro<br />
muy promisorio para el <strong>de</strong>sarrollo <strong>de</strong> este tipo <strong>de</strong> tecnología.<br />
Como se m<strong>en</strong>cionó anteriorm<strong>en</strong>te, para satisfacción <strong>de</strong> los usuarios se necesita<br />
<strong>de</strong> aplicaciones que reúnan <strong>la</strong> especial <strong>de</strong>manda que últimam<strong>en</strong>te se esta<br />
creando <strong>en</strong> el mercado universal <strong>de</strong> <strong>la</strong>s aplicaciones multim<strong>en</strong>dia. Los bajos<br />
costos se lograrán por medio <strong>de</strong> incorporar nuevas tecnologías <strong>en</strong> circuitos<br />
integrados y por un <strong>de</strong>sarrollo funcional tanto <strong><strong>de</strong>l</strong> hardware como <strong><strong>de</strong>l</strong> software.<br />
Se <strong>de</strong>be tomar <strong>en</strong> cu<strong>en</strong>ta también que se <strong>de</strong>be crear productos que estén<br />
regu<strong>la</strong>dos bajo estándares globales <strong>de</strong> comunicación, que jugarán un rol<br />
importante para <strong>la</strong> intercomunicación e interconexión con difer<strong>en</strong>tes aparatos<br />
<strong>de</strong> difer<strong>en</strong>tes tecnologías como <strong>la</strong>s máquinas <strong>de</strong> Fax, <strong>la</strong>s PCs, los VCRs, etc..<br />
Para los sigui<strong>en</strong>tes años el <strong>de</strong>sarrollo y características <strong>de</strong> los ví<strong>de</strong>o teléfonos<br />
será tal que se continuará <strong>en</strong> <strong>la</strong> mejora <strong>de</strong> tecnología y los precios <strong>de</strong> estos<br />
115
RED PRINCIPAL<br />
SIMBOLIGIA<br />
MC = Main Cross-Connect<br />
1C = Intermedíate Cross-Connecct<br />
~ Conector Mecánico<br />
= Conector
6 Piso<br />
SIMBOLOGIA<br />
EH3 = Hub<br />
= Eiem<strong>en</strong>ío Activo<br />
Vv = Conector<br />
DIAGRAMA VERTICAL<br />
TC<br />
Área <strong>de</strong> Trabajo<br />
Cableado Horizontal<br />
'—Equipos Activos<br />
'—Cableado <strong>de</strong> backbone
EQUIPOS Y COMPONENTES DEL SISTEMA<br />
TC
Calle Japón<br />
MAPA DE UBICACIÓN<br />
0<br />
9<br />
CÑ C<br />
Manhole.<br />
0<br />
e<br />
15m I ^Jj<br />
50m<br />
38m<br />
<strong>de</strong> V<br />
sp<br />
0<br />
SIMBQLQGIA<br />
Poste Exist<strong>en</strong>te<br />
<strong>de</strong><br />
Au. Amazonas<br />
Is<strong>la</strong> Isabe<strong>la</strong><br />
Is<strong>la</strong> San Cristoba<br />
Pozo <strong>de</strong> acometida (por construir)<br />
Manhote (punto <strong>de</strong> empalmes)<br />
Cable <strong>de</strong> Fibra Óptica
INTERNATIONAL TELECOMMUNICATION UNION<br />
ITU-T H.221<br />
TELECOMMUNICATION (03/93)<br />
STANDARDIZARON SECTOR<br />
OF ITU<br />
LINE TRANSMISSION OF NON-TELEPHONE<br />
SIGNÁIS<br />
FRAME STRUCTURE FOR<br />
A 64 TO 1920 kbit/s CHANNEL<br />
IN AUDIOVISUAL TELESERVICES<br />
ITU-T Recomm<strong>en</strong>dation H.221<br />
{Previously "CCITT Recomm<strong>en</strong>dation")
FOREWORD<br />
e ITU Telecommunication Standardization Sector (ITU-T) is a perman<strong>en</strong>t organ of the International Telecommícation<br />
Union. The ITU-T is responsible for study ing technical, operatíng and tariff questions and issuing<br />
comm<strong>en</strong>dations on them with a víew to standardizing telecommunications on a worldwi<strong>de</strong> basis.<br />
e World Telecommunication Standardization Confer<strong>en</strong>ce (WTSC), which meets every four years, establlshed the topics<br />
• study by the ITU-T Study Groups whích, in their tum, produce Recomm<strong>en</strong>dations on these topics.<br />
U-T Recomm<strong>en</strong>dation H.221 was revised by the ITU-T Study Group XV (1988-1993) and was approved by the WTSC<br />
elsinkí, March 1-12, 1993).<br />
NOTES<br />
As a consequ<strong>en</strong>ce of a reform process within the International Telecommunication Union (ITU), the CCITT<br />
ised to exist as of 28 February 1993. In its p<strong>la</strong>ce, the ITU Telecommunication Standardization Sector (ITU-T) was<br />
;ated as of 1 March 1993. Simi<strong>la</strong>rly, in this reform process, the CCIR and the IFRB have be<strong>en</strong> rep<strong>la</strong>ced by the<br />
diocommunication Sector.<br />
or<strong>de</strong>r not to <strong>de</strong><strong>la</strong>y publicatíon of this Recomm<strong>en</strong>dation, no change has be<strong>en</strong> ma<strong>de</strong> in the text to refer<strong>en</strong>ces containing<br />
acronyms "CCITT, CCIR or IFRB" or their associated <strong>en</strong>tities such as Pl<strong>en</strong>ary Assembly, Secretariat, etc. Future<br />
tions of this Recomm<strong>en</strong>dation will contain the proper terminology re<strong>la</strong>ted to the new ITU structure.<br />
In this Recomm<strong>en</strong>dation, the expression "Administration" ís used for concis<strong>en</strong>ess to indícate both a<br />
¡communícation administration and a recognized operating ag<strong>en</strong>cy.<br />
© ITU 1994<br />
, rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or<br />
chanical, including photocopying and microfilm, wilhout perraission in writing from the ITU.
CONTENTS<br />
Basic principie • • 1<br />
1.1 Frame alignm<strong>en</strong>t signal (FAS) 1<br />
1.2 Bit-rate allocatíon signal (BASJ 2<br />
1.3 Encryption control signal (ECS) 2<br />
1.4 Remaining capacíty 2<br />
Frame alignm<strong>en</strong>t 3<br />
2.1 G<strong>en</strong>eral..,, 3<br />
2.2 Multiframe structure 4<br />
2.3 Loss and recovery of frame alignm<strong>en</strong>t 5<br />
2.4 Loss and recovery of multiframe alignm<strong>en</strong>t.....í 6<br />
2.5 Procedure to recover octet timing from frame alignm<strong>en</strong>t 6<br />
2.6 Description of the CRC4 procedure ." 7<br />
2.7 Synchronízation of múltiple connectlons , 8<br />
Bit-rate allocatíon signal 9<br />
3.1 Encoding of the BAS 9<br />
3.2 Valúes of the BAS 10<br />
3.3 Procedures for the use of BAS 15<br />
,nex A — Definitions and tables of BAS valúes 15<br />
A.l Audiocommand valúes (000) 17<br />
A.2 Transfer-rate command valúes (001) 18<br />
A.3 Vi<strong>de</strong>o, <strong>en</strong>cryption, loop and other commands (010) 19<br />
A.4 LSD/MLP commands (Olí) 20<br />
A.5 Audio capabilities (100) 21<br />
A.6 Vi<strong>de</strong>o, MEE and <strong>en</strong>cryption capabilities (101) 21<br />
A.7 Transfer-rate capabilities (100) 22<br />
A.S LSD/MLP capabilities (101) 22<br />
A.9 Escape table valúes (111)......... 23<br />
A.10 HSD/H-MLP capabilities (111) [10000]-(101) 23<br />
A.11 HSD/H-MLP commands (111) [10000]-(0 11) 24<br />
A.12 Applications within LSD/HSD channels- capabilities (111) [10010]-(101) 25<br />
A.13 Applications within LSD/HSD channels - commands (111) [10010]-(011) 25<br />
nex B — Frame structure for ínterworking betwe<strong>en</strong> a 64 kbit/s terminal and a 56 kbit/s terminal 26<br />
B.l Sub-channel arrangem<strong>en</strong>t. 26<br />
B.2 Operation of the 64 kbit/s terminal 26<br />
B.3 Restriction against some communicatíon mo<strong>de</strong>s 26<br />
B.4 Audio command co<strong>de</strong>s (000) 26<br />
Recomm<strong>en</strong>dation H.221 (03/93)<br />
Page
INTRODÜCTION<br />
e purpose of this Recomm<strong>en</strong>dation Ís to <strong>de</strong>fine a frame structure for audiovisual teleservices ín single or múltiple B or<br />
channels or a single HH or H12 channel whích makes the best use of the characteristics and properties of the audio and<br />
Íeo <strong>en</strong>coding algorithms, of the transmissíon frame structure and of the exísting Recomm<strong>en</strong>datíons. It offers several<br />
vantages:<br />
— It takes into account Recomm<strong>en</strong>datíons such as G.704, X.30/1,461, etc. It may allow the use of existing<br />
hardware or software.<br />
- It is simple, economic and flexible. It may be implem<strong>en</strong>ted on a simple microprocessor using well-known<br />
hardware principies.<br />
— It ís a synchronous procedure. The exact tíme of a configuration change is the same in the transmitter and<br />
the receiver. Configuratíons can be changed at20 ms ¡ntervals.<br />
- Itneeds no retum Hnk for audiovisual signal transmissíon, sínce a configuration is signalled by repeatedly<br />
Iransmitted co<strong>de</strong>words.<br />
- It is very secure in case of transmission errors, since the co<strong>de</strong> controlling the multiplex is protected by a<br />
double-error correcting co<strong>de</strong>.<br />
- It allows the synchronization of múltiple 64 kbit/s or 384 kbit/s connections and the control of the<br />
multiplexing of audio, vi<strong>de</strong>o, data and other signáis within the synchronized multiconnection structure in<br />
the case of multimedia servíces such as ví<strong>de</strong>oconfer<strong>en</strong>ce.<br />
- It can be used to <strong>de</strong>rive octet synchronization in networks where this is not provi<strong>de</strong>d by other means.<br />
— It can be used in multipoint configurations, where no dialogue is nee<strong>de</strong>d to negotiate the use of a data<br />
channel.<br />
- It pro vi<strong>de</strong>s a varíety of data bit-rates (from 300 bit/s up to almost2 Mbit/s) to the user.<br />
Recomm<strong>en</strong>dation H.221 (03/93)
comm<strong>en</strong>dation H.221<br />
FRAME STRÜCTÜRE FOR A 64 TO 1920 kbit/s CHANNEL<br />
IN AUDIOVISUAL TELESERVICES1'<br />
Basic principie<br />
(revised 1990 and'at Helsinki, 1993)<br />
is Recomra<strong>en</strong>datíon provi<strong>de</strong>s for dynamically subdívlding an overall transmission channel of 64 to 1920 kbít/s into<br />
ver rates suitable for audio, ví<strong>de</strong>o, data and telematics purposes. The overall transmission channel is <strong>de</strong>rived by<br />
ichronízing and or<strong>de</strong>ring transmissíons overfrom 1 to 6 B-connections, from 1 to 5 H0-connect¡ons, or an HH or H¡2<br />
imection. The first connectíon established is the initial connection and carries the inítial channel in each dírection. The<br />
iitional connectíons carry additional channels,<br />
e total rate of transmitted information is called the "transfer rale"; it is possible to fíx the transfer rate less than the<br />
jacity of the overall transmission channel (valúes listed in Annex A).<br />
¡ingle 64 kbit/s channel is structured into octets transmitted at 8 kHz. Each bit position of the octets may be regar<strong>de</strong>d as<br />
ub-channel of 8 kbit/s (see Figure 1). The eíghth sub-channel is called the service channel (SCJ, consístíng of several<br />
ts as <strong>de</strong>scribed in 1.1 to 1.4.<br />
HO, HH or H|,2 channel may be regar<strong>de</strong>d as consisting of a number of 64 kbit/s time-slots (TS) (see Figure 2). The<br />
fest numbered time-slot Ís structured exactly as <strong>de</strong>scribed for a single 64 kbit/s channef, while the other TS have no<br />
¡h structure. In the case of múltiple B or Hg channels, all channels have a frame structure; that in the initial channel<br />
itrols most functions across the overall transmission, while the frame structure ín the additional channels is used for<br />
ichronization, channel numbering and re<strong>la</strong>ted controls.<br />
e térra "I-channel" is applied to the initial or only B channel, to TS1 of initial or only HQ channel, and to TS 1 of HI \,<br />
2 channels.<br />
Frame alignm<strong>en</strong>t signal (FAS)<br />
is signal structures the I-channel and other framed 64 kbit/s channels into frames of 80 octets each and multlframes<br />
F) of 16 frames each. Each multlframe is divi<strong>de</strong>d into eight 2-frame sub-multiframes (SMF). The term "frame<br />
jnm<strong>en</strong>t signal" (FAS) refers to bits 1-8 of the SC ¡n each frame. In addition to framíng and multiframing information,<br />
itrol and a<strong>la</strong>rm information may be inserted in the FAS, as well as error check information lo control <strong>en</strong>d-to-<strong>en</strong>d error<br />
formance and to check frame alignm<strong>en</strong>t validity. Other time-slots are allgned to the first.<br />
s bits are transmitted to Une in or<strong>de</strong>r, bit 1 first.<br />
l<strong>en</strong> an 8 kHz network clock is provi<strong>de</strong>d, FAS is transmitted and received in the leasl signifícant bit of the octei within<br />
h 125 microsecond, e.g. in an ISDN basic or primary rate interface. It should be noted that, where ínterworking<br />
we<strong>en</strong> the audiovisual terminal and the telephone is required, transmission using the network timing is ess<strong>en</strong>tial. In the<br />
eíver si<strong>de</strong>, FAS should be sought in all bit positions. If received FAS position conflícts with the network octet timing,<br />
FAS position is gív<strong>en</strong> priority. This may happ<strong>en</strong> wh<strong>en</strong> the receiver utilizes network octet timing while the transmitter<br />
is not as in a terminal using co<strong>de</strong>es sepárate wíth ISDN terminal adaptor, or wh<strong>en</strong> Ínterworking betwe<strong>en</strong> 64 kbit/s and<br />
kbit/s termináis takes p<strong>la</strong>ce.<br />
; FAS can be used to <strong>de</strong>rive receive octet timing wh<strong>en</strong> it is nol provi<strong>de</strong>d by the network. However, in the <strong>la</strong>tter case,<br />
terminal cannot transmít FAS wíth correct alígnm<strong>en</strong>t into the octet timed part of the network and cannot<br />
¡rcommunicate with termináis which rely only on network timing for octet alignm<strong>en</strong>t.<br />
TTiis Recomm<strong>en</strong>dation completely rep<strong>la</strong>ces the text of Recoram<strong>en</strong>dations H.221 and H.222 pubíished in Fascicle III.6 of the Biue<br />
Book.<br />
Recomm<strong>en</strong>dation H.221 (03/93) 1
Bit number<br />
1 2 3 * 5 6 7 8 (SC)<br />
S<br />
u<br />
b<br />
.<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
I<br />
#<br />
1<br />
S<br />
U<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
I<br />
*<br />
2<br />
S<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
I<br />
#<br />
3<br />
S<br />
U<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
I<br />
#<br />
4<br />
FAS Frame alignm<strong>en</strong>t signal<br />
BAS Bit-rate allocalion signal<br />
ECS Encryptíon control signal<br />
Bit-rate allocation signal (BAS)<br />
S<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
B<br />
]<br />
#<br />
5<br />
S<br />
U<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
I<br />
#<br />
6<br />
S<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
I<br />
tí<br />
7<br />
FIGURE 1/H.221<br />
FAS<br />
BAS<br />
ECS<br />
Frame structure of a single 64 kbit/s (B-channel<br />
#<br />
&<br />
1 Octet number<br />
:<br />
8<br />
9<br />
:<br />
16<br />
17<br />
:<br />
24<br />
25<br />
-<br />
•<br />
•<br />
80<br />
ts 9-16 of the SC in each frame are referred to as BAS. This sígnal allows the transmission of co<strong>de</strong>words to <strong>de</strong>scribe<br />
! capabílity of a terminal to structure the capacity of the channel or synchronized múltiple channels ¡n various ways, and<br />
command a receiver to <strong>de</strong>multiplex and make use of the constítu<strong>en</strong>t signáis in such structures. This signal is also used<br />
• controls and indications.<br />
NOTE - For some countries havíng 56 kbit/s channels, the net avai<strong>la</strong>ble bit rales will be 8 kbit/s less, Interworking betwe<strong>en</strong><br />
4 kbit/s terminal and a 56 kbít/s terminal ís establíshed accordíng to the frame structure in Annex B.<br />
Encryption control signal (ECS)<br />
future <strong>en</strong>cryptíon capability may require a <strong>de</strong>dicated transmission channel. It is anticípated that 800 bit/s should be<br />
>vi<strong>de</strong>d wh<strong>en</strong> required by allocatíng the bits 17-24 of the service channel. This reduces variable data and vi<strong>de</strong>o<br />
nsmission rales herein by 800 bit/s, The 800 bit/s ¡s referred to as the ECS channel.<br />
I Remaining capacity<br />
e remaining capacity (including the rest of the service channel), carried in bits 1-8 of each ocíet in the case of a single<br />
kbit/s connection, may convey a variety of signáis within the framework of a multimedia service, un<strong>de</strong>r the control of<br />
i BAS. Some examples follow:<br />
- voice <strong>en</strong>co<strong>de</strong>d at 56 kbit/s using a truncated form of PCM of Recomm<strong>en</strong>dation G.711 (A-<strong>la</strong>w or u-Iaw);<br />
- voice <strong>en</strong>co<strong>de</strong>d at 16 kbit/s and vi<strong>de</strong>o at 46.4 kbit/s;<br />
Recomm<strong>en</strong>dation H.221 (03/93)
- voice <strong>en</strong>co<strong>de</strong>d at 56 kb¡t/s with a bandwidth 50 to 7000 Hz (subband ADPCM according to<br />
Recomm<strong>en</strong>dation G.722); the coding algorithm is also able to work at 48 kbit/s - data can th<strong>en</strong> be<br />
dynaraically inserted at up to 14.4 kbit/s;<br />
— still pictures co<strong>de</strong>d at 56 kbit/s;<br />
- data at 56 kbit/s insi<strong>de</strong> an audiovisual session (e.g. file transfer for communicating betwe<strong>en</strong> personal<br />
computéis).<br />
125 microseconds<br />
1 2 3 4 S 6 7 6/1-2 6/7-1 6/1<br />
1<br />
S<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
I<br />
#<br />
1<br />
2<br />
S<br />
U<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
I<br />
Frame alignm<strong>en</strong>t<br />
#<br />
2<br />
Audio 4- serv¡C9 channel<br />
3 4 5 6<br />
S<br />
U<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
I<br />
*<br />
3<br />
S<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
I<br />
*<br />
4<br />
S<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
I<br />
*<br />
5<br />
S<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
I<br />
#<br />
6<br />
7<br />
S<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
1<br />
*<br />
7<br />
8<br />
FAS<br />
BAS<br />
S<br />
U<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
1<br />
#<br />
8<br />
H0 n- 1<br />
Ht1 n - 4<br />
H12 n = 5<br />
1 Oclel number<br />
:<br />
8<br />
9<br />
:<br />
16<br />
FIGURE 2/H.221<br />
Frame structure of higher-rate single channels<br />
(H0, Hll3 H12 channels)<br />
G<strong>en</strong>eral<br />
80-octet frame l<strong>en</strong>gth produces an 80-bÍt word in the service channel. These 80 bits are numbered 1-80. Bits 1-8 of<br />
service channel in every frame constitute the FAS (see Figure 3), whose cont<strong>en</strong>í ís as follows:<br />
- multíframe structure (see 2.2);<br />
- Frame alignm<strong>en</strong>t word (FAW);<br />
- A-bit;<br />
E- and C-bits (see 2.6).<br />
17<br />
•<br />
•<br />
•<br />
80<br />
Recomm<strong>en</strong>dation H.221 (03/93)
e FAW consists of "0011011" ¡n bits 2-8 of the FAS in ev<strong>en</strong> frames, complem<strong>en</strong>ted by an "1" in bit 2 of the<br />
:ceedíng oda frame.<br />
e "A-bit" of the I-channel is set to zero wh<strong>en</strong>ever the receiver is in multiframe alignm<strong>en</strong>t, and is set lo "1" otherwise<br />
¡e 2.3); for additional channels, see 2.7.1.<br />
Successive<br />
frames<br />
v<strong>en</strong> frames<br />
dd frames<br />
[OTES<br />
See 2.2 and Figure 4.<br />
1<br />
{Note 1)<br />
(Note 1)<br />
2<br />
0<br />
1<br />
(Note 2)<br />
Bitnumber<br />
3 4 5 6 7 8<br />
0 1 1 0 1 1<br />
A<br />
(Note 3)<br />
Frame alignm<strong>en</strong>t word (Note 2)<br />
E<br />
(Note 4)<br />
Cl C2 C3 C4<br />
The fírst sev<strong>en</strong> bits of the frarae alignm<strong>en</strong>t word are ín the ev<strong>en</strong> frames. The eighth bit of the FAW in the odd frame ís the<br />
omplem<strong>en</strong>t of the first FAW bit in or<strong>de</strong>r to avoid símu<strong>la</strong>tion of FAW by a frarae-repetitive pattem.'<br />
A-bít: loss of multiframe alignra<strong>en</strong>t indication (O = alignra<strong>en</strong>t; 1 = loss).<br />
The use of bits E and C1-C4 ís <strong>de</strong>scribed ín 2.6 (O =no error or cyclic redundancy check (CRC) not in use; 1 = error).<br />
t Multiframe structure<br />
; Figure 4,<br />
FIGURE 3/H.221<br />
Assignm<strong>en</strong>t of bits 1-8 of the service channel in each frame<br />
:h multiframe contains 16 consecutive frames numbered O to 15 dívi<strong>de</strong>d into eighl sub-multiframes of two frames each<br />
e Figure 4). The multiframe alignm<strong>en</strong>t signal is located in bit 1 of frames 1-3-5-7-9-11 and has the form 0010] 1. Bit 1<br />
frame 15 remains reserved forfuture use. The valué Ís fixed at 0.<br />
1 of frames 0-2-4-6 may be used for a modulo 16 counter to number multiframes in <strong>de</strong>sc<strong>en</strong>díng or<strong>de</strong>r, The least<br />
níficant bit is transmitted in frame O, and the most signifícant bit ín frame 6. The receiver uses the multiframe<br />
nberíng to equalize out the dlffer<strong>en</strong>tial <strong>de</strong><strong>la</strong>y of sepárale conneclions, and to synchronize ihe received signáis,<br />
; multiframe numbering is mandatory both in inítial and additional channels for múltiple B or múltiple HQ<br />
nmunications, bul it may or may not be inserted for single B or single H0 or HU/H¡2 for other Communications where<br />
chronization betwe<strong>en</strong> múltiple channels is not required.<br />
1 of frame 8 Ís setto 1 wh<strong>en</strong> multiframes are numbered and is set to O wh<strong>en</strong> they are not.<br />
1 of frames 10-12-13 must be used to number each channel Ín a multiconnection structure so that the distant receiver<br />
p<strong>la</strong>ce the octets received ¡n each 125 microseconds Ín the correct or<strong>de</strong>r.<br />
Drmation bits in the multiframe should be validated by, for example, being received consist<strong>en</strong>lly for three multiframes.<br />
Recomm<strong>en</strong>dation H.221 (03/93)
Multhrame<br />
Sub-multi frame<br />
(SMF)<br />
SMF1<br />
SMF2<br />
SMF3<br />
SMF4<br />
SMF5<br />
SMF6<br />
SMF7<br />
SMF8<br />
Frame<br />
0<br />
1<br />
2<br />
3<br />
4<br />
5<br />
6<br />
7<br />
8<br />
9<br />
10<br />
11<br />
12<br />
13<br />
14<br />
15<br />
Channel number, least significan! bit in L1<br />
Channel 13 L2 L1<br />
Initial O O 1<br />
Second<br />
Third<br />
Sixth 1 1 0<br />
Reserved forfuture use set lo 0.<br />
E, C1-C4 As in Figure 3.<br />
1<br />
N1 '<br />
0<br />
N2<br />
0<br />
N3<br />
1<br />
N4<br />
0<br />
N5<br />
1<br />
L1<br />
1<br />
L2<br />
L3<br />
TEA<br />
R<br />
2<br />
. 0<br />
1<br />
0<br />
1<br />
0<br />
1<br />
0<br />
1<br />
0<br />
1<br />
0<br />
1<br />
0<br />
1<br />
0<br />
1<br />
Bits 1 to 8 of the service channel in every frame<br />
-N4 Used for multiframe numbering as <strong>de</strong>scríbed in 2.2; set to O while numbering Ís inactive.<br />
3<br />
0<br />
A<br />
0<br />
A<br />
0<br />
A<br />
0<br />
A<br />
0<br />
A<br />
0<br />
A<br />
0<br />
A<br />
0<br />
A<br />
N4 N3 N2 N1<br />
Uultiframe number 0 0 0 0 0 (or numbaring inactiva)<br />
1 0 0 0 1<br />
2 0 0 1 0<br />
15 1 1 1 1<br />
Indícales whether muliíframe numbering ¡s active (N5 = 1} or inactive (N5 = 0).<br />
A The termina! equipm<strong>en</strong>t a<strong>la</strong>rm ís set to 1 in the outgoing signa! while an internal terminal equipm<strong>en</strong>t fault exists such that it<br />
cannot receíve and act on Ihe incoming signa!. Otherwise it Ís set to 0.<br />
FIGURE 4/H.221<br />
Assigntn<strong>en</strong>t of bits 1-8 of the service channel in each frame in a multiframe<br />
i Loss and recovery of frame alignm<strong>en</strong>t<br />
irae alignm<strong>en</strong>t is <strong>de</strong>fined to have be<strong>en</strong> lost wh<strong>en</strong> three consecutive frame alignm<strong>en</strong>t words have be<strong>en</strong> received with an<br />
4<br />
1<br />
E<br />
1<br />
E<br />
1<br />
E<br />
1<br />
E<br />
1<br />
E<br />
1<br />
E<br />
1<br />
E<br />
1<br />
E<br />
5<br />
1<br />
C1<br />
1<br />
C1<br />
1<br />
C1<br />
1<br />
C1<br />
1<br />
Cl<br />
1<br />
C1<br />
1<br />
C1<br />
1<br />
C1<br />
6<br />
0<br />
C2<br />
0<br />
C2<br />
0<br />
C2<br />
0<br />
C2<br />
0<br />
C2<br />
0<br />
C2<br />
0<br />
C2<br />
0<br />
C2<br />
Recomm<strong>en</strong>dation H.221 (03/93)<br />
7<br />
1<br />
C3<br />
1<br />
C3<br />
1<br />
C3<br />
1<br />
C3<br />
1<br />
C3<br />
1<br />
C3<br />
1<br />
C3<br />
1<br />
C3<br />
8<br />
1<br />
C4<br />
1<br />
C4<br />
1<br />
C4<br />
1<br />
C4<br />
1<br />
C4<br />
1<br />
C4<br />
1<br />
C4<br />
1<br />
C4
ime alignm<strong>en</strong>t is <strong>de</strong>fined to have be<strong>en</strong> recovered wh<strong>en</strong> the followíng sequ<strong>en</strong>ce is <strong>de</strong>tected:<br />
- for ihe fírst time, the pres<strong>en</strong>ce of the correct fírst se v<strong>en</strong> bits of the frame alignm<strong>en</strong>t word;<br />
- the eighth bit of the frame alignm<strong>en</strong>t word in the foliowing frame is <strong>de</strong>tecled by verifying that bit 2 is a 1;<br />
- for the second time, the pres<strong>en</strong>ce of the correct first sev<strong>en</strong> bits of the frame alignm<strong>en</strong>t word ín the next<br />
frame.<br />
Frame alignm<strong>en</strong>t is achieved but muitiframe alignm<strong>en</strong>t cannot be achieved, th<strong>en</strong> frame alignm<strong>en</strong>t should be sought at<br />
other position.<br />
íi<strong>en</strong> the frame alignm<strong>en</strong>t is lost, A-bit of the next odd frame is set to 1 in the transmit direction.<br />
[ Loss and recovery of muitiframe alignm<strong>en</strong>t<br />
iltiframe alignm<strong>en</strong>t is nee<strong>de</strong>d to number and synchronize two or more channels, and possibly also for <strong>en</strong>cryptíon.<br />
rminals such as those having only single-channel capabilities whích have no use for the muitiframe structure must<br />
Qsmit the muitiframe structure, but need not check for multiframe alignm<strong>en</strong>t on the incoming signal: they may transmit<br />
:going A = O wh<strong>en</strong> frame alignm<strong>en</strong>t ís recovered.<br />
NOTE - Such a terminal cannot transmit TEA (see Figure 4).<br />
!er multiframe alignm<strong>en</strong>t has be<strong>en</strong> validated the other functions repres<strong>en</strong>ted by bit 1 of the service channel can be used.<br />
t<strong>en</strong> multiframe alignm<strong>en</strong>t of the distant terminal has be<strong>en</strong> signalled (A — O receíved) the distant terminal is expected to<br />
fe validated BAS co<strong>de</strong>s and to be able to interpret BAS co<strong>de</strong>s.<br />
.Mírame alignm<strong>en</strong>t is <strong>de</strong>fined to have be<strong>en</strong> lost wh<strong>en</strong> three consecutive multiframe alignm<strong>en</strong>t signáis have be<strong>en</strong><br />
eived with an error. It is <strong>de</strong>fined to have be<strong>en</strong> recovered wh<strong>en</strong> the multiframe alignm<strong>en</strong>t signal has be<strong>en</strong> received with<br />
error in the next multiframe. Wh<strong>en</strong> multiframe alignm<strong>en</strong>t is lost, ev<strong>en</strong> wh<strong>en</strong> an unframed mo<strong>de</strong> is received, the A-bit of<br />
next odd frame is setto 1 in the transmit directíon. It is reset to O wh<strong>en</strong> multiframe alignm<strong>en</strong>t is regained. It is reset in<br />
litional channels wh<strong>en</strong> multiframe alignm<strong>en</strong>t and synchronism with the iniual channel ¡s regaíned.<br />
Procedure to recover octet timing from frame alignm<strong>en</strong>t<br />
,<strong>en</strong> the network does not provi<strong>de</strong> octet timing, the terminal may recover octet timing Ín the receive direction from bit<br />
¡ng and from the frame alignm<strong>en</strong>t. The octet timing in the transmit direction may be <strong>de</strong>rived from the network bit<br />
ing and an internal octet timing.<br />
,1 G<strong>en</strong>eral rule<br />
receive octet tlming is normally <strong>de</strong>termined from the FAS position. But at the start of the cali and before the frame<br />
nm<strong>en</strong>t is gained, the receive octet timing may be tak<strong>en</strong> to be the same as the internal transmit octel timing. As soon as<br />
•st frame alignm<strong>en</strong>t is gained, the receive octet tímíng ¡s initialized at the new bít position, but it is not yet validated. It<br />
be validated only wh<strong>en</strong> frame alignm<strong>en</strong>t is not lost during the next 16 frames.<br />
2 Particu<strong>la</strong>r cases<br />
a) Wh<strong>en</strong>, ai the initiation of a cali, the terminal is in a forced reception mo<strong>de</strong>, or wh<strong>en</strong> the frame alignm<strong>en</strong>t<br />
has not yet be<strong>en</strong> gained, the terminal may temporarily use the transmit octet timing,<br />
b) Wh<strong>en</strong> frame alignm<strong>en</strong>t is lost after being gained, the receive octet timing should not change until frame<br />
alignm<strong>en</strong>t is recovered.<br />
c) As soon as frame and multiframe alignm<strong>en</strong>t have be<strong>en</strong> gained once, the octet timing is consi<strong>de</strong>red as valid<br />
for the rest of the cali, unless frame alignm<strong>en</strong>t is lost and a new frame alignm<strong>en</strong>t is gained at another bit<br />
position.<br />
Recomro<strong>en</strong>dation H.221 (03/93)
'.1 Múltiple B-connections<br />
S and BAS are transmitted in each B-channel (see Note).<br />
NOTE - The actual bit rates allowed by this Recoram<strong>en</strong>dation for these audio codíngs within a 64 kbít/s I-channel are 64<br />
156 Icbit/s, coramands (000) [4/5 and 18/19], respectively. Thus in a 2B audiovisual cali ít !s not permitted to transrait fraraed G.711<br />
lio in the I-channel and vi<strong>de</strong>o ín the additíonal channel. The two. channels raust be synchronüsed, the audio raust be set to 56 kbit/s,<br />
I wh<strong>en</strong> the vi<strong>de</strong>o is ON it musí occupy the reraaining 68.8 kbit/s.<br />
S operatíon is as follows:<br />
- multiframe numbering ís used to <strong>de</strong>termine re<strong>la</strong>tíve transmission <strong>de</strong><strong>la</strong>y betwe<strong>en</strong> B-channels as <strong>de</strong>scríbed in<br />
2.2;<br />
- the channel numbers are transmitted as <strong>de</strong>scribed in 2.2 with the channel of the initial connection being<br />
numbered 1 and there being up to five additional connections;<br />
- the outgoing A-bit is set to 1 Ín the additional B-channel of the same connecüon wh<strong>en</strong>ever the received<br />
additional channel is not synchronízed to the initial channel;<br />
- wh<strong>en</strong> receive synchronlzation is achíeved betwe<strong>en</strong> the initial and additional channels by introducing <strong>de</strong><strong>la</strong>y<br />
to align their respective multiframe signáis, the transmitted A-bit is set to 0;<br />
- the E-bit for each additional B-channel is transmitted in the additional B-channel in the same connection,<br />
because Ít re<strong>la</strong>tes to a physical condition of the transmission path.<br />
; BAS operatíon in additional connections is restricted to the transmission of the additional channel number and TTX<br />
e Recomm<strong>en</strong>datíon H.230) (thus the channel numbering of any additional connection must be s<strong>en</strong>t both in BAS<br />
ording to Annex A and in the FAS as in 2.2), while channel numbering of the initial channel is s<strong>en</strong>t only in FAS.<br />
i dístant terminal, upon receíving the A-bÍt set to zero with respect to sequ<strong>en</strong>tially numbered channels, can add their<br />
•acity to the initial connection by s<strong>en</strong>ding the transfer rate BAS in Annex A. The or<strong>de</strong>r of the bits transmílted in the<br />
.nnels is in accordance wilh the examples giv<strong>en</strong> in Figure 5.<br />
.2 Múltiple H0 connections<br />
S and BAS are transmitted Ín the fírst time-slot of each HQ-<br />
S operation is as in 2.7.1 except that the channel number is used to or<strong>de</strong>r the six octets received each 125 microonds<br />
with respect to the six octet groups received Ín other channels.<br />
¡ BAS operation in additional channels is as specified in 2.7.1.<br />
Bit-rate allocation signal<br />
Encoding of the BAS<br />
: bit-rate allocation signal (BAS) occupies bits 9-16 of the service channel in every frame. An eight bit BAS co<strong>de</strong> (b0,<br />
b2, b3l 64, b5, bg, b7) Ís complem<strong>en</strong>ted by eíght error correctíon bits (p0, p¡, P2- P3- P-4- P5< P6- P?) to ¡mplem<strong>en</strong>t a (16,8)<br />
ble error correcting co<strong>de</strong>. This error correcting co<strong>de</strong> is obtained by short<strong>en</strong>ing ine (17,9) cyclic co<strong>de</strong> wlth g<strong>en</strong>erator<br />
("normal:<br />
g(x) = x8 + xl + x6 -f x4 + x2 + x + 1<br />
The error correction bits are calcu<strong>la</strong>led as coefficí<strong>en</strong>ts of the remain<strong>de</strong>r polynomíal in the following equation:<br />
PQX1 "f píX6 = p2X5 + /73X4 + p4X3 + pSX2 + p6X + p1<br />
re RESg(x-)[f(x)'] repres<strong>en</strong>ts the residue obtained by dividing/(x) by g(x).<br />
BAS co<strong>de</strong> Ís s<strong>en</strong>t in the ev<strong>en</strong>-numbered frame, whíle ihe assoc<strong>la</strong>ted error correction bits are s<strong>en</strong>t Ín the subsequ<strong>en</strong>t<br />
•numbered frame. The bits of the BAS co<strong>de</strong> or the error correction are transmitted Ín the or<strong>de</strong>r shown in Table 2 lo<br />
¡d emu<strong>la</strong>tion of the frame alígnm<strong>en</strong>t word.<br />
Recomm<strong>en</strong>dation H.221 (03/93) 9
le <strong>de</strong>co<strong>de</strong>d BAS valué is valíd if:<br />
Bít posítion<br />
9<br />
10<br />
11<br />
12<br />
13<br />
14<br />
15<br />
16<br />
TABLE 2/H.221<br />
Ev<strong>en</strong> f rara e<br />
bo<br />
- b3<br />
- the receiver is in frame and multiframe alignm<strong>en</strong>t, and<br />
b2<br />
bi<br />
b5<br />
b4<br />
b6<br />
b7<br />
Odd frame<br />
- the FAW In the same sub-multiframe was received with two or fewerbits in error,<br />
h<strong>en</strong>víse the <strong>de</strong>co<strong>de</strong>d BAS valué is ignored.<br />
h<strong>en</strong> the receiver actually looses frame alignm<strong>en</strong>t, it may be advisable to undo any changes caused by the three<br />
;vÍously <strong>de</strong>co<strong>de</strong>d valúes as they may well have be<strong>en</strong> erroneous ev<strong>en</strong> after correction.<br />
I Valúes of íheBAS<br />
,e <strong>en</strong>coding of BAS is ma<strong>de</strong> accordíng to a hierarchical attribute method. This consists of attribute c<strong>la</strong>ss (8 c<strong>la</strong>sses),<br />
ribute family (8 famílies), attríbute (8 attributes) and valué (32 valúes). The first three bits of an attribute repres<strong>en</strong>t its<br />
mber <strong>de</strong>scnbing the g<strong>en</strong>eral command or capability, and the other five bits i<strong>de</strong>ntífy the "valué" - the specific command<br />
capability.<br />
e followíng attributes are <strong>de</strong>fíned in the C<strong>la</strong>ss (000) and Family (000):<br />
Attribute<br />
000<br />
001<br />
010<br />
Olí<br />
100<br />
101<br />
110<br />
111<br />
Significarme<br />
Audio coding comrnand<br />
Transfer rate comrnand<br />
Vi<strong>de</strong>o and other comrnand<br />
Data command<br />
Terminal capability ]<br />
Terminal capability 2<br />
Reserved<br />
Escape co<strong>de</strong>s<br />
e valúes of these attributes are Usted and <strong>de</strong>fined Ín Ahnex A. They provi<strong>de</strong> for the followíng facilities:<br />
— transmission at various total rates and on single and múltiple channels, on clear channels and on networks<br />
subject lo restrictions to 56 kbit/s and its múltiples;<br />
— audio transmission, digitally <strong>en</strong>co<strong>de</strong>d to various recomm<strong>en</strong><strong>de</strong>d algorithms;<br />
- vi<strong>de</strong>o transmission, digitally <strong>en</strong>co<strong>de</strong>d to a recomm<strong>en</strong><strong>de</strong>d algorilhm, wilh provisión for future recomm<strong>en</strong><strong>de</strong>d<br />
ímprovem<strong>en</strong>t;<br />
- Low-speed data (LSD) within the I-channel, or TS 1 of a higher rale inilial channel;<br />
- High-speed data (HSD) in the highest-numbered 64 kbit/s channel or time-slots (excluding ihe I-channel):<br />
Recomm<strong>en</strong>dation H-221 (03/93)<br />
P2<br />
Pl<br />
Po<br />
p¿<br />
P3<br />
P3<br />
P6<br />
P7
- data transmission within a multi<strong>la</strong>yer protocol, either in the I-channel (MLP) or in capacity other than the Ichannel<br />
(H-MLP);<br />
- an <strong>en</strong>cryption control signal;<br />
- loopback towards the network for maint<strong>en</strong>ance purposes;<br />
— signalling for control and indícations;<br />
- a message system for, Ínter alia, conveying information concerning equipm<strong>en</strong>t manufacturer and lype.<br />
,e command BAS attributes have the following signifícance: on receipt of a BAS command co<strong>de</strong> in one (ev<strong>en</strong>) frame<br />
d its error-correcting co<strong>de</strong> ín the next (odd), the receiver prepares to accept the stated mo<strong>de</strong> change beginning from the<br />
bsequ<strong>en</strong>t (ev<strong>en</strong>) frame; thus a mo<strong>de</strong> change can be effected in 20 milliseconds. The command remains in forcé until<br />
unterman<strong>de</strong>d (see 12/H.242), The bit positions occupied by combinations of BAS commands are exemplified in Figures<br />
to 5g.<br />
e capability BAS attributes have the following signifícance: they indícate the ability of a terminal to receíve and<br />
)perly treat the various types of signal. It follows that having received a set of capability valúes from the remote terminal<br />
terminal X must not transmít signáis lying outsi<strong>de</strong> that <strong>de</strong>c<strong>la</strong>red range.<br />
ilues [0-7] ofthe attribute (111) are reserved for setting the c<strong>la</strong>ss, and [8-15] for setting the family; the <strong>de</strong>fault valué is<br />
30) for both,<br />
ie next eight attribute valúes ofthe attribute (111) are temporary escape BAS co<strong>de</strong>s of single byte ext<strong>en</strong>sión (SBE). The<br />
¡t three bits ofthe temporary escape BAS form a pointer to one of eight possible escape BAS tables of 224 <strong>en</strong>tries each<br />
)<strong>de</strong>s beginning wíth 111 are not used in the escape BAS tables). Th<strong>en</strong> the next received BAS indícales the specific<br />
try Ín the escape BAS table.<br />
e valué (111) [24] is the capability marker (see 2/H.242) which is followed by normal BAS co<strong>de</strong>s, not by any escape<br />
lúes.<br />
e <strong>la</strong>st sev<strong>en</strong> attribute valúes ofthe attribute (111) are of múltiple byte ext<strong>en</strong>sión (MBE) and are used lo s<strong>en</strong>d messages<br />
specified in the Notes to Tables A.2 and A.3.<br />
Bit number<br />
7 8<br />
1<br />
2<br />
8<br />
9<br />
16<br />
17<br />
19<br />
143<br />
:<br />
:<br />
FAS<br />
BAS<br />
18<br />
20<br />
144<br />
FIGURE 5a/H.221<br />
Oc<strong>la</strong>l<br />
number<br />
Bit numbering and positíon for 14.4 kbit/s LSD<br />
16<br />
17<br />
18<br />
80<br />
Recomm<strong>en</strong>dation H.221 (03/93) 11
ec. G.711<br />
50<br />
1<br />
:<br />
57<br />
:<br />
106<br />
113<br />
120<br />
554<br />
50<br />
57<br />
106<br />
113<br />
121<br />
617<br />
:<br />
1<br />
:<br />
Audío bít rate<br />
ec. G.722, 64 kbit/s<br />
ec. G.722, 56 kbit/s<br />
ec. G.722, 48 kbit/s<br />
ec. G.728, 16 kbit/s<br />
Hígh-band audio<br />
low-band audío<br />
51<br />
2<br />
:<br />
58<br />
:<br />
107<br />
114<br />
121<br />
555<br />
51<br />
58<br />
107<br />
114<br />
122<br />
618<br />
:<br />
2<br />
:<br />
:<br />
52<br />
3<br />
:<br />
59<br />
:<br />
108<br />
115<br />
122<br />
:<br />
556<br />
52<br />
3<br />
:<br />
59<br />
:<br />
108<br />
115<br />
123<br />
619<br />
1<br />
MSB<br />
H<br />
H<br />
H<br />
Bit number<br />
4 5 6<br />
53<br />
4<br />
:<br />
60<br />
:<br />
109<br />
116<br />
123<br />
557<br />
53<br />
:<br />
4<br />
:<br />
60<br />
;<br />
109<br />
116<br />
124<br />
620<br />
See below<br />
Recomm<strong>en</strong>cUtion H.221 (03/93)<br />
54<br />
5<br />
:<br />
61<br />
:<br />
110<br />
117<br />
124<br />
;<br />
558<br />
55<br />
62<br />
111<br />
118<br />
6<br />
:<br />
:<br />
125<br />
:<br />
559<br />
FIGURE 5b/H.221<br />
56 kbit/s LSD<br />
5<br />
54<br />
:<br />
61<br />
:<br />
110<br />
117<br />
125<br />
621<br />
55<br />
6<br />
:<br />
62<br />
:<br />
111<br />
118<br />
126<br />
622<br />
FIGURE 5c/H.221<br />
62.4 kbit/s LSD<br />
2<br />
...<br />
H<br />
H<br />
H<br />
3<br />
...<br />
L<br />
L<br />
L<br />
-<br />
56<br />
63<br />
112<br />
119<br />
7<br />
:<br />
:<br />
126<br />
;<br />
560<br />
56<br />
63<br />
112<br />
119<br />
127<br />
623<br />
7<br />
:<br />
Bit nuraber<br />
4<br />
...<br />
L<br />
L<br />
L<br />
-<br />
FAS<br />
BAS<br />
Sub-channel 8<br />
FAS<br />
BAS<br />
120<br />
128<br />
624<br />
5<br />
...<br />
L<br />
L<br />
L<br />
-<br />
6<br />
L<br />
L<br />
L<br />
-<br />
Oclel<br />
numbQr<br />
1<br />
2<br />
16<br />
17<br />
18<br />
16<br />
17<br />
18<br />
7<br />
...<br />
L<br />
L<br />
-<br />
-<br />
8<br />
LSB<br />
L<br />
-<br />
_<br />
-
728 audio<br />
e LD-CELP 2.5 ms frarae consiste of the following 40 numbered bits:<br />
Co<strong>de</strong>word O, bit 9 (MSB) to bit O (LSB): 09,08,07,06,05,04,03,02,01,00<br />
Co<strong>de</strong>word 1, bit 9 (MSB) to bit O (LSB): 19,18,17,16,15,14,13,12,11,10<br />
Co<strong>de</strong>word 2, bit 9 (MSB) to bit O (LSB): 29,28,27,26,25,24,23,22,21,20<br />
Co<strong>de</strong>word 3, bit 9 (MSB) to bit O (LSB): 39,38,37,36,35,34,33,32,31,30<br />
iese are pacfced into two 8 kbit/s H.221 sub-channels by putting odd nurabered bits in the first sub-channel and ev<strong>en</strong> nurabered bits<br />
the second. This structure ¡s repeated four limes ¡n each 10 ras H.221 frarae as shown below. The ftrst co<strong>de</strong>word in each H.221<br />
une is th<strong>en</strong> always the first co<strong>de</strong>word in the speech co<strong>de</strong>r frarae also. The speech co<strong>de</strong>r synchronization can th<strong>en</strong> be <strong>de</strong>rived frora<br />
221 FAS (frame alignm<strong>en</strong>t slgnal). '<br />
Bit number<br />
Speech<br />
co<strong>de</strong>r<br />
frame 0<br />
Speech<br />
co<strong>de</strong>r<br />
trame 1<br />
Speech<br />
co<strong>de</strong>r<br />
frame 2<br />
Spaech<br />
co<strong>de</strong>r<br />
frame 3<br />
1<br />
09<br />
07<br />
05<br />
03<br />
01<br />
19<br />
17<br />
•<br />
11<br />
29<br />
•<br />
21<br />
39<br />
•<br />
31<br />
09<br />
07<br />
•<br />
33<br />
31<br />
09<br />
07<br />
-<br />
33<br />
31<br />
09<br />
07<br />
•<br />
33<br />
31<br />
2<br />
08<br />
06<br />
04<br />
02<br />
00<br />
18<br />
16<br />
•<br />
10<br />
28<br />
•<br />
20<br />
38<br />
•<br />
30<br />
08<br />
06<br />
"<br />
32<br />
30<br />
08<br />
06<br />
•<br />
32<br />
30<br />
08<br />
06<br />
"<br />
32<br />
30<br />
The 10 ms H.221 frame<br />
3 4 5 6 7 8<br />
FIGURE 5d/H.221<br />
Bit positions for audio<br />
F<br />
A<br />
S<br />
Octel<br />
number<br />
1<br />
2<br />
3<br />
-<br />
-<br />
-<br />
•<br />
•<br />
-<br />
-<br />
-<br />
•<br />
•<br />
•<br />
-<br />
•<br />
-<br />
•<br />
-<br />
•<br />
•<br />
-<br />
-<br />
•<br />
-<br />
•<br />
-<br />
•<br />
79<br />
80<br />
Recomm<strong>en</strong>dation H.221 (03/93) 13
B!t1<br />
M<br />
\<br />
A2<br />
3<br />
A3<br />
TS1<br />
Inltial channel .<br />
4<br />
A4<br />
5<br />
AS<br />
• •<br />
A<br />
6<br />
A6<br />
A A A A A A A F<br />
A<br />
S<br />
B<br />
A<br />
S<br />
V<br />
V<br />
\n H.221 (03/93)<br />
A<br />
•<br />
,7<br />
V1<br />
V9<br />
' V121<br />
•<br />
•V129<br />
V139<br />
V759<br />
8<br />
FAS<br />
BAS<br />
V130<br />
i<br />
i<br />
. « ;<br />
V2<br />
1<br />
V10<br />
V122<br />
V131<br />
2<br />
V3<br />
3<br />
V4<br />
FIGU5Í.E. 5e/H.221<br />
Bit positions for yi<strong>de</strong>o in two B-channels<br />
V<br />
TS2 '<br />
VI V.8<br />
V25 !<br />
V361<br />
V386 :. '<br />
V411<br />
V1961 • •'<br />
TS3<br />
V9 V16<br />
FIGURE 5Í/H.221<br />
128 kbit/s HSD ín H0 channel<br />
Additional channel , .<br />
TS4<br />
4<br />
V5<br />
V17 V24<br />
V48<br />
V3fl4<br />
V409<br />
• • V1984<br />
5<br />
V6<br />
6<br />
V7<br />
TS5<br />
D1 D8<br />
D17<br />
D241<br />
D257<br />
D1265 -•<br />
7<br />
-V81<br />
V1'6<br />
V128<br />
VÍ37<br />
•<br />
TS6<br />
8<br />
FAS<br />
BAS<br />
V138<br />
V148<br />
•<br />
V768<br />
D9 D16<br />
D32<br />
0256<br />
• • D1280
Initial B-channel<br />
A A A A A A F<br />
A<br />
S<br />
B<br />
A<br />
S<br />
V<br />
V<br />
V<br />
2nd channel<br />
V1 V7<br />
V29<br />
V421<br />
V450<br />
V483<br />
V2529 • •<br />
.3 Procedures for the use of BAS<br />
F<br />
A<br />
S<br />
B<br />
A<br />
S<br />
3rd channel<br />
V8 V14 F<br />
A<br />
S<br />
B<br />
A<br />
S<br />
4th channnel<br />
V15 V21<br />
V42<br />
FIGURE 5g/H.221<br />
64 kbit/s HSD in 6 x 64 kbit/s channels<br />
he use of BAS co<strong>de</strong>s is specified in Recomm<strong>en</strong>dation H.242.<br />
Annex A<br />
Definitions and tables of BAS valúes<br />
(Thís annex fonns an integral part of this Recomm<strong>en</strong>dation)<br />
F<br />
A<br />
S<br />
B<br />
A<br />
S<br />
Sth channel<br />
V22 V28 F<br />
V56 A<br />
S<br />
B<br />
A<br />
V448 s<br />
V481<br />
V514<br />
• • V2560<br />
6lh channel<br />
DI D8 F<br />
D9 D16 A<br />
D121 D128 s<br />
S<br />
B<br />
A<br />
D129 D136<br />
D137 D144<br />
D633 • • D640<br />
he <strong>de</strong>finitions of BAS valúes are giv<strong>en</strong> below, and the corresponding numerical valúes are Usted in Tables A.l and A.2.<br />
Recomm<strong>en</strong>dation H.221 (03/93) 15
[0]<br />
[1]<br />
[2]<br />
[3]<br />
[4]<br />
[5]<br />
[6]<br />
[7]<br />
[8]<br />
[9]<br />
[10]<br />
[11]<br />
[12]<br />
[13]<br />
[14]<br />
[15]<br />
[16]<br />
[17]<br />
[18]<br />
[19]<br />
[20]<br />
[21]<br />
[22]<br />
[23]<br />
[24]<br />
[25]<br />
[26]<br />
[27]<br />
[28]<br />
[29]<br />
[30]<br />
31]<br />
(000)<br />
Audio coramand<br />
Neutral*'<br />
A-<strong>la</strong>w, OU<br />
Jl-Uw, OU<br />
G.722, mi*1<br />
Au-off.U1'<br />
(Note 2)<br />
(Note 2)<br />
Au-ISO-64<br />
Au-ISO-128<br />
Au-ISO-192<br />
Au-ISO-256<br />
Au-ISO-384<br />
A-<strong>la</strong>w, OF'><br />
Jl-Iaw, OF1'<br />
0<br />
•)<br />
G.722, ra2<br />
(Note 3)'><br />
G.722, m3<br />
(Note 3)'><br />
(Au-40k)<br />
(Au-32k)<br />
(Au-24k)<br />
G.728*><br />
(Au-
[0]<br />
[1]<br />
[2]<br />
[3]<br />
[4]<br />
[5]<br />
[6]<br />
[7]<br />
[8]<br />
[9]<br />
[10]<br />
[U]<br />
[12]<br />
[13]<br />
[14]<br />
[15]<br />
[16]<br />
[17]<br />
[18]<br />
[19]<br />
[20]<br />
[21]<br />
[22]<br />
[23]<br />
[24]<br />
[25]<br />
[26]<br />
[27]<br />
[28]<br />
[29]<br />
[30]<br />
[31]<br />
Var-HSD(R)<br />
H-MLP-62,4<br />
H-MLP-64<br />
H-MLP-128<br />
H-MLP-192<br />
H-MLP-256<br />
H-MLP-320<br />
H-MLP-384<br />
Var-H-MLP(R)<br />
64k<br />
128k<br />
192k<br />
256k<br />
3201c<br />
384Ic<br />
512k(R)<br />
768k(R)<br />
1152k(R)<br />
1536Ic(R)<br />
TABLE A.2/H.221<br />
HSD/H-MLP numerical valúes<br />
Capabilities (101)<br />
HSD-off<br />
Var-HSD(R)<br />
H-MLP-62.4<br />
H-MLP-64<br />
H-MLP-12S<br />
H-MLP-192<br />
H-MLP-256<br />
H-MLP-320<br />
H-MLPH-384<br />
Var-H-MLP(R)<br />
H-MLP-off<br />
64k<br />
128k<br />
192k<br />
256k<br />
320k<br />
384k<br />
512k(R)<br />
768k(R)<br />
11521c(R)<br />
1536k(R)<br />
Comraands (Olí)<br />
NOTES<br />
1 The column hea<strong>de</strong>r gives the attnbute <strong>de</strong>signation as bus (bfl, b]t b2); the left-hand<br />
column gíves the <strong>de</strong>cimal valué of bits [b3, b¿, bs, b^, b7], AI1 assigned valúes are<br />
reserved, as are valúes marked (R).<br />
2 Escape <strong>la</strong>ble reached by BAS (111) [16].<br />
1 Audio command valúes (000)<br />
r bilposition illustrations see Figure 4. Abbreviations "G.711" and "G.722" refer to Recomm<strong>en</strong>dations.<br />
Recomm<strong>en</strong>dation H.221 (03/93) 17
;utral<br />
j-off, U<br />
a-off, F<br />
-<strong>la</strong>w, OU<br />
•<strong>la</strong>w, OF<br />
<strong>la</strong>w, OU<br />
<strong>la</strong>w, OF<br />
722, mi<br />
722, m2<br />
722, ra3<br />
i-40k<br />
i-32k<br />
i-2 4k<br />
i-l 6k<br />
i-
i Signal occupies one 64 kbit/s channel.<br />
X 64 Signal occupies two 64 kbit/s channels, with FAS and BAS in each.<br />
[06 X 64 Signal occupies three to six 64 kbit/s channels, with FAS and BAS ín each.<br />
14 Signal occupies 384 kbit/s, with FAS and BAS Ín the first 64 kbit/s lime-slot; the<br />
effective channel may be the whole of an HQ channel or the lowest numbered time-slots of<br />
an HU orHi2 channel.<br />
X 384 Signal occupies two channels of 384 kbit/s, with FAS and BAS in each.<br />
to 5 X 384 Sígnal occupies three to five 384 kbit/s channels, with FAS and BAS Ín each.<br />
136 Signal occupies 1536 kbit/s, with FAS and BAS in the first 64 kbit/s time-slot. The<br />
effective channel occupies the whole of an HU channel or the lowest numbered tíme-slots<br />
of an H¡2 channel.<br />
120 Signal occupies 1920 kbit/s, with FAS and BAS in the first 64 kbit/s time-slot. The<br />
effective channel occupies the whole of an H¡2 channel.<br />
18/192/256 Signal occupies 128/192/256 kbit/s, with FAS and BAS in the first 64 kbit/s time-slot.<br />
The effective channel occupies the lowest numbered time-slots of a channel with<br />
correspondíng or hígher capacity.<br />
2/768/1152/1472 Signal occupies 512/768/1152/1472 kbit/s, with FAS and BAS in the first 64 kbit/s tímeslol.<br />
The effective channel occupies the lowest numbered time-slots of a channel with<br />
corresponding orhigher capacity.<br />
jss-i.c. Designated "Initial channel", especially used following loss of the channel previously so<br />
<strong>de</strong>signated (see 7.2.3/H.242).<br />
<strong>la</strong>nnel No. 2-6 Numbering of additional channels - see 2.7.1.<br />
.3 Vi<strong>de</strong>o, <strong>en</strong>cryptioiij loop and other commands (010)<br />
¡<strong>de</strong>o-off No vi<strong>de</strong>o; vi<strong>de</strong>o switched off.<br />
,261 Vi<strong>de</strong>o on, to Recomm<strong>en</strong>dation H.261: vi<strong>de</strong>o occupies all capacity not otherwise allocated<br />
by other commands; vi<strong>de</strong>o cannot be inserted in the I-channel wh<strong>en</strong> var-LSD or var-MLP<br />
¡s in forcé; examples are giv<strong>en</strong> Ín Figure 5e.<br />
Specifically, the vi<strong>de</strong>o rate in initial B-channel (framed) or TS 1 is: 62.4 kbit/s - audio rate<br />
- (800 bít/s if ECS is ON} - {MLP rate if ON} - [LSD rate if ON}.<br />
¡d-imp,(R) Reserved for vi<strong>de</strong>o on, to ¡mproved recomm<strong>en</strong><strong>de</strong>d algorithm.<br />
,<strong>de</strong>o-ISO Vi<strong>de</strong>o on, to ISO standard: vi<strong>de</strong>o occupies the same capacity as stipu<strong>la</strong>ted above for the<br />
case of H.261 vi<strong>de</strong>o.<br />
I/-ISO Composite audio/ví<strong>de</strong>o to ISO standard: the composíte signal occupies the same capacity<br />
as stipu<strong>la</strong>ted above for the case of H.261 ví<strong>de</strong>o.<br />
eeze-pic. Freeze-picture request (see Recomm<strong>en</strong>dation H.230, VCF).<br />
st-update Fast-update request (see Recomm<strong>en</strong>dation H.230, VCU).<br />
icryp-on ECS Channel active.<br />
icryp-off ECS channel off.<br />
NOTE - Wh<strong>en</strong> <strong>en</strong>cryption is active, ít applíes to all Information bits in all channels of the<br />
connection, except bits 1-24 of the SG in the I-channel and the FAS and BAS posltions of the other<br />
channels; use of <strong>en</strong>cryption in conjunction with MLP ís for further study.<br />
4-loop Audio loop request (see Recomm<strong>en</strong>dation H.230, LCA).<br />
Recomm<strong>en</strong>dation H.221 (03/93) 19
id-Ioop Vi<strong>de</strong>o loop request (see Recomm<strong>en</strong>dation H.230, LCV).<br />
ig-loop Digital loop request (see Recomm<strong>en</strong>dation H.230, LCDJ.<br />
oop-off Loop off request (see Recomm<strong>en</strong>dation H.230, LCO).<br />
NOTE — Loopback requests are ¡at<strong>en</strong><strong>de</strong>d for use by maint<strong>en</strong>ance staff.<br />
B-Hrj-comp To provi<strong>de</strong> for compatibility betwe<strong>en</strong> termináis connected to single HQ channel and six Bchannel<br />
accesses, the least significant bits of the first 16 octets of all tíme-slots of the HQ<br />
channel, except TS1, are not used; the HQ terminal must discard these bits from the<br />
incoming signal on receipt of this co<strong>de</strong>, and must set the same bits to "1" ín the outgoing<br />
signal.<br />
ot-6B-H0 Negates the command "6B-H0-comp".<br />
NOTE — Used, for example, Ín testíng.<br />
estrict To provi<strong>de</strong> for operation on a restricted network, and for interconnectíon betwe<strong>en</strong> a<br />
terminal on restricted and unrestricted networks: on receipt of this co<strong>de</strong>, a terminal must<br />
treat the SC as being in bit 7 of the 1-channel, and discard bit 8 of every other channel<br />
and/or time-slot; in the outgoing direction these bits are set to "1".<br />
erestrict On receipt of this co<strong>de</strong>, a terminal must revert to "unrestricted network" operation,<br />
treatíng the SC as being in bit 8 of the I-channel.<br />
,.4 LSD/MLP coraraands (Olí)<br />
or bit position illustrations see Figure 5.<br />
SD off LSD switched off.<br />
These LSD rates are not allowed if ECS channel is in use.<br />
In restricted cases, the starred bit numbers are reduced by one.<br />
)0 Low-speed data at 300 bit/s in SC, octets 38-40.<br />
200 Low-speed data at 1200 bit/s in SC, octets 29-40.<br />
300 Low-speed data at 4800 bit/s ¡n SC, octets 33-80.<br />
100 Low-speed data at 6400 bit/s in SC, octets 17-80#.<br />
)00 Low-speed data at 8000 bit/s in bit 7*.<br />
500 Low-speed data at 9600 bit/s in bil 7* and octets 25-40 of SC.<br />
MOO Low-speed data at 14400 bit/s in bit 7* and octets 17-80 of SC#.<br />
ík Low-speed data at 16 kbít/s Ín bit 6* and bit 7*.<br />
tk Low-speed data at 24 kbit/s ¡n bits 5*. 6* and 7*.<br />
lk Low-speed data at 32 kbít/s Ín bits 4*-71*.<br />
)k Low-speed data at 40 kbit/s in bits 3*-l*.<br />
!k Low-speed data at 48 kbit/s in bits 2*-7*.<br />
ík Low-speed data at 56 kbit/s in bits 1-7 (no framing in restricted case).<br />
L4k Low-speed data at 62.4 kbit/s in bits 1-7 and octets 17-80 of SC. If ECS channel is Ín use,<br />
the data rale is reduced to 61.6 kbit/s, but returns to 62.4 kbit/s if ECS channel is closed.<br />
I Recomro<strong>en</strong>dation H.221 (03/93)
ík Low-speed data at 64 kbit/s in bits 1-8, no framing.<br />
ar-LSD Low-speed data occupying all I-channel capacíty not allocated un<strong>de</strong>r other fixed-rate<br />
commands; cannot be invoked wh<strong>en</strong> other LSD is on, or wh<strong>en</strong> varíable-MLP ¡s on (may<br />
also be impractical wh<strong>en</strong> vi<strong>de</strong>o is on in I-channel alone).<br />
Exact var-LSD rate: 62.4 kbit/s - audio rate - {800 bit/s if ECS is ON} - {fixed-MLP<br />
ifONJ.<br />
TI(R) Three co<strong>de</strong>s reserved for cornmunicatíng the status of the data terminal equipm<strong>en</strong>t<br />
ínterfaces.<br />
[LP-off MLP off in all channels.<br />
[LP-4k MLP on at 4 kbit/s in octets 41-80 of SC.<br />
ÍLP-6.4k MLP on at 6.4 kbit/s in octets 17-80 of SC; if ECS channel is in use, the data rate is<br />
reduced to 5.6 kbit/s in octets 25-80, but returns to 6.4 kbit/s if ECS channel is closed.<br />
ar-MLP MLP occupying all I-channel capacity not allocated un<strong>de</strong>r other fixed-rate commands:<br />
cannot be invoked wh<strong>en</strong> other MLP is on, or wh<strong>en</strong> variable-LSD is on (may also be<br />
impractical wh<strong>en</strong> ví<strong>de</strong>o is on in I-channel alone).<br />
.5 Audio capabilities<br />
Exact var-MLP rate: 62.4 kbit/s - audio rate- {800 bit/s if ECS is ON} - (fixed-LSD<br />
if ON}.<br />
eutral Neutral capability: no change in the curr<strong>en</strong>t capabiliiies of ihe terminal.<br />
-<strong>la</strong>w Capable of <strong>de</strong>coding audio to Recomm<strong>en</strong>dation G.711, A-<strong>la</strong>w.<br />
<strong>la</strong>w Capable of <strong>de</strong>coding audio to Recomm<strong>en</strong>dation G.711, u-<strong>la</strong>w.<br />
.722-64 Capable of <strong>de</strong>coding audio to Recomm<strong>en</strong>dation G.722 (mo<strong>de</strong> 1) and to Recomm<strong>en</strong>dation<br />
G.711.<br />
.722-48 Capable of <strong>de</strong>coding audio to Recomm<strong>en</strong>dation G.722 (mo<strong>de</strong>s 1, 2, 3) and to Recomm<strong>en</strong>dation<br />
G.711.<br />
u-16k Capable of <strong>de</strong>coding audio, both to Recomm<strong>en</strong>dation G.728 and Recomm<strong>en</strong>datión G.711.<br />
u-ISO Capable of <strong>de</strong>coding audio to ISO standard al all rates up lo 384 kbít/s.<br />
.6 Vi<strong>de</strong>Oj MBE and <strong>en</strong>cryption capabilities (101)<br />
CIF Can <strong>de</strong>co<strong>de</strong> ví<strong>de</strong>o to QCIF picture format, but not CIF (see Recomm<strong>en</strong>dation H.261) -<br />
this co<strong>de</strong> must be followed by one of the four minimum piciure interval (MPI) valúes<br />
below.<br />
7 Can <strong>de</strong>co<strong>de</strong> ví<strong>de</strong>o to CIF and QCIF formáis (see Recomm<strong>en</strong>dation H.261) - this co<strong>de</strong><br />
must be followed by two MPI valúes, the first applicable to QCIF and the other to CIF<br />
format.<br />
Minimum picture interval (MPI) co<strong>de</strong>s are as follows:<br />
29.97 Can <strong>de</strong>co<strong>de</strong> vi<strong>de</strong>o, having a minimum picture interval . of 1/29.97 seconds, to<br />
Recomm<strong>en</strong>dation H.261.<br />
29.97 Can <strong>de</strong>co<strong>de</strong> vi<strong>de</strong>o, having a minimum picture interval of 2/29.97 seconds, to<br />
Recomm<strong>en</strong>dation H.261.<br />
Recomm<strong>en</strong>dation H.221 (03/93) 21
'29.97<br />
'29.97<br />
id-imp(R)<br />
i<strong>de</strong>o-ISO<br />
V-ISO<br />
[BE-cap<br />
ic-CF(R)<br />
[icryp.<br />
.7<br />
X H0<br />
H0<br />
18/192/256<br />
12/768/1152/1472<br />
.8 LSD/MLP capabilities (101)<br />
10 (to 64k)<br />
ir-LSD<br />
LP-4k<br />
LP-6.4k<br />
ar-MLP<br />
Recomm<strong>en</strong>dation H.221 (03/93)<br />
Can <strong>de</strong>co<strong>de</strong> vi<strong>de</strong>o, having a mínimum picture interval of 3/29.97 seconds, to<br />
Recomm<strong>en</strong>datlon H.261.<br />
Can <strong>de</strong>co<strong>de</strong> vi<strong>de</strong>o, having a mínimum picture interval of 4/29.97 seconds, to<br />
Recomm<strong>en</strong>dation H.26 I.<br />
Reserved for future ímproved recomm<strong>en</strong><strong>de</strong>d vi<strong>de</strong>o algorithm.<br />
Can <strong>de</strong>co<strong>de</strong> vi<strong>de</strong>o to ISO standard.<br />
Can <strong>de</strong>co<strong>de</strong> composite audío/vi<strong>de</strong>o signal to ISO standard.<br />
Can handle multíple-byte ext<strong>en</strong>sions messages in the BAS posltion, those beginning with<br />
co<strong>de</strong>s in the range (111) [25-31], in addition to other valúes.<br />
Reserved for capability to accept non-zero c<strong>la</strong>ss/family escape co<strong>de</strong>s.<br />
Capable of handling signáis on the ECS channel.<br />
Transfer-rate capabilities (100)<br />
Can accept signáis only on one 64 kbit/s channel, one 384 kbit/s channel.<br />
Can accept signáis on one or two 64 kbit/s channels, and synchronize them.<br />
Can accept signáis on one to six 64 kbit/s channels, and synchronize them.<br />
Can accept signáis on one or two 384 kbit/s channels, and synchronize them.<br />
Can accept signáis on one to five 384 kbít/s channels, and synchronize them.<br />
Can accept signáis on a 1536 kbit/s channel, a 1920 kbít/s channel.<br />
Can work only at p X 56 kbit/s, rate-adapted to p X 64 kbit/s by moving the SC to bit<br />
posilion 7 and setting bit 8 to "one" in every channel or lime-slot; a constant "one",<br />
however, may be sel in bit 8 íf ít is known by out-of-band signalling prior to the<br />
connectíon that the restriction exists; this co<strong>de</strong> has the effect of forcing the remote<br />
terminal to work in the p x 56 kbít/s mo<strong>de</strong> (see Annex BJ.<br />
Capable of acting upon the corresponding command.<br />
Capable of accepting the transfer rate specífied by the corresponding command.<br />
Capable of accepting the transfer rate specífíed by the corresponding command.<br />
Can accept LSD at 300 blt/s (to 64 kbit/s) in the bit positions specífied against the<br />
corresponding commands.<br />
Can accept LSD variable rate ín the bit positions specífíed againsl the corresponding<br />
command.<br />
Can accept MLP at 4 kbit/s in the SC.<br />
Can accept MLP at up to 6.4 kbit/s in the SC,<br />
Can accept MLP at up to 64 kbit/s ¡n the I-channel.
.9 Escape table valúes (111)<br />
SD Hígh-speed data: a 32-co<strong>de</strong> table contamíng HSD capabílities and commands.<br />
,230 Control and indications: see <strong>de</strong>finitions in Recomm<strong>en</strong>dation H.230.<br />
art-MBE First byte of (N +2) octet BAS message; the message format is;<br />
start-MBE//value of N (max = 255)//W bytes.<br />
S-cap First byte of non-CCITT capabílities message; the message format is:<br />
NS-cap//value of N (max = 255)//country co<strong>de</strong>5V/manufacturer co<strong>de</strong>*//(jV- 4) bytes,<br />
S-comm Fírst byte of non-CCITT command message; the message format ís:<br />
NS-comm//value of N (max — 255)//country co<strong>de</strong>5V/manufacturer co<strong>de</strong>*//(N - 4) bytes.<br />
ip-mark Capability marker - the first Ítem in a capability set- see 2/H.242.<br />
ata-apps Applications within LSD/HSD channels: a 32-co<strong>de</strong> table - see Table A.3.<br />
NOTES<br />
1 The valué of N is co<strong>de</strong>d by its binary repres<strong>en</strong>tatíon.<br />
1 The raost signifícant bit of each MBE message byte is transraitted as the b0 bit of BAS.<br />
.10 HSD/H-MLP capabilities (111) [lOOOO)-(lOl)<br />
•k to 1536k Can accept HSD at the specified rate in the bit positions specified against the<br />
corresponding commands.<br />
SD-other Reserved for otherHSD rates.<br />
ir-HSD Can accept HSD variable rate in the bit positions specified against the corresponding<br />
command.<br />
•MLP-62.4k Can accept MLP at 62.4 kbit/s in the bit positions specified against the corresponding<br />
command.<br />
•MLP-r Can accept MLP at r= 64/128/192/256/320/384 kbit/s in the bit positions specified<br />
against the corresponding command.<br />
ir-H-MLP Reserved for capability to accept H-MLP variable rate in the bit positions specified against<br />
the corresponding command.<br />
Country co<strong>de</strong> consists of two bytes, the first being according to Recomm<strong>en</strong>dation T.35; ihe second byte and the terminal<br />
manufacturer co<strong>de</strong> of two bytes are assigned nationally.<br />
Recomm<strong>en</strong>dation H.221 (03/93) 23
[0]<br />
[1]<br />
[2]<br />
[3]<br />
[4]<br />
[5]<br />
[6]<br />
m [8]<br />
[9]<br />
[10]<br />
[11J<br />
[12]<br />
[13]<br />
[14]<br />
[15]<br />
[16]<br />
[17]<br />
[ÍS]<br />
[19]<br />
[20]<br />
[21]<br />
[22]<br />
[23]<br />
[24]<br />
[25]<br />
[26]<br />
[27]<br />
[28]<br />
[29]<br />
[30]<br />
[31]<br />
TABLE A.3/H.221<br />
Numerical valúes for applicalions in LSD/HSD channels<br />
Capabílities (101)<br />
i. •<br />
IS"O-SP baseline on LSD '' .<br />
ISO-SP.baseline on HSD<br />
ISO-SP, spatial<br />
ISO-SP progressive<br />
ISO-SP aríthraetic<br />
-<br />
Süirimage (Rec. H.261)<br />
Graphics" cursor<br />
Group 3 fax<br />
Group 4 fax<br />
y, i 20 LSD<br />
V.120 HSD<br />
,<br />
ISO-SP on in LSD<br />
ISO-SP on in HSD<br />
Coramands (Olí)<br />
Cursor data on in LSD<br />
Fax on ín LSD<br />
Fax on in HSD<br />
V.120 LSD<br />
V.120 HSD<br />
NOTES<br />
1 The column.hea<strong>de</strong>r gíves the attribute <strong>de</strong>signalion as bits (b0, bj, b2); the left-hand<br />
colurnn gíves the <strong>de</strong>cimal valué of bits [b3, b¿, b^, bfi' b7], AII assígned valúes are reserved,<br />
as are valúes raa'rked (R).<br />
2 Escape tableVeachedby BAS (111) [18].<br />
11 HSD/H-MLP commands (111) [10000]-(011)<br />
NOTE — In the cases of múltiple channels, the térra "highest-numbered líme-slot" refers to the highest-numbered channel.<br />
D-off HSD switched off; FAS and BAS restored in additional channels,<br />
k HSD on, Ín highest numbered channel/time-slot: FAS and BAS are removed Ín the case of<br />
múltiple B-channels.<br />
8/192/256k<br />
Dk<br />
Recorom<strong>en</strong>dation H.221 (03/93)<br />
HSD on in highest-numbered time-slots of an HQ or greater channel.<br />
HSD on in highest-numbered time-slots of an HQ or greater channel.
D-other<br />
r-HSD<br />
MLP-off<br />
MLP-62,4k<br />
MLP-28k<br />
MLP-192k<br />
MLP-256k<br />
MLP-320k<br />
MLP-384k<br />
r-H-MLP<br />
HSD on in highest-numbered H0 channel, or highest-numbered time-slots of a greater<br />
channel; FAS and BAS are removed in the case of muItiple-H(j channels.<br />
Reserved for other HSD rates.<br />
Reserved for high-speed data occupying all capacity, other than in the I-channel, not<br />
allocated un<strong>de</strong>r other commands: cannot be invoked wh<strong>en</strong> other HSD is on, or wh<strong>en</strong> var-<br />
H-MLP is on (may also be impractical wh<strong>en</strong> vi<strong>de</strong>o ¡s on, the <strong>la</strong>tter th<strong>en</strong> being confined to<br />
the I-channel).<br />
H-MLP switched off (this does not affect I-channel MLP).<br />
H-MLP on at 62.4 kbit/s, occupying second 64 kbit/s channel except FAS and BAS<br />
positions.<br />
H-MLP on at 64/128/192/256/320 kbít/s in the lowest-numbered time-slots, (other than<br />
TS3) of an HQ or greater channel.<br />
H-MLP on at 384 kbit/s in time-slots 2-7 of a greater channel than HQ.<br />
Reserved for MLP occupying all capacity, other than in the I-channel, not allocated un<strong>de</strong>r<br />
other commands: cannot be invoked wh<strong>en</strong> other MLP is on, or wh<strong>en</strong> var-HSD ¡s on.<br />
NOTE- Wh<strong>en</strong> the "restrict" comrnand is in forcé the least significant bit of all octets covered by the HSD and H-MLP<br />
arnands is set to " 1", so the effective data rale is less than that indícated by the command.<br />
12 Applications within LSD/HSD channels - capabilities (111) [10010]-(101)<br />
)~SP baseline on on LSD Can accepl ISO-still picture (SP) baseline mo<strong>de</strong> on specified LSD rale.<br />
D-SP baseline on HSD Can accept ISO-still picture baseline mo<strong>de</strong> on specified HSD rate.<br />
3-SP spatial<br />
)-SP progressive<br />
3-SP aríthmetic<br />
llimage (H.261)<br />
aphics cursor<br />
aup 3 fax<br />
oup 4 Fax<br />
[20 LSD<br />
120 HSD<br />
Can accept ISO-still picture baseline and spatial mo<strong>de</strong>s.<br />
Can accept ISO-still picture baseline and progressive mo<strong>de</strong>s.<br />
Can accept ISO-still picture baseline and amhmetic mo<strong>de</strong>s.<br />
Can accepl s til I images <strong>en</strong>co<strong>de</strong>d by the method <strong>de</strong>fined in Annex D/H.261 (See Note)<br />
NOTE - Admínistrations may use this optional procedure as a simple and inexp<strong>en</strong>sive method<br />
to transmit still images. However, Recomm<strong>en</strong>dation T.S 1 (JPEG) is preferred wh<strong>en</strong> the procedures<br />
for usíng T.81 wíthin audiovisual systems are standardized.<br />
Can handle graphics cursor data.<br />
Can accept group 3 fax.<br />
Can accept group 4 fax.<br />
Can accept V.120 terminal adaptation within an LSD channel.<br />
Can accepl V.120 terminal adaptation within an HSD channei.<br />
13 Applications within LSD/HSD channels - commands (111) [10010]-(OU)<br />
)-SP on in LSD ISO-still picture switched on in specified LSD.<br />
)-SP on in HSD ISO-still picture switched on in specified HSD.<br />
Recomm<strong>en</strong>dation H.221 (03/93) 25
irsor data on in LSD Cursor data swítched on in specified LSD.<br />
x on in LSD Fax switched on in specified LSD.<br />
x on in HSD Fax switched on in specified HSD.<br />
,120 LSD V.120 switched on in specified LSD.<br />
120 HSD V.120 switched on in specified HSD.<br />
Annex B<br />
Frame structure for interworking betwe<strong>en</strong> a 64 kbit/s terminal<br />
1 Sub-channel arrangem<strong>en</strong>t<br />
ie sub-channel arrangem<strong>en</strong>t is giv<strong>en</strong> in Table B.l.<br />
2 Operation of the 64 kbit/s terminal<br />
and a 56 kbit/s terminal<br />
(Thís annex forms an integral part of this Recomm<strong>en</strong>dation)<br />
le transmitter filis the eighth sub-channel wlth "1", whíle the receiver searches FAS at every sub-channel. lt should be<br />
ted that at the receiver si<strong>de</strong> stuffing bits "1" appear always at Bit number 8, but FAS and BAS appear at any of Bit<br />
mbers 1-7.<br />
3 Restriction against some communication mo<strong>de</strong>s<br />
nce the interworkmg bit rate becomes 56 kbit/s, the transmission mo<strong>de</strong>s using more than 56 kbil/s are forbid<strong>de</strong>n<br />
ceívers ignore these command BAS co<strong>de</strong>s). Facilities using the original sev<strong>en</strong>th sub-channel move to the sixth subannel.<br />
4 Audio command co<strong>de</strong>s (000)<br />
,e following are applicable ínstead of those in Annex A.<br />
¡utral Neutralizad ]-channel, contaíning only FAS and BAS; all other bits are to be Ignored at<br />
the receiver.<br />
i-off, U No audio signal, no framíng; bits 1-7 of the I-channel are avai<strong>la</strong>ble.<br />
i-off, F No audio signal, FAS and BAS in use; 54.4 kbit/s avai<strong>la</strong>ble for use un<strong>de</strong>r other<br />
commands.<br />
<strong>la</strong>w, U7 G.711 audio at 56 bit/s, A-<strong>la</strong>w truncated to 7 bits, no framing (mo<strong>de</strong> OU).<br />
<strong>la</strong>w, F6 G.711 audio at 48 kbit/s, A-<strong>la</strong>w truncated to 6 bits, with FAS and BAS in bit 7.<br />
<strong>la</strong>w, U7 G.711 audio at 56 kbit/s, Ji-<strong>la</strong>w truncated to 7 bits, no framing (mo<strong>de</strong> OU).<br />
|aw, F6 G.711 audio at 48 kbit/s, ]I-<strong>la</strong>w truncated lo 6 bits, with FAS and BAS ín bit 7.<br />
722, U8 Not possíble to transmit 8 bits per octet.<br />
722, U7 G.722 7 kHz audio in bits 1-7, 56 kbít/s (unframed).<br />
722, F6 G.722 7 kHz. audio at 48 kbit/s, in bits 1-6 (mo<strong>de</strong> 3).<br />
1-16 kbit/s Audio at 16 kbit/s to Recomm<strong>en</strong>dation G.728 in bits 1,2 (mo<strong>de</strong> 7).<br />
ther] All other valúes reserved.<br />
Recomm<strong>en</strong>dation H.221 (03/93)
TABLE B.1/H.221<br />
Transmitter of the 64 kbit/s terminal<br />
Bit nuraber<br />
1 2 3 4 5 6 7 (SC) 8<br />
S<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
1<br />
#<br />
1<br />
S<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
1<br />
#<br />
2<br />
S<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
1<br />
#<br />
3<br />
S<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
1<br />
#<br />
4<br />
S<br />
u<br />
b _<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
1<br />
#<br />
5<br />
S<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
1<br />
#<br />
6<br />
FAS<br />
BAS<br />
(ECS)<br />
#<br />
7<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1 Octet number<br />
:<br />
8<br />
9<br />
16<br />
17<br />
:<br />
24<br />
25<br />
NOTE- Cl, C2, C3 and G4 ¡n the FAS arecomputedforthe 160 septets.or 1120 bits.<br />
A frame<br />
structured<br />
by the<br />
56 kbit/s<br />
terminal<br />
Receiver of the 64 kbit/s terminal<br />
Bit number*'<br />
1 2 3 4 5 6 7 8<br />
s<br />
u<br />
b -<br />
c<br />
h a<br />
n<br />
n<br />
e<br />
1<br />
#<br />
3<br />
s<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
1<br />
#<br />
4<br />
s<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
1<br />
#<br />
5<br />
s<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
1<br />
#<br />
6<br />
pb)<br />
A<br />
S<br />
B<br />
A<br />
S<br />
#7<br />
•<br />
80<br />
s<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
I<br />
#<br />
1<br />
*) Synchronized with the octet tirning of the network.<br />
b' FAS may appear at any of Bit number 1-7.<br />
s<br />
u<br />
b<br />
-<br />
c<br />
h<br />
a<br />
n<br />
n<br />
e<br />
!<br />
#<br />
2<br />
Recomm<strong>en</strong>dation H.221 (03/93) 27<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1
INTERNATIONAL TELECOMMUNICATION UNION<br />
CCITT<br />
THE INTERNATIONAL<br />
TELEGRAPH AND TELEPHONE<br />
CONSULTATIVE COMMITTEE<br />
LINE TRANSMISSION<br />
OF NON-TELEPHONE SIGNÁIS<br />
FRAME-SYNCHRONOUS CONTROL<br />
AND INDICATION SIGNALS<br />
FOR AUDIOVISUAL SYSTEMS<br />
Recomm<strong>en</strong>dation H.230<br />
G<strong>en</strong>eva, 1990<br />
H.230
FOREWORD<br />
The CCITT (the International Telegraph and Telephone Consultative Committee) is the perman<strong>en</strong>t organ of<br />
the International Telecommunication Union (ITU). CCITT is responsible for studying technical, operáting and tariff<br />
questions and issuing Recomm<strong>en</strong>dations on them with a view to staudardizing telecomnaunications on a worldwi<strong>de</strong><br />
basis.<br />
The Pl<strong>en</strong>ary Assembly of CCITT which meets every four years, establishes the topics for study and approves<br />
Recomm<strong>en</strong>dations prepared by its Study Groups. The approval of Recomm<strong>en</strong>dations by the members of CCITT<br />
betwe<strong>en</strong> Pl<strong>en</strong>ary Assemblies is covered by the procedure <strong>la</strong>id down in CCITT Resolution No. 2 (Melbourne, 1988).<br />
Recomm<strong>en</strong>dation H.230 was prepared by Study Group XV and was approved un<strong>de</strong>r the Resolution No. 2<br />
procedure on the 14 of December 1990.<br />
CCITT NOTE<br />
In this Recomm<strong>en</strong>dation, the expressíon "Administration" is used for concis<strong>en</strong>ess to indícate both a<br />
telecommunication Administration and a recognized prívate operáting ag<strong>en</strong>cy.<br />
© ITU 1990<br />
All ríghts reserved. No part of this publication may be reproduced or utüized in any fonn or by any means, electronic<br />
or mechanical, including photocopying and microfilm, without permission in writing from the ITU.
Recomm<strong>en</strong>dation H.230<br />
1 Introduction<br />
FRAME-SYNCHRONOUS CONTROL AND INDICATION SIGNALS<br />
FOR AUDIOVISUAL SYSTEMS<br />
Digital audiovisual services are provi<strong>de</strong>d by a transmission system in which the relevant signáis are<br />
multiplexed onto a digital path. In addition to the audio, vi<strong>de</strong>o, user data and telematic information, these signáis<br />
inclu<strong>de</strong> information for the proper functioning of the system. The additional information has be<strong>en</strong> named control and<br />
indication (C&I) to reflect the fact that while some bits are g<strong>en</strong>uinely for "control", causing a state change somewhere<br />
else in the system, others provi<strong>de</strong> for indications to the users as to the functioning of the system.<br />
The C&I may be categorized into three groups:<br />
a) cali control — these are treated in Recomm<strong>en</strong>dations of the Q-Series;<br />
b) transmission frame-synchronous, or otherwise requiring rapid response;<br />
c) confer<strong>en</strong>ce, data, and Telematic control not requiring frame synchronism, governed by the multi<strong>la</strong>yer<br />
protocol (MLP) of Recomm<strong>en</strong>dation H.200/AV.270.<br />
This Recomm<strong>en</strong>dation conceras only those C&I coming in category b) which inclu<strong>de</strong>s a simplified set of<br />
confer<strong>en</strong>ce C&I for multipoint connections of simple termináis.<br />
2 Procedures<br />
There are two procedures: some frame-synchronous C&I are provi<strong>de</strong>d for directly as a bit-rate allocation<br />
signal (BAS) co<strong>de</strong>s in Recomm<strong>en</strong>dation H.221, while the remain<strong>de</strong>r require the use of an escape co<strong>de</strong>.<br />
2.1 C&I co<strong>de</strong>s provi<strong>de</strong>d in Recomm<strong>en</strong>dation H. 221<br />
The foliowing co<strong>de</strong>s, whose functions are <strong>de</strong>fined in section 3, are provi<strong>de</strong>d in Recomm<strong>en</strong>dation H.221:<br />
•— VCF, VCU (procedures for use in multipoint calis according to Recomm<strong>en</strong>dation H.200/AV.243);<br />
— LCV, LCD, LCA, LCO (for maint<strong>en</strong>ance — no standardized procedures).<br />
In each case the co<strong>de</strong> is transmitted in the BAS position at an appropriate time.<br />
2.2 Other C&I co<strong>de</strong>s<br />
All frame-synchronous C&I co<strong>de</strong>s not Usted in section 2.1 are transmitted by a sequ<strong>en</strong>ce involving the BAS<br />
positions in two consecutivo sub-multiframes. In the fírst, the co<strong>de</strong> (111)[10001] is transmitted. In the second, the<br />
co<strong>de</strong> <strong>de</strong>fíned in Table 1/H.230 is transmitted.<br />
It should be noted that only one symbol is transmitted by this method — the co<strong>de</strong> in the subsequ<strong>en</strong>t<br />
sub-multiframe is again treated as a normal BAS co<strong>de</strong>.<br />
3 Befinitions of C&I symbols<br />
The ftill <strong>de</strong>fínitions of these symbols are set out below and co<strong>de</strong> valúes in Table 1/H.230. (The first letter of<br />
the alphabetic co<strong>de</strong>-name indicates the type; the second is C for command, I for indication; the third is for the specific<br />
function.)
3.1 C&I re<strong>la</strong>ted to vi<strong>de</strong>o<br />
VIS Vi<strong>de</strong>o Indícate Suppressed: mis symbol is used to indícate that the cont<strong>en</strong>t of the vi<strong>de</strong>o channel<br />
does not repres<strong>en</strong>t a normal camera image. The vi<strong>de</strong>o <strong>en</strong>co<strong>de</strong>r may be without vi<strong>de</strong>o input or an<br />
electronically-g<strong>en</strong>erated pattefn may nave be<strong>en</strong> substituted.<br />
VÍA Vi<strong>de</strong>o Indícate Active: complem<strong>en</strong>tary to VIS. The vi<strong>de</strong>o source is the only one, or, in the case<br />
that more vi<strong>de</strong>o sources are to be distinguished, it is that <strong>de</strong>signated "vi<strong>de</strong>o No. 1".<br />
VIA2 Equival<strong>en</strong>t to VIA, but <strong>de</strong>signating "vi<strong>de</strong>o No. 2" as the source.<br />
VIA3 Equival<strong>en</strong>t to VTA, but <strong>de</strong>signating "vi<strong>de</strong>o No. 3" as the source.<br />
VIR Vi<strong>de</strong>o Indícate Ready-to-Activate: mis symbol is transmitted by a terminal whose user has <strong>de</strong>ci<strong>de</strong>d<br />
not to s<strong>en</strong>d vi<strong>de</strong>o unless he will also receive vi<strong>de</strong>o from the other <strong>en</strong>d.<br />
VCF Vi<strong>de</strong>o Command "Freeze-Picture Request": this symbol may be transmitted prior to the<br />
"vi<strong>de</strong>o-off" mo<strong>de</strong> switch, to prepare the vi<strong>de</strong>o <strong>de</strong>co<strong>de</strong>r for this ev<strong>en</strong>t. This symbol is also<br />
transmitted by a multipoint control unit (MCU) prior to vi<strong>de</strong>o switching. On receipt, a terminal<br />
vi<strong>de</strong>o <strong>de</strong>co<strong>de</strong>r should complete updating of the curr<strong>en</strong>t vi<strong>de</strong>o frame but subsequ<strong>en</strong>tly disp<strong>la</strong>y the<br />
froz<strong>en</strong> picture until receipt of the freeze-picture reléase control which is embed<strong>de</strong>d in the vi<strong>de</strong>o.<br />
VCU Vi<strong>de</strong>o Command "Fast Update Request": this symbol is transmitted by an MCU after performing a<br />
vi<strong>de</strong>o switch. It may also be transmitted by a terminal at the start of communication wh<strong>en</strong> the vi<strong>de</strong>o<br />
<strong>de</strong>co<strong>de</strong>r is first ready to receive. On receipt, the terminal vi<strong>de</strong>o <strong>en</strong>co<strong>de</strong>r should <strong>en</strong>ter the fast-update<br />
mo<strong>de</strong> at its earliest opportunity.<br />
3.2 C&I re<strong>la</strong>ted to audio<br />
AIM Audio Indícate Muted: this symbol is used to indícate that the cont<strong>en</strong>t of the audio channel<br />
does not repres<strong>en</strong>t a normal audio signa!. The audio <strong>en</strong>co<strong>de</strong>r may be without audio input or an<br />
electronically-g<strong>en</strong>erated tone may have be<strong>en</strong> substituted.<br />
AIA Audio Indícate Active: complem<strong>en</strong>tary to AIM.<br />
3.3 C&I for maim<strong>en</strong>ance parpases<br />
LCV Loopback Command, "Vi<strong>de</strong>o Loop Request": on receipt of this symbol, a terminal must connect<br />
the output of the vi<strong>de</strong>o <strong>de</strong>co<strong>de</strong>r to the input of the vi<strong>de</strong>o <strong>en</strong>co<strong>de</strong>r.<br />
LCD Loopback Command, "Digital Loop Request": on receipt of this symbol, the terminal must<br />
disconnect the output of the multiplexer from the outgoing path, rep<strong>la</strong>cing it with the input to the<br />
<strong>de</strong>multiplexer. In the case of múltiple B or HO connections, loopback is activated in each<br />
connection.<br />
LCA Loopback Command, "Audio Loop Request": on receipt of this symbol, the terminal should if<br />
possible connect the output of the audio <strong>de</strong>co<strong>de</strong>r to the input of the audio <strong>en</strong>co<strong>de</strong>r.<br />
LCO Loopback Command Off: on receipt of this symbol, the terminal must disconnect all loops and<br />
restore audio and data paths to their normal conditíon.
3.4 C&I re<strong>la</strong>ted to simple multipoint confer<strong>en</strong>ces not using MLP<br />
Note — Some of the following co<strong>de</strong>s may be cancelled by transmission of appropríate co<strong>de</strong>s as listed in<br />
Table 1/H.230 but not separately <strong>de</strong>fined here.<br />
MCV Multipoint Command Visualization-Forcing: transmitted by a terminal to forcé an associated MCU<br />
to broadcast its vi<strong>de</strong>o signal. (Used to transmit the picture of a chairman or VIP, alternatively to<br />
hold a picture source during the transmission of graphics.)<br />
MIV Multipoint Indication Visualizaron: transmitted by an MCU to indícate to a terminal that its<br />
vi<strong>de</strong>o signal is being se<strong>en</strong> by other termináis (otherwise known as "On-air" indication).<br />
MCC Multipoint Command Confer<strong>en</strong>ce: transmitted by an MCU to a terminal. The terminal receiving<br />
MCC must make its outgoing transfer rate equal to its incoming transfer rate, and its outgoing<br />
audio rate equal to its incoming audio rate.<br />
Note — The command could also be used to invoke an on-scre<strong>en</strong> user indication.<br />
MCS Multipoint Command Symmetrical Data-transmission: transmitted by an MCU wh<strong>en</strong> setting up<br />
data broadcasting. On receipt, a terminal must prepare itself for data reception and <strong>en</strong>sure, by<br />
mo<strong>de</strong> change if necessary, that its outgoing data channel occupies the same capacity as its incoming<br />
data channel. A terminal in receipt of MCS cannot initiate data broadcasting.<br />
MCN Multipoint Command Negating MCS: transmitted by an MCU at the completion of data<br />
broadcasting. On receipt, a terminal must cióse any outgoing data channel which it has op<strong>en</strong>ed as a<br />
result of the previous reception of MCS. Following the <strong>en</strong>d of data reception and the receipt of<br />
MCN, a terminal is permitted to initiate data broadcasting.<br />
MIL Multipoint Indication Loop: an MCU has had iís ports extemally looped. The topic is for further<br />
study.<br />
MIZ Multipoint Indication Zero-communication: transmitted by an MCU to a terminal for information,<br />
with the meaning that no other termináis are yet connected to the MCU.<br />
MIS Multipoint Indication Secondary-status: transmitted by an MCU to a terminal for information, with<br />
the meaning that since other termináis of higher capability are participating in the confer<strong>en</strong>ce-call,<br />
this terminal will not necessarily receive all the signáis that are s<strong>en</strong>t to those other termináis (see<br />
Recomm<strong>en</strong>dation H.200/AV.243).<br />
MCA Multipoint Command Assign-tok<strong>en</strong>: possession of the tok<strong>en</strong> gives the holding terminal the right to<br />
give the MCU certain cornmands (see Recomm<strong>en</strong>dation H.200/AV.243).<br />
MCT Multipoint Command Tok<strong>en</strong>-c<strong>la</strong>im: s<strong>en</strong>t by a terminal to the MCU. The MCU acce<strong>de</strong>s to this<br />
c<strong>la</strong>im if the tok<strong>en</strong> is unassigned or has be<strong>en</strong> released.<br />
MCR Multipoint Command Reléase-tok<strong>en</strong>: s<strong>en</strong>t to the MCU by the terminal holding the tok<strong>en</strong> to give the<br />
MCU the authority to reassign the tok<strong>en</strong> to another terminal wh<strong>en</strong>/if it receives MCT.
4 Requirem<strong>en</strong>ts for C&I<br />
The C&I functions are <strong>de</strong>fined such that, un<strong>de</strong>r various appropnate circumstances, the audiovisual system<br />
will opérate in a fauH-free manner and also such that sympathetic pres<strong>en</strong>tation to users is possible. Some functions<br />
must therefore be mandatory, others optional. This section, together with the categorization in Table 1/H.230,<br />
c<strong>la</strong>rines the circumstances un<strong>de</strong>r which C&I functions are mandatory.<br />
CM <strong>de</strong>notes "conditionally mandatory": if the terminal (or MCU) is capable of <strong>en</strong>tering the giv<strong>en</strong> state,<br />
th<strong>en</strong> it must transmit the giv<strong>en</strong> co<strong>de</strong> and, wh<strong>en</strong> leaving that state, the complem<strong>en</strong>tary co<strong>de</strong>. If it has<br />
no such capabiiity it can ignore both.<br />
M <strong>de</strong>notes "mandatory" for all equipm<strong>en</strong>ts of either terminal or MCU type.<br />
X <strong>de</strong>notes "non-mandatory": on receipt of such a co<strong>de</strong>, it may be unrecognized, or recognized but<br />
not acted upon, or recognized and acted upon, <strong>en</strong>tirely at the discretion of the manufacturer or<br />
NA <strong>de</strong>notes that the co<strong>de</strong> is not applicable in that case.<br />
It will be noted that there are only a few mandatory requirem<strong>en</strong>ts on most terruinals. All audiovisual<br />
termináis must recognize and obey the command to make or break the digital loopback, and vi<strong>de</strong>o loopback if they<br />
have vi<strong>de</strong>o capabiiity. All termináis having a vi<strong>de</strong>o capabiiity must also obey fast-update, freeze-picture, and<br />
MCS/MCN, otherwise there will be system misoperation on a multipoint cali.
TABLE 1/H.230<br />
Co<strong>de</strong><br />
First 3 bits<br />
(000)<br />
(001)<br />
Last 5 bits in<br />
<strong>de</strong>cimal form<br />
[0,1]<br />
[2]<br />
[3]<br />
[4-15]<br />
[16]<br />
[17]<br />
[18]<br />
[19]<br />
[20-30]<br />
[31]<br />
[0]<br />
[1]<br />
[2]<br />
[3]<br />
[4]<br />
[5]<br />
[6,7]<br />
[8]<br />
[9]<br />
[10]<br />
[11-15]<br />
[16]<br />
[17]<br />
[18]<br />
[19]<br />
[20]<br />
[21]<br />
[22-30]<br />
[31]<br />
Valué<br />
Reserved<br />
AIM<br />
AIA<br />
Reserved<br />
VIS<br />
VÍA<br />
VIA2<br />
VIA3<br />
Reserved<br />
VIR<br />
MCC<br />
Cancel-MCC<br />
MIZ<br />
Cancel-MIZ<br />
MIS<br />
Cancel-MIS<br />
Reserved<br />
MCT<br />
MCR<br />
MCA<br />
Reserved<br />
MCV<br />
Cancel-MCV<br />
MIV<br />
Cancel-MIV<br />
MCS<br />
MCN<br />
Reserved<br />
MIL<br />
Transmit<br />
Terminal<br />
CM<br />
CM<br />
CM<br />
CM<br />
X<br />
X<br />
X<br />
NA<br />
NA<br />
NA<br />
NA<br />
NA<br />
NA<br />
X<br />
X<br />
X<br />
X<br />
X<br />
NA<br />
NA<br />
NA<br />
NA<br />
NA<br />
MCU<br />
CM<br />
CM<br />
CM<br />
CM<br />
NA<br />
NA<br />
NA<br />
M<br />
M<br />
M<br />
M<br />
M<br />
M<br />
NA<br />
NA<br />
NA<br />
NA<br />
NA<br />
M<br />
M<br />
M<br />
M<br />
NA<br />
Receive<br />
Terminal<br />
X<br />
X<br />
X<br />
X<br />
X<br />
X<br />
X<br />
M<br />
M<br />
X<br />
X<br />
X<br />
X<br />
NA<br />
NA<br />
NA<br />
NA<br />
NA<br />
X<br />
X<br />
M<br />
M<br />
NA<br />
MCU<br />
X<br />
X<br />
X<br />
X<br />
X<br />
X<br />
NA<br />
NA<br />
NA<br />
NA<br />
NA<br />
NA<br />
NA<br />
M<br />
M<br />
M<br />
M<br />
M<br />
NA<br />
NA<br />
NA<br />
NA<br />
M<br />
Refer<strong>en</strong>ce for<br />
procedures<br />
Section 3.2<br />
Section3.1<br />
Section 3.1<br />
H.320/AV.312<br />
H.320/AV.312<br />
H.320<br />
H.200/AV.243<br />
H.200/AV.243<br />
H.200/AV.243<br />
H.200/AV.243<br />
H.200/AV.243<br />
H.200/AV.243<br />
H.200/AV.243<br />
H.200/AV.243<br />
H.200/AV.243<br />
H.200/AV.243<br />
H.200/AV.243<br />
H.200/AV.243<br />
H.200/AV.243<br />
H.200/AV.243<br />
H.200/AV.243<br />
(111) All co<strong>de</strong>s forbid<strong>de</strong>n<br />
Co<strong>de</strong> valúes listed<br />
in Recomm<strong>en</strong>dation H.221,<br />
Annex A<br />
VCF<br />
VCU<br />
LCV<br />
LCA<br />
LCD<br />
LCO<br />
X<br />
X<br />
NA<br />
NA<br />
NA<br />
NA<br />
M<br />
M<br />
NA<br />
NA<br />
NA<br />
NA<br />
M<br />
M<br />
CM<br />
X<br />
M<br />
M<br />
NA<br />
NA<br />
NA<br />
X<br />
X<br />
X<br />
H.221<br />
H.221<br />
H.221<br />
H.221<br />
H.221<br />
H.221
INTERNATIONAL TELECOMMUNICATION UNION<br />
ITU-T H.242<br />
TELECOMMUNICATION (03/93)<br />
STANDARDIZARON SECTOR<br />
OF ITU<br />
LINE TRANSMISSION OF NON-TELEPHONE<br />
SIGNÁIS<br />
SYSTEM FOR ESTABLISHING<br />
COMMUNICATION BETWEEN AUDIOVISUAL<br />
TERMINALS USING DIGITAL CHANNELS<br />
UP TO 2 Mbit/s<br />
ITU-T Recomm<strong>en</strong>dation H.242<br />
(Previously "CCITT Recomm<strong>en</strong>dation")
FOREWORD<br />
Che ITU Telecommunication Standardizatíon Sector (ITU-TJ is a perman<strong>en</strong>t organ of the International Telecomnunication<br />
Union. The ITU-T ís responsible for studying technical, operating and tariff questions and issuing<br />
ilecomm<strong>en</strong>dations on them with a view to standardizing telecommunications on a worldwi<strong>de</strong> basis.<br />
rhe World Telecommunication Standardizaron Confer<strong>en</strong>ce (WTSC), which meets every four years, established the<br />
opics for study by the ITU-T Study Groups which, in their turn, produce Recomm<strong>en</strong>dations on these topics.<br />
TU-T Recomm<strong>en</strong>dation H.242 was revísed by the'ITU-T Study Group XV (1988-1993) and was approved by the<br />
VTSC (Helsinki, March 1-12, 1993).<br />
NOTES<br />
As a consequ<strong>en</strong>ce of a reform process withín the International Telecommunication Union (ITU), the CCITT<br />
eased to exist as of 28 February 1993. In íts p<strong>la</strong>ce, the ITU Telecommunication Standardízation Sector (ITU-T) was<br />
reated as of 1 March 1993. Simi<strong>la</strong>rly, in this reform process, the CCIR and the IFRB have be<strong>en</strong> rep<strong>la</strong>ced by the<br />
Ladiocommunication Sector.<br />
El or<strong>de</strong>r not to <strong>de</strong><strong>la</strong>y publícation of this Recomm<strong>en</strong>dation, no change has be<strong>en</strong> ma<strong>de</strong> in the texí to refer<strong>en</strong>ces containing<br />
le acronyms "CCITT, CCIR or IFRB" or their associated <strong>en</strong>tities such as Pl<strong>en</strong>ary Assembly, Secretariat, ele. Future<br />
ditions of this Recomm<strong>en</strong>dation will contain the proper terminology re<strong>la</strong>ted to the new ITU structure.<br />
In this Recomm<strong>en</strong>dation, the expression "Administration" is used for concis<strong>en</strong>ess to indícate both a<br />
ilecommunication administration and a recognized operating ag<strong>en</strong>cy.<br />
© ITU 1994<br />
11 rights reserved. No part of this publication may be reproduced or utilized in any form or by any ineans, electronic or
CONTENTS<br />
Introduction 1<br />
Terminal capabilíties ,. 1<br />
2.1 Audio capabilities „ 2<br />
2.2 Vi<strong>de</strong>o capabilities 2<br />
2.3 Transfer rate capabilities 2<br />
2.4 Data capabilities , 2<br />
2.5 Termináis on restricted networks: capability 2<br />
2.6 Encryption and ext<strong>en</strong>síon-BAS capabilities........... 3<br />
Transmission 3<br />
3.1 Transmission mo<strong>de</strong>s 3<br />
3.2 Establishm<strong>en</strong>t of compatible mo<strong>de</strong>s of operatíon 3<br />
Frame structure , 3<br />
Basic sequ<strong>en</strong>ces for in-channel procedures ., 4<br />
5.1 Capability exchange sequ<strong>en</strong>ce A 4<br />
5.2 Mo<strong>de</strong> switching sequ<strong>en</strong>ce B 5<br />
5.3 Frame reinstatem<strong>en</strong>t sequ<strong>en</strong>ce C 5<br />
Mo<strong>de</strong> inítialízation, dynamic mo<strong>de</strong> switching and mo<strong>de</strong> O forcíng 7<br />
6.1 Mo<strong>de</strong> ínitialization procedure 7<br />
6.2 Dynamic mo<strong>de</strong> switching...... 9<br />
6.3 Mo<strong>de</strong> O forcing procedure , 9<br />
6.4 Mo<strong>de</strong> mismatch recovery procedure , 12<br />
Recovery from fault conditions , 12<br />
7.1 Unexpected loss of synchronízation or frame alignm<strong>en</strong>t 12<br />
7.2 Recovery from loss of connection(s) 13<br />
Network consi<strong>de</strong>ration: cali connection, disconnection and calí transfer....... 14<br />
8.1 Cali connection............. 14<br />
8.2 Terminal disconnection ., 15<br />
8.3 Cali transfer 15<br />
8.4 Confer<strong>en</strong>cing , ,. 15<br />
8.5 PCM format conversión 15<br />
Procedure for activation and <strong>de</strong>-activation of data channels 16<br />
9.1 Data equipm<strong>en</strong>t not conforming to Recomm<strong>en</strong>dation H.200/AV.270 16<br />
9.2 Equipm<strong>en</strong>t operating with an MLP according to Recomm<strong>en</strong>dation H.200/AV.270 16<br />
9.3 Simultaneous transmission of low-speed data and MLP 16<br />
Procedures for operation of termináis in restricted networks 17<br />
10.1 Network aspects ., 17<br />
10.2 Refer<strong>en</strong>ce connections 17<br />
10.3 Transmission formats .• 17<br />
10.4 Interworking betwe<strong>en</strong> 56 kbit/s and 64 kbií/s termináis 19<br />
10.5 Interworking betwe<strong>en</strong> HQ orHn termináis in restricted and unrestricted networks 19<br />
Procedure for use of BAS-ext<strong>en</strong>sion co<strong>de</strong>s 19<br />
Bit occupancy and the sequ<strong>en</strong>cing of BAS co<strong>de</strong>s 20<br />
Procedure for <strong>de</strong>aling with 6B-Ho ínterconnection 22<br />
Page
¿Vnnex A — I<strong>de</strong>ntification of the <strong>en</strong>d of sequ<strong>en</strong>ce A , , , 23<br />
(\pp<strong>en</strong>dix I — Initialization: Case of vi<strong>de</strong>ophone-to Recomm<strong>en</strong>dation H.320, type Xb2/s 24<br />
íVpp<strong>en</strong>dix II — Mo<strong>de</strong>-0 forcing: Case of vi<strong>de</strong>ophone to Recomm<strong>en</strong>dation H.320, type Xbj/3 „ 27<br />
\pp<strong>en</strong>dix III - Example of use of message structure , 28<br />
111.1 Initial capability exchange, including MBE-cap 28<br />
111.2 Subsequ<strong>en</strong>t capability exchange, including MBE capabílíty message 29<br />
111.3 Mo<strong>de</strong> switch to non-standard mo<strong>de</strong> using MBE command , 29<br />
\pp<strong>en</strong>dix IV — Examples of symmetrical and unsymmetricaí transmission mo<strong>de</strong>s , 30<br />
IV. 1 Example of symmetrical transmission mo<strong>de</strong> 30<br />
IV.2 Example of unsymmetricaí transmission mo<strong>de</strong> 30<br />
Vpp<strong>en</strong>dix V — Examples re<strong>la</strong>ting to data transmissíons 30<br />
V.l Transfer-rate IB, audio at 48 kbit/s, no vi<strong>de</strong>o or vi<strong>de</strong>o off 30<br />
V.2 Transfer-rate IB, audio at 16 kbit/s, no vi<strong>de</strong>o or vi<strong>de</strong>o off.... 31<br />
V.3 Transfer-rate IB, audio at 16 kbit/s, vi<strong>de</strong>o on 31<br />
V.4 Transfer-rate 2B, audio at 48 kbit/s, vi<strong>de</strong>o on....... 31<br />
V.5 Transfer-rate 2B, audio at 16 kbit/s, vi<strong>de</strong>o on 31<br />
Vpp<strong>en</strong>dix VI - Hierarchical capability BAS co<strong>de</strong>s....... 32<br />
\.pp<strong>en</strong>dix VII - Interpretation of received audio capability BAS co<strong>de</strong>s 32<br />
^.pp<strong>en</strong>dix VIII — Examples of legal and illegal capability BAS sequ<strong>en</strong>ces 33<br />
Page
lecomm<strong>en</strong>dation H.242<br />
[ Introduction<br />
SYSTEM FOR ESTABLISHING COMMÜNICATION<br />
BETWEEN AUDIOVISUAL TERMINALS<br />
ÜSING DIGITAL CHANNELS ÜP TO 2 Mbit/s<br />
(G<strong>en</strong>ova, 1990; revised at Helsinki, 1993)<br />
^his Recomm<strong>en</strong>dation should be associated with Recomm<strong>en</strong>dations G.725 (System aspects for the use of the 7 kHz<br />
udio co<strong>de</strong>e within 64 kbit/s), H.221 (Frame structure for 64 to 1920 kbít/s channels in audiovisual teleservices)<br />
nd H.230 (Frame-synchronous control and indication signáis for audiovisual systems).<br />
L number of applications utilizing narrow (3 kHz) and wi<strong>de</strong>band (7 kHz) speech together with vi<strong>de</strong>o and/or data have<br />
ic<strong>en</strong> i<strong>de</strong>ntified, including high quality telephony, audio and vi<strong>de</strong>oconfer<strong>en</strong>cing (with or without various kinds of<br />
elematic aids), audíographic confer<strong>en</strong>cing and so on. More applications will undoubtedly emerge in the future.<br />
'o proví<strong>de</strong> these services, a scheme ís recomm<strong>en</strong><strong>de</strong>d ín which a channel accommodates speech, and optionally vi<strong>de</strong>o<br />
nd/or data at several rates, in a number of díffer<strong>en</strong>t mo<strong>de</strong>s. Signalling procedures are required to establish a<br />
ompatible mo<strong>de</strong> upon cali set-up, to switch betwe<strong>en</strong> mo<strong>de</strong>s during a cali and to allow for cali transfer.<br />
lome services wíll require only a single channel, which could according to the procedures in this Recomm<strong>en</strong>dation be<br />
! (64 kbit/s), H0 (384 kbit/s), Hn (1536 kbit/s) or H12 (1920 kbit/s). Other services will require the establishm<strong>en</strong>t of<br />
tvo or more connections providíng B or HQ channels: in such cases the first established is called hereafter the initial<br />
hannel while the others are called addítíonal channels, Unless otherwise specified, all refer<strong>en</strong>ces to frame alignm<strong>en</strong>t<br />
ignal (FAS), bit rate allocation signal (BAS) and service channel (SC) refer to the initial channel or, in the case of a<br />
ígher-or<strong>de</strong>r channel, to the time-slot No. 1 of this channel.<br />
Lll audio and audiovisual termináis using G.722 audio coding and/or G.711 speech coding or other standardizad audio<br />
odings at lower bit rales should be compatible to permit connection betwe<strong>en</strong> any two termináis. This implies that a<br />
ommon mo<strong>de</strong> of operation has to be established for the cali. The initial mo<strong>de</strong> might be the only one used during a cali<br />
r, altematively, switching to another mo<strong>de</strong> can occur as nee<strong>de</strong>d <strong>de</strong>p<strong>en</strong>ding on the capabilities of the termináis. Thus,<br />
or these termináis an in-channel procedure for dynamic mo<strong>de</strong> switching is required,<br />
'he followíng c<strong>la</strong>uses <strong>de</strong>velop these consi<strong>de</strong>rations and <strong>de</strong>scribe recomm<strong>en</strong><strong>de</strong>d in-channel procedures.<br />
Terminal capabilities<br />
'he procedures in this Recomm<strong>en</strong>dation are mt<strong>en</strong><strong>de</strong>d to <strong>en</strong>sure that only those signáis are transmitted which can be<br />
;ceived and appropriately treated by the remote terminal, without ambiguity. This requíres that the capabilities of each<br />
¡rminal to receive and <strong>de</strong>co<strong>de</strong> be known to the other terminal. Some capabilities are <strong>de</strong>fined with a hierarchical<br />
Eructure; a terminal with capability valué N is th<strong>en</strong> also capable of all lower valúes. Where there Ís no hierarchy, th<strong>en</strong><br />
¡vo or more co<strong>de</strong>s of the same type may have to be transmitted in successive frames.<br />
'he following subc<strong>la</strong>uses <strong>de</strong>fine audio, ví<strong>de</strong>o, transfer rate, and data rate capabilities of a terminal. It Ís not necessary<br />
<strong>la</strong>t a terminal un<strong>de</strong>rstand or store all íncoming capabilities. Those which are not un<strong>de</strong>rstood, or which cannot be used<br />
secause the terminal has no means to transmít corresponding information), can be ignored.
'he total capability of a terminal to receive and <strong>de</strong>co<strong>de</strong> various signáis is ma<strong>de</strong> known to the other terminal by<br />
ransmission (see 5.1) of its capability set, consisting of the BAS-capability marker followed by all of the curr<strong>en</strong>t<br />
apabilities. The co<strong>de</strong>s are specified in Annex A/H.221; Table 1 (see 12) summarizes the capabilities which may be<br />
aclu<strong>de</strong>d in a valid set. The transmission or<strong>de</strong>r is immaterial with the exception that ví<strong>de</strong>o picture format valúes must<br />
e followed by mínimum picture interval valúes.<br />
NOTE - G.725 termináis s<strong>en</strong>d only a single capability valué without a marker. The valué is valid only if repeated at least<br />
nce: this may be used to í<strong>de</strong>ntify a G.725 terminal. Having so i<strong>de</strong>ntified, the H.242 terminal should foliow the procedures of<br />
¡ecomm<strong>en</strong>dation G.725.<br />
,.1 Audio capabilities<br />
Ludio capability valúes are <strong>de</strong>fined in Annex A/H.221.<br />
ill audiovisual termináis int<strong>en</strong><strong>de</strong>d for interregional operation should be capable of transmítting and receivlng<br />
i- and jl-<strong>la</strong>w G.711.<br />
formally, it is not necessary to transmit G.711 capabilities in a set containing other audio capabilities. Inclusión of just<br />
ne valué (A or (I) must be interpreted as a request not to s<strong>en</strong>d audio <strong>en</strong>co<strong>de</strong>d signáis to the other <strong>la</strong>W (see 6.3.1).<br />
.2 Vi<strong>de</strong>o capabilities<br />
'i<strong>de</strong>o capabilities are <strong>de</strong>fined in Recomm<strong>en</strong>daííon H.221, including:<br />
— picture format: quarter-CIF, or both quarter-CIF and CIF;<br />
minimum picture interval (MPI): 1/29.97, 2/29.97, 3/29.97, 4/29.97 seconds.<br />
he quarter-CIF valué must be followed by one MPI valué. The full-CIF valué must be followed by two MPI valúes, the<br />
rst applicable to quarter-CIF and the other to CIF.<br />
.3 Transfer rate capabilities<br />
ransfer-rate capabilities are <strong>de</strong>fined in Recomm<strong>en</strong>dation H.221.<br />
he capability to receive a giv<strong>en</strong> number of múltiple 64 kbit/s channels inclu<strong>de</strong>s the capability to receive fewer 64 kbit/s<br />
tiannels. Simí<strong>la</strong>rly, the capability to receive a gív<strong>en</strong> number of H0 channels inclu<strong>de</strong>s the capability to receive fewer<br />
'.Q channels. In both cases the receiving terminal will synchronize the connected additional channels to the initial<br />
nannel and maintain that synchronism throughout the period of connection.<br />
11 other ranges of capability must be sígnalled by inclusión ín the capability seí of more than one transfer rate<br />
ipability co<strong>de</strong>. For example, a terminal may list its transfer-rate capabilities as [2B and HQ and Hj \d H¡2]; in this<br />
tse IB capability is also ímplied.<br />
,4 Data capabilities<br />
ata capabilities are <strong>de</strong>fined Ín Recomm<strong>en</strong>dation H.221.<br />
a terminal is able to accept more than one data rate of whatever type (LSD, HSD, MLP, H-MLP), th<strong>en</strong> all relevant<br />
ilues must be inclu<strong>de</strong>d in the capability sel. Statem<strong>en</strong>t of one valué does not inclu<strong>de</strong> any other valúes.<br />
,5 Termináis on restricted networks: capability<br />
terminal connected to a network whose B-channels are effectively restricted to p X 56 kbit/s (p = J to 6), or whose<br />
Lannels at HQ or higher are restricted by ones-<strong>de</strong>nsity consi<strong>de</strong>rations, must <strong>de</strong>c<strong>la</strong>re the capability valué (100) [22] as<br />
v<strong>en</strong> in Recomm<strong>en</strong>dation H.221. All termináis int<strong>en</strong><strong>de</strong>d for interworking with termináis on restricted networks must<br />
tve the cauabilitv to resuond to this co<strong>de</strong> accordíne to Annex B.
1.6 Encryption and ext<strong>en</strong>sion-BAS capabilities<br />
lie capabilities are <strong>de</strong>fined in Recomm<strong>en</strong>dation H.221.<br />
I Transmission<br />
1.1 Transmission mo<strong>de</strong>s<br />
Ludio mo<strong>de</strong>s of operation are <strong>de</strong>fíned in Annex A/H.221 audio commands.<br />
!or analogue telephone termináis, it may be assumed that the speech sígnal is converted to PCM to G.711 at a digital<br />
etwork interface. These termináis are viewed as workíng in mo<strong>de</strong> OU wh<strong>en</strong> connected to wi<strong>de</strong>band speech termináis.<br />
'he ví<strong>de</strong>o transmission is govemed by the vi<strong>de</strong>o-on and vi<strong>de</strong>o-off commands. Wh<strong>en</strong> switched on, the vi<strong>de</strong>o signal<br />
ccupies all of the capacily, both in the initial channel and in any additional channels, whích is not specifícally<br />
Ilocated to other signáis by other commands. Thus differ<strong>en</strong>t vi<strong>de</strong>o bit rates will result from audio, transfer-rate, ECS<br />
nd data commands, the resultant vi<strong>de</strong>o bit rate being: [transfer rate, less audio rate, less data rate if pres<strong>en</strong>t, less<br />
ncryption control channel if pres<strong>en</strong>t, less FAS and BAS in all the channels/tíme-slots where they are pres<strong>en</strong>t}.<br />
'ransfer-rate mo<strong>de</strong>s are <strong>de</strong>fined in Recomm<strong>en</strong>dation H.221, and specify the total capacity of íhe communication<br />
ffectíve in the following the BAS command sub-multiframe.<br />
<strong>la</strong>ta mo<strong>de</strong>s are <strong>de</strong>fined in Recomm<strong>en</strong>dation H.221, and specify only the bit rate and bit positíons used for a user data<br />
ígnal. The protocol used for data applications is <strong>de</strong>fined by the termináis, but see also 9.<br />
.2 Establishm<strong>en</strong>t of compatible mo<strong>de</strong>s of operation<br />
,t the beginníng of the communication phase of a cali, all termináis start to work in mo<strong>de</strong> OF (outgoing signal<br />
•amed). Termináis other than those limited to G.71 1 capability will th<strong>en</strong> begin an initialization procedure.<br />
his procedure (further <strong>de</strong>scribed in 6) consists of;<br />
— the transmission of Information conceming the capabilities of the respective termináis for receiving and<br />
<strong>de</strong>coding audio, vi<strong>de</strong>o, transfer rate, data rates and other capabilities;<br />
— the <strong>de</strong>termination of a suitable transmission mo<strong>de</strong>, consist<strong>en</strong>t with the known capabilities of both<br />
termináis. An example is giv<strong>en</strong> ín IV.1, in which the transmission mo<strong>de</strong> is the same in both dírections,<br />
but the H.242 procedures are equally applicable to systems in which asymmetric bidirectional<br />
communication is optimal (examples are surveil<strong>la</strong>nce — see IV.2 — and retrieval servicesj;<br />
— switching to thís mo<strong>de</strong>; and establishing additional channels if relevant.<br />
he termináis connected to a cali may change during the cali. This may require re-initialization in or<strong>de</strong>r to i<strong>de</strong>ntify the<br />
rminal type and to re-establish the <strong>de</strong>sired mo<strong>de</strong> of operation. In panicu<strong>la</strong>r, this feature is used in mo<strong>de</strong> O forcing,<br />
hich is necessary Ín the case of a cali transfer (see 8).<br />
Frame structure<br />
ti e frame structure <strong>de</strong>scribed in Recomm<strong>en</strong>dation H.221 is used for mo<strong>de</strong> initialization and dynamic mo<strong>de</strong> switching<br />
ee the following subc<strong>la</strong>uses) and more g<strong>en</strong>erally to <strong>de</strong>fine the multiplex of the various bit streams (audio, ví<strong>de</strong>o, data,<br />
icryption control signal, frame structure) wíthin the frame.<br />
ecomm<strong>en</strong>dation H.221 <strong>de</strong>fines a bit rate allocation signal (BAS) whích is used ínter alia to allocate sub-channels and<br />
indícate the coding algorithm(s).<br />
A.S co<strong>de</strong>s are c<strong>la</strong>ssified by the valué of the first three bits which repres<strong>en</strong>t the BAS attribute; each attribute may<br />
e.refore have, un to 32 <strong>de</strong>fíned valúes.
ourBAS attríbutes are commands: they <strong>de</strong>fine the multiplex within the next and following sub-multiframes, as well as<br />
ludio coding algoríthm, and therefore command the distant receiver to treat the signáis accordingly. The four attributes<br />
iré ín<strong>de</strong>p<strong>en</strong><strong>de</strong>nt; that is, a valué of one attribute does not modify that of another.<br />
i-urther BAS attributes are <strong>de</strong>fined to signal terminal capabilities to the distant terminal. Wh<strong>en</strong> received, these<br />
tttributes do not directly affect the curr<strong>en</strong>t transmission mo<strong>de</strong>, However, they may lead to the ínitiation of a specific<br />
tctíon to be carried out by the terminal, Thís feature is utilized in the mo<strong>de</strong> initialization procedure and in the mo<strong>de</strong> O<br />
'orcing procedure (see 6).<br />
rhe third bit of the H.221 frame alignm<strong>en</strong>t signal (FAS) in odd frames of the initíal channel, called the A-bit, is set to 1<br />
m loss of frame or multiframe alignm<strong>en</strong>t, and is set to O on acquiring both frame and multiframe alignm<strong>en</strong>t (see Note).<br />
Üonsequ<strong>en</strong>tly, a terminal which is receiving a framed signal with the A-bit set to O can assume that the distant terminal<br />
s able to act upon a change of BAS.<br />
NOTE — A terminal having capabilities only for single-channel workíng, and wíthout <strong>en</strong>crypnon capabilíty, does not need<br />
Q seek and gain multíframe alignm<strong>en</strong>t since the <strong>la</strong>tter serves for numbering and synchronizing múltiple channels.<br />
í Basic sequ<strong>en</strong>ces for in-channel procedures<br />
'hree sígnalling sequ<strong>en</strong>ces are <strong>de</strong>fined in this c<strong>la</strong>use. These sequ<strong>en</strong>ces are used as the building blocks for the<br />
rocedures <strong>de</strong>fined in 6 and 7.<br />
.1 Capability exchange sequ<strong>en</strong>ce A<br />
'he capability exchange sequ<strong>en</strong>ce forces framing in both dírections of transmission and the exchange of terminal<br />
apability co<strong>de</strong>s. Either terminal may initiate the sequ<strong>en</strong>ce and there is no problem caused by boíh doing so<br />
ímultaneously or nearly simultaneously. Capability BAS should not be s<strong>en</strong>t unnecessarily wh<strong>en</strong> the incoming signal is<br />
nframed.<br />
'he terminal X which initíates the capability exchange sequ<strong>en</strong>ce must fírst reínsíate framing by using sequ<strong>en</strong>ce C<br />
¡ee 5.3) if previously transmitting unframed; ít th<strong>en</strong> sets a timer TI (valué 10 seconds) and transmits its curr<strong>en</strong>t<br />
apability set (see 2) repetitively, or at least one complete set followed by the marker co<strong>de</strong> (to indícate completion of<br />
le set); these capabilities will be one or more of the set Usted in Table 1.<br />
/"h<strong>en</strong> Y first <strong>de</strong>tects any incoming capability co<strong>de</strong> except neutral (see 5.3), it begins transmission of its own set of<br />
ipability co<strong>de</strong>s. This, of course, requires switching to a framed mo<strong>de</strong> if transmission had be<strong>en</strong> unframed. To <strong>en</strong>sure<br />
<strong>la</strong>t each receives the complete set of capabilities of the other, they must continué repetitive transmission beyond the<br />
me they <strong>de</strong>tect incoming A = O by at least one complete set and the marker co<strong>de</strong>.<br />
NOTE-See Note on G.725 termináis in 2.<br />
here are three possible outcomes:<br />
Outcome I: Within the timer expiratíon period, multiframe alignm<strong>en</strong>t has be<strong>en</strong> gained, the A bit is<br />
received with a valué of zero and the complete set of capability BAS co<strong>de</strong>s of the distant<br />
terminal has be<strong>en</strong> validated. In this case the sequ<strong>en</strong>ce ¡s completed successfully.<br />
NOTE 1 - If sequ<strong>en</strong>ce A ís inítiated whíle incoming A = O, repetítion of the set is not necessary.<br />
Outcome II: The timer has expíred without multiframe alignm<strong>en</strong>t. In this case, the sequ<strong>en</strong>ce failed,<br />
NOTE 2 — Thís Ís the expected case of connection to a PCM tclephony terminal, so the<br />
communícation should proceed verbally from here.<br />
Outcome III: The timer has expired with multiframe alignm<strong>en</strong>t achieved, but without either the validation of<br />
the A bit as O or the receiving of the complete set of the distant terminal1 s capability BAS co<strong>de</strong>s<br />
(or both). In this case, the sequ<strong>en</strong>ce is restarted. Outcome III should be notífíed to the user as a<br />
pot<strong>en</strong>tial fault condition (whích might, however, be in the remote terminal).<br />
E any time during a cali, the terminal can initiate sequ<strong>en</strong>ce A, which may inclu<strong>de</strong> a capability set díffer<strong>en</strong>t from the<br />
¡e used at the cali setup for changing communication mo<strong>de</strong>s (e.g, from mo<strong>de</strong> a0 to mo<strong>de</strong> bj, from mo<strong>de</strong> b2 to mo<strong>de</strong> a¡<br />
per Recomm<strong>en</strong>dation H.320). Wh<strong>en</strong> a terminal has received such a capability set of the remote terminal during the
it shall respond by s<strong>en</strong>ding its own capability set, but the set need not be changed in response to the remote terminal's<br />
iew capability.<br />
ÍVh<strong>en</strong> a terminal activates sequ<strong>en</strong>ce A during a cali, it must maintain the curr<strong>en</strong>t mo<strong>de</strong> of multimedia multiplexing,<br />
¡ncluding FAS and BAS ín additional channels if relevant.<br />
Dscil<strong>la</strong>tion of the capability exchange can be avoi<strong>de</strong>d by the arrangem<strong>en</strong>ts to i<strong>de</strong>ntify the <strong>en</strong>d of sequ<strong>en</strong>ce A as giv<strong>en</strong> in<br />
Annex A.<br />
5.2 Mo<strong>de</strong> switching sequ<strong>en</strong>ce B<br />
Mo<strong>de</strong> switching is performed using BAS command co<strong>de</strong>s, each being effective from the begínning of the ev<strong>en</strong> frame<br />
following the sub-multiframe in which the co<strong>de</strong> is first transmitted. Mo<strong>de</strong> switching is possible at any time during a<br />
iommunication, after the initialization procedure has be<strong>en</strong> completed.<br />
Wh<strong>en</strong> the transmitting terminal signáis the mo<strong>de</strong> of operation, this is valid from the next sub-multiframe. It is ess<strong>en</strong>tial<br />
:o note that transmitted signáis must always be in accordance with the known capabilhies of the remote terminal to<br />
receive and <strong>de</strong>co<strong>de</strong>; in the abs<strong>en</strong>ce of such knowledge, only mo<strong>de</strong> OF or OU (audio to Recomm<strong>en</strong>dation G.711) may be<br />
¡<strong>en</strong>t. If a change of capability, indicated in performing sequ<strong>en</strong>ce A, has the result that the curr<strong>en</strong>í mo<strong>de</strong> is no longer<br />
•eceivable/<strong>de</strong>codable, there must be a swltch as soon as possible to a mo<strong>de</strong> which can be received and <strong>de</strong>co<strong>de</strong>d.<br />
3AS commands other than <strong>de</strong>fault ones (IB transfer rate, A/JI <strong>la</strong>w audio, vi<strong>de</strong>o off, etc. in Table 2) shall not be<br />
;ransmitted before the sequ<strong>en</strong>ce A is finished at the start of the communication.<br />
BAS commands whích exceed the curr<strong>en</strong>t transmission capacity must not be transmitted (e.g. transmission of 2B<br />
;ransfer rate command before the second channel is established).<br />
rhe receiving terminal <strong>de</strong>co<strong>de</strong>s and valídales the BAS co<strong>de</strong>, and switches its receive mo<strong>de</strong> of operation accordingly. If<br />
for any reason a terminal receives a BAS command it cannot obey, a mo<strong>de</strong> mísmatch may result (see 6.3).<br />
ji addition to switching of the audio mo<strong>de</strong>, mo<strong>de</strong> switching inclu<strong>de</strong>s turning vi<strong>de</strong>o off or on; the adoption/cessaüon of<br />
ise of additional channels; the op<strong>en</strong>ing/closing of the <strong>en</strong>cryption control channel; the op<strong>en</strong>ing/closing of a data<br />
¡hannel.<br />
rhe mo<strong>de</strong> switching is in principie performed in<strong>de</strong>p<strong>en</strong><strong>de</strong>níly for the two transmission directions; some applications may<br />
)e fundam<strong>en</strong>tal!y asymmetric. For conversational servíces the terminal procedures will g<strong>en</strong>erally be such as to proví<strong>de</strong><br />
lymmetrical transmission, though this is not mandatory (see Notes 1, 2).<br />
NOTES<br />
1 See App<strong>en</strong>dix IV for some examples of symmetrical and asymmetrical transmission mo<strong>de</strong>s.<br />
2 Desígn of H.221/H.242 equipm<strong>en</strong>t should avoíd any ¡nsist<strong>en</strong>ce on symmetry, though H.320 poirits in that direction.<br />
t is for the termináis to talce such <strong>de</strong>císíons at theír servíce/application <strong>la</strong>yer. If a terminal supplier or user wíshes that his terminal<br />
idopt the same mo<strong>de</strong> as the incoming signal, the interna! software may do this without further recourse to standards, There is an<br />
ibvious risk that two such termináis remain in mo<strong>de</strong> O, though, so the algorithm should contaín "if the selected mo<strong>de</strong> is within the<br />
ange i<strong>de</strong>ntified as suítable for the applicatíon".<br />
¡.3 Frame reinstatem<strong>en</strong>t sequ<strong>en</strong>ce C (see Figure 1)<br />
f terminal A is transmilting unframed but receiving framed, frame reinstatem<strong>en</strong>t consists in the insertion of FAS<br />
,nd BAS into the first 16 bits of the service channel, waiting for incoming A = 0; the over<strong>la</strong>id frame can contain<br />
icutral BAS capability to avoid triggering a full capacity exchange.<br />
í cutral capability (100)[0] shall always be <strong>en</strong>closed betwe<strong>en</strong> BAS capability markers. It should be noted that the<br />
leutral-cap shall never be inclu<strong>de</strong>d in a capability set.<br />
^ terminal A which ís receiving unframed may wish the remote terminal B to reinstale framing: to do this, A must first<br />
tself reinstale framing if it is not already transmitting framed and th<strong>en</strong> s<strong>en</strong>d the neutral BAS capability; B must<br />
espond by reinstatíng framing in or<strong>de</strong>r to return the neutral BAS capability and A = O, and continuíng this at íeast<br />
intil it receives A = O itself.
ame reínstatem<strong>en</strong>t sequ<strong>en</strong>ce C<br />
¡thout consí<strong>de</strong>ration of restricted networks)<br />
Is curr<strong>en</strong>t outgoing signal<br />
64 kbit/s data? (mo<strong>de</strong> 10)<br />
Is curr<strong>en</strong>t outgoing signal<br />
64 kbit/s vi<strong>de</strong>o?<br />
Is curr<strong>en</strong>t outgoing signal<br />
audío mo<strong>de</strong> 1 ?<br />
Fhe curr<strong>en</strong>t mo<strong>de</strong> must be<br />
PCM audio.<br />
ime reinstatem<strong>en</strong>t sequ<strong>en</strong>ce C<br />
)p!Ícat¡on to restricted networks)<br />
!s curr<strong>en</strong>t outgoing signal<br />
56 kbit/s data?<br />
s curr<strong>en</strong>t outgoing signal<br />
56 kbit/s vi<strong>de</strong>o?<br />
N<br />
s curr<strong>en</strong>t outgoing signal 56<br />
tbit/s<br />
)f Recomm<strong>en</strong>dation G.722?<br />
fhe curr<strong>en</strong>t mo<strong>de</strong> must be<br />
3CM audio.<br />
Y<br />
Y<br />
S<strong>en</strong>d framed signal wíth suitable data command (62.4 kbit/s or less) — note<br />
that data is corrupted in the receiver until FAS is recovered at other <strong>en</strong>d;<br />
interleave neutral-cap if relevant.<br />
S<strong>en</strong>d framed signal with (000) [31] and (010) [1]- note thatvi<strong>de</strong>o is corrupted<br />
in the receiver until FAS is recovered at other <strong>en</strong>d; ¡nterleave neutral-cap if<br />
relevant.<br />
Over<strong>la</strong>y framing without mo<strong>de</strong> change; use (000) [6] and interleave neutralcap<br />
if relevant.<br />
S<strong>en</strong>d mo<strong>de</strong> OF with (000) [18 or 1 9] and interleave neutral-cap if relevant.<br />
S<strong>en</strong>d framed signal with suitable data command (54.4 kbit/s or less) — note<br />
that data is corrupted in the receiver until FAS is recovered at other <strong>en</strong>d;<br />
interleave neutral-cap if relevant.<br />
S<strong>en</strong>d framed signal with (000) [31] and (010) [1 or 2] - note that vi<strong>de</strong>o is<br />
corrupted ¡n the receiver until FAS is recovered at other <strong>en</strong>d; interleave<br />
neutral-cap ¡f relevant.<br />
Over<strong>la</strong>y framíng without mo<strong>de</strong> change and interleave neutral-cap if relevant.<br />
S<strong>en</strong>dmo<strong>de</strong>OF with (000) [18 or 1 9] and interleave neutral-cap ¡f relevant.<br />
FIGURE 1/H.242
í Mo<strong>de</strong> initialization, dynamic mo<strong>de</strong> switching and mo<strong>de</strong> O forcing<br />
Audiovisual termináis will be connected to digital networks where other kinds of termináis will also be connected:<br />
3.711 termináis but also data termináis, telematic termináis, servéis, etc. Wh<strong>en</strong> compatibility betwe<strong>en</strong> the differ<strong>en</strong>t<br />
.ervices involving those termináis is required, an initialization procedure is necessary.<br />
Vh<strong>en</strong> automatic compatibility ís required, a procedure based on the sequ<strong>en</strong>ces <strong>de</strong>fined in 5 is used.<br />
;or cali transfer or mo<strong>de</strong> mismatch recovery, it is necessary for termináis to opérate ín the common mo<strong>de</strong> OF and a<br />
ao<strong>de</strong> O forcíng procedure Ís required, again based on the sequ<strong>en</strong>ces <strong>de</strong>fined in 5.<br />
\.t the comm<strong>en</strong>cem<strong>en</strong>t of the cali, after cali transfer and after the procedure of 6.3, there is a need for an initialization<br />
irocedure to <strong>en</strong>sure that the two connected termináis can opérate in the most suitable common mo<strong>de</strong>.<br />
í.l Mo<strong>de</strong> initialization procedure<br />
¡.1.1 Single cnannel<br />
^he initialization procedure begins as soon as a connection message is received from the network, or any indication<br />
aeaning that the physical connection is established.<br />
U the beginning of mo<strong>de</strong> initialization, each terminal wíll start to transmit Ín mo<strong>de</strong> OF.<br />
?he receive part of the terminal should be in frame search and the receive audio is mo<strong>de</strong> OF. Sequ<strong>en</strong>ce A is started.<br />
Jpon completion of sequ<strong>en</strong>ce A according to outcome I (see Figure 2 outcome <strong>la</strong>), sequ<strong>en</strong>ce B will comm<strong>en</strong>ce.<br />
?he BAS co<strong>de</strong> which is s<strong>en</strong>t in sequ<strong>en</strong>ce B is calcu<strong>la</strong>ted from the knowledge of the capabilitíes of the local and distant<br />
erminals and is used to switch to a suitable working mo<strong>de</strong>. Thís process may involve terminal procedures effecting<br />
hoices ma<strong>de</strong> by the user or preset in the terminal. An example illustrating conformance to a <strong>de</strong>fined teleservice is<br />
;Ív<strong>en</strong> in Recomm<strong>en</strong>dation H.320.<br />
n the ev<strong>en</strong>t of outcome II, the terminal will switch ¡ts transmíssíon and reception to mo<strong>de</strong> OU. The receive part of the<br />
erminal should remain in frame search throughout the cali.<br />
n the ev<strong>en</strong>t of outcome III, íimer TI Ís reset and the terminal remains within sequ<strong>en</strong>ce A.<br />
'he initialization procedure is completed wh<strong>en</strong> both termináis have switched to the <strong>de</strong>sired working mo<strong>de</strong>(s).<br />
.1.2 Additional channels<br />
L possibility of addíng more channels is established from the capability exchange sequ<strong>en</strong>ce, The calling terminal may<br />
a<strong>en</strong> immediately begin establíshing the additional connections. Wh<strong>en</strong> each is established, ít transmits only FAS<br />
nd BAS on that channel, setting a timer Ta of valué 10 seconds. Synchronization with the ínitial channel is performed<br />
ccording to 2.7/H.221, Wh<strong>en</strong> the incoming A bits on additional channels are observed to be O, mo<strong>de</strong> swítchíng to<br />
ccupy sequ<strong>en</strong>tially numbered channels is initiated by an appropriate transfer-rate command BAS. If the timer Ta has<br />
xpired without receiving A = O, it Ís <strong>de</strong>alt with as a fault condition.<br />
LS the buffering process may involve the insertion of additional <strong>de</strong><strong>la</strong>y in the initial channel, which may already<br />
e carrying user information (speech, vi<strong>de</strong>o, data), it may be necessary to make some provisión for this interruption<br />
E.g. short-term muting of audio output).<br />
Ls additional channels achieve synchronization they are sequ<strong>en</strong>tially numbered using both FAS and BAS numbering as<br />
rovi<strong>de</strong>d in Recomm<strong>en</strong>dation H.221.<br />
tn example of mo<strong>de</strong> initialization on two channels is giv<strong>en</strong> in App<strong>en</strong>dix I.
4WOA<br />
TX m mo<strong>de</strong> OF,<br />
BAS-cap.cycle<br />
FAS searchfl)<br />
Yes ^ \o \1 expired? Recovered<br />
frame alignm<strong>en</strong>t?<br />
Outcome Il<strong>la</strong><br />
(indícate faült<br />
Note incommg<br />
BAS-cap.cycle<br />
Incoming FAS<br />
in bits?<br />
Put outgomg SC<br />
bit?<br />
All-ones ¡n bit 8<br />
Incoming FAS<br />
in bit??<br />
Shift mcommg<br />
bits 1-7 til! FAS<br />
in bit?<br />
:Óütcomé:lá:;.. Outcome: Ib<br />
a' Unless there is octettiming and It is certain that a restricted network is notinvolved, FAS should atthis pomt be<br />
sought throughoutthe ¡ncoming signal.<br />
' Outcome IV: communication is ¡mpossible, because it is notknown which bit is lost or stuffed; the terminal should<br />
so indícate to the user and wait<br />
c) If the cali is known to be ínter-regional, itis advisableto mute the loudspeaker(s) until the audio <strong>de</strong>co<strong>de</strong>r ¡s setto<br />
the corred coding <strong>la</strong>w.<br />
FIGURE 2/H.242<br />
Initial capability exchange - G<strong>en</strong>eral case<br />
TlS14250-93/dDl
¡.2 Dynamic mo<strong>de</strong> switching (see Figure 3)<br />
'he mo<strong>de</strong> switching procedure makes use of the frame structure specifíed in 4 and of the sequ<strong>en</strong>ces <strong>de</strong>fíned in 5. It<br />
hould be noted that all terminal receivers musí remain in frame search throughout the cali.<br />
Vh<strong>en</strong> the terminal ís receiving ín a framed mo<strong>de</strong>, that is, it ís capable of <strong>de</strong>coding bit A, mo<strong>de</strong> switching should be<br />
le<strong>la</strong>yed if the A bit is set to 1; ev<strong>en</strong>tually the mo<strong>de</strong> mismatch recovery procedure as <strong>de</strong>scribed in 6.4 míght be used.<br />
Vh<strong>en</strong> the terminal X wishing to make a mo<strong>de</strong> switch is receiving unframed signáis, the capability exchange sequ<strong>en</strong>ce<br />
nay be used fírst to forcé the other terminal Y to a framed mo<strong>de</strong>; h<strong>en</strong>ee terminal X can check for incoming A — 0. This<br />
ise of sequ<strong>en</strong>ce A Ís particu<strong>la</strong>rly necessary if X was previously transmítting unframed signáis, since Y would not be in<br />
i posítion to <strong>de</strong>al wíth a mo<strong>de</strong> change from X until it had regaíned frame alignm<strong>en</strong>t (see 6.2.3). If X had previously<br />
le<strong>en</strong> transmitting framed signáis, the capability exchange sequ<strong>en</strong>ce may be omitted on the assumption that if Y had<br />
tnexpectedly lost frame alignm<strong>en</strong>t it would already have attempted a recovery procedure (see 7),<br />
[.2.1 Dynamic mo<strong>de</strong> switching from a framed mo<strong>de</strong> to another framed mo<strong>de</strong><br />
'he basic sequ<strong>en</strong>ce mo<strong>de</strong> switching <strong>de</strong>scribed Ín 5.2 is used.<br />
U the transmitting terminal, if a BAS command is transmitted to sígnal a new mo<strong>de</strong>, the transmitter must opérate in<br />
he appropriate mo<strong>de</strong> from the first octet of the next sub-multiframe.<br />
¡imi<strong>la</strong>rly, at the receiving terminal, if the received BAS signáis a new mo<strong>de</strong>, the receiver must opérate in the<br />
ppropriate rno<strong>de</strong> from the first octet of the next sub-multiframe.<br />
1.2.2 Dynamic mo<strong>de</strong> switching from a framed mo<strong>de</strong> to an unframed mo<strong>de</strong><br />
^.s in 6.2.1, the basíc sequ<strong>en</strong>ce mo<strong>de</strong> switching <strong>de</strong>scribed in 5.2 is used.<br />
íowever, as the BAS for sígnalling an unframed mo<strong>de</strong> is transmitted for a single sub-multiframe, a mo<strong>de</strong> mismatch<br />
nay occur in drastic error condítions. Optionally, a method may be used to improve the reliability of the switching: the<br />
iew BAS valué Ín the basic sequ<strong>en</strong>ce mo<strong>de</strong> switching is repeated three times; this WÍ11 cause a temporary corruption of<br />
Ele least significant bit of the received ínformation.<br />
1.2.3 Dynamic mo<strong>de</strong> switching from an unframed mo<strong>de</strong> to another mo<strong>de</strong> (framed or unframed)<br />
?he basic sequ<strong>en</strong>ces frame reínstatem<strong>en</strong>t and mo<strong>de</strong> switching are sequ<strong>en</strong>tially transmitted, the former including<br />
apabilíty exchange if necessary.<br />
1.3 Mo<strong>de</strong> O forcing procedure<br />
¡ee Figure 4<br />
>.3.1 Single channel<br />
Vhere ít is necessary to <strong>en</strong>sure that both termináis are operating in mo<strong>de</strong> O (for ínstance before cali transfer), tras<br />
irocedure is used.<br />
'he forcing terminal uses dynamic mo<strong>de</strong> switching (see 6.2) with BAS audio command to switch to mo<strong>de</strong> OF, followed<br />
ly sequ<strong>en</strong>ce A using BAS (100) indicating only G.711 audio capability. The valué [1 or 2] appropriate to the terminal"s<br />
iwn región Ís used in case.the cali is to be transferred to a local G.725 type-0 terminal. On receipt of this, the remote<br />
erminal Ís obliged to switch to mo<strong>de</strong> OF also using the indicated <strong>la</strong>w for its <strong>en</strong>co<strong>de</strong>r and <strong>de</strong>co<strong>de</strong>r. The procedure is<br />
omplete wh<strong>en</strong> the forcing terminal <strong>de</strong>tects incomíng mo<strong>de</strong> OF. Changes of network configuraron can now be<br />
mplem<strong>en</strong>ted (see 8).
TERMINAL X<br />
Incommg<br />
signal<br />
ramed<br />
Outgoing<br />
signa!<br />
ramed<br />
Frame relnst. wltn Frame reinst. wlth<br />
SetTl;<br />
FAS search in bit8)<br />
S<strong>en</strong>d BAS-cap.b)<br />
Incoming FAS <strong>de</strong>tected<br />
S<strong>en</strong>d A = O<br />
Swltch to <strong>de</strong>s red mo<strong>de</strong><br />
Is it a framed<br />
mo<strong>de</strong> that has be<strong>en</strong><br />
wítched to<br />
If byte-timed, search in appropríate bit.<br />
Incoming<br />
A = 1 after<br />
2 sec.7<br />
Prpc'edüre cdrrüpted<br />
]- i;Répeat attempt .<br />
Neutral or complete BAS-cap cycle, <strong>de</strong>p<strong>en</strong>ding on received BAS-cap.<br />
Incoming BAS-cap causes outgoing frame reinstatem<strong>en</strong>t.<br />
FIGURE 3/H.242<br />
No \i<br />
SetT1<br />
FAS search, In bit3)<br />
TERMINAL Y<br />
\ffa<br />
^^^ i<br />
Frame re¡nst.with<br />
neutral BAS-cap.b)<br />
and A = O<br />
No<br />
Procedure corrupted:<br />
Awaít BAS.command or'<br />
mismatch recov. proc.<br />
Mo<strong>de</strong> switching - Terminal X initiates the mo<strong>de</strong> switch<br />
Yes<br />
Acl on Incomlng mo<strong>de</strong> swltch<br />
Is it a (rameo ^--^^ No<br />
moda thal has be<strong>en</strong><br />
swftched to?<br />
COMPLETION; ;<br />
•can-drop-outgoing :".<br />
fráming:if dasired- •-,<br />
Tl51í260-93/d02
«i-<br />
Frame reinst, with<br />
neutral BAS-cap.<br />
SetT1<br />
FAS search<br />
S<strong>en</strong>d neutral BAS-cap.<br />
Incoming FAS <strong>de</strong>tected<br />
S<strong>en</strong>d A = O<br />
TERMINALX<br />
Frame reinst.<br />
wlthout BAS-cap<br />
;-:-:Fáülí;<br />
:téll<br />
Swltch te- mo<strong>de</strong>OF<br />
S<strong>en</strong>d cap. setwith (100) [1 or 2]<br />
.3.2 Two or more channels<br />
^<br />
No<br />
: "Pracédúre.corrupted '<br />
~--\t anempt •<br />
FIGURE 4/H.242<br />
Frame reinst. wlth<br />
neutral BAS-cap.<br />
and A =0<br />
Mo<strong>de</strong> zero forcing — Terminal X initiates the forcing<br />
TERMINALY<br />
START<br />
Acton incoming mo<strong>de</strong> OF<br />
Note Incoming (1 00}<br />
Switch to mo<strong>de</strong> OF<br />
Procedure corrupted:<br />
iMismatch recovery<br />
.procedur.e<br />
T15U27o.93/d03<br />
n this case the mo<strong>de</strong> O forcing is applied to the initial channel only, and sepárate consi<strong>de</strong>rations apply to treatm<strong>en</strong>t of<br />
ti e additional channels. Three cases are consi<strong>de</strong>red here by way of guidance for the multiple-B case:<br />
a) Additional channels dropped — This would be necessary, for example, prior to dísconnection. The<br />
procedure is as for one channel, the forcing terminal <strong>de</strong>c<strong>la</strong>ring capabílity of PCM audio only wíth<br />
transfer rate capabilíty of 1 X 64 kbit/s; this wíll result in mo<strong>de</strong> switches successively to "data OFF",<br />
"vi<strong>de</strong>o OFF" and audio mo<strong>de</strong> OF or OU, such that all additional channels are vacated and can be<br />
disconnected;
) Additional channels idle — This is the same as a), except that the forcing terminal makes no move to<br />
disconnect; the channels carry FAS, the multiframe number and the BAS indicatíng channel number; the<br />
cont<strong>en</strong>t of the remain<strong>de</strong>r of the idle channels ís irrelevant;<br />
c) Additional channels ma'mtained active — This might be b<strong>en</strong>eficial in some recovery procedures. The<br />
forcing terminal <strong>de</strong>c<strong>la</strong>res a capability of PCM audio plus transfer rate unchanged from its previous valué,<br />
and th<strong>en</strong> itself switches to the appropriate mo<strong>de</strong>.<br />
^.n example of mo<strong>de</strong> O forcing a) is giv<strong>en</strong> in App<strong>en</strong>dix II.<br />
5.3.3 Ad<strong>de</strong>nda<br />
1) Mo<strong>de</strong> zero forcing may be activated at any time during the cali. Since mo<strong>de</strong> zero forcing comprises<br />
sequ<strong>en</strong>ce B (mo<strong>de</strong> switching) followed by sequ<strong>en</strong>ce A (capability exchange), the forced terminal may not<br />
recognize "forcing", but must respond to the capability exchange first by returning its ordinary capability<br />
set and th<strong>en</strong> reduced commands correspondíng to the capability set of the forcing terminal. Sophisticated<br />
terminal <strong>de</strong>sign may inclu<strong>de</strong> recognition of "being forced", thus returns reduced commands first and th<strong>en</strong><br />
responds to the capability exchange.<br />
2) If IB transfer rate is inclu<strong>de</strong>d in the capability set of the forcing terminal, the cont<strong>en</strong>t of the additional<br />
channel(s) are not concemed; they may inclu<strong>de</strong> only FAS and BAS with any bits ín other bit positions, or<br />
may ev<strong>en</strong> become vacant wíthout FAS and BAS.<br />
3) After mo<strong>de</strong> zero forcing is activated and both termináis start to opérate at the int<strong>en</strong><strong>de</strong>d forcing mo<strong>de</strong>, reactivation<br />
of sequ<strong>en</strong>ce A for ordinary capability exchange, actívation of additional channel(s),<br />
disconnection, or other mo<strong>de</strong> changes may take p<strong>la</strong>ce according to the terminal procedure.<br />
&.4 Mo<strong>de</strong> mismatch recovery procedure<br />
[n the case where mo<strong>de</strong> mismatch has occurred, the mo<strong>de</strong> O forcing procedure may be used to establish a common<br />
tvorking mo<strong>de</strong>. Following this procedure, re-ínítialization can be achieved by using the mo<strong>de</strong> initialization procedure.<br />
7 Recovery from fault conditions<br />
Fhe provisions of this c<strong>la</strong>use are not wholly mandatory. In g<strong>en</strong>eral it is expected that fault conditions will be rare and it<br />
may be uneconomical to provi<strong>de</strong> e<strong>la</strong>bórate recovery procedures to cover all ev<strong>en</strong>tualities. It is mandatory that proper<br />
indications of fault conditions be transmitted on the outgoing channel(s) - in particu<strong>la</strong>r, A must be set to 1 where<br />
ippropriate conditions for A — O are not met. Other action to be tak<strong>en</strong> on losing frame alignm<strong>en</strong>t, multiframe<br />
alignm<strong>en</strong>t, synchronism, or a connectíon, or on receiving incoming A = 1, is pres<strong>en</strong>ted here for guidance.<br />
7.1 Unexpected loss of synchronixation or frame alignm<strong>en</strong>t<br />
7.1.1 Loss of frame alignm<strong>en</strong>t in the initial channel<br />
[f a terminal unexpectedly loses frame alignm<strong>en</strong>t on its receive path, a timer T^ is set (valué for example 1 second) and<br />
incoming information is discar<strong>de</strong>d if unintelligíble. During this time the status of the framing in the receive direction is<br />
monitored:<br />
a) If framing is recovered before the timer expires, the normal operation is resumed,<br />
/<br />
b) If framing is not recovered before the tirner expires, the terminal goes to the mo<strong>de</strong> O forcing procedure<br />
followed by re-Ínitialization.
M.2 Loss of frame alignm<strong>en</strong>t or synchronization in an additional channel<br />
f a terminal unexpectedly loses synchronization (including that due to loss of frame alignm<strong>en</strong>t) on an additional<br />
ihannel, a timer T3 is set, outgoing A-bít is set to 1 and incoming information díscar<strong>de</strong>d if unintelligible; íf the loss of<br />
his information also causes information on other channels to become meaningless that also is discar<strong>de</strong>d,<br />
a) if synchronization is recovered before the timer expires, normal operation ís resumed; this takes into<br />
account recoverable synchronization loss due to bit or synchronization errors on the transmission Une;<br />
b) if synchronization is not recovered before the timer expires, the mo<strong>de</strong> O forcing procedure may be used.<br />
f.2 Recovery from loss of connection(s)<br />
L.OSS of a connection means that <strong>en</strong>d-to-<strong>en</strong>d transmission on that channel has be<strong>en</strong> díscontinued, so that al] appar<strong>en</strong>tly<br />
•eceived bits are meaningless. The receíver will, of course, lose frame alignm<strong>en</strong>t and may follow the procedures of 7.1.<br />
íowever, an indícation may be avai<strong>la</strong>ble from the network (D-channel or otherwise) that the connection has be<strong>en</strong> lost;<br />
n this case the procedures of this subc<strong>la</strong>use are followed. It is assumed that connection loss is bidirectional; the case of<br />
oss in one direction only is for further study.<br />
r.2.1 R<strong>en</strong>umbering of channels<br />
Phis procedure is used for reconstnicting the remaining normal additional channels wh<strong>en</strong> one additional channel<br />
treaks down.<br />
i) make the transmission mo<strong>de</strong> of all channels into "framed";<br />
U) vacate the s<strong>en</strong>ding additional channel(s);<br />
iii) r<strong>en</strong>umber the additional channel(s);<br />
ív) wait for the synchronization establishm<strong>en</strong>t of the remote terminal and th<strong>en</strong> expand communication onto<br />
the additional channels.<br />
r.2.2 Loss of an additional connection<br />
f any remaining channels are unframed (for example, data transmission) they must immedíately have frame structure<br />
according to Recomm<strong>en</strong>dation H.221) reimposed and maintained until conditíons have returned to normal. The<br />
mtgoing A-bit on additional channels is set to 1 if the incoming direction Ís unframed or out of sequ<strong>en</strong>ce, or if<br />
ynchronism has be<strong>en</strong> lost.<br />
f the lost channel was carrying part of a signal (such as <strong>en</strong>co<strong>de</strong>d vi<strong>de</strong>o) which also involved other channels, so that íts<br />
oss r<strong>en</strong><strong>de</strong>rs the information in those other channels meaningless, th<strong>en</strong> by dynamic mo<strong>de</strong> switching those channels are<br />
•acated.<br />
'he next step is to r<strong>en</strong>umber the avai<strong>la</strong>ble channels if appropriate, to obtain a continuous sequ<strong>en</strong>ce; this ís done using<br />
;ie procedure of 7.2.1.<br />
)ynamíc mo<strong>de</strong> switching is applíed to re-establish the vi<strong>de</strong>o or other transmission on the channels for which<br />
iicoming A-bíts are zero.<br />
El the ev<strong>en</strong>t that the lost channel be reconnected, it is ad<strong>de</strong>d to the capacity in the same way as at the start of the cali.<br />
.2.3 Loss of the initial connection<br />
'his results in the loss of the initial channel in both directions. Both termináis immediately regard #2 as the initial<br />
hannel and transmit thereon the foílowing BAS:<br />
i) reinstatem<strong>en</strong>t of FAS and BAS in any unframed channels;<br />
ü) transfer rate (001) [O or 6] — co<strong>de</strong> havíng the effect of vacating all additional channels; also audio<br />
command (000) unchanged from previous valué;<br />
iii) transfer rate (001) [17] on original second channel, indicating loss of original channel, and from next<br />
sub-multiframe original second channel substitutes for original initial channel; simultaneously any
iv) wait for confirmation that the synchronism at the remote terminal is retained/regained (all incoming<br />
An=0);<br />
v) expand communication onto all channels using appropriate transfer-rate command;<br />
NOTE — As a result of this procedure, s<strong>en</strong>ding and receiving initial channels may not be on the same<br />
connection.<br />
vi) the terminal tries to re-establish the lost channel.<br />
£ Network consi<strong>de</strong>ration: cali connection, disconnectíon and cali transfer<br />
i.l Cali connection<br />
í.1.1 Initial channel<br />
t is assumed that the termináis for switched network operation will have a signalling arrangern<strong>en</strong>t for originating calis<br />
i ver the network.<br />
n the case that the network provi<strong>de</strong>s an indication that the connection is established (CONNECT-ACK message), the<br />
iriginating terminal will set its transmit and receive audio mo<strong>de</strong>s to PCM and begín the mo<strong>de</strong> initialization procedure<br />
bllowing the connection establishm<strong>en</strong>t indication. V/here the network does not provi<strong>de</strong> an indication of connection<br />
¡stablishm<strong>en</strong>t, the originating terminal wíll begin the mo<strong>de</strong> initialization procedure immediately.<br />
Jpon answering a cali, the terminal will begin the mo<strong>de</strong> initialization procedure,<br />
termináis for use on leased circuits may have a means for s<strong>en</strong>ding the alerting signal to the distant terminal and for<br />
nsweríng the alerting signal. In this case, the s<strong>en</strong>ding of the alerting signal is equival<strong>en</strong>t to dialling and the foregoing<br />
irocedures apply,<br />
Vh<strong>en</strong>ever a terminal is manually reset, or recovers from a fault condition, íhe terminal will begin the mo<strong>de</strong> O forcing<br />
irocedure of 6.3. Th<strong>en</strong> the terminal wíll begin mo<strong>de</strong> initialization.<br />
1.1.2 Additional channels<br />
?all connection to provi<strong>de</strong> additional channels may be initiated by one of the following:<br />
a) manually (in<strong>de</strong>p<strong>en</strong><strong>de</strong>ntly of the capability exchange through the initial channel);<br />
b) on completion of the capability exchange sequ<strong>en</strong>ce indícating mutual additional-channel capability;<br />
c) at some time <strong>la</strong>ter than in b), prompted by user action.<br />
'he cholee betwe<strong>en</strong> these will <strong>de</strong>p<strong>en</strong>d on servíce provisión and/or terminal procedures.<br />
'or ISDN switched services, the initial connection shall be first established and according to the outcome of the in-band<br />
apability exchange the additional connectíons shall be established, This means that only choices b) and c) are allowed<br />
i this case.<br />
lonsequ<strong>en</strong>tly, for2B or2Ho Communications, the initial connection contaíns the initial channels of both directions, and<br />
le additional connection contains the additional channels of both directions as well. Another implication of this<br />
;qu<strong>en</strong>cing of in-channel negotiation and additional cali establishm<strong>en</strong>t is that the capability set should inclu<strong>de</strong> "2B (or<br />
HQ) or hígher transfer rate" at the first capability exchange, otherwíse íhe additional cali seíup ¡s not activated at the<br />
tart of communication.<br />
'he capabilities in both directions should indicate the ínt<strong>en</strong>tions/ability of the two to use the additional B/Ho-channel:<br />
— the calling terminal signáis both its abílíty to <strong>de</strong>co<strong>de</strong> and (implicitly) íts int<strong>en</strong>íion to make a second cali<br />
request;<br />
— the called terminal signáis its ability (or inability) to cope with a second B-channel, and (implicitly) its<br />
intp.ntion tn «nswp.r stn ínr.nrmnp r.all rfiniiRKt if nnp. ís forfhcrunin cr.
Wh<strong>en</strong> two or more connections are to be established betwe<strong>en</strong> two units (termináis or MCUs or one of each), one unit<br />
nust make all of the cali requests — it is not permitted that a terminal, havíng answered an íncoming cali, make a<br />
request for a connection back to the same unit, To put this another way, we are not expecting to <strong>de</strong>sign termináis which,<br />
iiaving ma<strong>de</strong> a cali to Y, will th<strong>en</strong> accept an incoming cali from Y and not from any other address.<br />
[f more than two connections are used, all the additional connections may be called simultaneously, Each additional<br />
íhannel number is allocated in the or<strong>de</strong>r of cali establishm<strong>en</strong>t at the transmitter, a connection may convey two channels<br />
tvith differ<strong>en</strong>t channel numbers in each dírection.<br />
Wh<strong>en</strong> the establishm<strong>en</strong>t of connection is known to the terminal, the mo<strong>de</strong> initialization procedure of 6.1,2 is applied.<br />
During cali establishm<strong>en</strong>t, an originating terminal should reserve additional channels by not answering incoming calis<br />
on those channels until it is <strong>de</strong>termined whether the additional channels will be used in the connection. This prev<strong>en</strong>ts<br />
múltiple cali collisions and cont<strong>en</strong>tion for the avai<strong>la</strong>ble channels. A network solution is un<strong>de</strong>rstudy.<br />
5.2 Terminal disconnection<br />
Wh<strong>en</strong> a terminal disconnects from a cali, the terminal must fírst initiate the mo<strong>de</strong> O forcing procedure, await<br />
;ompletion of the procedure and th<strong>en</strong> allow the actual disconnection of the cali to occur.<br />
Ef for any reason a terminal can no longer use one (or more) of the additional connections, it should first itself switch to<br />
i mo<strong>de</strong> occupying the <strong>de</strong>síred lower capacity, th<strong>en</strong> s<strong>en</strong>d a capset showing the lower transfer rate as máximum, th<strong>en</strong> wait<br />
;or the incoming signáis to drop to the lower rate, and finally disconnect the unwanted connections. The capset is to<br />
prev<strong>en</strong>t any attempt from the other <strong>en</strong>d to remake the connection. There ís no need to forcé all the way down to mo<strong>de</strong>-0<br />
n thís case, though un<strong>de</strong>r some fault condilions it may still turn out to be the best route to recovery.<br />
3.3 Cali transfer<br />
\ a consequ<strong>en</strong>ce of the above, the terminal which continúes to particípate in a transferred cali will be receiving ín<br />
i PCM-forced state and therefore will be transmitting its capability sel in framed PCM. Wh<strong>en</strong> the transferred-to<br />
;erminal answers, mo<strong>de</strong> initialization will occur in both directions.<br />
í.4 Confer<strong>en</strong>cing<br />
^onfer<strong>en</strong>cing will be accomplished by means of a multipoint control unit (MCU). Each terminal will be connected to a<br />
>ort of the MCU by a switched connection or a leased círcuit. Each connection betwe<strong>en</strong> the terminal and the MCU ¡s<br />
¡onsi<strong>de</strong>red to be a point-to-point connection as far as cali connection, terminal disconnection and cali transfer<br />
jrocedures are concerned.<br />
5.5 PCM format conversión<br />
n the above procedures, no automatic method for establishing A-<strong>la</strong>w or u,-<strong>la</strong>w compatible PCM operation was <strong>de</strong>fined.<br />
U the beginning of the cali, <strong>en</strong>coding and <strong>de</strong>coding by each terminal is according to the <strong>la</strong>w prevailing in its own<br />
egion. The <strong>de</strong>co<strong>de</strong>r must adapt to the coding <strong>la</strong>w of the incoming signáis. In a framed signal this wilí be clear from the<br />
3AS command; for unframed audio, signal analysis or local knowledge should be applied, and if this indicates that the<br />
ither terminal is using a differ<strong>en</strong>t coding <strong>la</strong>w th<strong>en</strong> the H.242 terminal should switch both its <strong>en</strong>co<strong>de</strong>r and <strong>de</strong>co<strong>de</strong>r to the<br />
¡oding <strong>la</strong>w of the other terminal.<br />
n the case where both termináis transmit framed signáis, once the capability exchange is completed they may transmit<br />
n either PCM mo<strong>de</strong> if <strong>de</strong>sired.<br />
3efore cali transfer, in the case where both termináis can transmit framed audio, the distant termmal's <strong>en</strong>co<strong>de</strong>r and<br />
[eco<strong>de</strong>r must be forced .by the relevant BAS capabüities and commands to the coding <strong>la</strong>w of íhe región where the<br />
ransferis lo take p<strong>la</strong>ce.
) Procedurefor activation and <strong>de</strong>-activation of data channels<br />
1.1 Data equipm<strong>en</strong>t not conforming to Recomm<strong>en</strong>dation H.200/AV.270<br />
Hach terminal must transmit a data-rate capability co<strong>de</strong> (see Recomm<strong>en</strong>dation H.221) for each data rate it Ís able to<br />
'eceive. This may be done during the capability exchange sequ<strong>en</strong>ce at the start of the cali or at a <strong>la</strong>ter time by initíatíng<br />
i new capability exchange,<br />
\ terminal may transmit data at any rate which has be<strong>en</strong> indícated in the data-rate capability co<strong>de</strong>s it has received from<br />
he other terminal (see Note). The appropriate data command (see Recomm<strong>en</strong>dation H.221) is s<strong>en</strong>t and in the following<br />
lub-multiframe the data transmission is comm<strong>en</strong>ced, occupying the bits within each frame <strong>de</strong>fined in Recomm<strong>en</strong><strong>la</strong>tion<br />
H.221. However, at the time the data command is first s<strong>en</strong>t, these bits must be unoccupied or contain only vi<strong>de</strong>o<br />
nformation; therefore audio or any other signáis must be removed from this part of the frame with the prior<br />
ransmission of an appropriate command. In the case of occupancy by vi<strong>de</strong>o ínformation, commands are not avaí<strong>la</strong>ble to<br />
educe the vi<strong>de</strong>o rate, but the vi<strong>de</strong>o <strong>de</strong>co<strong>de</strong>r continúes to opérate correctly on the lower ñow of Ínformation. However, if<br />
he vi<strong>de</strong>o rate is being ma<strong>de</strong> very low (for example, less than 30.4 kbit/s) or slopped altogether by the introduction of a<br />
[ata stream, itis advisable fírstto s<strong>en</strong>d freeze-picture request, followed by the ví<strong>de</strong>o OFF command.<br />
NOTE 1 — Sometimes symmetrícal data transmission ís required, e.g. in data transmission th'rough the V.24/V.28<br />
tlterface. If more than one data rates have be<strong>en</strong> i<strong>de</strong>ntified as common betwe<strong>en</strong> two termináis, asymmetrical data transmission may<br />
ake p<strong>la</strong>ce according to differ<strong>en</strong>t terminal procedures. This can be avoi<strong>de</strong>d by using the highest common rate.<br />
Phe command variable LSD i<strong>de</strong>ntifies as a data path the whole of the I-channel capacity not otherwise allocated by<br />
ither commands; ít must not be used wh<strong>en</strong> variable MLP is on,-or wh<strong>en</strong> another LSD valué is in forcé. If used while<br />
á<strong>de</strong>o is on, vi<strong>de</strong>o is exclu<strong>de</strong>d from the I-channel.<br />
U the conclusión of the data Transmission the data OFF command is s<strong>en</strong>t. If vi<strong>de</strong>o is ON, Ít will th<strong>en</strong> occupy the freed<br />
lits in the next sub-multiframe and thereafter; otherwise those bits remain unoccupied until another command is s<strong>en</strong>t.<br />
U any time during data transmission the rate may be changed by an appropriate data command, subject to the<br />
irovisions gív<strong>en</strong> above.<br />
NOTE 2 — In the case where 64 kbit/s HSD, for example, has be<strong>en</strong> transmitted ín the highest-numbered channel of a<br />
nultiple-B channel connection, a slip during this data transmission would leave a misalígnm<strong>en</strong>t wh<strong>en</strong> the HSD is mrned off. To<br />
voíd corruption of vi<strong>de</strong>o un<strong>de</strong>r these circumstances, it may be advisable to switch off the vi<strong>de</strong>o stream before s<strong>en</strong>díng HSD-off,<br />
witchíng it on again as soon as A = O is received on the erstwhile data channel.<br />
L2 Equipm<strong>en</strong>t operating with an MLP according to Recomm<strong>en</strong>dation H.200/AV.270<br />
iach terminal capable of operating with an MLP must transmit one of the MLP-capability co<strong>de</strong>s. This may be done<br />
luring the capability exchange sequ<strong>en</strong>ce at the start of the cali, or at a <strong>la</strong>ter time by ínitiating a new capability<br />
xchange.<br />
Vh<strong>en</strong> terminal X wishes to transmit MLP, it transmits MLP ON at the appropriate rate. Receiving the <strong>la</strong>tter, terminal<br />
f, must establish an MLP channel at an appropriate rate (not necessarily íhe same rate) Ín the return direction.<br />
'he above provisions apply equally to the use of MLP on the I-channel, or in other channels or time-slots. Normally<br />
nly one of these is required; however if both are in forcé, with appropriaíe commands, th<strong>en</strong> a single MLP sub-channel<br />
t the combíned rate may be interpreted — this would be specified within the appropriate service Recomm<strong>en</strong>dation<br />
s.g. MLP rates of about 100 kbit/s on a 2B cali).<br />
'o change the MLP rate, an appropriate MLP command Ís s<strong>en</strong>t.<br />
'o discontinué use of the MLP, this matter may first be negotiated within the MLP itself; th<strong>en</strong> one or both termináis<br />
ransmitMLP-OFF.<br />
1.3 Simultaneous transmission of low-speed data and MLP<br />
i<br />
,SD and MLP may be active simultaneously, provi<strong>de</strong>d that no over<strong>la</strong>p is implied by the commands in forcé; however,<br />
ariable LSD and variable MLP cannot coexist. No more than one LSD channel and one MLP channel may be active at<br />
nvtimp. fse.R also 1 2X
LO Procedures for operation of termináis in restricted networks<br />
Jn<strong>de</strong>r study; the followíng subc<strong>la</strong>uses give preliminary consi<strong>de</strong>rations.<br />
rerminals connected to a restricted network shall transmit the BAS capability "restricted" (100) [22] continuously<br />
ivh<strong>en</strong> receiving an incomíng A = 1 at the start of a cali.<br />
LO.l Network aspects<br />
!n this Recomm<strong>en</strong>datlon the term "restricted network" applies to a network havíng restricted 64 kbit/s transfer<br />
;apability, <strong>de</strong>fined m Recomm<strong>en</strong>datlon 1.464 as 64 kbit/s octet-structured capability with the restriction that an all-zero<br />
>ctet is not permitted.<br />
LO.2 Refer<strong>en</strong>ce connections<br />
LO.2.1 Case 1: 56 kbit/s, V.35 interfaces<br />
Diagram a) of Figure 5 shows a refer<strong>en</strong>ce connection by a 56 kbit/s data service using V.35 interfaces. A 56 kbit/s clock<br />
s avai<strong>la</strong>ble at the V.35 ínterface; 8 kHz clock is not assumed. Diagram c) of Figure 5 shows a refer<strong>en</strong>ce connection,<br />
¡onnected by 56 kbit/s network service with network clock.<br />
LO.2.2 Case 2: n X 56 kbit/s, V.35 interfaces<br />
Diagram b) of Figure 5 shows a refer<strong>en</strong>ce connection with more than two 56 kbit/s connections. Frame alignm<strong>en</strong>t wíll<br />
>e according to Recomm<strong>en</strong>dation H.221. Neither septet timing ñor septet alignm<strong>en</strong>t is assumed. Diagram d) of Figure 5<br />
ihows a múltiple n X 56 kbit/s without septet alignm<strong>en</strong>t or septet tirning.<br />
LO.2.3 Case 3: n X 64 kbit/s with octet timing and alignm<strong>en</strong>t<br />
Diagram e) of Figure 5 shows a refer<strong>en</strong>ce connection consisting of two visual telephones connected by facilities<br />
>perating in a private Une <strong>en</strong>vironm<strong>en</strong>t. Unrestricted mo<strong>de</strong> of operation is not assumed.<br />
LO.2.4 Case 4: H0 (384 kbit/s) operation<br />
ÍVh<strong>en</strong> working in a restricted network a "1" shall be p<strong>la</strong>ced ín the eighth bit position of every octet of every time-slot;<br />
he service channel is th<strong>en</strong> in the sev<strong>en</strong>th bit.<br />
LO.2.5 Case 5: 56 kbit/s satellite operation<br />
"or further study.<br />
LO.2.6 Case 6: 56 kbit/s interconnecting a 64 kbit/s network<br />
V 64 kbit/s terminal will interwork with a 56 kbit/s terminal as a rate adapted data cali over a 64 kbit/s bearer channel,<br />
rhe terminal connected to the 64 kbit/s connection will rate adapt according to Recomm<strong>en</strong>dation H.221. In the case of<br />
[64 kbit/s terminal connected to ISDN, the terminal may optionally be equipped to intercommunicate through an<br />
SDN V.35 terminal adaptor, In any case, because the 56 kbit/s terminal cannot transmit correctly aligned septets, the<br />
erminal at the 64 kbit/s <strong>en</strong>d cannot assume septet timing.<br />
L0.3 Transmission formáis<br />
LO.3.1 Framing signal (56 kbit/s)<br />
rhe transmission shall be arranged in 80 septet frames as specified in Recomm<strong>en</strong>dation H.221.<br />
LO.3.2 Transmission formats (56 kbit/s operation)<br />
n 56 kbit/s operation the septets of each 7 X 80 bit frame will be transmitted in or<strong>de</strong>r, most significant bit first at the<br />
i6 kbit/s rate. Septet alignm<strong>en</strong>t will be recovered from the frame alignm<strong>en</strong>t signal as specified in Recomm<strong>en</strong>iot;~n<br />
TJ 001
Rec. V.35<br />
VT DSU<br />
VT<br />
0 sil<br />
E g DDS or PSDS<br />
W\i|<br />
Rec. V.35<br />
DSU VT<br />
Rec. V.35 Rec, V.35<br />
DSU<br />
oíNI<br />
Rec. V.35 rMi •<br />
E g DDS or PSDS<br />
DSU E.g. DDS or PSDS<br />
1^ NI<br />
CNI<br />
DSU<br />
b) Refer<strong>en</strong>ce connection wlth morethan two 56 kblt/a connectlons<br />
VT<br />
VT<br />
VT<br />
VT Vi<strong>de</strong>o telephone<br />
DSU Data service unit<br />
CNI Customer network ínterface<br />
DDS Digital data s<strong>en</strong>/ice<br />
PSDS Public swltched digital service<br />
LO.3.3 n x 56 kbit/s operation<br />
E.g. ISDN<br />
56/64 kbit/s<br />
c) Refer<strong>en</strong>ce connection by 56 kblt/s network service<br />
E.g, ISDN<br />
56/64 kbií/s<br />
C fJ i *<br />
I<br />
E.g. ISDN<br />
56/64 kbit/s<br />
d) Múltiple n x 56 kblt/s connection<br />
E.g.Hu<br />
prívate une<br />
e) Refer<strong>en</strong>ce connection conslstlng of two visual telephones<br />
connected by facllltles operating In a prívate Une <strong>en</strong>vlronm<strong>en</strong>t<br />
FIGURE 5/H.242<br />
VT<br />
VT<br />
VT<br />
Rec. V .35<br />
VT<br />
T1502420-89/dOÍ<br />
ii n X 56 kbit/s operation each 56 kbit/s connection will be framed and transmitted separately. Sepíet timíng will be<br />
•ecovered in<strong>de</strong>p<strong>en</strong><strong>de</strong>ntly from the frame alignm<strong>en</strong>t signal of each channel, and the differ<strong>en</strong>tial <strong>de</strong>jay betwe<strong>en</strong> the<br />
¡hannels will be comp<strong>en</strong>sated for on the basis of the multiframe numbering method specified in Recomm<strong>en</strong>-<br />
<strong>la</strong>tínn W 901
The voice signal will be carried in the initial connection and ví<strong>de</strong>o, graphics and auxiliary data may be carried in the<br />
initial and/or other connections.<br />
10.3.4 n X H0 operation<br />
In n X HQ operation, each connection will he framed separately and differ<strong>en</strong>t<strong>la</strong>l <strong>de</strong><strong>la</strong>y betwe<strong>en</strong> the channels will be<br />
comp<strong>en</strong>sated accordíng to Recomm<strong>en</strong>dation H.221.<br />
10.3.5 Dynamic allocation within a primary-rate connection<br />
Intellig<strong>en</strong>t termináis may have a means for dynamically increasing or <strong>de</strong>creasing the bit rate during a connection. The<br />
means for controlling these allocations will be performed according to Recomm<strong>en</strong>dation H.221. There may be a need to<br />
recover framing by extractíon from the received signal in<strong>de</strong>p<strong>en</strong><strong>de</strong>ntly.<br />
10.4 Interworking betwe<strong>en</strong> 56 kbit/s and 64 kbit/s termináis<br />
In the worst case it must he assumed that neither terminal is aware (by means of a D-channel message or otherwise)<br />
that it is connected to a terminal of the other type; furthermore septet timing cannot be assumed at the 56 kbit/s <strong>en</strong>d. At<br />
the 64 kbít/s <strong>en</strong>d, byte timíng is indisp<strong>en</strong>sable, since without this it cannot be known which bit (1 in every 8) will not be<br />
transmitted to the remote <strong>en</strong>d (see Figure 2, outcome IV).<br />
Initially, terminal X (at 64 kbií/s) transmits FAS and capability-BAS on bit 8, on the false assumption that the remote<br />
terminal ís also at 64 kbit/s. Frame search is carried out on the whole incoming signal; clearly, searching only on bit 8<br />
will result in outcome II (see Figure 2).<br />
If frame alignm<strong>en</strong>t is found, and this may be in any bit posítion, giv<strong>en</strong> the <strong>la</strong>ck of septet timing at the other <strong>en</strong>d, th<strong>en</strong><br />
the fact of interworking with a 56 kbit/s terminal immediately becomes known from the capability BAS, which terminal<br />
Y must inclu<strong>de</strong> in its capability BAS cycle. Terminal X immediately changes to transmhting FAS and BAS on bit 7,<br />
since bit 8 is the one whích is not transmitted through the restricted networks. Initialization should th<strong>en</strong> proceed as in<br />
6.1, with outcome Ib in Figure 2.<br />
In the ev<strong>en</strong>t that no frame alignm<strong>en</strong>t is found in any sub-channel, outcome II of 6.1.1 applies.<br />
NOTES<br />
1 All 56 kbit/s audiovisual termináis must transmit the appropriate capability BAS (100J [22] ¡n every capability<br />
exchange.<br />
2 Unless it Ís sure that they will never be requíred to interwork with 56 kbit/s networks, termináis manufacturad for<br />
use on 64 kbit/s networks should preferably have the capability to search for frame alignm<strong>en</strong>t in all bit positíons.<br />
3 It may be advisable to mute audio output untíl incoming frame alignrn<strong>en</strong>t has be<strong>en</strong> achíeved or a switch to<br />
unframed PCM has be<strong>en</strong> <strong>de</strong>ci<strong>de</strong>d upon.<br />
10.5 Interworking betwe<strong>en</strong> HQ or HU termináis in restricted and unrestricted networks<br />
At the start of the communication, the terminal on the restricted network transmits framed signáis with the service<br />
channel in.bit 7 of the I-channel and all "1 "s in bit 8 of every time-slot; the restricted capability BAS (100) [22] is s<strong>en</strong>t.<br />
In the terminal on the unrestricted network, frame search is carried out on the whole incoming signal (or incoming TSI<br />
if synchronizatíon betwe<strong>en</strong> H0/Hn framing and H.221 framing is maintained). Wh<strong>en</strong> BAS (100) [22] Ís <strong>de</strong>tected, a<br />
terminal immediately shifts the outgoíng service channel to bit 7 and sets all "l"s on bit 8 of every time-slot.<br />
It should be noted that the re<strong>la</strong>tive posítion betwe<strong>en</strong> received FAS and stuffing T Ís fixed for the restricted HQ/H ¡ i<br />
or 128/192/256/512/768/1152/1472 kbit/s case, while it is variable for the 56 kbit/s case (see Annex B/H.221).<br />
All termináis int<strong>en</strong><strong>de</strong>d for interworking with termináis connected to restricted networks must be capahle of performing<br />
thís procedure.<br />
11 Procedure for use of BAS-ext<strong>en</strong>sion co<strong>de</strong>s<br />
Recomm<strong>en</strong>dation H.221 provi<strong>de</strong>s for the attribute (111) for ext<strong>en</strong>sión of the use of the BAS position in the subsequ<strong>en</strong>t<br />
sub-multlframe(s) for other purposes. There are 32 valúes of this attribute, the meanings of these being <strong>de</strong>fined in
Note that the valué (111) [24] is the capability marker (see 2) which is followed by normal BAS co<strong>de</strong>s, not by any<br />
:scape valúes.<br />
Valúes [0-15] are reserved for future ext<strong>en</strong>sión of the scheme to úiclu<strong>de</strong> attribute c<strong>la</strong>ss and family.<br />
Valúes [16-23] are <strong>de</strong>fined as single-byte ext<strong>en</strong>sión (SBE); co<strong>de</strong>s of SBE type may be transmitted at any time and to any<br />
terminal.<br />
Valué [18] gives access to a table of valúes specifying applícations of a data channel (LSD or HSD), The application is<br />
active from the sub-multiframe following that in which the relevant specific application command BAS is transmitted.<br />
The closure of the data channel (usíng LSD/HSD-off) effectively closes the application.<br />
A1I termináis must recognize the SBE attributes, at least to the ext<strong>en</strong>t of ignoring the subsequ<strong>en</strong>t co<strong>de</strong>, whose meaning<br />
ís not prescribed in this Recomm<strong>en</strong>dation. However, wh<strong>en</strong> (111) [17] is received, the subsequ<strong>en</strong>í co<strong>de</strong> may be one of the<br />
mandatory valúes specified ín Recomm<strong>en</strong>dation H.230. The abilíty of a terminal to use the cont<strong>en</strong>í of other such co<strong>de</strong>s<br />
¡s governed by other Recomm<strong>en</strong>dations. For example, Recomm<strong>en</strong>dation H.320 <strong>de</strong>fines the requirem<strong>en</strong>ts for visual<br />
telephone termináis to act upon some of the control and indication valúes.<br />
Valúes [25-31] are of múltiple byte ext<strong>en</strong>sión (MBE); co<strong>de</strong>s of MBE may only be transmitted to a terminal which has<br />
previously indicated its capability to receive MBE. It follows that a non-CCITT capabilities message may not be<br />
transmitted Ín the ínitial capability exchange, until the MPE-cap has be<strong>en</strong> received. An example of the structure of<br />
MBE messages is giv<strong>en</strong> in App<strong>en</strong>dix III.<br />
12 Bit occupancy and the sequ<strong>en</strong>cing of BAS co<strong>de</strong>s<br />
[n g<strong>en</strong>eral, wh<strong>en</strong> there is no set procedure governing the sequ<strong>en</strong>ce of BAS co<strong>de</strong>s, priorities may be <strong>de</strong>termined by the<br />
;<strong>en</strong>ding terminal. Wh<strong>en</strong> there Ís no other <strong>de</strong>mand for use of the BAS positíon, it is advisable to cycle through all the<br />
valid BAS commands, so that in the ev<strong>en</strong>t of a temporary disturbance the proper mo<strong>de</strong> will be restored as soon as<br />
possible thereafter.<br />
Pable 1 summarizes the BAS capabílitíes that can be simultaneously valid.<br />
Phe capability set consists of the capability marker (111) [24] followed by all curr<strong>en</strong>tly valid valúes, in any or<strong>de</strong>r; this<br />
may in turn be followed by a repetition of the set, or by the marker alone to indícale completion of the set prior to<br />
s<strong>en</strong>ding commands. L<strong>en</strong>gth of the capability set ¡teration is not Hmited, but the <strong>la</strong>st capability set shall be followed by a<br />
:ap-marker and at least one command Usted in Table 2. No valúes should be repeated within a set. If it is <strong>de</strong>síred to<br />
shange the capability set during íts transmission, the existing set must first be completed without change, followed by<br />
the marker alone and at least one BAS command before the new, changed set is starled. See App<strong>en</strong>dix VIII for some<br />
sxamples of legal and ¡Ilegal BAS sequ<strong>en</strong>ces.<br />
rhe inclusión of more than a very occasional "<strong>de</strong>fault" command in the initial capability sequ<strong>en</strong>ce might not be a good<br />
;hing, since the receiver would be loa<strong>de</strong>d with many "new but unchanged" capsets each requiring a response, and there<br />
night ev<strong>en</strong> be instability betwe<strong>en</strong> two termináis doing this. It should be noted that the purpose of including the<br />
5ccasÍonaI "<strong>de</strong>fault" command is to establish PCM audio communicatíon as soon as a cali has be<strong>en</strong> set up.<br />
Fable 2 summarizes the BAS commands that can be simultaneously valid.<br />
Dnly one valué in each row can be in forcé at any one instant, up to 17 valúes on the initial channel (all the above<br />
falues except (001) [18-22] apply only to the ínitial channel); however in practíce many of the combinations are<br />
íreclu<strong>de</strong>d by the fact that they would affect the same bits of the channel (for example, (Olí) [31] and (Olí) [19] cannot<br />
loexist).<br />
f\d remains Ín forcé until another from the same row is transmitted. A command must not be transmitted if to<br />
obey it would cause a simultaneous mo<strong>de</strong> change on another row; in such a case the other row valué must be changed<br />
first (for this purpose, a change of bit-rate of vi<strong>de</strong>o or any of the variable data valúes does not constitute a mo<strong>de</strong><br />
;hange). /<br />
[n g<strong>en</strong>eral, unless specified otherwíse, a BAS co<strong>de</strong> which is invalid or which contrav<strong>en</strong>es the provisions of this table, or<br />
ítherwise indicates an imüossible frame structure or svstem status, must not be transmilted.
Audio<br />
Vi<strong>de</strong>o<br />
Transfer rate<br />
Restricted network<br />
Low-speed data (LSD)<br />
High-speed data (HSD)<br />
Low-speed MLP<br />
High-speed MLP<br />
Applications in data channel<br />
Capabilitíes <strong>de</strong>fíned in Rec. H.230<br />
Encryption<br />
Multíple-byte ext<strong>en</strong>sión<br />
TABLE 1/H.242<br />
Capability summary*)<br />
One or more valúes from A-<strong>la</strong>w, fl-<strong>la</strong>w, G.725-T1,<br />
G.725-T2, Au-16 kbit/s, Au-ISOb)<br />
Abs<strong>en</strong>t, or (QGIF plus one MPI valué), or (QCIF + CIF<br />
plus two MPI valúes), and/or vi<strong>de</strong>o-ISO and/or AV-ISO<br />
Abs<strong>en</strong>t (meaning rate = 64 kbit/s onlyc*) or up to four<br />
valúes: max. no. of 64, 384 kbít/s channels, 1536,<br />
1920 kbit/s; and optíonally any relevanl valúes from<br />
(128, 192,256,512,768, 1152, 1472 kbit/s)<br />
Abs<strong>en</strong>t or pres<strong>en</strong>t<br />
Abs<strong>en</strong>t or all relevant valúes<br />
Abs<strong>en</strong>t or all relevant valuesd^<br />
Abs<strong>en</strong>t or al! relevant valúes<br />
Abs<strong>en</strong>t or all relevant valúes<br />
Abs<strong>en</strong>t or all relevant valúes<br />
Abs<strong>en</strong>t or all relevant valúes<br />
Abs<strong>en</strong>t or pres<strong>en</strong>t<br />
Abs<strong>en</strong>t or pres<strong>en</strong>t<br />
*' See App<strong>en</strong>dix VI for híerarchical capability BAS co<strong>de</strong>s.<br />
b' See App<strong>en</strong>dix VII for interpretaron of received audio capabilities.<br />
c' Wh<strong>en</strong> reducing the transfer-rate capability to 64 kbit/s from a higher valué, the valué transfer-capacíty<br />
= 64 kbit/s must be inclu<strong>de</strong>d.<br />
d) A capability set must not inclu<strong>de</strong> any HSD capabilíties whose bit rates exceed the transfer rate<br />
capability (e.g. 256 kbit/s HSD for 2B transfer ratej.<br />
[n g<strong>en</strong>eral, a terminal is not required to recognize BAS-commands which do not correspond to capabilities it has<br />
previously <strong>de</strong>c<strong>la</strong>red; however it is better practice to recognize such commands and c<strong>la</strong>ssify them as (i) those which can<br />
be ignorad, and (n) those which may result in a mo<strong>de</strong> mismatch (see 6.4). It is important not to initiate a recovery<br />
arocedure unnecessarily (for example, on receipt of LSD-off wh<strong>en</strong> it had never be<strong>en</strong> <strong>de</strong>c<strong>la</strong>red or turned on), since this<br />
;ould result in system lock-up betwe<strong>en</strong> two termináis having differ<strong>en</strong>t intemal procedures,<br />
Fhe following notes serve To c<strong>la</strong>rify the application of these rules to the multiplexing of audío, ví<strong>de</strong>o and the various<br />
;orms of data. Some examples re<strong>la</strong>ting to data transmission are giv<strong>en</strong> in App<strong>en</strong>dix V.<br />
a) Audio cannot p<strong>en</strong>étrate into fixed rate data (LSD or MLP) bit positíons. It can expand its capacity into<br />
vacant or vi<strong>de</strong>o or variable data bh positions. It can reduce its capacity withín the audio bit positions<br />
curr<strong>en</strong>tly occupíed.<br />
b) Vi<strong>de</strong>o occupies all bit positions which are not assigned by other commands (ECS, audio, LSD/MLP<br />
regardless of being fixed rate or variable rate).<br />
Vi<strong>de</strong>o can be turned on at any time ev<strong>en</strong> if the avai<strong>la</strong>ble capacity for vi<strong>de</strong>o is zero at íhe corresponding<br />
sub-multiframe; (it may happ<strong>en</strong>, for example, that ví<strong>de</strong>o is switched on just before the variable rate LSD<br />
or MLP channel is closed); the <strong>de</strong>co<strong>de</strong>r must not ignore "vi<strong>de</strong>o on" ev<strong>en</strong> ín this case, otherwise a mo<strong>de</strong><br />
mismatch occurs. However, if vi<strong>de</strong>o capacity is less than about 30 kbit/s averaged over several submultiframes,<br />
itmay not be practical.<br />
Ti c>iíMilH K*> nrvtoH fViot (Ti 1 fU nr»fpraMxi rvr^r ai\r*r\f f rf>í"/p-n í/'tlt rf> rf»nuopt ffl 1 (\\
To <strong>en</strong>sure that the picture builds up quickly wh<strong>en</strong> the vi<strong>de</strong>o transmíssion first begins, the <strong>en</strong>co<strong>de</strong>r should<br />
transmit in INTRA mo<strong>de</strong> (see Recomm<strong>en</strong>dation H.261).<br />
Since thís INTRA can be received completely oníy if the remote <strong>de</strong>co<strong>de</strong>r ís ready, the <strong>en</strong>co<strong>de</strong>r should<br />
estímate wh<strong>en</strong> it should start INTRA. One way may be to repeat INTRA appropríate times or s<strong>en</strong>d<br />
Fill bits (<strong>de</strong>fined in 5.4.3/H.261) before s<strong>en</strong>ding INTRA. Another way may be that the <strong>de</strong>co<strong>de</strong>r estimates<br />
wh<strong>en</strong> the remote <strong>en</strong>co<strong>de</strong>r is ready and issues a VCU command at an appropriate timing.<br />
c) Fixed rate LSD/MLP cannot p<strong>en</strong>étrate into audio bitpositions ñor into fixed rate MLP/LSD bit positions.<br />
It can expand íts capacity into vacant or ví<strong>de</strong>o or variable MLP/LSD bit positions. It can reduce its<br />
capacity within the data bit positions curr<strong>en</strong>tly occupied. As a combination, fixed rate LSD/MLP can<br />
occupy new bit positions which have previously be<strong>en</strong> either vacant, vi<strong>de</strong>o, variable rate MLP/LSD or<br />
occupied by the same type of fixed rate data.<br />
d) Variable rate LSD/MLP occupies all bit positions which are not assigned by other fixed rate commands<br />
(ECS, audio, fixed rate MLP/LSD). If vi<strong>de</strong>o has be<strong>en</strong> on, it is exclu<strong>de</strong>d wh<strong>en</strong> variable rate LSD or MLP<br />
is turned on. If variable rate LSD/MLP has be<strong>en</strong> on, op<strong>en</strong>ing a variable rate MLP/LSD channel should be<br />
prece<strong>de</strong>d by closing the existing variable rate LSD/MLP channel.<br />
Variable rate LSD or MLP can be turned on at any time ev<strong>en</strong> if the avai<strong>la</strong>ble capacity for it is zero at the<br />
corresponding sub-multíframe; (it may happ<strong>en</strong>, for example, that the variable MLP is switched on just<br />
before closing the LSD channel which has be<strong>en</strong> occupying all the capacity other than audio); the <strong>de</strong>co<strong>de</strong>r<br />
must not ignore "variable rate LSD or MLP on" ev<strong>en</strong> in this case, otherwise a mo<strong>de</strong> mismatch occurs.<br />
e) LSD/MLP rate may be changed without first closing the data channel — this applies equally to changes<br />
betwe<strong>en</strong> fixed and variable rate. It is emphasized that there can only be one LSD and one MLP channel<br />
at any mstant.<br />
f) Capacity of ví<strong>de</strong>o or variable LSD/MLP can be temporarily reduced to zero in a sub-multíframe as part of<br />
dynamíc bit rate allocations. It is ¡mpractical, however, if that sítuation continúes for a long time.<br />
g) The rules for the use of HSD and H-MLP (in other than the I-channel) are i<strong>de</strong>ntical to those giv<strong>en</strong> above<br />
for LSD and MLP in the I-channel.<br />
h) Any co<strong>de</strong> can be s<strong>en</strong>t in bit positions which have not yet be<strong>en</strong> op<strong>en</strong>ed by the BAS commands, In a 2B<br />
communication, for example, the additional channel may s<strong>en</strong>d "O" or "1" or any combinations in bit<br />
positions except those for FAS and BAS till a 2B transfer rate command Ís s<strong>en</strong>t. It should be noted that<br />
although the terminal may set the "unop<strong>en</strong>ed" bits to any valúes, there is no assurance that those bits wíll<br />
be <strong><strong>de</strong>l</strong>ívered to other termináis in a multipoint confer<strong>en</strong>ce by the MCU.<br />
L3 Procedure for <strong>de</strong>aling with 6B-H0 interconnection<br />
por further study.<br />
L4 Procedure for use of <strong>en</strong>cryption control signal channel<br />
Each terminal must transmit the <strong>en</strong>cryption capabilhy co<strong>de</strong> if il Ís able to handle the ECS channel. No terminal may<br />
ictivate the channel without first receiving the corresponding capabílity co<strong>de</strong>. Once an ECS capability co<strong>de</strong> has be<strong>en</strong><br />
iransmitted it cannot be cancelled by omission from a subsequ<strong>en</strong>t capability exchange. That is to say, a terminal having<br />
anee received, stored and ma<strong>de</strong> use of an ECS capability co<strong>de</strong> should assume continued valídity until cancelled by the<br />
[ocal user. Thus <strong>en</strong>cryption can be discontinued by the users themselves but not by a third party tampering with<br />
the BAS-capability exchange,<br />
The initiating terminal transmits the command "ECS channel ON"; from the next multiframe it op<strong>en</strong>s the 800 bit/s<br />
ECS channel <strong>de</strong>fined in /Recomm<strong>en</strong>dation H.221, whose use is specified in the Recomm<strong>en</strong>dation <strong>de</strong>fining the<br />
sncryption system (FAS, BAS and the ECS channel itself are in any case not <strong>en</strong>crypled).<br />
V/h<strong>en</strong> <strong>en</strong>cryption has be<strong>en</strong> turned off, the BAS command "ECS channel OFF" is used ío cióse the ECS channel.
Attribute<br />
Audio (000)<br />
Transfer<br />
rate (001)<br />
Vi<strong>de</strong>o and<br />
other(OlO)<br />
LSD and<br />
MLP(Oll)<br />
HSD and<br />
H-MLP<br />
Altemative valúes<br />
(<strong>la</strong>st valué only is valid)<br />
[0,4-7, 13-19,24-31]<br />
[0-15,23,24,26,29]<br />
D7]<br />
[18-22]<br />
[0-4]<br />
[6,7]<br />
[16]<br />
[17]<br />
[18,21]<br />
[19,21]<br />
[20,21]<br />
[25, 26]<br />
[27,28]<br />
[0-15,31]<br />
[16-19]<br />
[0, 17-22]<br />
[2-8, 13, 14]<br />
TABLE 2/H.242<br />
Command summary<br />
Default<br />
assumed<br />
[18 or 19]<br />
[0]<br />
[0]<br />
[7]<br />
[21]<br />
[21]<br />
[21]<br />
[26]<br />
[28]<br />
[OJ<br />
[16]<br />
[OJ<br />
[14]<br />
Annex A<br />
Comm<strong>en</strong>ts<br />
See 7.2.3<br />
additíonal channels only<br />
Cancelled by command ¡n vi<strong>de</strong>o frame<br />
Expires after fast update completed<br />
Escape rabie (11 1)[16]<br />
I<strong>de</strong>ntification of the <strong>en</strong>d of sequ<strong>en</strong>ce A<br />
(This annex forms an integral part ofthis Recomm<strong>en</strong>dation)<br />
fhe following arrangem<strong>en</strong>ts are effective forboth of normal and neutral capability exchanges.<br />
Perminal X activating sequ<strong>en</strong>ce A<br />
terminal X s<strong>en</strong>ds capability sets until both the following conditions are met:<br />
i) one complete set has be<strong>en</strong> transmitted since receivíng A = 0;<br />
ii) it <strong>de</strong>tects an incoming cap-mark followed by at least one normal capability co<strong>de</strong> (to await a full cap-set is<br />
better as it increases robustness).<br />
t th<strong>en</strong> completes transmission of the curr<strong>en</strong>t set (since partial cap-sets are not allowed) followed by cap-mark and at<br />
east one command. If condítion ii) does not materialize within 10 seconds of i), the sequ<strong>en</strong>ce must be terminated, a<br />
ommand s<strong>en</strong>t, and th<strong>en</strong> a new sequ<strong>en</strong>ce started.<br />
t i<strong>de</strong>ntifies the <strong>en</strong>d of sequ<strong>en</strong>ce A wh<strong>en</strong> it receives a command following the capability sets from the remote terminal<br />
(. At any time after the <strong>en</strong>d of sequ<strong>en</strong>ce A, the terminal X should respond to reception of a capability set by returning<br />
ts own capability set.<br />
ferminal Y responding to the reception of capability co<strong>de</strong><br />
'h<strong>en</strong> it starts to s<strong>en</strong>d commands. The <strong>en</strong>d of sequ<strong>en</strong>ce A is i<strong>de</strong>ntifíed by receiving a command from the remote<br />
srminal X. At any time after the <strong>en</strong>d of sequ<strong>en</strong>ce A, the terminal Y should respond to reception of a capability set by
App<strong>en</strong>dix I<br />
Initialization: Case of vi<strong>de</strong>ophone to Recomm<strong>en</strong>dation H.320, type<br />
(This app<strong>en</strong>dix does not form an integral part of this Recomm<strong>en</strong>dation)<br />
Jn<strong>de</strong>rlined letters in the comm<strong>en</strong>ts column correspond topoints in the associated Figure I.l.<br />
FAS,<br />
A-bit<br />
XX<br />
F.l<br />
F,l<br />
F.l<br />
F.l<br />
F.l<br />
F,l<br />
F,l<br />
F,l<br />
F,l<br />
F.l<br />
F.l<br />
F.l<br />
F.l<br />
F,l<br />
F,l<br />
F.l<br />
F,0<br />
F.O<br />
F,0<br />
F,0<br />
F.O<br />
F,0<br />
F.O<br />
F,0<br />
F,0<br />
F.O<br />
F,0<br />
F.O<br />
F.O<br />
F.O<br />
F.O<br />
F.O<br />
F.O<br />
F.O<br />
F,0<br />
F.O<br />
F,0<br />
F.O<br />
BAS<br />
Attr.<br />
XX<br />
Transmítted<br />
Valué<br />
XX<br />
Audio<br />
mo<strong>de</strong><br />
XX<br />
(UD [24] 0<br />
(100) [5] 0<br />
(100) [4] 0<br />
(101) [20] 0<br />
(101)<br />
(100)<br />
(111)<br />
[24]<br />
rni<br />
[24]<br />
0<br />
0<br />
0<br />
(100) [5] 0<br />
(Continué to cycle caps)<br />
(101) [24] 0<br />
(100)<br />
(111)<br />
[17]<br />
[24]<br />
0<br />
0<br />
(100) [5] 0<br />
(100) [4] 0<br />
(101) [20] 0<br />
(101) [24] 0<br />
(100) [17] 0<br />
(101)<br />
(100)<br />
(100)<br />
(111)<br />
(100)<br />
(100)<br />
(101)<br />
(101)<br />
(100)<br />
(111)<br />
(000)<br />
fOlO)<br />
(000)<br />
(010)<br />
(000)<br />
(010)<br />
(010)<br />
(000)<br />
(010)<br />
(000)<br />
(010)<br />
(000)<br />
[24]<br />
[17]<br />
[17]<br />
[24]<br />
[5J<br />
[4]<br />
[20]<br />
[24]<br />
[17]<br />
[24]<br />
[29J<br />
ÜJ<br />
[29]<br />
[U<br />
[29]<br />
[1]<br />
[1J<br />
[29]<br />
U ,<br />
[29]<br />
[1]<br />
[29]<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
7.<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
Successive sub-multiframes at terminal "X" only<br />
Ví<strong>de</strong>o<br />
rate<br />
XX<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
46.4<br />
46.4<br />
46.4<br />
46.4<br />
46.4<br />
46.4<br />
46.4<br />
46.4<br />
46.4<br />
46.4<br />
FAS.<br />
A-bít<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
BAS<br />
Atrr.<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
Valué<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
Audio<br />
mo<strong>de</strong><br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
(Searching forframe alígnm<strong>en</strong>t)<br />
Receíved<br />
Vi<strong>de</strong>o<br />
rate<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
XX<br />
F.l<br />
XX<br />
ÜJJJ<br />
XX<br />
[24]<br />
XX<br />
0<br />
XX<br />
(off)<br />
F,l (100) [5] 0 (off)<br />
F,l (100) [4] 0 (off)<br />
F,l (10Í) [20] 0 (off)<br />
F,l (101) [24J 0 (off)<br />
F,l (100) [17] 0 (off)<br />
F.l (111) [24] 0 (off)<br />
(Searching for multiframe align.)<br />
F,l<br />
F,l<br />
(100)<br />
(111)<br />
[17]<br />
[24]<br />
0<br />
0<br />
(off)<br />
(off)<br />
(Waitíng for íncoming A = 0)<br />
F,l (111) [24] 0 (off)<br />
F.O (100) [5] 0 (off)<br />
F,0 (100) [4] 0 (off)<br />
F.O (101) [20] 0 (off)<br />
F.O (101) [24] 0 (off)<br />
F.O<br />
F.O<br />
(100)<br />
(111)<br />
[17]<br />
[24]<br />
0<br />
0<br />
toff)<br />
(off)<br />
F.O (100) [5] 0 (off)<br />
F,0 (100) [4] 0 (off)<br />
F.O (101) [20] 0 (off)<br />
F.O (101) [24] 0 (off)<br />
F.O<br />
F.O<br />
(100)<br />
(111)<br />
[17]<br />
[24]<br />
0<br />
0<br />
(off)<br />
(off)<br />
F.O (100) [5] 0 (off)<br />
(Waítíng for Íncoming mo<strong>de</strong> changes)<br />
F.O (101) [24] 0 (off)<br />
F.O (000) [29] 0 (off)<br />
F.O (010) [U 7 (off)<br />
F.O (000) [29] 7 46.4<br />
F.O (010) [1] 7 46.4<br />
F.O (000) [29] 7 46.4<br />
Comm<strong>en</strong>ts<br />
A.cap-mark<br />
Audío BAS-cap 1<br />
Audio BAS-cap 2<br />
Vi<strong>de</strong>o CapabiÜty-QCIF<br />
MPI 3/29.97<br />
Transfer rate capability 2B<br />
Repeat capabÍIÍty-set<br />
About one transit?<br />
B_ incomíng capability-set<br />
Capabílity-set complete<br />
up to 320 ms<br />
C.mfa achieved, A = 0<br />
D Íncoming A = 0<br />
Capability-set complete<br />
E.start mo<strong>de</strong> switch<br />
(see Note)<br />
Eíncoming switch<br />
16 kbit/s audio<br />
vi<strong>de</strong>o ON<br />
repeat valid commands
FAS,<br />
A-bit<br />
XX<br />
BAS<br />
Attr.<br />
XX<br />
Transmitted<br />
Valué<br />
XX<br />
Audio<br />
mo<strong>de</strong><br />
XX<br />
Vi<strong>de</strong>o<br />
rate<br />
XX<br />
FAS,<br />
A-bÍt<br />
XX<br />
BAS<br />
Attr.<br />
XX<br />
Valué<br />
XX<br />
Audio<br />
mo<strong>de</strong><br />
XX<br />
Received<br />
Ví<strong>de</strong>o<br />
rate<br />
(Now <strong>de</strong>al with second B-channel, once connection is completed)<br />
FF.OI (010) U] 7 46.4 Fx.Ox (000) [29] 7 46.4<br />
FF.Ol (000) [29] 7 46.4 Fx.Ox (010) [i] 7 46.4<br />
(Searching for frame alígnm<strong>en</strong>t on<br />
channel No. 2)<br />
FF.01 (010) ni 7 46.4 FF.OI (000) [29] 7 46.4<br />
FF.01 (000) [29] 7 46.4 FF.01 (010) [1] 7 46.4<br />
(Finding multiframe alignm<strong>en</strong>t and<br />
buffering to synchronize)<br />
FF.OO. (010) [1] 7 46.4 FF.01 (000) [29] 7 46.4<br />
FF.OO (000) [29] 7 46.4 FF.01 (010) [1]<br />
-7 46.4<br />
(Waíting for incoming A2 = 0)<br />
FF.OO (010) DI 7 46.4 FF.OO. (000) [29] 7 46.4<br />
FF.OO (001) ÜJ 7 46.4 FF.OO (010) [1] 7 46.4<br />
FF.OO (001) [1] 7 108.8 FF.OO (000) [29] 7 46.4<br />
FF.OO (010) [1] 7 108.8 FF.OO (010) [U 7 46.4<br />
FF.OO<br />
FF.OO<br />
(000)<br />
(001)<br />
[29]<br />
m<br />
7<br />
7<br />
108.8<br />
108,8<br />
FF.OO<br />
FF.OO<br />
(000)<br />
(010)<br />
[29]<br />
[1]<br />
7<br />
7<br />
46.4<br />
46.4<br />
(Continué to cycle BAS commands)<br />
FF.OO (010) [u 7 108.8<br />
(Waiting for incomíng mo<strong>de</strong> changes)<br />
FF.OO foon ÜJ 7 46.4<br />
FF.OO (000) [29] 7 108.8 FF.OO (001) fi] 7 108,8<br />
(Initialízation completed)<br />
XX<br />
G.<br />
Comm<strong>en</strong>ts<br />
H.alignm<strong>en</strong>t recovered<br />
Is<strong>en</strong>d A = 0 on channel No. 2<br />
J incomíng A2 = 0<br />
start mo<strong>de</strong> switch to expand<br />
vi<strong>de</strong>o (see Note)<br />
EC incoming mo<strong>de</strong> sw.<br />
NOTE — The mo<strong>de</strong>s selected for switching are governed by terminal procedures which in g<strong>en</strong>eral <strong>de</strong>p<strong>en</strong>d on the application;<br />
in the pres<strong>en</strong>t case of vi<strong>de</strong>ophone service. the procedure is specífíed ¡n Recomm<strong>en</strong>datíon H.320,
TERMINAL-X-<br />
Start<br />
communication<br />
TERMINAL "X"<br />
Start<br />
communication<br />
Searching for FAS<br />
Detect FAS<br />
Store BAS capability-set<br />
Recover MFA<br />
D etect A = O,<br />
S<strong>en</strong>d one more capabllity-set<br />
S<strong>en</strong>d mo<strong>de</strong> switch<br />
Swilch receive mo<strong>de</strong><br />
Searching for FAS No. 2<br />
Detect FAS No. 2<br />
Recover MFA No. 2 and<br />
buff er to synchronization<br />
with inítial<br />
channel... completed<br />
D'etectA2 = O<br />
S<strong>en</strong>d mo<strong>de</strong> switch<br />
D eco<strong>de</strong><br />
108.8 kbit/s vi<strong>de</strong>o<br />
First connection estabiished<br />
Second connection established<br />
FIGURE I.1/H.242<br />
TERMINAL-Y-<br />
Start<br />
(a littíe <strong>la</strong>ter}<br />
TERMINAL "Y-<br />
Start<br />
(alittle<strong>la</strong>ter)<br />
Searching for FAS<br />
Detect FAS<br />
Store BAS capability-set<br />
Recover MFA<br />
Detect A = O, s<strong>en</strong>d<br />
one more capability-set<br />
S<strong>en</strong>d mo<strong>de</strong>-switch<br />
Switch receive mo<strong>de</strong><br />
Searching for FAS No. 2<br />
Detect FAS No. 2<br />
Recover MFA<br />
and synchronization<br />
D etect A = O<br />
S<strong>en</strong>d mo<strong>de</strong> switch<br />
Deco<strong>de</strong><br />
108.8 kbit/s vi<strong>de</strong>o<br />
T15D6a70-9D/dOS
App<strong>en</strong>dix II<br />
Mo<strong>de</strong>-0 forcing: Case of vi<strong>de</strong>ophone to Recomm<strong>en</strong>dation H.320, type<br />
(This app<strong>en</strong>dix does not form an integral part of this Recomm<strong>en</strong>dation)<br />
Lín<strong>de</strong>rlined letters in the comm<strong>en</strong>ts column correspond to points in the associated Figure 11.2.<br />
FAS,<br />
A-bit<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
BAS<br />
Attr.<br />
(010)<br />
(000)<br />
(001)<br />
(OH)<br />
(010)<br />
(P_1D<br />
(0101<br />
(001)<br />
COPO)<br />
(000)<br />
(010)<br />
(000)<br />
OÍD<br />
ÜOO)<br />
flOO)<br />
OÍD<br />
Transmítted<br />
Valué<br />
[1J<br />
[29J<br />
[U<br />
[2]<br />
tu<br />
[OJ<br />
[01<br />
[OJ<br />
[18]<br />
[18]<br />
[0]<br />
[18J<br />
[24]<br />
06J<br />
ÜJ<br />
[24]<br />
Audio<br />
mo<strong>de</strong><br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
OF<br />
OF<br />
OF<br />
OF<br />
OF<br />
OF<br />
OF<br />
(continué to cycle these capabilities)<br />
(100)<br />
(100)<br />
(111)<br />
(100)<br />
(100)<br />
(010)<br />
(001)<br />
(000)<br />
.(Olí)<br />
(010)<br />
(001)<br />
(000)<br />
Successive sub-multiframes at terminal "X" only<br />
Vi<strong>de</strong>o<br />
rate<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
108.8<br />
(off)<br />
(ofí)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
FAS.<br />
A-bit<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
BAS<br />
Attr.<br />
(000)<br />
(001)<br />
(Olí)<br />
(010)<br />
(000)<br />
(001)<br />
(Olí)<br />
(010)<br />
(000)<br />
(001)<br />
(Olí)<br />
(010)<br />
(000)<br />
(001)<br />
(Olí)<br />
(010)<br />
Valué<br />
[29]<br />
[1J<br />
[2]<br />
[1J<br />
[29]<br />
[1]<br />
[2J<br />
[1]<br />
[29]<br />
[1]<br />
[2]<br />
[1]<br />
[29J<br />
[1]<br />
[2]<br />
[1]<br />
Audio<br />
mo<strong>de</strong><br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
7<br />
(Awaiting incoming mo<strong>de</strong> change<br />
and capabílity set)<br />
[16] OF (off) FF.OO (000) [29]<br />
[1] OF (off) FF.OO (Olí) [0]<br />
[24] OF (off) FF.OO (010) [PJ<br />
[16]<br />
[1]<br />
[0]<br />
OF<br />
OF<br />
OF<br />
(off)<br />
(off)<br />
(off)<br />
FF.OO<br />
FF.OO<br />
FF.OO<br />
(001)<br />
(000)<br />
(111)<br />
[OJ<br />
US]<br />
[24]<br />
[OJ OF (off) FF.OO (100) [5]<br />
[18] OF (off) FF.OO (100) [4J<br />
[OJ OF (off) FF.OO (101) [20]<br />
[0] OF (off) FF.OO (101) [24]<br />
[OJ<br />
[18J<br />
OF<br />
OF<br />
(off)<br />
(off)<br />
FF.OO<br />
FF.OO<br />
(100)<br />
(111)<br />
[17J<br />
[24J<br />
(Continué to cycle all valid BAS commands)<br />
7<br />
7<br />
7<br />
7<br />
7<br />
OF<br />
OF<br />
OF<br />
OF<br />
OF<br />
OF<br />
OF<br />
Received<br />
Ví<strong>de</strong>o<br />
rate<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
107.6<br />
108.8<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
(off)<br />
Comm<strong>en</strong>ts<br />
Vi<strong>de</strong>o isON (Rec. H.261)<br />
Audio is 16 kbit/s<br />
Transfer rate is 2 X 64<br />
Data is ON at 1.2 kbit/s<br />
L dta to go off<br />
Vi<strong>de</strong>o to go off<br />
Transfer rate 64 kbít/s<br />
Audío A-<strong>la</strong>w, OF<br />
M. capabílity mark<br />
64 kbít/s-capability only<br />
A-Iaw capabílity only<br />
Capabilíty mark<br />
N. incoming data to go off<br />
Incoming vi<strong>de</strong>o to go off<br />
Incoming channel No. 2 off<br />
Incoming audio co be OF<br />
he mo<strong>de</strong>-0 forcing procedure is not complete: subsequ<strong>en</strong>t action <strong>de</strong>p<strong>en</strong>ds on the terminal procedure, according to the<br />
;ason for performing the switch to mo<strong>de</strong> O
TERMINAL "X-<br />
Swltch to mo<strong>de</strong> OF L<br />
S<strong>en</strong>d capabllity-set y<br />
FIGURE II.2/H.242<br />
App<strong>en</strong>dix III<br />
T1S0608l-00/d06<br />
Example of use of message structure<br />
(This app<strong>en</strong>dix does not form an integral part of this Recomm<strong>en</strong>dation)<br />
S<strong>en</strong>d Receive<br />
Initial capability exchange, including MBE-cap<br />
(111) [24] Capabllity-mark<br />
(100) [4] Audio type 2 (G.722, 56 kbit/s)<br />
(100) [17] 2 X 64 kbií/s transfer rate<br />
(101) [21] CIF vi<strong>de</strong>o capability<br />
(101) [22] 1729.97 MPIforQCIF<br />
(101) [23] 2/29.97 MPI for CIF<br />
TERMINAL "Y"<br />
Switch receive mo<strong>de</strong><br />
Detect reduced capability<br />
s<strong>en</strong>d mo<strong>de</strong> swItch<br />
S<strong>en</strong>d capab¡l¡ry-set
S<strong>en</strong>d Receive<br />
(101) [31] MBE-capability<br />
(111) [16] Set to escape tableforHSD .<br />
(101) [17] 64kbit/s HSD-capability<br />
(111) [24] Capability-mark, repetition of capability set<br />
(100) [4] Audio type 2 (Rec. G.722, 56 kbit/s)<br />
Subsequ<strong>en</strong>t capability exchange, including MBE capability message<br />
(111) [24] Capability-mark<br />
(100) [4] Audio type 2 (Rec. G.722, 56 kbit/s)<br />
(100) [17] 2 X 64 kbit/s transfer rate<br />
(101) [21] CIF vi<strong>de</strong>o capability<br />
(101) [22] 1/29.97 MPI for QCIF<br />
(101) [23] 2/29.97 MPI for CIF<br />
(101) [31] MBE-capability<br />
(111) [16] Set to escape tableforHSD<br />
(101) [17] 64 kbit/s HSD-capability<br />
(111) [30] Startofnon-CCITT capability message<br />
(M) Information will be M-bytes<br />
[byte 1) Country co<strong>de</strong> according to Rec.T.35<br />
{byte 2] Country co<strong>de</strong><br />
[bytes 3, 4] Manufacturer co<strong>de</strong> (Company XYZ)<br />
[bytes 5-M) Type i<strong>de</strong>ntity<br />
(111) [24] Capability-mark, repetition of capability set<br />
(100) [4] Audio type 2 (Rec. G.722, 56 kbit/s)<br />
Mo<strong>de</strong> switch to non-standard mo<strong>de</strong> using MBE command<br />
(111) [30] Start of non-CCITT command message<br />
{N} Information will be N-bytes<br />
{byte 1} Country co<strong>de</strong> according to Rec. T.35<br />
[byte 2} Country co<strong>de</strong><br />
{bytes 3, 4) Manufacturer co<strong>de</strong> (Company XYZ)<br />
{bytes 5-N} Type i<strong>de</strong>ntity<br />
Deco<strong>de</strong> incoming BAS capabilities;<br />
these inclu<strong>de</strong> (101) [31], so remóte <strong>en</strong>d<br />
can handle MBE co<strong>de</strong>s<br />
Incoming capability cycle now inclu<strong>de</strong>s<br />
the same non-standard mo<strong>de</strong>
App<strong>en</strong>dix IV<br />
Examples of symmetrical and unsymmetrical transmission mo<strong>de</strong>s<br />
(This app<strong>en</strong>dix does not form an integral part of this Recomm<strong>en</strong>dation)<br />
[V.l Example of symmetrical transmission mo<strong>de</strong><br />
Capabilities of Terminal X<br />
Capabilities of Terminal Y<br />
Mo<strong>de</strong> ín X-to-Y direction<br />
Mo<strong>de</strong> in Y-to-X direction<br />
Audio<br />
16 kbít/s<br />
Type2<br />
+ 16 kbit/s<br />
16kbít/s<br />
16kbÍt/s<br />
Vi<strong>de</strong>o<br />
Yes<br />
Yes<br />
[V.2 Example of unsymmetrical transmission mo<strong>de</strong><br />
-<br />
Capabilities of Terminal X<br />
Capabilities of Terminal Y<br />
Mo<strong>de</strong> Ín X-to-Y direction<br />
Mo<strong>de</strong> in Y-to-X direction<br />
Audio<br />
PCM<br />
16 kbít/s<br />
OFF<br />
OFF<br />
ON<br />
ON<br />
Vi<strong>de</strong>o<br />
Yes<br />
No<br />
OFF<br />
ON<br />
Transfer<br />
rate<br />
IB<br />
2B<br />
IB<br />
IB<br />
Transfer<br />
rate<br />
2B<br />
2B<br />
2B<br />
2B<br />
LSD<br />
1.2 kbit/s<br />
1.2kbit/s<br />
1.2 kbit/s<br />
1.2 kbit/s<br />
LSD<br />
1 .2 kbít/s<br />
56 kbit/s<br />
56 kbit/s<br />
1.2 kbit/s<br />
App<strong>en</strong>dix V<br />
Examples re<strong>la</strong>ting to data transmissions<br />
(This app<strong>en</strong>dix does not form an integral part of this Recomm<strong>en</strong>dation)<br />
NOTE- For the examples giv<strong>en</strong> below:<br />
* These rates are reduced by 800 bit/s wh<strong>en</strong> the ECS is active;<br />
# "Vi<strong>de</strong>o-on" may not be practica! in these cases.<br />
Transfer-rate IB, audio at 48 kbit/Sj no vi<strong>de</strong>o or vi<strong>de</strong>o off<br />
MLP LSD Forbld<strong>de</strong>n next commands (example)<br />
4k<br />
4k<br />
1200/<br />
Sk<br />
#, LSD = 4.8k/6.4k/14.4k and over, MLP = 6.4k<br />
Au = 56k, #, LSD = 4.8k/6.4k/14.4k and over<br />
HSD<br />
-<br />
-<br />
-<br />
- -<br />
HSD<br />
No<br />
No<br />
-<br />
-<br />
MLP<br />
. No<br />
Yes<br />
OFF<br />
OFF<br />
MLP<br />
No<br />
No<br />
OFF<br />
OFF
6.4*k 8k<br />
var 1200<br />
var 6.4k<br />
var 9.6k<br />
Au = 56k, #, LSD = 300/1200/4.8k/6.4k/9.6k/14.4k and over<br />
#, LSD = 16k and over/var, MLP = 6.4k<br />
#, LSD = 16k and over/var, MLP = 4k/6.4k<br />
Au = 56k,#,'LS-D = 16k and over/var, MLP = 6.4k<br />
Transfer-rate IBj audio at 16 kbit/Sj no vi<strong>de</strong>o or vi<strong>de</strong>o off<br />
MLP LSD Forbid<strong>de</strong>n next commands (example)<br />
4k<br />
4k<br />
4k<br />
4k<br />
6.4*k<br />
6.4*k<br />
var<br />
var<br />
var<br />
300<br />
8k<br />
16k<br />
var<br />
8k<br />
40k<br />
4.8k<br />
9.6k<br />
16k<br />
Transfer-rate IBj audio at 16 kbit/Sj vi<strong>de</strong>o on<br />
LSD = 4.8k/6.4k/14.4k/48k and over, MLP = 6,4k<br />
Au = 56k, LSD = 4.8k/6.4k/14.4k/48k and over<br />
Au = 48k/56k, #, LSD = 4.8k/6.4k/14.4k/48k and over<br />
#, LSD = 4.8k/6.4k/14.4k/48k and over, MLP = var<br />
Au = 56k, LSD = 300/1200/4.8k/6.4k/9.6k/14.4k/48k and over<br />
Au = 48k/56k, #, LSD = 300/1200/4.Sk/6,4k/9.6k,/14.4k/4Sk and over<br />
#, LSD = 48k and over/var, MLP = 4k/6.4k<br />
Au = 56k, #, LSD = 48k and over/var, MLP = 6.4k<br />
Au = 4Sk/56k, #, LSD = 48k and over/var<br />
MLP LSD Forbld<strong>de</strong>n next commands (example)<br />
4k 1200<br />
4k Sk<br />
6.4*k 8k<br />
Transfer-rate 2B, audio at 48 kbit/Sj vi<strong>de</strong>o on<br />
LSD = 4.8k/6.4k/14.4k/48k and over, MLP = 6.4k<br />
Au = 56k, LSD = 4.8k/6.4k/14.4k/48k and over<br />
Au = 56k, LSD = 300/1200/4.8k/6.4k/9.6k/14.4k/48k and over<br />
MLP LSD Forbid<strong>de</strong>n next commands (example)<br />
var<br />
var<br />
var<br />
4k<br />
1200<br />
4.8k<br />
9.6k<br />
8k<br />
LSD = 16k and over/var, MLP = 6.4k<br />
Transfer-rate 2Bj audio at 16 kbit/s, vi<strong>de</strong>o on<br />
LSD= 16kand over/var, MLP = 4k/6.4k<br />
Au = 56k, LSD = I6k and over/var, MLP = 6.4k<br />
Au = 56k, LSD = 4.8k/6.4k/14.4k/16k and over<br />
MLP LSD Forbid<strong>de</strong>n next commands (example)<br />
var<br />
var<br />
var<br />
var<br />
4k<br />
var<br />
LSD<br />
HSD<br />
1200<br />
4.8k<br />
8k<br />
16k<br />
8k<br />
Variable<br />
i<br />
Low speed data<br />
Hígh speed data<br />
LSD = 48k and over/var, MLP = 6.4K<br />
LSD = 48k and over/var, MLP 4k/6.4k<br />
Au = 56k, LSD = 48k and over/var<br />
Au = 48k/56k, LSD = 48k and over/var<br />
Au = 56k, LSD = 4.8k/6.4k/14.4k/48kand over
App<strong>en</strong>dix VI<br />
Hierarchical capability BAS co<strong>de</strong>s<br />
(This app<strong>en</strong>dix does not form an integral part of this Recomm<strong>en</strong>dation)<br />
Che following capability co<strong>de</strong>s are hierarchically structured:<br />
G.711 (A or ü. or both) < G.722-64 < G.722-48<br />
IB
App<strong>en</strong>dix VIII<br />
Examples of legal and illegal capability BAS sequ<strong>en</strong>ces<br />
(This app<strong>en</strong>dix does not form an integral part of this Recomm<strong>en</strong>dation)<br />
To be clear about legal and illegal capability BAS sequ<strong>en</strong>ces, it may be appropriate to quote the following examples<br />
(Al, A2 repres<strong>en</strong>t audio capabilíties):<br />
Allowed:<br />
S'ot allowed:<br />
{cap-mark, Al, A2, QCIF, 2/29.97} cap-mark<br />
(cap-mark, Al, A2, QCIF, 2/29.97] [repeat} [repeat] {repeat] cap-mark<br />
{cap-mark, Al, A2, QCIF, 2/29.97] cap-mark;<br />
command; [cap-mark, Al, A2, QCIF, 2/29.97, 2B] cap-mark<br />
[cap-mark, neutral} cap-mark<br />
{cap-mark, Al, A2, QCIF, 2/29.97]; command (final cap-mark missíng)<br />
{cap-mark, Al, A2, QCIF, 2/29.97] [repeat} {repeat] [repeat]; command<br />
(final cap-mark missing)<br />
[cap-mark, Al, A2, Al, QCIF, 2/29.97] cap-mark (repeated valué)<br />
{cap-mark, neutral}; command (final cap-mark missing)<br />
command; neutral; command (both cap-marks missing)<br />
{cap-mark, Al, A2, QCIF, 2/29.97}<br />
cap-mark {cap-mark Al, A2, QCIF,<br />
2/29.91', 2B] cap-mark (changad capset without command betwe<strong>en</strong>)<br />
{cap-mark, Al, A2, QCIF, 1/29.97, 2/29.97} cap-mark (two MPI valúes)<br />
[cap-mark, Al, A2, GIF. 2/29,97} cap-mark (one MPI valué)<br />
cap-mark, cap-mark (no capabüities)<br />
command; {Al, A2, QCIF, 2/29.97); command (no cap-marks)
or! Error!<br />
ot a valid<br />
leñame.<br />
INTERNATIONAL TELECOMMUNICATION UNION<br />
CCITT<br />
5rror!Error!<br />
Jot a valid<br />
lléname. G<strong>en</strong>eva,,1990<br />
THE INTERNATIONAL<br />
TELEGRAPH AND TELEPHONE<br />
CONSULTATIVE COMMITTEE<br />
LINE TRANSMISSION<br />
ON NON-TELEPHQNE SIGNALS<br />
H.261<br />
VIDEO CODEC FOR AUDIOVISUAL SERVICES<br />
AT p x 64 kbit/s<br />
Recomm<strong>en</strong>dation H.261
FOREWORD<br />
The CCITT (the International Telegraph and Telephone Consultative Committee) is the perman<strong>en</strong>t organ of<br />
le International Teleconrmunicatión Union (ITU). CCITT is responsible for studying technical, operating and tarüT<br />
uestions and issuing Recomm<strong>en</strong>dations on them with a view to standardizing telecommunications on a worldwi<strong>de</strong><br />
asís.<br />
The Pl<strong>en</strong>ary Assembly of CCITT which meets every four years, establishes the topics for study and approves<br />
lecomm<strong>en</strong>dations prepared by its Study Groups. The approval of Recomm<strong>en</strong>dations by the members of CCITT betwe<strong>en</strong><br />
leñar}' Assernblies is covered by the procedure <strong>la</strong>id down in CCITT Resolution No. 2 (Melbourne, 1988).<br />
Recomm<strong>en</strong>dation H.261 was prepared by Study Group XV and was approved un<strong>de</strong>r the Resolution No. 2<br />
rocedure on the 14 of December 1990.<br />
ccm NOTE<br />
In this Recomm<strong>en</strong>dation, the expression "Administration" is used for concis<strong>en</strong>ess to indícate both a<br />
¡lecommunication Administration and a recognized prívate operating ag<strong>en</strong>cy.<br />
© ITU 1990<br />
ai rights reserved. No part of this publication may be reproduced or utilized in any fonn or by any rneans, electronic or<br />
aechanical, including photocopying and microfilm, without permission in writing from the ITU.
iecomm<strong>en</strong>dation H.261<br />
'onsi<strong>de</strong>ríng<br />
ervices;<br />
The CCITT,<br />
VIDEO CODEC FOR AUDIOVISUAL SERVICES AT p x 64 kbit/s<br />
(revised 1990)<br />
(a) that there is signifícant customer <strong>de</strong>mand for vi<strong>de</strong>ophone, vi<strong>de</strong>oconfer<strong>en</strong>ce and other audiovisual<br />
(b) that circuits to meet this <strong>de</strong>mand can be provi<strong>de</strong>d by digital transmission using the B, HO rates or their<br />
lultiples up to the primary rate orHn/Hi2 rates;<br />
(c) that ISDNs are likely to be avai<strong>la</strong>ble in some countries that provi<strong>de</strong> a switched transmission service at<br />
íieB, HQ orHn/Hi2 rate;<br />
(d) that the exist<strong>en</strong>ce of difíer<strong>en</strong>t digital hierarchies and differ<strong>en</strong>t televisión standards in difíer<strong>en</strong>t parts of<br />
íie world complicates the problems of specifying coding and transmission standards for international connections;<br />
(e) that a number of audiovisual services are likely to appear using basic and primary rate ISDN accesses<br />
nd that some means of intercommunication among these termináis should be possible;<br />
(f) that the vi<strong>de</strong>o co<strong>de</strong>e provi<strong>de</strong>s an ess<strong>en</strong>tial elem<strong>en</strong>t of the inírastructure for audiovisual services which<br />
llows such intercommunication in the framework of Recomm<strong>en</strong>dation H.200;<br />
(g) that Recomm<strong>en</strong>dation H.120 for vi<strong>de</strong>oconfer<strong>en</strong>cing using primary digital group transmission was the<br />
irst in an evolving series of Recomm<strong>en</strong>dations,<br />
ppreciating<br />
that advances have be<strong>en</strong> ma<strong>de</strong> in research and <strong>de</strong>velopm<strong>en</strong>t of vi<strong>de</strong>o coding and bit rate reduction techniques<br />
i'hich lead to the use of lower bit rates down to 64 kbit/s so that this may be consi<strong>de</strong>red as the second in the evolving<br />
sries of Recomm<strong>en</strong>dations,<br />
nd noñng<br />
zcomm<strong>en</strong>ds<br />
that it is the basic objective of the CCITT to recomm<strong>en</strong>d unique solutions for international connections,<br />
that in addition to those co<strong>de</strong>es complying to Recomm<strong>en</strong>dation H.120, co<strong>de</strong>es having signa! processing and<br />
'ansmission coding characteristics <strong>de</strong>scribed below should be used for international audiovisual services.<br />
Note 1 — Co<strong>de</strong>es of this type are also suitable for some televisión services where full broadcast quality is not<br />
iquired.<br />
Note 2 — Equipm<strong>en</strong>t for transcoding from and to co<strong>de</strong>es according to Recomm<strong>en</strong>dation H. 120 is un<strong>de</strong>r study.<br />
Scope<br />
This Recomm<strong>en</strong>dation <strong>de</strong>scribes the vi<strong>de</strong>o coding and <strong>de</strong>coding methods for the moving picture compon<strong>en</strong>t of<br />
udiovisual services at the rates ofp x 64 kbit/s, where/? is in the range 1 to 30.
Brief specification<br />
An outline block diagram of the co<strong>de</strong>e is giv<strong>en</strong> in Figure 1/H.261.<br />
1.1 Vi<strong>de</strong>o input and output<br />
FIGURE 1/H.261<br />
To permit a single Recomm<strong>en</strong>dation to cover use in and betwe<strong>en</strong> regions using 625- and 525-line televisión<br />
¡tandards, the source co<strong>de</strong>r opérales on pictures based on a common intermedíate format (CIF). The standards of the<br />
nput and output televisión signáis, which may, for example, be composite or compon<strong>en</strong>t, analogue or digital and the<br />
nethods of performing any necessary conversión to and from the source coding format are not subject to<br />
lecomm<strong>en</strong>dation.<br />
1.2 Digital output and input<br />
The vi<strong>de</strong>o co<strong>de</strong>r provi<strong>de</strong>s a self-contained digital bit stream which may be combined with other multi-facüity<br />
¡ignals (for example as <strong>de</strong>fined in Recomm<strong>en</strong>dation H.221). The vi<strong>de</strong>o <strong>de</strong>co<strong>de</strong>r performs the reverse process.<br />
1.3 Samplingfrequ<strong>en</strong>cy<br />
Pictures are sampled at an intcger múltiple of the vi<strong>de</strong>o line rate. This sampling clock and the digital<br />
letwork clock are asynchronous.
,4 Source codirig olgorithm<br />
. . ,, *TÍ-«^*^T~ ^<br />
A hybnd of inter-picture prediction to uühze temporal redundancy and transform coding of the remaining<br />
ignal to reduce spatial redundancy is adopted. The <strong>de</strong>co<strong>de</strong>r has motion comp<strong>en</strong>sation capability, allowing optional<br />
icorporation of this technique in the co<strong>de</strong>r.<br />
.5 Bitrate<br />
This Recomm<strong>en</strong>dation is primarily int<strong>en</strong><strong>de</strong>d for use at vi<strong>de</strong>o bit rates betwe<strong>en</strong> approximately 40 kbit/s and<br />
Mbit/s.<br />
.6 Symmetry oftronsmission<br />
The co<strong>de</strong>e may be used for bidirectional or unidirectional visual communication.<br />
.7 Error handlmg<br />
The transmitted bit-stream contains a BCH1) (511,493) ibrward error correction co<strong>de</strong>. Use of this by the<br />
eco<strong>de</strong>r is optional.<br />
.8 Multipoint operation<br />
Features necessary to support switched rnultipoint operation are inclu<strong>de</strong>d.<br />
Source co<strong>de</strong>r<br />
.1 Source format<br />
The source co<strong>de</strong>r operates on non-inter<strong>la</strong>ced pictures occurring 30 000/1001 (approximately 29.97) times per<br />
econd. The tolerance on picture frequ<strong>en</strong>cy is ± 50 ppm.<br />
Pictures are co<strong>de</strong>d as luminance and two colour difíer<strong>en</strong>ce compon<strong>en</strong>ts (Y, CB and CR). These compon<strong>en</strong>ts<br />
nd the co<strong>de</strong>s repres<strong>en</strong>ting their sampled valúes are as <strong>de</strong>fined in CCIR Recomm<strong>en</strong>dation 601.<br />
B<strong>la</strong>ck = 16<br />
White = 235<br />
Zero colour differ<strong>en</strong>ce = 128<br />
Peak colour differ<strong>en</strong>ce =16 and 240.<br />
These valúes are nominal ones and the coding algoritiim functions with input valúes of 1 through to 254.<br />
Two picture scanning formats are specifíed.<br />
In the fírst format (CIF), the luminance sampling structure is 352 pels per Une, 288 Unes per picture in an<br />
rthogonal arrangem<strong>en</strong>t. Sampling of each of the two colour differ<strong>en</strong>ce compon<strong>en</strong>ts is at 176 pels per Une, 144 Unes per<br />
icture, orthogonal, Colour differ<strong>en</strong>ce samples are sited such that their block boundaries coinci<strong>de</strong> with luminance block<br />
oundaries as shown in Figure 2/H.261. The picture área covered by these numbers of pels and Unes has an aspect ratio<br />
f 4:3 and correspondí to the active portion of the local standard vi<strong>de</strong>o input.<br />
Note - The number of pels per Une is compatible with sampling the active portions of the luminance and<br />
olour differ<strong>en</strong>ce signáis from 525- or 625-line sources at 6.75 and 3.375 MHz respectively. These frequ<strong>en</strong>cies have a<br />
imple reíationship to those in CdRUecomm<strong>en</strong>dation 601.<br />
BCH = Bose, Chaudhuri and Hocqu<strong>en</strong>ham (co<strong>de</strong>).
FIGURE 2/H.261<br />
The second format, quarter-CIF (QCIF), has half the number of pels and half the number of Unes stated<br />
bove. All co<strong>de</strong>es must be able to opérate using QCIF. Some co<strong>de</strong>es can also opérate with CIF.<br />
Means shall be provi<strong>de</strong>d to restrict the máximum picture rate of <strong>en</strong>co<strong>de</strong>rs by having at least O, 1, 2 or 3 nonransmitted<br />
pictures betwe<strong>en</strong> transmitted ones. Selection of this minimum number and CIF or QCIF shall be by external<br />
neans (for example via Recomrn<strong>en</strong>dation H.221).<br />
-.2 Vi<strong>de</strong>o source coding algorithm<br />
The source co<strong>de</strong>r is shown in g<strong>en</strong>eralized form in Figure 3/H.261. The main elem<strong>en</strong>ts are prediction, block<br />
ransformation and quantization.<br />
The prediction error (ÍNTER mo<strong>de</strong>) or the input picture (INTRA mo<strong>de</strong>) is subdivi<strong>de</strong>d into 8 peí by 8 line<br />
docks which are segm<strong>en</strong>ted as transmitted or non-transmitted. Further, four luminance blocks and the two spatially<br />
orresponding colour differ<strong>en</strong>ce blocks are combined to form a macroblock as shown in Figure 10/H.261.<br />
The criteria for choice of mo<strong>de</strong> and transmitting a block are not subject to recomm<strong>en</strong>dation and may be varied<br />
[ynamically as part of the coding control strategy. Transmitted blocks are transformed and resulting coeffici<strong>en</strong>ts are<br />
[uantized and variable l<strong>en</strong>gth co<strong>de</strong>d.<br />
.2.1 Prediction<br />
The prediction is inter-picture and may be augm<strong>en</strong>ted by motion comp<strong>en</strong>sation (see § 3.2.2) and a spatial<br />
ilter (see § 3.2.3).
3.2.2 Motion comp<strong>en</strong>sation<br />
FIGURE 3/H.261<br />
Motion comp<strong>en</strong>sation (MC) is optional in the <strong>en</strong>co<strong>de</strong>r. The <strong>de</strong>co<strong>de</strong>r will accept one vector per macroblock.<br />
Both horizontal and vertical compon<strong>en</strong>ts of these motion vectors have integer valúes not exceeding ± 15. The vector is<br />
used for all four luminance blocks in the macroblock. The motion vector for both colour differ<strong>en</strong>ce blocks is <strong>de</strong>rived by<br />
halving the compon<strong>en</strong>t valúes of the macroblock vector and truncating the magnitu<strong>de</strong> parts towards zero to yield<br />
integer compon<strong>en</strong>ts.<br />
A positive valué of the horizontal or vertical compon<strong>en</strong>t of the motion vector signiñes that the prediction is<br />
formed frorn pels in the previous picture which are spatially to the right or below the pels being predicted.
Motion vectors are restrícted such that all pels refer<strong>en</strong>ced by them are witiiin the co<strong>de</strong>d picture área.<br />
3.2.3 Loopfüter<br />
The prediction process may be modiñed by a two-dim<strong>en</strong>sional spatial fílter (FIL) which operates on pels<br />
within a predicted 8 by 8 block.<br />
The filter is separable into one-dim<strong>en</strong>sional horizontal and vertical functions. Both are non-recursive with<br />
coeffici<strong>en</strong>ts of 1/4, 1/2, 1/4 except at block edges where one of the taps would fall outsi<strong>de</strong> the block. In such cases the<br />
1-D filter is changed to nave coeffici<strong>en</strong>ts of O, 1, 0. Full aríthmetic precisión is retained with rounding to 8 bit integer<br />
valúes at the 2-D filter output. Valúes whose fractional part is one half are roun<strong>de</strong>d up.<br />
The filter is switched on/oíffor all six blocks in a macroblock according to the macroblock type (see § 4.2.3<br />
MTYPE).<br />
3.2.4 Transforme?<br />
Transmitted blocks are first processed by a separable two-dim<strong>en</strong>sional discrete cosine transform of size 8 by 8.<br />
The output from the inverse transform ranges írom -256 to +255 añer clipping to be repres<strong>en</strong>ted with 9 bits. The<br />
transfer function of the inverse transform is giv<strong>en</strong> by:<br />
with u, v,x,y = O, 1, 2, . , ., 7<br />
7 7<br />
Jfy, y) - 1/4 Z 2 C(i/) C(v) F(u, v) eos [TT (2x + 1) u/16] eos [TI (2y + 1) v/16]<br />
u=0 v=0<br />
where xj> = spatial coordinates in the peí domain,<br />
u,v = ccordinates in the transform domain,<br />
C(w) = II A/2 for u = O, otherwise 1,<br />
C(v) = I/ V2 for v = O, oth<strong>en</strong>vise 1.<br />
Note — Within the block being transformed, x = O and y = O refer to the peí nearest the leít and top edges of<br />
the picture respectively,<br />
The arithmetic procedures for computmg the transforms are not <strong>de</strong>fined, but the inverse one should meet the<br />
error tolerance specified in Annex A.<br />
3.2.5 Ouantization<br />
The number of quantízers is 1 for the INTRA <strong>de</strong> coeffici<strong>en</strong>t and 31 for all other coefEíci<strong>en</strong>ts. Within a<br />
macroblock the same quantizer is used for all coefEíci<strong>en</strong>ts except the INTRA <strong>de</strong> one. The <strong>de</strong>cisión levéis are not<br />
<strong>de</strong>fined. The INTRA <strong>de</strong> coeffici<strong>en</strong>t is nominally the transform valué linearly quantized with a stepsize of 8 and no<br />
<strong>de</strong>ad-zone. Each of the other 31 quantizers is also nominally linear but with a c<strong>en</strong>tral <strong>de</strong>ad-zone around zero and with a<br />
step size of an ev<strong>en</strong> valué in the range 2 to 62.<br />
The reconstruction levéis are as <strong>de</strong>ñned in § 4.2.4.<br />
Note — For the smaller quantization step sizes, the full dynamic range of the transform coeffici<strong>en</strong>ts cannot be<br />
repres<strong>en</strong>ted.
3.2.6 Clipping of reconstructed picture<br />
To prev<strong>en</strong>t quantizaüon distortion of transform coeffici<strong>en</strong>t amplitu<strong>de</strong>s causing arithmetic overflow in the<br />
<strong>en</strong>co<strong>de</strong>r and <strong>de</strong>co<strong>de</strong>r loops, clipping functíons are inserted. The clipping function is applied to the reconstructed picture<br />
which is formed by summing the prediction and the prediction error as modifled by the coding process. This clipper<br />
operates on resulting peí valúes less than O or greater than 255, changing them to O and 255 respectively.<br />
3.3 Codlng control<br />
Severa! parameters may be varied to control the rate of g<strong>en</strong>eration of co<strong>de</strong>d vi<strong>de</strong>o data. These inclu<strong>de</strong><br />
processing prior to the source co<strong>de</strong>r, the quantizer, block significance criterion and temporal subsampling. The<br />
proportions of such measures in the overall control strategy are not subject to recomm<strong>en</strong>dation.<br />
Wh<strong>en</strong> invoked, temporal subsampling is performed by discarding complete pictures.<br />
3.4 Forcea updating<br />
This function is achieved by forcing the use of the INTRA mo<strong>de</strong> of the coding algorithm. The update pattern<br />
is not <strong>de</strong>fined. For control of accumu<strong>la</strong>üon of inverse transform mismatch error a macroblock should be forcibly<br />
updated at least once per every 132 times it is transmitted.<br />
Vi<strong>de</strong>o múltiples co<strong>de</strong>r<br />
4.1 Data structure<br />
Urüess speciñed oth<strong>en</strong>vise the most significant bit is transmitted first. This is bit 1 and is the leftmost bit in<br />
the co<strong>de</strong> tables in this Recomm<strong>en</strong>dation. Unless specifíed oth<strong>en</strong>vise all unused or spare bits are set to "1". Spare bits<br />
must not be used until their functions are specified by the CCITT.<br />
4.2 Vi<strong>de</strong>o multiplex arrangem<strong>en</strong>t<br />
The vi<strong>de</strong>o multiplex is arranged in a hierarchical structure with four <strong>la</strong>yers. From top to bottom the <strong>la</strong>yers are:<br />
— Picture.<br />
- Group of blocks (GOB).<br />
- Macroblock (MB).<br />
- Block.<br />
A syntax diagram of the vi<strong>de</strong>o multiplex co<strong>de</strong>r is shown in Figure 4/H.261. Abbreviations are <strong>de</strong>ñned in <strong>la</strong>ter<br />
sections.
Level<br />
-127<br />
-126<br />
-2<br />
-1<br />
0<br />
1<br />
2<br />
3<br />
4<br />
5<br />
56<br />
57<br />
58<br />
59<br />
60<br />
125<br />
126<br />
127<br />
1<br />
-255<br />
-253<br />
-5<br />
-3<br />
0<br />
3<br />
5<br />
7<br />
9<br />
11<br />
113<br />
115<br />
117<br />
119<br />
121<br />
251<br />
253<br />
255<br />
2<br />
-509<br />
-505<br />
—9<br />
-5<br />
0<br />
5<br />
9<br />
13<br />
17<br />
21<br />
225<br />
229<br />
233<br />
237<br />
241<br />
501<br />
505<br />
509<br />
3<br />
-765<br />
-759<br />
-15<br />
-9<br />
0<br />
9<br />
15<br />
21<br />
27<br />
33<br />
339<br />
345<br />
351<br />
357<br />
363<br />
753<br />
759<br />
765<br />
4<br />
-1019<br />
-1011<br />
-19<br />
-11<br />
0<br />
11<br />
19<br />
27<br />
35<br />
43<br />
451<br />
459<br />
467<br />
475<br />
483<br />
1003<br />
1011<br />
1019<br />
Recoustruction levéis (REC)<br />
QUANT<br />
8<br />
-2039<br />
-2023<br />
-39<br />
-23<br />
0<br />
23<br />
39<br />
55<br />
71<br />
87<br />
903<br />
919<br />
935<br />
951<br />
967<br />
2007<br />
2023<br />
2039<br />
9<br />
-2048<br />
-2048<br />
-45<br />
-27<br />
0<br />
27 •<br />
45<br />
63 -<br />
81<br />
99<br />
1017 •<br />
1035<br />
1053<br />
1071<br />
1089<br />
2047<br />
2047<br />
2047<br />
17<br />
-2048<br />
-2048<br />
-85<br />
-51<br />
0<br />
51<br />
85<br />
119<br />
153<br />
187<br />
1921<br />
1955<br />
1989<br />
2023<br />
2047<br />
2047<br />
2047<br />
2047<br />
18<br />
-2048<br />
-2048<br />
-89<br />
-53<br />
0<br />
53 •<br />
89 •<br />
125 •<br />
161<br />
197<br />
2033<br />
2047<br />
2047 •<br />
2047<br />
2047<br />
2047 -<br />
2047<br />
2047<br />
Note -Reconstruction levéis are symmetrical \vith respect to the sign of level except for 2047/-2048.<br />
For INTRA blocks the íirst coeffici<strong>en</strong>t is<br />
30<br />
-2048<br />
-2048<br />
-149<br />
-89<br />
0<br />
89<br />
149<br />
209<br />
269<br />
329<br />
2047<br />
2047<br />
2047<br />
2047<br />
2047<br />
2047<br />
2047<br />
2047<br />
31<br />
-2048<br />
-2048<br />
-155<br />
-93<br />
0<br />
93<br />
155<br />
217<br />
279<br />
341<br />
2047<br />
2047<br />
2047<br />
2047<br />
2047<br />
2047<br />
2047<br />
2047<br />
nominally the transform <strong>de</strong> valué linearly quantized<br />
with a step size of 8 and no <strong>de</strong>ad-zone. The resulting valúes are repres<strong>en</strong>ted with 8 bits. A nominally b<strong>la</strong>ck block will<br />
give 0001 0000 and a nominally white one 1110 1011, The co<strong>de</strong> 0000 0000 is not used. The co<strong>de</strong> 1000 0000 is not<br />
used, the reconstruction level of 1024being co<strong>de</strong>d as 1111 1111 (see Table 6/H.261).<br />
Coeffici<strong>en</strong>ts añer the <strong>la</strong>st non-zero one are not transmitted. EOB (<strong>en</strong>d of block co<strong>de</strong>) is always the <strong>la</strong>st Ítem in<br />
blocks for which coeffici<strong>en</strong>ts are transmitted.<br />
4.3 Mitltipoint consi<strong>de</strong>rations<br />
The following facilities are provi<strong>de</strong>d to support switched multipoint operation.<br />
4.3,1 Freeze picture request<br />
Causes the <strong>de</strong>co<strong>de</strong>r to freeze its disp<strong>la</strong>yed picture until a freeze picture reléase signa! is received or a timeout<br />
period of at least six seconds has expired. The transmission of this signal is via external means (for example by<br />
Recomm<strong>en</strong>dation H.221).
4.2.1 Picture ¡ayer<br />
Data for each picture consists of a picture hea<strong>de</strong>r followed by data for GOBs. The structure is shown in<br />
Figure 5 /H.261. Picture hea<strong>de</strong>rs for dropped pictures are not transmitted.<br />
PSC TR PTYPE PEÍ PSPARE PEÍ GOB Data<br />
4.2.1.1 Picture Start Co<strong>de</strong> (PCS) (20 bits)<br />
FIGURE 5/H.261<br />
Structure of picture <strong>la</strong>yer<br />
A word of 20 bits. Its valué is 0000 0000 0000 0001 0000.<br />
4.2. 1.2 Temporal Refer<strong>en</strong>ce (TR) (5 bits)<br />
A 5-bit number which can have 32 possible valúes. It is formed by increm<strong>en</strong>ting its valué in the previously<br />
transmitted picture hea<strong>de</strong>r by one plus the number of non-transmitted pictures (at 29.97 Hz) since that <strong>la</strong>st transmitted<br />
one. The arithmetic is performed with only the fíve LSBs.<br />
4.2.1.3 Type Information (PTYPE) (6 bits)<br />
Information about the complete picture:<br />
Bit 1 Split scre<strong>en</strong> indicator, "O" off, "1" on.<br />
Bit 2 Docum<strong>en</strong>t camera indicator. "O" off, "1" on.<br />
Bit 3 Freeze Picture Reléase. "O" off, "1" on.<br />
Bit 4 Source Format. "O" QCEF, "1" CIF.<br />
Bits 5 to 6 Spare.<br />
1.2.1.4 Extra Jnsertion Information (PEÍ) (1 bit)<br />
A bit which wh<strong>en</strong> set to "1" signáis the pres<strong>en</strong>ce of the following optional data field.<br />
[.2.1.5 Spare Information (PSPARE) (0/8/16 . . . bits)<br />
If PEÍ is set to "1", th<strong>en</strong> 9 bits follow consisting of 8 bits of data (PSPARE) and th<strong>en</strong> another PEÍ bit to<br />
.ndicate if a further 9 bits follow and so on. Enco<strong>de</strong>rs must not insert PSPARE until specified by the CCITT. Deco<strong>de</strong>rs<br />
cnust be <strong>de</strong>signed to discard PSPARE if PEÍ is set to 1. This will allow the CCITT to specify future backward<br />
:ompaüble additions in PSPARE.<br />
4.2.2 Group of blocks <strong>la</strong>yer<br />
i<br />
Each picture is divi<strong>de</strong>d into groups of blocks (GOBs). A group of blocks (GOB) compases one twelfth of the<br />
[TF or one third of the QCIF picture áreas (see Figure 6/H.261). A GOB re<strong>la</strong>tes to 176 pels by 48 lines of Y and the<br />
rpatially corresponding 88 pels by 24 lines of each of CB and CR.
1<br />
3<br />
5<br />
7<br />
9<br />
11<br />
CIF<br />
2<br />
4<br />
6<br />
8<br />
10<br />
12<br />
QCIF<br />
FIGURE 6/H.261<br />
Arrangem<strong>en</strong>t of GOBs in a picture<br />
Data for each group of blocks consists of a GOB hea<strong>de</strong>r followed by data for macroblocks. The structure is<br />
shown in Figure 7/H.261. Each GOB hea<strong>de</strong>r is transmitted once betwe<strong>en</strong> picture start co<strong>de</strong>s in the CEF or QCIF<br />
sequ<strong>en</strong>ce numbered in Figure 6/H.261, ev<strong>en</strong> if no macroblock data is pres<strong>en</strong>t in that GOB.<br />
4.2.2.1 Group of blocks start co<strong>de</strong> (GBSC) (16 bits)<br />
A word of 16 bits, 0000 0000 0000 0001.<br />
GBSC GN GQUANT GEI GSPARE GEI MB Data<br />
4.2.2.2 Group number (GN) (4 bits)<br />
HGURE 7/H.261<br />
Structure of group of blocks <strong>la</strong>yer<br />
Four bits indicating the position of the group of blocks. The bits are the binary repres<strong>en</strong>tation of the number<br />
in Figure 6/H.261. Group numbers 13, 14 and 15 are reserved for future use. Group number O is used in the PSC.<br />
4.2.2.3 Quantizer Information (GQUANT) (5 bits)<br />
A fixed l<strong>en</strong>gth co<strong>de</strong>word of 5 bits which indicates the quantizer to be used in the group of blocks until<br />
overrid<strong>de</strong>n by any subsequ<strong>en</strong>t MQUANT. The co<strong>de</strong>words are the natural binary repres<strong>en</strong>tations of the valúes of<br />
QUANT (§ 4.2.4) which, being half the step sizes, range from 1 to 31.<br />
4.2.2.4 Extra insertion Information (GEI) (1 bit)<br />
A bit which wh<strong>en</strong> set to "1" signáis the pres<strong>en</strong>ce of the following optional data íleld.
4.2.2.5 Spare Information (GSPARE) (0/8/16 . . . bits)<br />
If GEI is set to "1", th<strong>en</strong> 9 bits follow consisting of 8 bits of data (GSPARE) and th<strong>en</strong> another GEI bit to<br />
indícate if a further 9 bits follow and so on. Enco<strong>de</strong>rs must not inserí GSPARE until specifíed by the CCITT. Deco<strong>de</strong>rs<br />
must be <strong>de</strong>signed to discard GSPARE if GEI is set to 1. This will allow the CCITT to specify future "backward"<br />
compatible additions in GSPARE.<br />
Note - Emu<strong>la</strong>tion of start co<strong>de</strong>s may occur if the future speciflcation of GSPARE has no restríctions on the<br />
final GSPARE data bits.<br />
4.2.3 Macroblock <strong>la</strong>yer<br />
Each GOB is divi<strong>de</strong>d into 33 macroblocks as shown in Figure 8/H.261. A macroblock re<strong>la</strong>tes to 16 pels by 16<br />
lines of Y and the spatially corresponding 8 pels by 8 lines of each of CB and CR.<br />
1<br />
12<br />
23<br />
2<br />
13<br />
24<br />
3<br />
14<br />
25<br />
4<br />
15<br />
26<br />
5<br />
16<br />
27<br />
6<br />
17<br />
28<br />
7<br />
18<br />
29<br />
8<br />
19<br />
30<br />
9<br />
20<br />
31<br />
10<br />
21<br />
32<br />
FIGURE 8/H.261<br />
11<br />
22<br />
33<br />
Arrangem<strong>en</strong>t of macroblocks in a GOB<br />
Data for a macroblock consists of a MB hea<strong>de</strong>r followed by data for blocks (see Figure 9/H.261). MQUANT,<br />
MVD and CBP are pres<strong>en</strong>t wh<strong>en</strong> indicated by MTYPE.<br />
MBA MTYPE MQUANT MVD CBP Block data<br />
4.2.3.1 Macroblock address (MBA) (Variable l<strong>en</strong>gth)<br />
FIGURE 9/H.261<br />
Structure of macroblock <strong>la</strong>yer<br />
A variable l<strong>en</strong>gth co<strong>de</strong>word indicating the position of a macroblock within a group of blocks. The<br />
transmission or<strong>de</strong>r is as shown in Figure 8/H.261. For the first transmitted macroblock in a GOB, MBA is the absolute<br />
address in Figure 8/H.261. For subsequ<strong>en</strong>t macroblocks, MBA is the differ<strong>en</strong>ce betwe<strong>en</strong> the absolute addresses of the<br />
macroblock and the <strong>la</strong>st transmitted macroblock. The co<strong>de</strong> table for MBA is giv<strong>en</strong> in Table 1/H.261.
An extra co<strong>de</strong>word is avai<strong>la</strong>ble In the table for bit stufñng immediately after a GOB hea<strong>de</strong>r or a co<strong>de</strong>d<br />
macroblock (MBA stuffing). This co<strong>de</strong>word should be discar<strong>de</strong>d by <strong>de</strong>co<strong>de</strong>rs.<br />
MBA<br />
1<br />
2<br />
3<br />
4<br />
5<br />
6<br />
7<br />
8<br />
9<br />
10<br />
11<br />
12<br />
13<br />
14<br />
15<br />
16<br />
The VLC for start co<strong>de</strong> is also shown in Table 1/H.261.<br />
Co<strong>de</strong><br />
1<br />
Olí<br />
010<br />
0011<br />
0010<br />
0001 1<br />
0001 0<br />
0000 111<br />
0000 110<br />
0000 1011<br />
0000 1010<br />
0000 1001<br />
0000 1000<br />
0000 0111<br />
0000 0110<br />
0000 0101 11<br />
TABLE 1/H.261<br />
VLC table for macroblock addressing<br />
MBA<br />
17<br />
18<br />
19<br />
20<br />
21<br />
22<br />
23<br />
24<br />
25<br />
26<br />
27<br />
28<br />
29<br />
30<br />
31<br />
32<br />
33<br />
MBA stufFing<br />
Start co<strong>de</strong><br />
MBA is always inclu<strong>de</strong>d in transmitted macroblocks.<br />
Co<strong>de</strong><br />
0000 0101 10<br />
0000 0101 01<br />
0000 0101 00<br />
0000 0100 11<br />
0000 0100 10<br />
0000 0100 Olí<br />
0000 0100 010<br />
0000 0100 001<br />
0000 0100 000<br />
0000 0011 111<br />
0000 0011 110<br />
0000 0011 101<br />
0000 0011 100<br />
0000 0011 Olí<br />
0000 0011 010<br />
0000 0011 001<br />
0000 0011 000<br />
0000 0001 111<br />
0000 0000 0000 0001<br />
Macroblocks are not transmitted wh<strong>en</strong> they contain no information for that part of the pictiire.<br />
4.2.3.2 Type information (MTYPE) (Variable l<strong>en</strong>gth)<br />
Variable l<strong>en</strong>gth co<strong>de</strong>words giving information about the macroblock and which data elem<strong>en</strong>ts are pres<strong>en</strong>t.<br />
Macroblock types, inclu<strong>de</strong>d elem<strong>en</strong>ts and VLC words are listed in Table 2/H.261.<br />
MTYPE is always inclu<strong>de</strong>d in transmitted macroblocks.
Intra<br />
Intra<br />
Prediction<br />
ínter<br />
ínter<br />
ínter + MC<br />
ínter -í-MC<br />
ínter + MC<br />
ínter -fMC + HL<br />
Inter + MC + HL<br />
ínter + MC + FIL<br />
MQUANT<br />
X<br />
X<br />
X<br />
X<br />
MVD<br />
X<br />
X<br />
X<br />
X<br />
X<br />
X<br />
TABLE2/H.261<br />
VLCtable for MTYPE<br />
CBP<br />
X<br />
X<br />
X<br />
X<br />
X<br />
X<br />
TCOEFF<br />
X<br />
X<br />
X<br />
X<br />
X<br />
X<br />
X<br />
X<br />
0001<br />
VLC<br />
0000001<br />
1<br />
0000 1<br />
0000 0000 1<br />
0000 0001<br />
0000 0000 01<br />
001<br />
01<br />
000001<br />
Note J — "x" means that íhe Ítem is pres<strong>en</strong>t in the macroblock.<br />
Note 2 — It is possible to apply the fílter in a non-motion comp<strong>en</strong>sated macroblock by <strong>de</strong>c<strong>la</strong>ring it as MC + F1L<br />
but wíth a zero vector.<br />
4.2.3.3 Quantizer (MQUANT) (5 bits)<br />
MQUANT is pres<strong>en</strong>t only if so indicated by MTYPE.<br />
A co<strong>de</strong>word of 5 bits signifying the quantizer to be used for this and any following blocks in the group of<br />
blocks until overrid<strong>de</strong>n by any subsequ<strong>en</strong>t MQUANT.<br />
Co<strong>de</strong>words for MQUANT are the same as for GQUANT.<br />
4.2.3.4 Motion vector dato (MVD) (Variable l<strong>en</strong>gth)<br />
Motion vector data is inclu<strong>de</strong>d for all MC macroblocks. MVD is obtained from the macroblock vector by<br />
subtracting the vector of the preceding macroblock. For this calcu<strong>la</strong>tion the vector of the preceding macroblock is<br />
regar<strong>de</strong>d as zero in the following three situations:<br />
1) evaluating MVD for macroblocks 1, 12 and 23;<br />
2) evaluating MVD for macroblocks in which MBA does not repres<strong>en</strong>t a differ<strong>en</strong>ce of 1;<br />
3) MTYPE of the previous macroblock was not MC.<br />
MVD consists of a variable l<strong>en</strong>gth co<strong>de</strong>word for the horizontal compon<strong>en</strong>t followed by a variable l<strong>en</strong>gth<br />
co<strong>de</strong>word for the vertical compon<strong>en</strong>t. Variable l<strong>en</strong>gth co<strong>de</strong>s are giv<strong>en</strong> in Table 3/H.261.<br />
Advantage is tak<strong>en</strong> of the fact that the range of motion vector valúes is constrained. Each VLC word<br />
repres<strong>en</strong>ts a pair of differ<strong>en</strong>ce valúes. Only one of the pair will yield a macroblock vector falling witliin the permitted<br />
range.
-- '*.'• . : '^VV'- ;: ' '' .'.'í.V ~-'~^Sp&'( - '' ' ',"' "'v í&-^ >:ü;?'1''^- "
CBP<br />
60<br />
4<br />
8<br />
16<br />
32<br />
12<br />
48<br />
20<br />
40<br />
28<br />
MVD<br />
-16 & 16<br />
-15 & 17<br />
-14 & 18<br />
-13 & 19<br />
-12 & 20<br />
-11 & 21<br />
-10 & 22<br />
-9& 23<br />
-8& 24<br />
-7& 25<br />
-6& 26<br />
-5 «fe 27<br />
-4& 28<br />
-3& 29<br />
-2& 30<br />
-1<br />
0<br />
1<br />
2&-30<br />
3&-29<br />
4&-2S<br />
5&-27<br />
6&-26<br />
7&-25<br />
S&-24<br />
9&-23<br />
10&-22<br />
11 &-21<br />
12 & -20<br />
13&-19<br />
14&-18<br />
15&-17<br />
Co<strong>de</strong><br />
111<br />
1101<br />
1100<br />
1011<br />
1010<br />
1001 1<br />
1001 0<br />
1000 1<br />
1000 0<br />
0111 1<br />
Co<strong>de</strong><br />
0000 0011 001<br />
0000 0011 Olí<br />
0000 0011 101<br />
0000 0011 111<br />
0000 0100 001<br />
0000 0100 Olí<br />
0000 0100 11<br />
0000 0101 01<br />
0000 0101 11<br />
0000 0111<br />
0000 1001<br />
0000 1011<br />
0000 111<br />
0001 1<br />
0011<br />
Olí<br />
1<br />
010<br />
0010<br />
0001 0<br />
0000 110<br />
0000 1010<br />
0000 1000<br />
0000 0110<br />
0000 0101 10<br />
0000 0101 00<br />
0000 0100 10<br />
0000 0100 010<br />
0000 0100 000<br />
0000 0011 110<br />
0000 0011 100<br />
0000 0011 010<br />
CBP<br />
35<br />
13<br />
49<br />
21<br />
41<br />
14<br />
50<br />
22<br />
42<br />
15<br />
TABLE 3/H.261<br />
VLCtableforMVD<br />
TABLE 4/H.261<br />
VLC table for CBP<br />
Co<strong>de</strong><br />
0001 1100<br />
0001 1011<br />
0001 1010<br />
0001 1001<br />
0001 1000<br />
0001 0111<br />
0001 0110<br />
0001 0101<br />
0001 0100<br />
0001 0011
4.2.4 Block <strong>la</strong>yer<br />
A macroblock compases four luminance blocks and one of each of the two colour differ<strong>en</strong>ce blocks (see<br />
Figure 10/H.261).<br />
Data for a block consists of co<strong>de</strong>words for transform coeffici<strong>en</strong>ts followed by an <strong>en</strong>d of block marker (see<br />
Figure 11/H.261). The or<strong>de</strong>r of block transmission is as in Figure 10/H.261. '•r*-¿---- - -<br />
TCOEFF EOB<br />
CB CR<br />
4.2.4.1 Transform coeffici<strong>en</strong>ts (TCOEFF)<br />
HGURE 10/H.261<br />
Arrangem<strong>en</strong>t of blocks in a macroblock<br />
HGURE 11/H.261<br />
Structure of block <strong>la</strong>yer<br />
Transform coeffici<strong>en</strong>t data is always pres<strong>en</strong>t for all six blocks in a macroblock wh<strong>en</strong> MTYPE indicates<br />
INTRA. In other cases MTYPE and CBP signal which blocks have coeffici<strong>en</strong>t data transmitted for them. The quantized<br />
transform coeffici<strong>en</strong>ts are sequ<strong>en</strong>tially transmitted according to the sequ<strong>en</strong>ce giv<strong>en</strong> in Figure 12/H.261.
1<br />
3<br />
4<br />
10<br />
11<br />
21<br />
22<br />
36<br />
2<br />
5<br />
9<br />
12<br />
20<br />
23<br />
35<br />
37<br />
6<br />
8<br />
13<br />
19<br />
24<br />
34<br />
38<br />
49<br />
7<br />
14<br />
18<br />
25<br />
33<br />
.39<br />
48<br />
50<br />
15<br />
17<br />
26,<br />
,32.<br />
40<br />
47<br />
51 :<br />
58<br />
16<br />
27<br />
31:<br />
-41". '<br />
46<br />
52<br />
57<br />
"30<br />
•42<br />
:45<br />
;'!53:'<br />
•;56<br />
-^P<br />
59\8-63<br />
'29<br />
-43<br />
44<br />
54<br />
55<br />
61<br />
62<br />
64<br />
—>>lncreasing cycles per<br />
picture width<br />
Increasing cycles per<br />
picture heíght<br />
FÍGURE 12/H.261<br />
Transmission or<strong>de</strong>r for transform coeffici<strong>en</strong>ts
The most commoiüy occurríng combinations of successive zeros (RUN) and the following valué (LEVEL) are<br />
<strong>en</strong>co<strong>de</strong>d with variable l<strong>en</strong>gth co<strong>de</strong>s. Other combinations of (RUN, LEVEL) are <strong>en</strong>co<strong>de</strong>d with a 20-bit word consisting<br />
of 6 bits ESCAPE, 6 bits RUN and 8 bits LEVEL. For the variable l<strong>en</strong>gth <strong>en</strong>coding there are two co<strong>de</strong> tables, one being<br />
used for the first transmitted LEVEL in ÍNTER, INTER+MC and INTER+MC+FIL blocks, the second for all other<br />
LEVELs except the first one in INTRA blocks which is fixed l<strong>en</strong>gth co<strong>de</strong>d with 8 bits.<br />
Co<strong>de</strong>s are giv<strong>en</strong> in Table 5/H.261.<br />
The most commonly occurring combinations of zero-run and the following valué are <strong>en</strong>co<strong>de</strong>d with variable<br />
l<strong>en</strong>gth co<strong>de</strong>s as Usted in the table below. End of block (EOB) is in this set. Because CBP indícales those blocks with no<br />
coefñci<strong>en</strong>t data, EOB cannot occur as the first coeffici<strong>en</strong>t. H<strong>en</strong>ee EOB can be removed írom the VLC table for the first<br />
coeffici<strong>en</strong>t.<br />
The <strong>la</strong>st bit "s" <strong>de</strong>notes the sign of the level, "O" for positive and "1" for negativo.<br />
Run<br />
EOB<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
1<br />
1<br />
1 1<br />
1 11<br />
Level<br />
1<br />
1<br />
2<br />
3<br />
4<br />
5<br />
6<br />
7<br />
8<br />
9<br />
10<br />
11<br />
12<br />
13<br />
14<br />
15<br />
1<br />
2<br />
3<br />
4<br />
5<br />
6<br />
7<br />
a) Neveroised in INTRA macroblocks.<br />
TABLE 5/H.261<br />
VLC table for TCOEFF<br />
Co<strong>de</strong><br />
10<br />
lsa) If first coeíííci<strong>en</strong>t in block<br />
lis Not first coeffici<strong>en</strong>t in block<br />
0100 s<br />
0010 Is<br />
0000 11 Os<br />
0010 0110 s<br />
0010 0001 s<br />
0000 0010 lOs<br />
0000 0001 1101 s<br />
0000 0001 1000 s<br />
0000 0001 0011 s<br />
0000 0001 0000 s<br />
0000 0000 1101 Os<br />
0000 0000. 1100 Is<br />
0000 0000 1100 Os<br />
0000 0000 1011 Is<br />
Olls<br />
0001 lOs<br />
0010 0101 s<br />
0000 0011 OOs<br />
oooo 0001 ion s<br />
0000 0000 1011 Os<br />
0000 0000 1010 Is
Run<br />
2<br />
2<br />
2<br />
2<br />
2<br />
3<br />
3<br />
3<br />
3<br />
4<br />
4<br />
4<br />
5<br />
5<br />
5<br />
6<br />
6<br />
7<br />
7<br />
8<br />
8<br />
9<br />
9<br />
10<br />
10<br />
11<br />
12<br />
13<br />
14<br />
15<br />
16<br />
17<br />
18<br />
19<br />
20<br />
21<br />
22<br />
23<br />
24<br />
25<br />
26<br />
Escape<br />
Level<br />
1<br />
2<br />
3<br />
4<br />
5<br />
1<br />
2<br />
3<br />
4<br />
1<br />
2<br />
3<br />
1<br />
2<br />
3<br />
1<br />
2<br />
1<br />
2<br />
1<br />
2<br />
1<br />
2<br />
1<br />
2<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
1<br />
TABLE 5/H.261 (Cont)<br />
Co<strong>de</strong><br />
0101 s<br />
0000 lOOs<br />
0000 0010 lis<br />
0000 0001 0100 s<br />
0000 0000 1010 Os<br />
0011 Is<br />
0010 0100 s<br />
0000 0001 1100 s<br />
0000 0000 1001 Is<br />
0011 Os<br />
0000 0011 lis<br />
0000 0001 0010 s<br />
0001 lis<br />
0000 0010 Oís<br />
0000 0000 1001 Os<br />
0001 Oís<br />
0000 0001 1110 s<br />
0001 OOs<br />
0000 0001 0101 s<br />
0000 llls<br />
0000 0001 0001 s<br />
0000 lOls<br />
0000 0000 1000 Is<br />
0010 0111 s<br />
0000 0000 1000 Os<br />
0010 0011 s<br />
0010 0010 s<br />
0010 0000 s<br />
0000 0011 lOs<br />
0000 0011 Oís<br />
0000 0010 OOs<br />
0000 0001 1111 s<br />
0000 0001 1010 s<br />
0000 0001 1001 s<br />
0000 0001 0111 s<br />
0000 0001 0110 s<br />
0000 0000 1111 Is<br />
0000 0000 1111 Os<br />
0000 0000 1110 Is<br />
0000 0000 1110 Os<br />
0000 0000 1101 Is<br />
0000 01
The remaining combinations of (run, level) are <strong>en</strong>co<strong>de</strong>d with a 20-bit word consisüng of 6 bits escape 6 bits<br />
run and 8 bits level. Use of this 20-bit word form <strong>en</strong>coding the combinations Usted in the VLC table is not prohibited.<br />
Run is a 6 bit fixed l<strong>en</strong>gth co<strong>de</strong> Level is an 8 bit fixed l<strong>en</strong>gth co<strong>de</strong><br />
Run<br />
0<br />
1<br />
2<br />
63<br />
Co<strong>de</strong><br />
0000 00<br />
0000 01<br />
0000 10<br />
1111 11<br />
Level<br />
-128<br />
-127<br />
-2<br />
—1<br />
O<br />
1<br />
2<br />
127<br />
Co<strong>de</strong><br />
FORBIDDEN<br />
1000 0001<br />
1111 1110<br />
1111 1111<br />
FORBIDDEN<br />
0000 0001<br />
0000 0010<br />
0111 1111<br />
For all coeffici<strong>en</strong>ts other than the INTRA <strong>de</strong> one, the reconstruction levéis (REC) are in the range —2048<br />
to 2047 and are giv<strong>en</strong> by clipping the results of thefollowing formules:<br />
REC = QUANT • (2 • level + 1); level > O<br />
REC = QUANT • (2 • level - 1); level < O<br />
REC = QUANT • (2 • level + !)-!; level > O<br />
REC = QUANT • (2 • level + !) + !; level < O<br />
REC = 0; level = O<br />
QUANT = "odd"<br />
QUANT = "ev<strong>en</strong>"<br />
Note - QUANT ranges írom 1 to 31 and is transmitted by either GQUANT or MQUANT.
FLC<br />
0000 0001 (1)<br />
0000 0010 (2)<br />
0000 0011 (3)<br />
0111 1111 (127)<br />
1111 1111 (255)<br />
1000 0001 (129)<br />
1111 1101 (253)<br />
1111 1110 (254)<br />
4.3.2 Fast update request<br />
TABLE6/H.261<br />
Recoustruction levéis for INTRA-mo<strong>de</strong> <strong>de</strong> coeffici<strong>en</strong>t<br />
Reconstruction level unto<br />
inversa transform<br />
16<br />
24<br />
1016<br />
1024<br />
1032<br />
2024<br />
2032<br />
Note - The <strong>de</strong>co<strong>de</strong>d valué corresponding to FLC "n" is 8n<br />
exceptFLC 255 gives 1024.<br />
Causes the <strong>en</strong>co<strong>de</strong>r to <strong>en</strong>co<strong>de</strong> its next picture in INTRA mo<strong>de</strong> with coding parameters such as to avoid buffer<br />
overílow. The transrnission method for this signal is vía external means (for example by Recomm<strong>en</strong>datíon H.221).<br />
4.3.3 Freeze picture reléase<br />
A signal from an <strong>en</strong>co<strong>de</strong>r which has respon<strong>de</strong>d to a fast update request and allows a <strong>de</strong>co<strong>de</strong>r to exit from its<br />
freeze picture mo<strong>de</strong> and disp<strong>la</strong>y <strong>de</strong>co<strong>de</strong>d pictures in the normal manner. This signa! is transmitted by bit 3 of PTYPE<br />
(see § 4.2.1) in the picture hea<strong>de</strong>r of the first picture co<strong>de</strong>d in response to the fast update request.<br />
5 Transmission co<strong>de</strong>r<br />
5.1 Bit r ate<br />
The transmission clock is provi<strong>de</strong>d externally (for example from an 1.420 interface).<br />
5.2 Vi<strong>de</strong>o data bitffering<br />
<strong>de</strong>co<strong>de</strong>r <strong>de</strong>fined in Annex B.<br />
\e <strong>en</strong>co<strong>de</strong>r must control its output bitstream to comply with the requirem<strong>en</strong>ts o<br />
Wh<strong>en</strong> operating with CIF the number of bits created by coding any single picture must not exceed<br />
256 -Kbits. ,£=1024.
Wh<strong>en</strong> operating with QCIF the number of bits created by coding any single picture musí not exceed<br />
64 -K bits.<br />
In both the above cases the bit count inclu<strong>de</strong>s the picture start co<strong>de</strong> and all other data re<strong>la</strong>ted to Üiat picture<br />
including PSPAKE, GSPARE and MBA stuffíng. The bit count does not inclu<strong>de</strong> error correction framing bits, fill<br />
indicator (Fi), fill bits or error correction parity'information <strong>de</strong>scribed in § 5.4 below.<br />
Vi<strong>de</strong>o data must be provi<strong>de</strong>d on every valid clock cycle. This can be <strong>en</strong>sured by the use of either the fill bit<br />
indicator (Fi) and subsequ<strong>en</strong>t fill all l's bits in the error corrector block framing (see Figure 13/H.261) or MBA stuffing<br />
(§ 4.2.3) or both.<br />
5.3 Vi<strong>de</strong>o coding <strong>de</strong><strong>la</strong>y<br />
HGURE 13/H.261<br />
This item is inclu<strong>de</strong>d in this Recomm<strong>en</strong>dation because the vi<strong>de</strong>o <strong>en</strong>co<strong>de</strong>r and vi<strong>de</strong>o <strong>de</strong>co<strong>de</strong>r <strong>de</strong><strong>la</strong>ys need to be<br />
known. to allow audio comp<strong>en</strong>sation <strong>de</strong><strong>la</strong>ys to be íixed wh<strong>en</strong> H.261 is used to form part of a conversational service. This<br />
will afíow lip synchronization to be maintained. Annex C recomm<strong>en</strong>ds a method by which the <strong>de</strong><strong>la</strong>y figures are<br />
established. Other <strong>de</strong><strong>la</strong>y measurem<strong>en</strong>t methods may be used but they must be <strong>de</strong>signed in a way to produce simi<strong>la</strong>r<br />
results to the method giv<strong>en</strong> in Annex C.<br />
5.4 Fonvard error correction for co<strong>de</strong>a vi<strong>de</strong>o signa!<br />
5.4,1 Error correctlng co<strong>de</strong><br />
!<br />
The transmitted bitstream contains a BCH (511,493) fonvard error correction co<strong>de</strong>. Use of this by the <strong>de</strong>co<strong>de</strong>r<br />
is optional.
5.4.2 G<strong>en</strong>erator polynomial<br />
g(x) = (x* + x^<br />
Example: for the input data of "01111 . . . 11" (493 bits) the resulting correction parity bits are<br />
"011011010100011011" (18 bits).<br />
5.4.3 Error correction framing<br />
To allow the vi<strong>de</strong>o data and error correction parity information to be i<strong>de</strong>ntifíed by a <strong>de</strong>co<strong>de</strong>r an error<br />
correction framing pattern is inclu<strong>de</strong>d. This consistí of a multiframe of eight frames, each frame comprising 1 bit<br />
framing, 1 bit fill indicator (Fi), 492 bits of co<strong>de</strong>d data (or £11 all Is) and 18 bits parity. The frame alignm<strong>en</strong>t pattern<br />
is:<br />
(SiS2S3S4S5S6S7S8) = (00011011).<br />
See Figure 13/H.261 for the frame arrangem<strong>en</strong>t. The parity is calcú<strong>la</strong>te*! against the 493-bits including fill<br />
indicator (Fi).<br />
The fill indicator (Fi) can be set to zero by an <strong>en</strong>co<strong>de</strong>r. In this case only 492 consecutive fill bits (fill all Is)<br />
plus parity are s<strong>en</strong>t and no co<strong>de</strong>d data is transmitted. This may be used to meet the requirem<strong>en</strong>t in § 5.2 to provi<strong>de</strong><br />
vi<strong>de</strong>o data on every valid clock cycle.<br />
5.4.4 Relock time for error corrector framing<br />
Three consecutive error correction framing sequ<strong>en</strong>ces (24 bits) should be received before frame lock is<br />
<strong>de</strong>emed to have be<strong>en</strong> aclüeved. The <strong>de</strong>co<strong>de</strong>r should be <strong>de</strong>signed such that frame lock will be re-established within<br />
34 000 bits after an error corrector framing phase change. ¿Cuo o<<br />
Note — This assumes that the vi<strong>de</strong>o data does not contain three correctly phased emu<strong>la</strong>tions of the error<br />
correction framing sequ<strong>en</strong>ce during the relocking period.<br />
ANNEXA<br />
(to Recomm<strong>en</strong>datíon H.261)<br />
Inverse transform accuracy specifícation<br />
A.l G<strong>en</strong>érate random integer peí data valúes in the range -L to +H according to the random number g<strong>en</strong>erator<br />
giv<strong>en</strong> below ("C" versión). Arrange into 8 by 8 blocks. Data set of 10 000 blocks should each be g<strong>en</strong>erated for (L = 256,<br />
H=255), (L = H=5)and(L = H =<br />
A.2 'For each 8 by 8 block, perform a separable, orthonormal, matrix multiply, fonvard discrete cosine transform<br />
using at least 64-bit floating point accuracy.<br />
\\ith u, v^x,y~ O, 1,2,.. .,7<br />
7 7<br />
F(u, v) = 1/4 C(w) C(v) £ Z A*, y) eos [n (2x + 1) u/16] eos [n (2y + 1) v/16]<br />
x=0 y=0<br />
where xy = spatial coordinates in the peí domain,<br />
u,v = coordinates in the transform domain,<br />
C(w) = I/ -v/2 for u = O, oth<strong>en</strong>vise 1,<br />
C(v) = I/ ^2 for v = O, oth<strong>en</strong>vise 1.
A.3 For each block, round the 64 resulting transformed coeffici<strong>en</strong>ts to the nearest integer valúes. Th<strong>en</strong> clip them<br />
to the range -2048 to +2047. This is the 12-bit input data to the inverse transfonn.<br />
A.4 For each 8 by 8 block of 12-bit data produced by § A.3, perform a separable, orthonormal, matrix multiply,<br />
inverse discrete transfonn (IDCT) using'at least 64-bit floating point accuracy. Round the resulting pels to the nearest<br />
integer and clip to the range —256 to +255. These blocks of 8 x 8 pels are tiie refer<strong>en</strong>ce IDCT input data.<br />
A.5 For each 8 by 8 block produced by § A.3, apply the IDCT un<strong>de</strong>r test and clip the output to the range -256 to<br />
4-255. These blocks of 8 x 8 pels are the test IDCT output data.<br />
A.6 For each of the 64 IDCT output pels, and for each of the 10,000 block data sets g<strong>en</strong>erated above, measure the<br />
peak, mean and mean square error betwe<strong>en</strong> the refer<strong>en</strong>ce and the test data.<br />
A.7 For any peí, the peak error should not exceed 1 in magnitu<strong>de</strong>.<br />
For any peí, the mean square error should not exceed 0.06.<br />
Overall, the mean square error should not exceed 0.02.<br />
For any peí, the mean error should not exceed 0.015 in magnitu<strong>de</strong>.<br />
Overall, the mean error should not exceed 0.0015 in magnitu<strong>de</strong>.<br />
A.8 All zeros in must produce all zeros out.<br />
A.9 Re-run the measurem<strong>en</strong>ts using exactly the same data valúes of step 1, but change the sign on each peí.<br />
"C" programfor random number g<strong>en</strong>eration<br />
/* L and H must be long, that is 32 bits */<br />
long rand (L,H)<br />
long L,H;<br />
static long randx = 1; /* long is 32 bits */<br />
static double z = (double)<br />
long ij;<br />
double x; /* double is 64 bits */<br />
randx = (randx * 1103515245) + 12345;<br />
i = randx & OxVffiffife; /* keep 30 bits */<br />
x = ( (double)i ) / z; /* range O to 0.99999 ... */<br />
x * = (L+H+1); /* range O to < L+H+1 */<br />
j = x; ' /* trúncate to integer */<br />
return( j - L); /* range -L to H */
ANNEXB<br />
(to Recomm<strong>en</strong>dation H.261)<br />
Hypothetical refer<strong>en</strong>ce <strong>de</strong>co<strong>de</strong>r<br />
The Hypothetical refer<strong>en</strong>ce <strong>de</strong>co<strong>de</strong>r (HRD) is <strong>de</strong>fíned as follows:<br />
B. 1 The HRD and the <strong>en</strong>co<strong>de</strong>r have the same clock frequ<strong>en</strong>cy as well as the same CIF rate, and are operated<br />
synchronously.<br />
B.2 The HRD receiving buffer size is (B + 256 • ¿Tbits). The valué of B is <strong>de</strong>fined as follows:<br />
B = 4Rmax/29.97 where R^iax is the máximum vi<strong>de</strong>o bit rate to be used in the connection.<br />
B.3 The HRD buffer is initially empty.<br />
B.4 The HRD buffer is examined at CIF intervals (=33 ms). If at least one complete co<strong>de</strong>d picture is in the buffer<br />
th<strong>en</strong> all the data for the earliest picture is instantaneously removed (e.g. at tn+i in Figure B-1/H.261). Immediately after<br />
removing the above data the buffer occupancy must be less than B. This is a requirem<strong>en</strong>t on the co<strong>de</strong>r output bitstream<br />
including co<strong>de</strong>d picture data and MBA stuffing but not error correction framing bits, fíll mdicator (Fi), fíll bits or error<br />
correction parity Information <strong>de</strong>scribed in § 5.4.<br />
FIGURE B-1/H.261
where:<br />
To meet this requirem<strong>en</strong>t the number of bits for the (w+l)th co<strong>de</strong>d picture dn+± must satisfy:<br />
bn is buffer occupancy just after the time tn,<br />
tn is the time the nth co<strong>de</strong>d picture is removed from the HRD buffer,<br />
R(f) is the vi<strong>de</strong>o bit rate at the time t.<br />
ANNEX C<br />
(to Recomm<strong>en</strong>dation H.261)<br />
Co<strong>de</strong>e <strong>de</strong><strong>la</strong>y measurem<strong>en</strong>t method<br />
The vi<strong>de</strong>o <strong>en</strong>co<strong>de</strong>r and vi<strong>de</strong>o <strong>de</strong>co<strong>de</strong>r <strong>de</strong><strong>la</strong>ys wül vary <strong>de</strong>p<strong>en</strong>ding on implem<strong>en</strong>tation. The <strong>de</strong><strong>la</strong>y will also<br />
<strong>de</strong>p<strong>en</strong>d on the picture íbnnat (QCIF, CIF) and data rate in use. This annex specifies the method by which the <strong>de</strong><strong>la</strong>y<br />
figures are established for a particu<strong>la</strong>r <strong>de</strong>sign. To allow correct audio <strong>de</strong><strong>la</strong>y comp<strong>en</strong>satíon the overall vi<strong>de</strong>o <strong>de</strong><strong>la</strong>y needs<br />
to be established from a user perception point of view un<strong>de</strong>r typical viewing conditions.<br />
FIGURE C.1/H.261<br />
Point A is the vi<strong>de</strong>o input to the vi<strong>de</strong>o co<strong>de</strong>r. Point B is the channel output from the vi<strong>de</strong>o terminal<br />
(i.e. including any KEC, channel framing, etc.). Point C is the vi<strong>de</strong>o outputfrom the <strong>de</strong>co<strong>de</strong>r.
A vi<strong>de</strong>o sequ<strong>en</strong>ce <strong>la</strong>sting more than 100 seconds is connected to the vi<strong>de</strong>o co<strong>de</strong>r input (point A) in<br />
Figure C-1/H.261 above. The vi<strong>de</strong>o sequ<strong>en</strong>ce should nave the following characteristics:<br />
- it should contain a typical moving sc<strong>en</strong>e consist<strong>en</strong>t with the int<strong>en</strong><strong>de</strong>d purpose of the vi<strong>de</strong>o co<strong>de</strong>e;<br />
- it should produce a mínimum co<strong>de</strong>d picture rate of 7.5 Hz at the bit rate in use;<br />
it should contain a visible i<strong>de</strong>ntification mark at intervals throughout the l<strong>en</strong>gth of the sequ<strong>en</strong>ce. The<br />
visible i<strong>de</strong>ntification should change every 97 vi<strong>de</strong>o input frames and be located within the picture área<br />
repres<strong>en</strong>ted by the first GOB in the picture. For example, the first block in the picture could change from<br />
b<strong>la</strong>ck to white at intervals of 97 vi<strong>de</strong>o frame periods. The i<strong>de</strong>ntification mark should be chos<strong>en</strong> so that it<br />
can be <strong>de</strong>tected at point B and does not significantly contribute to the overall coding performance.<br />
The co<strong>de</strong>e and vi<strong>de</strong>o sequ<strong>en</strong>ce should be arranged so that the bitstream contains less than 10% stuffing (MBA<br />
stuffing + error correction fill bits).<br />
The <strong>en</strong>co<strong>de</strong>r <strong>de</strong><strong>la</strong>y is obtained by measuring the time from wh<strong>en</strong> the visible i<strong>de</strong>ntification changes at point A<br />
to the time that the change is <strong>de</strong>tected at point B. Simi<strong>la</strong>rly, the <strong>de</strong>co<strong>de</strong>r <strong>de</strong><strong>la</strong>y is obtained by taking measurem<strong>en</strong>ts at<br />
points B and C.<br />
Several measurem<strong>en</strong>ts should be ma<strong>de</strong> during the sequ<strong>en</strong>ce l<strong>en</strong>gth and the average period obtained. Several<br />
tests should be ma<strong>de</strong> to <strong>en</strong>sure that a consist<strong>en</strong>t average figure can be obtained for both <strong>en</strong>co<strong>de</strong>r and <strong>de</strong>co<strong>de</strong>r <strong>de</strong><strong>la</strong>y<br />
times.<br />
Average results should be obtained for each combination of picture format and bit rate within the capability of<br />
the particu<strong>la</strong>r co<strong>de</strong>e <strong>de</strong>sign.<br />
Note — Due to pre- and post-temporal processing it may be necessary to take a mid-level for establishing the<br />
transition of the i<strong>de</strong>ntification mark at points B and C.
INTERNATIONAL TELECOMMUNICATION UNION<br />
ITU-T H.320<br />
TELECOMMUNICATION (03/93)<br />
STANDARDIZARON SECTOR<br />
OF ITU<br />
LINE TRANSMISSION<br />
OF NON-TELEPHONE SIGNÁIS<br />
NARROW-BAND VISUAL TELEPHONE<br />
SYSTEMS AND TERMINAL EQUIPMENT<br />
ITU-T Recomm<strong>en</strong>dation H.320<br />
(Previously "CCITT RGComm<strong>en</strong>dation")
FOREWORD<br />
be ITU Telecommunication S<strong>la</strong>ndardixalinn Sector (ITU-T) is a perman<strong>en</strong>i organ of Ihe Inlemalionai Telecomunication<br />
Union. The ITU-T'is responsible for sludying technícal, operating and <strong>la</strong>rifí queslions and issuíng<br />
ecomm<strong>en</strong>dations on ihem wilh a view lo siandardr/ing teleeommunicalions on a worldwi<strong>de</strong> basis.<br />
he V/orld Telecommunícalion S<strong>la</strong>ndardhalíon Confer<strong>en</strong>ce (WTSC), whieh meéis every fuur years, established ihe topics<br />
ir study by ihe ITU-T Study Groups whích, in their lurn, produce Recomm<strong>en</strong>dations on these tupies. .<br />
PU-T Recomm<strong>en</strong>dation H.320 was revi.sed by the ITU-T Sludy Group XV (1988-1993) and was approved by Ihe WTSC<br />
ielsinki, March 1-12, 1993).<br />
NOTKK<br />
As a consequ<strong>en</strong>ce o¡' a rel'orm procesa wilhin ihe Iniernalional Te-lecommunicalion Unión (!TU), Ihe CC1TT<br />
sased to exist as of 28 Fehrtiary 1993. In iis p<strong>la</strong>ce, the 1TU Telecommunication Siandardizalion Secior (ITU-T) was<br />
reaied as of 1 March 1993. Simi<strong>la</strong>rly, in this reform process, ihe CC1K and the 1FRB have he<strong>en</strong> rep<strong>la</strong>ced by the<br />
.adiocommunication Sector.<br />
\r not to <strong>de</strong><strong>la</strong>y publícation of ihis Recomm<strong>en</strong>dation, no change has he<strong>en</strong> ma<strong>de</strong> in ihe lexl to refer<strong>en</strong>ces con<strong>la</strong>ining<br />
le acronyms "CCITT, CC1R or 1FRB" or Iheir associated <strong>en</strong>iíties suuh as Pl<strong>en</strong>ary Assembly, Secre<strong>la</strong>riai, etc. Fuiure<br />
dílions of this Recomm<strong>en</strong>dation will uomaín the proper lerrninology re.Iated lo the new 1TU slrunuire.<br />
In thís Recomm<strong>en</strong>dalion, ¡he. expres.sion "Adminüslralion" i.s u sed for üo n cus<strong>en</strong> es s lo indiuaie both a<br />
ílecommunication adminisiraiion and a recognixed operaling ag<strong>en</strong>cy.<br />
© ITU 1994<br />
Ul righls reserved. No parí of (his publicalion may he reproducid or ulilixed ¡n any ibrm or by any means, elecironíc or<br />
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ecomra<strong>en</strong>dation H.320<br />
Scope<br />
NARROW-BAND VISUAL TELEPHONE SYSTEMS<br />
AND TERMINAL EQÜIPMENT<br />
(G<strong>en</strong>eva, J990; revised ai Helsinki, ¡993)<br />
'hís Recomm<strong>en</strong>dation covers I he teehnical requirem<strong>en</strong>ls for narro w-band visual lelephone services <strong>de</strong>fined<br />
i H.200/AV.120-Series Recomm<strong>en</strong>dalions, where channel rales do nol exeeed 1920 kbtl/s.<br />
NOTE - It ís anlicipaled ihal ihis Recomm<strong>en</strong>dalion will he ext<strong>en</strong><strong>de</strong>d lo a numlier of Recomrn<strong>en</strong>dations each of whích would<br />
over a single vi<strong>de</strong>oconfer<strong>en</strong>clng or ví<strong>de</strong>ophone service (narrow-band, broadband, ele.). Ilowever, <strong>la</strong>rge parts of these<br />
ecomm<strong>en</strong>dalions would have i<strong>de</strong>nlical wordíny, while in ihe pninis of diverg<strong>en</strong>ce [he actual cholees beiwe<strong>en</strong> allcmaiives have not yel<br />
e<strong>en</strong> ma<strong>de</strong>; for the lime being, iherefore, ti is conv<strong>en</strong>i<strong>en</strong>i lo [real al! [he Icxl in a single Rtícomm<strong>en</strong>dalion.<br />
'he servíce requirem<strong>en</strong>l.s for visual Itikphone servicies are. pre.s<strong>en</strong>led \\\s F.72U for vi<strong>de</strong>olelephony and<br />
'.730 for ví<strong>de</strong>oconferttiuití; vi<strong>de</strong>o and audio «oding sysiem.s and oiher ieL-.hnit:al sel aspcícts uommon lo audiovisual<br />
ervíces are covered in othcr Recomm<strong>en</strong>dalinns in Ihe H.20Ü/A V.2í)0-Series.<br />
\í<br />
:or the purpose of this Recomm<strong>en</strong>dalion, Ihe following <strong>de</strong>finilions apply:<br />
i¡t-rate allocatian sígnal (RAS): 8 hils posilioned wilhín ihe framt; slrueiure of H.221 lo transmil, e.g. commands,<br />
onlrol and indicaiion signáis, capabililitís.<br />
:ontrol and indication (C&I): <strong>en</strong>d-lo-e.nd signalling heiwe<strong>en</strong> I e rm i na I.s uonsisliiig of i: mitro I whic-h causes a si al e change<br />
n Ihe receiver and indicalion which pruvi<strong>de</strong>s for informal ion as lo ihe funclíoning of I he syslem, see also<br />
lecomm<strong>en</strong>dalion H.230.<br />
<strong>la</strong>ta port: ínpui/ouipui gaie for ihe ustír da<strong>la</strong> iransmiiled wilhin .service channel or suh-tih aunéis accordíng lo<br />
lecomm<strong>en</strong>dation H.221.<br />
ip synchronixation; operation lo provi<strong>de</strong> feeling ihal .speaking nioiion nf the di.sp<strong>la</strong>yed person i.s .synchronixed wilh the<br />
(he person makes.<br />
n-band signalling: sígnalling via BAS of Ihe H.221 j'ramt: .striuiiure.<br />
nultipoint control unit (MCU): a píete nf equipm<strong>en</strong>l localed ¡n a no<strong>de</strong> of Ihe neiwork or in a terminal which receives<br />
leveral channels from acces.s pnrls and, ar.cording 10 c'.ertain «rileria, proces.se.s audiovisual signáis and distrilniles ihem lo<br />
he connecled channels.<br />
nan-raachine interface (MMI): niuii-manhine inierfane helwe<strong>en</strong> user and lerniinal/syslem which consisls of a physícal<br />
leclíon (electro-acouslic, electro-opiic iransducer, key.s, ele.) and a logical seclitin <strong>de</strong>aling wilh runctional operalion staies.<br />
<strong>la</strong>rrow-band: bil rales ranging from 64 khil/s lo 192Ü khil/.s. Thís channel capacíly may be provi<strong>de</strong>d as a single<br />
3/Ho/Hn/H[2-channeI or muliiple B/Hf)-«hanneIs in KSDN.<br />
out-band signalling: signalling via a channel nol hc-ing parí of ihe B/H|)/H i i/H [2-channel (due lo !.400-Ser¡es<br />
Recomrn<strong>en</strong>dalions).<br />
visual telephone services: a group of audiovisual .serviles inHuding vi<strong>de</strong>ophone <strong>de</strong>fined in Recomm<strong>en</strong>dalion F.721 and<br />
vi<strong>de</strong>oconfer<strong>en</strong>cing <strong>de</strong>fined in (he Rettomm<strong>en</strong>daiion F.730.<br />
/<br />
Recomm<strong>en</strong>daEion H.320 (03/93) 1
System <strong>de</strong>scription<br />
1 ' Block diagram and I<strong>de</strong>ntification of elem<strong>en</strong>ts<br />
g<strong>en</strong>eric visual lelephone system is shown in Figure 1. !( consists of terminal equipm<strong>en</strong>t, nelwork, mullipoinl control<br />
lít (MCUJ and other system operalíon <strong>en</strong>iílies.<br />
configuration of the terminal equipm<strong>en</strong>i consisiing uf several funciional uníis is also shown in Figure 1. Vi<strong>de</strong>o I/O<br />
¡uipm<strong>en</strong>t ¡nclu<strong>de</strong>s cameras, monitors and vi<strong>de</strong>o processing unils lo provi<strong>de</strong> funciions such as splíl-scre<strong>en</strong> scheme. Audio<br />
0 equipm<strong>en</strong>l inclu<strong>de</strong>s microphones, loud-speakers and audio processing unils lo provi<strong>de</strong> such funciions as ac.oustíc echo<br />
incel<strong>la</strong>tion. Teletnaüc equipm<strong>en</strong>l are vi.sual aid.s such a.s electronic b<strong>la</strong>ckboard, slill piclure Iransceiver lo <strong>en</strong>hance basic<br />
isual lelephone communicalion. System conirol unil carries oul such functlions as nelwork access ihrough <strong>en</strong>d-to-network<br />
gnalling and <strong>en</strong>d-to-<strong>en</strong>d control lo e.s<strong>la</strong>blish common mo<strong>de</strong> of op<strong>en</strong>Umn and üignalling l'or proper operalion of ihe<br />
irminal through <strong>en</strong>d-lo-<strong>en</strong>d signalling. Vi<strong>de</strong>o co<strong>de</strong>e carnes oul rf.dimdaiu:y reducllon codmg and <strong>de</strong>coding for ví<strong>de</strong>o<br />
gnals. whíle audio co<strong>de</strong>e does ihe same ihing for atidio signáis. De<strong>la</strong>y in the atidiu path compénsales ví<strong>de</strong>o co<strong>de</strong>e <strong>de</strong><strong>la</strong>y<br />
1 maintain lip synchronizatíon. Mtix/dmux unil mulliplexes iransmilling ví<strong>de</strong>o, audio, data and control signáis inlo a<br />
ngle bit stream and <strong>de</strong>mulüplexes a receíved bit slream inlo consisling multimedia signáis. Network inlerface makes<br />
ecessary adaptation belwe<strong>en</strong> the nelwork and the lerminal according lo Ihe usf.r-network inlerface requírem<strong>en</strong>ls <strong>de</strong>fined<br />
t the 1.400-Seríes Recomm<strong>en</strong>dalions (see Note).<br />
NOTE - For leased line nelworks, the nelwork inlerface is <strong>de</strong>fined in Recomm<strong>en</strong>datíon G.703 for bit rales in lhe range of 64<br />
ait/s lo 2048 kbit/s. An altemalive inlerfacc is <strong>de</strong>fined ín Recnmmündation X.21. Fnr nxllo channels, timeslot allocalíon is giv<strong>en</strong><br />
i5/G.704 for ihe G.703 inlerface. ll Is stressed ihal inlerworking uiwards ISDN need synchronous operalion of the leased line<br />
iiwork.<br />
Rec.H.320<br />
onl/Ooqupmont <strong>de</strong>c<br />
Audlo |/O equlprn<strong>en</strong>t — Audio co<strong>de</strong>e<br />
System control<br />
MCU Multipoint control unil<br />
5.2 Signáis<br />
T-Serles, H.200/AV.270-Sertes Recs., etc<br />
Telernallc equipmsnt<br />
Recs. H.242.H.230. H.221<br />
Rec.H.261 Rec. H.221 [.400-Series Raes.<br />
H.200/AV.250-Series RQCS.<br />
End-to-<strong>en</strong>d signalling C&l<br />
— De<strong>la</strong>y<br />
— End-to-nerwork signalling<br />
1.400-SarÍes Raes.<br />
—<br />
F1GLJRH 1/H.320<br />
Vi.suul telepliune .syslum<br />
MUX/DMUX<br />
Visual telephone signáis are c<strong>la</strong>ssifíed inlo vi<strong>de</strong>o, audio, da<strong>la</strong> and control as follows:<br />
Audio signáis are continuou.s Iraffíc and require real-líme transmíssíon.<br />
Network<br />
interface<br />
NOTB - In or<strong>de</strong>r lo reduce the average híl rale nf audin signáis, voice actlvatinn can be inlroduced (in whích case<br />
Ihe audio signáis are no longer uoniinuous).<br />
Recomra<strong>en</strong>dation H.320 (03/93)
Vi<strong>de</strong>o signáis are also contiiuious iraffic. the hii rale allocated lo vi<strong>de</strong>o signáis should be as high as<br />
possible, in or<strong>de</strong>r lo maximi'/e ihe qualiiy wiihin Ihe avai<strong>la</strong>ble channel capacity.<br />
Da<strong>la</strong> signáis Inclu<strong>de</strong> fililí piulurtt.s, fae-simile and dmuim<strong>en</strong>ts, or olher facililies, this signal may occur only<br />
occasionally as requíred and may lemporarily disp<strong>la</strong>ce all or parí of Ihe audiovisual signal cont<strong>en</strong>í. It<br />
should be noled ihal da<strong>la</strong> signáis are associaied only wilh opiional <strong>en</strong>hanciem<strong>en</strong>ls lo (he basic visual<br />
lelephone sysiem, iherefore, Ihe op<strong>en</strong>ing of a palh 10 carry suah signáis ís prece<strong>de</strong>d hy negolialion betwe<strong>en</strong><br />
Ihe termináis.<br />
Control signáis are some sysiem control signáis hy <strong>de</strong>finilion. The palh for Ihe lermínal-lo-nelwork control<br />
signáis is provi<strong>de</strong>d in Ihe D-channel, while Ihe palh for the lerminal-lo-ierminal cunlrol signáis is provi<strong>de</strong>d<br />
in BAS orservice channel only wh<strong>en</strong> neee.ssary hy Ihe meehanism <strong>de</strong>fíned in Hec:omme.ndalion H.221.<br />
.3 Bit rate options and infrastructure<br />
.3.1 Comrnunication rno<strong>de</strong>s of visual telephime<br />
lommunicatíon mo<strong>de</strong>s of visual lelephone are <strong>de</strong>fined in Table 1 aceordíng lo iheir channel configuraron and coding.<br />
Visual<br />
lelephone<br />
mo<strong>de</strong><br />
a a0<br />
al<br />
b bj<br />
^2<br />
b3<br />
q qi<br />
Nole3) q2<br />
qs<br />
g<br />
h<br />
i<br />
JOTES<br />
j<br />
k<br />
I<br />
m '<br />
Channel<br />
rale<br />
(khit/s)<br />
1536<br />
1536<br />
1920<br />
TABLK 1/H.32Ü<br />
Communication mu<strong>de</strong>s urvisiuil telephime<br />
ISDN i ;<br />
channel - ISDN inlerface<br />
(Nou2)<br />
Codiny<br />
! . [ I<br />
i Basic ; Priniary rale Audio ¡ Vi<strong>de</strong>o<br />
64<br />
H • Rey. 0.711 (Nolc 4) ¡ Nal applícable<br />
Ret:. G .728 1<br />
128 211 ¡ RCK.CI .711 ¡<br />
Rec. C, .722 i<br />
': Rec. 0 .728 i<br />
! ¡ j Rec. Ü.7I1 j<br />
n XÓ4 ni! | : Rec-. C! .722<br />
Kec. G<br />
\8 ¡ . Rec. 11.261<br />
384<br />
768<br />
¡ l¡0 ' Appl¡c--ab!e Km-. (1.722<br />
;<br />
1152<br />
5) !<br />
j<br />
2n« - ;<br />
3II[, ' Niil ap- i ; (Noití<br />
; plit-ahle !<br />
4l»o ' ! i<br />
"n • !<br />
5"o ! 1<br />
1920 | u,, ; ;<br />
(Audio coding of mo<strong>de</strong> h^ In addilion ID O.728, higlier t¡ua|ily audin uodiny such as II.20Ü/AV.253 may be used for this<br />
no<strong>de</strong>.<br />
For muliiple channels of n/IIn, al! chaiinels are synwhroni/eiJ ai ihe lerminal auuording lo 2.7/11.22 1.<br />
q = c/d/e/f correspondí:; 10 n = 3/4/5/6, respeciively.<br />
If a visual telephone inlerwork wilh a G.725 terminal, G.722 audio may he used insiead of G.7 I 1 audio.<br />
Mo<strong>de</strong>s (G.711 and G.728 audio) other iban this recnmm<strong>en</strong><strong>de</strong>d mo<strong>de</strong> may b e in volved by 11.242 prncedures.<br />
Recnmm<strong>en</strong>dation H.320 (03/93)<br />
i
,3.2 Terminal types of visual telephtme<br />
able 2 lists terminal types of visual lelephmie. The. lerniinal lype i.s categorixed according lo Ihe. communicaiion mo<strong>de</strong>s<br />
id the type of communicalion channels with whiuh Ihe lermmal can commuaicale; mxB (type X with parameler a-f),<br />
xHo (lype Y wilh parameler 1-5; see Note), Hu/Hl2 Oype ^ with parameter Cí-B) or Iheir combinalions.<br />
NOTE - Type Y lerrninals musí have ihe II(j-6I3 cornpallbility mo<strong>de</strong> <strong>de</strong>fined in Recomrn<strong>en</strong>dalion 11.221 for inlerworking of<br />
networks.<br />
Transfer rale<br />
ao B<br />
al B<br />
bj 2B<br />
b2 2B<br />
b3 2B<br />
5<br />
X<br />
X<br />
i.2.1 Examples:<br />
a) type Xb2/3 is a [erminal uapuhle of operaling al mnatas &0, a}, b|, b2 and b3 ihrough B or 2xB-channe!;<br />
b) lype Xb2/3Yl ¡s a lerrnina! capable of operaling al mu<strong>de</strong>s a{1, a,, b], b2, b3 and g ihrough B, 2xB- or<br />
Hrj-chamiel.<br />
c) type Xf2/3Y4Za ¡s a leonina! capable oí' operating al mo<strong>de</strong>s arj-k ihrough (l-fi)xB, (l-4)xH0- or<br />
Htl-channel.<br />
jr MxB and NxHo calegories, ihe terminal shnuld be able lo opérale al all the valúes of m and n nol hígher iban M<br />
id N in principie. The [ype of remole [erminal is ¡<strong>de</strong>ntified through ihe [ransfer rale capabilily exchange <strong>de</strong>fined in<br />
Bcomm<strong>en</strong>dation H.242.<br />
3.3 Vi<strong>de</strong>o co<strong>de</strong>e<br />
s per Recomm<strong>en</strong>dation H.261.<br />
3.4 Audio cu<strong>de</strong>c<br />
sperRecomm<strong>en</strong>dalions G.711, G.722. G.72K, H.20U/AV.253 (see Table 1).<br />
,3.5 Frarae structure<br />
s per Recomm<strong>en</strong>dation H.22 1.<br />
,3.6 Control and indication (C&l)<br />
í<strong>en</strong>tifíed subsel of H.230 is u.sed (.see 4.4).<br />
.3.7 Cummunication procedí!re<br />
,s per "Recomm<strong>en</strong>dalion H.242.<br />
.4 Cali control arrangem<strong>en</strong>ts<br />
'o establish intercommunicaiíon beiwe<strong>en</strong> varitius audiovisual termináis, it is necessary to carry out ín-band and out-band<br />
rocedures according to Recomm<strong>en</strong>dalion H.242 and olher re-levaní Recomm<strong>en</strong>dalion.s.<br />
*he differ<strong>en</strong>t stages of Ihe cali are referred ¡utcording lo a poinl-lo-pninl confíguralion where terminal X is Ihe calüng<br />
erminal and Y Ihe called terminal.<br />
1.4.1 Estublishm<strong>en</strong>t of a vi.sual lelephone cali - Normal procedure<br />
l"he provisión of ihe communlcalion is madtj in lh« iiinin fnlUnving sieps:<br />
- phase. A: cali sut-up, oui-band HÍgn;illing',<br />
- phase Bl: mo<strong>de</strong> inilializalinn nn ¡nitial channel;<br />
- phase CA: cali set-up of addílional channel(.s), if relevaní:<br />
- phase CB1: initÍalÍ7.aiion on addilmnaí channel(s);<br />
- phase B2 (or CB2): esiablishme.nl of uimimnn parameters:<br />
- phase C: visual le.lephone communicalion;<br />
- phase D: te.rrninalion pha.se;<br />
— phase E: cali releuse.<br />
3.4.1.1 Phase A - Cali set-up<br />
After user ¡nilializalion, !he I erminal X pe-rform.s a cali set-up procedure (ihis procedurtí lo be <strong>de</strong>fined ín<br />
Recomm<strong>en</strong>dalion H.200/AV.420 is un<strong>de</strong>r siudy). As soon as the. [erminal receives an indicalion from Ihe nelwork that ihe<br />
connectíon is esiabüshed, a bidireciional channel is op<strong>en</strong>ed from <strong>en</strong>d lo ei\d, and it over<strong>la</strong>ys H.221 framing on the<br />
channel. .<br />
f<br />
Recnmm<strong>en</strong>dation H.320 (03/93)
)IIowing the conneclion esiahlishmmil, all Ihe lerminals will s<strong>la</strong>rl lo work in Ihe following mo<strong>de</strong>s <strong>de</strong>fined in Recom<strong>en</strong>daüon<br />
H.22I:<br />
- lype X: mo<strong>de</strong> OF (A-<strong>la</strong>w or p-Iaw):<br />
- type Y and type Z: mo<strong>de</strong> OF (A-<strong>la</strong>w or u-<strong>la</strong>w) audío only.<br />
i-band procedure is aciivaied.<br />
4.1.2 Ph.ase Bl - Mo<strong>de</strong> mitiuliy.atian<br />
,4.1.3 Phase Bl-1<br />
sing Ihe procedures proví<strong>de</strong>d in Recornrn<strong>en</strong>dalion H.242, l'ramed PCM audiu is iransmilled in holh direcuíons, afler<br />
ame and mulliframe altgnm<strong>en</strong>l is gained, le.rminal capabíliiies are extihanged.<br />
,4.1.4 Phase Bl-2 (terminal pro ce dure)<br />
lelertnination of ihe appropriale mo<strong>de</strong> lo he iransmilled. This will normally be Ihe highesl uotnmon mo<strong>de</strong> (see Table 3 for<br />
le case using a B or 2xB-channel), huí a lower compatible mo<strong>de</strong> could he chosea inslead.<br />
i Ihe case that both termináis have announced Ihe tiapahitily lo work on addiiíonal c'.haniiel(s), lerminal X iniliales Ihe<br />
¡quesl for supplem<strong>en</strong>iary cal! sel-up. Allernaiively, ihis aclion may be susp<strong>en</strong><strong>de</strong>d unül ihe user ai X has giv<strong>en</strong> ihe<br />
[t-ahead, (he Y user may also control Ihe addilional channul requesls. ll is for furlher sludy.<br />
NOTE- If ihe user ai eilher lerniina! dnes nol wish ihe cali 10 proceed lo two or more channels, ev<strong>en</strong> ihough his terminal<br />
is ihís capabiliiy, he musí sel the termina! sucli ihai nnly .single-channe! capabílily is <strong>de</strong>c<strong>la</strong>red in phase R 1-1. In ihat cass, we should<br />
islinguish ihe active capabilíiy, wíshed hy Ihe users, from ihe virtual capahilily of the terminal.<br />
al<br />
NOTES<br />
TABLE 3/H.320<br />
Cummon mo<strong>de</strong> (<strong>de</strong>fault) fnr ctimmunicutiim betwe<strong>en</strong> differ<strong>en</strong>t types<br />
of visual telephone.s using a B or 2xB-channeI<br />
Xb5 Terminal ivpe<br />
1 "b2/h:j" indícales Ihai eilher of b? or bi can be common. h? gives beiier audio, while b^<br />
gives belier vi<strong>de</strong>o.<br />
2 The communiciuion minies indicaled in Ihe <strong>la</strong>ble alnwe inclu<strong>de</strong> ihe possibiliiy lo use ihe<br />
GIF formal as weil as ihe QCIP formal. The C!F formal is used In both díreclions of<br />
iransmission, if both lerminals indícale ibis capubüily. In a!l olher cases the QCIP formal is<br />
used.<br />
Each terminal may use a mínimum <strong>de</strong>eoiiahle piclure inlervaí in iis s<strong>en</strong>ding dirección<br />
which makes hesl use oflhe f.apabiliiy iiulif-aled by (he olher terminal.<br />
This table iloes nol inulu<strong>de</strong> inierworking silualions belwe<strong>en</strong> visual lelephones and<br />
telephone lerminals. ¡f visual lelephone lerminals are c'onnecled lo telephones, rno<strong>de</strong> a() is used<br />
for the comrnunicaiion.<br />
3.4,1.5 Phase Bl-3 (mu<strong>de</strong> switfhing)<br />
Both lerminals swilch lo Ihe mo<strong>de</strong> Ihey have i<strong>de</strong>nlii'ied in pha.se Hl-2, using Ihe prociedure of Recomm<strong>en</strong>dalíon H.242.<br />
NOTE - íf ihe lerminals havc no| bolh adnpled the cununon rno<strong>de</strong>, an asymmetric communícalion may result.<br />
Recomm<strong>en</strong>datirin H.320 (03/93)<br />
Xb<br />
2/3<br />
Xb5
4.1.6 Phase CA - Cali set-up of the additional channel(s)<br />
>Ilowing phases Bl-3 and B2 ¡f relevan!, ihe communicatíon phase C proceeds on thal channel. If additional channels<br />
ive be<strong>en</strong> requesled ihese go also ihrough pha.se A (he.nt'.e ihe nom<strong>en</strong>e<strong>la</strong>iure "Phase CA"), exacüy as in phase A above,<br />
id addilional cali sel-ups are performed by Ihe termináis. On each of Ihe e.s<strong>la</strong>hlíshed channels H.221 framing i.s over<strong>la</strong>id<br />
ee Note).<br />
NOTE - During phase CA an imermediale audiovisual mo<strong>de</strong> could he offered on ihe ínílial channel used for inH¡aliy.ation,<br />
ilil ful! completion of inilia|Í7.a[inn phase.<br />
,4.1.7 Phase CB1 ~ Mu<strong>de</strong> ¡nitialr/.atiim on additional chunnel(s)<br />
.4.3.8 Phase CBl-11<br />
sing the procedure proví<strong>de</strong>d in "Recomm<strong>en</strong>daiion H.242, frame and mullU'rame alignm<strong>en</strong>ls are gainud.<br />
,4.1,9 Phase CB1-12<br />
ynchroiiization of the channels ís aehieved.<br />
,4.1.10 Phase CB1-2 (terminal procedure)<br />
'eterminatíon of ihe appropriate mu<strong>de</strong> ui be [ran.smitled. This WÍII normally be (he highest common mo<strong>de</strong>, bul a lower<br />
ampatible mo<strong>de</strong> could be chos<strong>en</strong> instead.<br />
.4.1.11 Phase CB1-3 (mo<strong>de</strong> switching)<br />
¡oth termináis sw i tch lo ihe mo<strong>de</strong> ihey ha ve. i<strong>de</strong>nlified in phase B 1-2 u.sing ihe. procedure of Recomm <strong>en</strong> dal<strong>la</strong>n H.242.<br />
¡sull.<br />
NOTE - Mere ngain, if Ihe lerminals have noi holh adopted the same comrnon mo<strong>de</strong>, an a^yriimelric communícation will<br />
.4.1.12 Phase B2 (or CB2) - Establíshmtínt of common paranieter.s<br />
'hís phase es<strong>la</strong>blishes common operalional parameler.s specífic to visual le.lephone (e.g. <strong>en</strong>eryplion) afler phase Bl<br />
rocess Ís finíshed. Capabililíes or requirerntínis ni' ihe ref.e.iving sí<strong>de</strong> are firsl indicaied Ih<strong>en</strong> ihe ü<strong>en</strong>ding si<strong>de</strong> <strong>de</strong>ci<strong>de</strong>s<br />
peral ío nal pararneiers and con i ruis Ihe rüceivmg si <strong>de</strong>.. HAS co<strong>de</strong>s i'or ihi.s purpure are <strong>de</strong>fined ¡n Recomm<strong>en</strong>alion<br />
H.221.<br />
.4.1.13 Phase C - Visual telephime communicatttin<br />
ti the case where more than one channel i.s usad, there will be intermedíale phases CA, CB], CB2 as <strong>de</strong>scribed ¡n this<br />
ubc<strong>la</strong>use. Ltkewise, if additional c.hannels are dropped during ihe cali there will be intermedíale phases CO, CE as<br />
lescribed ín 3.4.4. The provisimus of ihl.s .suhit<strong>la</strong>ustt apply lo any channel, Inil<strong>la</strong>l or addilional, for which phases Bl<br />
nd B2 have be<strong>en</strong> compleled and phase. I) nol yel siarted.<br />
1.4.1.13.1 Mo<strong>de</strong> ¡rvvitching<br />
^ccordíng lo action by either user (for example, siariing a facsimile machine) a differe.nl mo<strong>de</strong> from Ihe highesl common<br />
no<strong>de</strong> may become more appropriate. Swilching lo lilis mo<strong>de</strong>. i.s ma<strong>de</strong> acxording lo [he procedure of Recomn<strong>en</strong>dation<br />
H.242.<br />
1.4.1.13.2 Capability changu<br />
Phe ustir may change ihe capabilily of h¡.s terminal during Ihe cali (for examplc. by connecling or swílching-on auxiliary<br />
elemaiíc equipm<strong>en</strong>l); ihe terminal musí initialtí ihe capabiltly e.xchange. procedure. <strong>de</strong>fined in Recomm<strong>en</strong>dalion H.242.<br />
L4.1.14 Phase D - Terminatiim pha.ve<br />
L4.1.15 Phase Di (terminal procedure)<br />
iVh<strong>en</strong> one of ihe users hangs up, ihe terminal involve.s phase í)2 direclly.<br />
3.4.1.16 Phase D2 (mo<strong>de</strong> swítching)<br />
Mo<strong>de</strong> OF is forced according lo Recomni<strong>en</strong>dalion H.242 (or <strong>la</strong>king inlu acuinint Ihe result uf phase DI if differ<strong>en</strong>l; for<br />
further study).<br />
: ' Recomm<strong>en</strong>dation H.320 (03/93) 7
case the cali is dísconnecied for some rea.son, il may take a long lime ÍJ' il keeps awalling reacííon from ihe remole<br />
rminal, and receplíon or ¡niliaiíon of ihe ne.xl cali rnay líe blocked. 1SON disconneclion should he giv<strong>en</strong> príoriiy over ihe<br />
.242 cali terminaiíon procedure. A líme-oiu of lwo secuild.s is' suggesled.<br />
4.1.17 Phase E - Cali terminatiun (reléase)<br />
he terminal which has inilíated Ihe hang up .s<strong>en</strong>ds messages over Ihe D-channel wílh respecl In all channels and idles all<br />
'them (ihat means no more informalinn s<strong>en</strong>l over).<br />
t Ihe other lerminal, the firsl discnnnecí message receivr.d wíll idle all channels.<br />
he actual disconnection occurs al reeeplion of Ihe oiher di.sconneci message(s).<br />
,4.2 Exceptional procedures for phases A and K<br />
i case of unsuccessful oulcome during phases A and R (due lo many causes), excepiúmal proeedures are provi<strong>de</strong>d ín<br />
r<strong>de</strong>r to <strong>en</strong>sure a suítable service. The maner Í.s for furlher sludy.<br />
,4.3 Exceptional procedures during pha.se C<br />
uring Ihe actual exchange of audiovisual da<strong>la</strong>, pmhiems may oüeur in sorne channels. Fallhack procedures, managed hy<br />
e lerminal are aclivated. The <strong>de</strong>seriplion of Ihe procedures and ihe appropriale indícations are for furlher study.<br />
,4.4 Addition and drnpping of channels during u visual telephone culi<br />
.4,4.1 Addition<br />
ccordíng lo aclíon hy a user (for example Ihe aclivalion of auxiliary equipm<strong>en</strong>l) one or more addiiional channels are<br />
¡quested. The procedure follows those. <strong>de</strong>scrihed for phases CA and CB1.<br />
.4.4,2 Dropping<br />
'wo phases are <strong>en</strong>visaged.<br />
.4.4.2.1 Phase CD1<br />
'he common mo<strong>de</strong>, appropriale tu Ihe channel(s) which remains, is selecied.<br />
.4.4.2.2 Phase CD2<br />
'he mo<strong>de</strong> swiichlng procedure of Kecomni<strong>en</strong>dalíon H.242 is appliei! lo involve Ihe mo<strong>de</strong> idttnlified ¡n phase CD1; the<br />
ímainíng channel ¡s the channc! used for iniliali'/alinn (see phase A), ll suppiiris an appropriale fallhack mo<strong>de</strong>. The<br />
<strong>la</strong>tter is for furlher sludy.<br />
.4.S Transmission and disp<strong>la</strong>y of pictures at the síart nf a visual telephone cali<br />
iccording to ihe chos<strong>en</strong> lerminal procedures, píclures may or may nol he visihle lo liolh users as soon as ínitíalixalion is<br />
omplete. In the case thal eilher phase Rl-3 or phase ('Hl-3 has aclivuled a common mo<strong>de</strong>, including vi<strong>de</strong>o, muiual<br />
isibility of Ihe users is po.ssihle.<br />
'he followíng ilems collect allernalive procedures which can he used lo susp<strong>en</strong>d píclure disp<strong>la</strong>y unlil user inlerv<strong>en</strong>tion<br />
by mutual agreem<strong>en</strong>l orolherwise) causes piciures lo he disp<strong>la</strong>yed.<br />
1) No vi<strong>de</strong>o íransmitted - !n phase BÍ-2 and (if relevanl) phase CB1-2 ihe mo<strong>de</strong> selecled inclu<strong>de</strong>s vi<strong>de</strong>o<br />
OFF. During phase C eilher user may uni<strong>la</strong>lerally swilch lo vi<strong>de</strong>o ON, alternalively, his lerminal may s<strong>en</strong>d<br />
the C&I BAS co<strong>de</strong> V1R (vi<strong>de</strong>o indícale ready-to-aetivate), huí not swilch to vi<strong>de</strong>o-ON unlil ViR is also<br />
received from ihe olher terminal. While the incoming vi<strong>de</strong>o-OFF stale remains, the visual lelephone scre<strong>en</strong><br />
should disp<strong>la</strong>y a symbol or message ¡ndicaiing Ihi.s (i.e. there is no faull).<br />
As already noled ¡n 3.4.1, phase Bl-2, ihe requesl for addílional channel may, according lo terminal<br />
procedure, be <strong>de</strong><strong>la</strong>yed whilr, vi<strong>de</strong>o-OFF is mainiained; user aclion lo aelivate vi<strong>de</strong>o would ih<strong>en</strong> resull in<br />
procedure phases CA 1, CB1 (CB2 ¡f required).<br />
I Recomm<strong>en</strong>dation H.320 (03/93)
2) Vi<strong>de</strong>o paltern iransmiíted- An üleclroniüally g<strong>en</strong>eraled ur olher paltern is iransmilied inslead of Ihe sígnal<br />
/"rom a normal camera. Trie C&l RAS co<strong>de</strong> VIS (ví<strong>de</strong>o indicíale suppressed) is used lo indícale the situatian<br />
lo the remole party.<br />
3) Vi<strong>de</strong>o transmitted bal no! disp<strong>la</strong>yed - Terminal proeedures simply involve local action lo disp<strong>la</strong>y not ihe<br />
incomíng signal huí an exp<strong>la</strong>naiory symbol or me.ssage. User action would cause the incurrí ing signa! to be<br />
disp<strong>la</strong>yud, bul ií ihis should <strong>de</strong>p<strong>en</strong>d on mulual atuion by bolh usera ih<strong>en</strong> a new C£I BAS co<strong>de</strong> VRD (vi<strong>de</strong>o<br />
ready-lo-disp<strong>la</strong>y) musí be <strong>de</strong>fined. Thi.s poinl is for furlher siudy.<br />
,5 O p ti o nal <strong>en</strong>hancem<strong>en</strong>ts<br />
5.1 Bata porís<br />
ata porís as physical l/O ports oí' Ihe lermínal for lelemalk: and olher equípm<strong>en</strong>l are aclivaled/<strong>de</strong>aclíved by HAS<br />
immands. Dep<strong>en</strong>ding on Ihe transmission ciipahility of a eonneelion, e.g. mulliples of R/H(| channels, ele., varíous bit<br />
ites are avai<strong>la</strong>blt at (hese porís. AHocaiion ol'bii slmanis lo lh« pori(.s) is peri'ornied by in-band .signalling. Da<strong>la</strong> conveyed<br />
. the port(s) is iranspar<strong>en</strong>t, da<strong>la</strong> rales being lisliid in Atinex A/H.22!.<br />
,5.2 Encryption<br />
ncrypiion may be applíed on audio and ví<strong>de</strong>o sigoal.s separaiely (preferul>ly J'or mitltipoinl üonneulions) or on audio and<br />
<strong>de</strong>o signáis multiplexed. vSwíichíng-on and olí the <strong>en</strong>cryplion process has to be sigiialled beiwe<strong>en</strong> Ihe termináis (or<br />
rminal and MCU respeclively) vía ¡n-band signalling.<br />
Terminal requirem<strong>en</strong>ts<br />
,1 Environm<strong>en</strong>ts<br />
n<strong>de</strong>r sludy.<br />
.2 Audio and vi<strong>de</strong>o arrangem<strong>en</strong>ts<br />
ín<strong>de</strong>r study.<br />
.3 De<strong>la</strong>y comp<strong>en</strong>sation in the audio path<br />
lie H.261 vi<strong>de</strong>o co<strong>de</strong>es require so me. processing <strong>de</strong>.<strong>la</strong>y, while ihe H.200/A V.2.1t)-ííe.rÍes audio tm <strong>de</strong>cís involve. mucih le.s.s<br />
e<strong>la</strong>y. H<strong>en</strong>ee, if Üp synchroniy.alion is lo be rnain<strong>la</strong>ined, ihal vi<strong>de</strong>o proce.s.sing <strong>de</strong><strong>la</strong>y musí be ctomp<strong>en</strong>saled ¡n ihe audio<br />
aih. Since vi<strong>de</strong>o co<strong>de</strong>r and <strong>de</strong>c-.o<strong>de</strong>r <strong>de</strong><strong>la</strong>ys may vary acciording lo implern<strong>en</strong><strong>la</strong>iion, <strong>de</strong><strong>la</strong>y f.omp<strong>en</strong>salion musí be yarried<br />
ut indivídually al the co<strong>de</strong>r and <strong>de</strong>co<strong>de</strong>r. A refere.ncui me.asurtíme.iil melhod of vi<strong>de</strong>o co<strong>de</strong>r and <strong>de</strong>co<strong>de</strong>r <strong>de</strong><strong>la</strong>ys i.s <strong>de</strong>fined<br />
i Recomm<strong>en</strong>dalion H.261.<br />
A Control and indications (C&I)<br />
!&I are chos<strong>en</strong> from Ihe g<strong>en</strong>eral audiovisual .sel tumiained in Rr.üomni<strong>en</strong>dalion H.230. For visual lelephone systems,<br />
lose signáis m Table 4 are used mandalory, whe.re iheir sourcte., sink, synchroniy.alion wilh pif.lure, Iransmíssion channel<br />
nd co<strong>de</strong>words are ¡iidícaied.<br />
ill visual íelephone lerrninals have a vi<strong>de</strong>o .source providing a piciure o!' parlicipanls, and some termináis may nave<br />
ddítional ví<strong>de</strong>o sources; the. parlicipanl-piclure source ís <strong>de</strong>.signaied #1, havíng Ihe assoeialed symbol VÍA. Wh<strong>en</strong><br />
icoming vi<strong>de</strong>o is ON (BAS cornmand (OJO) [1 or 21) and VIA. V!A2, V1A3 have nol be<strong>en</strong> iransmiiled, sourceífl is<br />
ssumed.<br />
Jome supplem<strong>en</strong><strong>la</strong>ry services provi<strong>de</strong>d by sonie coni'ereiu:e bridges, servers, ele. may require termináis 10 s<strong>en</strong>d out the<br />
.ymbols on iheir diallíng keyboards. il ís highly <strong>de</strong>sirable. Ihai all visual lerminals be able lo irans<strong>la</strong>le ihese symbols inlo<br />
3AS co<strong>de</strong>s repres<strong>en</strong>líng Ihe Alphabel of Figure 2/T.fi] as provi<strong>de</strong>d for ín Recomm<strong>en</strong>t<strong>la</strong>iíon H.230.<br />
! Kecumm<strong>en</strong>dation H.320 (03/93) 9
f<strong>la</strong>co<br />
wcu<br />
Maint<strong>en</strong>ance<br />
Confer<strong>en</strong>ce<br />
Terminal<br />
C&Isignal<br />
Pícture formal<br />
Picture formal<br />
Mínimum <strong>de</strong>uodable<br />
picturtí inlerval<br />
Freeitt pícture<br />
requesl control,<br />
VCF<br />
Fas! update request<br />
control, VCD<br />
Freey,e picture<br />
reléase coptrol<br />
Multipoint<br />
command<br />
confer<strong>en</strong>ce, MCC<br />
and cancel-MCC<br />
Muliipoint<br />
cornmand<br />
symmelrical da<strong>la</strong><br />
transmission, MCS<br />
Multipoint<br />
command negalíng<br />
MCS, MCN<br />
Vi<strong>de</strong>o loop requesl<br />
control. LCV<br />
Digital loop reqnesl<br />
control, LCD<br />
Loop off reqtiest,<br />
LCO<br />
Splil «cre<strong>en</strong><br />
¡ndicalion<br />
Docum<strong>en</strong>i camera<br />
indication<br />
Audio active/rmiied<br />
indicalion, AIA/AÜM<br />
Vi<strong>de</strong>o active<br />
indication VÍA<br />
Vi<strong>de</strong>o suppressed<br />
indication VIS<br />
TAHLK 4/H.32Ü<br />
C&I signáis mündatury for visual telephnne<br />
C/I<br />
Recomm<strong>en</strong>dation H.320 (03/93)<br />
Source<br />
I Deco<strong>de</strong>r<br />
C Co<strong>de</strong>r<br />
1 Deco<strong>de</strong>r<br />
•<br />
:<br />
C Co<strong>de</strong>r or<br />
MCU<br />
C Deco<strong>de</strong>r or<br />
MCU<br />
C Co<strong>de</strong>r<br />
C MCU<br />
C MCU<br />
C MCU<br />
i<br />
C Terminal<br />
C. Terminal<br />
.<br />
C. Terminal<br />
¡<br />
1 .Semliny<br />
terminal<br />
1 i S<strong>en</strong>diny<br />
terminal<br />
1 j S<strong>en</strong>ding<br />
terminal<br />
1 S<strong>en</strong>ding<br />
terminal<br />
1 S<strong>en</strong>ding<br />
i terminal<br />
Co<strong>de</strong>r<br />
Sink<br />
Deco<strong>de</strong>r<br />
Co<strong>de</strong>r<br />
Deco<strong>de</strong>r<br />
Co<strong>de</strong>r<br />
Deco<strong>de</strong>r<br />
Terminal<br />
Terminal<br />
Terminal<br />
Terminal<br />
Termina!<br />
Terminal<br />
Rectíiving<br />
terminal<br />
Receiving<br />
terminal<br />
Rectíiving<br />
terminal<br />
Receíving<br />
lerrninal<br />
Receiving<br />
lerminal<br />
Sync.<br />
whh<br />
piclure<br />
No<br />
Yes<br />
No<br />
No<br />
No<br />
Yes<br />
No<br />
No<br />
No<br />
No<br />
No<br />
No<br />
Yes<br />
Yes<br />
No<br />
No<br />
No<br />
Transmission<br />
channel<br />
DAS<br />
Hmbed<strong>de</strong>d in<br />
vi<strong>de</strong>o<br />
RAS<br />
DAS<br />
BAS<br />
Brnhed<strong>de</strong>d in<br />
vi<strong>de</strong>o<br />
BAS<br />
HAS<br />
BAS<br />
BAS<br />
BA.S<br />
BAS<br />
Bmhed<strong>de</strong>d in<br />
vi<strong>de</strong>o<br />
P.mhed<strong>de</strong>d in<br />
vi<strong>de</strong>o<br />
BAS<br />
BAS<br />
BAS<br />
Co<strong>de</strong>word<br />
<strong>de</strong>finilion<br />
H.221<br />
11.261<br />
H.221<br />
11.221<br />
H.221<br />
11.261<br />
Í1.230<br />
11.230<br />
11.230<br />
H.221<br />
11.221<br />
11.221<br />
11.261<br />
11.261<br />
11.230<br />
11.230<br />
11.230
,5 Multipoint operation<br />
is híghly <strong>de</strong>sirable thal lermínals not having any auxiliar)' da<strong>la</strong> equípm<strong>en</strong>l be able lo op<strong>en</strong> LSD and/ur HSD channels so<br />
<strong>la</strong>t they could particípale in mullipoinl comnuinieatmns wiihoul losing vi<strong>de</strong>o. This could be ext<strong>en</strong><strong>de</strong>d 10 the use of MLP<br />
id/or H-MLP, bul is for furlher siudy.<br />
>ther aspects of muliipoinl operalion nf the terminal are un<strong>de</strong>r xludy.<br />
ínter Communications<br />
he mechanisms for iniercommunicalion wilh other .servicies are <strong>de</strong>scribed in Ihe H.200/AV.240-Series Recomm<strong>en</strong>atíons.<br />
.1 Intercommunication betwe<strong>en</strong> differ<strong>en</strong>t visual telephone terminal types<br />
, common mo<strong>de</strong> of aperalíon i.s <strong><strong>de</strong>l</strong>ermíned a.s <strong>de</strong>scribed in 3.4.1. I)-channel signalling shuuld inclu<strong>de</strong> new LLC and<br />
!LC which are appropriale for audiovisual services, bul Ihis .sul)u<strong>la</strong>use i.s for furlher sludy.<br />
.2 Intercommunication with telephony<br />
NOTE - Descriplion of this subu<strong>la</strong>uss is fnr Communications using a li-uhannel.<br />
.2.1 Intercommunication with ISDN telcphune.s<br />
L cali from a visual lelephone to an 1SDN lelephone is firsl p<strong>la</strong>r.ed as an audiovisual cali, bul Ihe ISDN lelephone reiurns<br />
icompaiible <strong>de</strong>siination orihe neiwurk reiurns recuivery on limerexpiry in case of no responses from Ihe called si<strong>de</strong>, ih<strong>en</strong><br />
le visual lelephone may .swíich lo a spee.ch or 7 kHx auiüii liearer .servicie cali.<br />
, cali from ISDN lelephone lo a visual [elf.plume is Mt'.uepted hy ihe visual leleplione. het:austi every audiovisual terminal is<br />
quípped with this lelephone capahilily as a minimum fuiíulion.<br />
'or both of Ihe abo ve cases, ihe ope rallona I mo<strong>de</strong> of com ni un lea I ion is 0.71 1 speech or G.722 audio.<br />
.2.2 ínter cora mun i catión with PSTN telephone.s<br />
t cali from visual lelephone 10 a PSTN lelephone may be ínilialed as an audiovisual cali, bul (he nelwork reiurns no roule<br />
D <strong>de</strong>stinatíon, ih<strong>en</strong> the visual lelephone may swilch lo a sptsetih or 3.1 kHv, audio bearer service t:all. The operalional<br />
of communication is G.711 audio iiti<br />
I cali from a PSTN lelephone is routed inlu ihe ISDN as a 3.1 kHv audio cali which can be respon<strong>de</strong>d by Ihe visual<br />
elephone for the same reason as <strong>de</strong>scribed in 5.2.1. The operalion a I mo<strong>de</strong> nf f.omniunicaikm ¡s 3.1 kHv, audio.<br />
1.3 Intercommunication with other audiovisual termináis<br />
^ common mo<strong>de</strong> of operalion is <strong><strong>de</strong>l</strong>ermined auunrdiny lo Ihe H.20()-.Se.rÍes Recomm<strong>en</strong>dalíons.<br />
í Maint<strong>en</strong>ance<br />
lome loop-back funclions are <strong>en</strong>visaged lo aílow verificalion of thtí i'unclional as p<strong>en</strong>is of Ihe terminal ín or<strong>de</strong>r lo <strong>en</strong>sure<br />
.orrect operalion of ihe sy.stem and .snlis<strong>la</strong>clory qualiiy o!' ihe serviue lo Ihe remole party. The f'ollowíng loop-back<br />
unctions (see Figure 2) are <strong>en</strong>visaged:<br />
a) Loop at lerminal-neiwork inle.r!'ace (imvard.s nelwork)<br />
Upon receíving ihe digital loop-back HAS, loop-back is aclivaled ai ihe digital mterfaee of the terminal<br />
lf.iM,V loward Ihe network si<strong>de</strong>. In ua.se of a múltiple. B/Ho channel arrangem<strong>en</strong>t, [oop-bauk is aclivaled in each<br />
connection.<br />
Kecomm<strong>en</strong>dation H.320 (03/93) 11
) Loop ai lerminal-nelwork inlerl'ace (lowards lerminal}<br />
The procedure is for furlher sludy.<br />
c) Loop ai analogue ]/Q iiilerface<br />
U pon receivíng ihe vi<strong>de</strong>o hiop-back or audio loup-hack BAS, loop-hack ís aciivaled al Ihe analogue<br />
ínlerface of ihe vi<strong>de</strong>o/audio co<strong>de</strong>e lowards Ihe vi<strong>de</strong>u/audio co<strong>de</strong>e.<br />
he opponuniíy of havíng a self-checking procedure al lerminal siage is for furlher sindy.<br />
a) Normal<br />
7 Human factor<br />
b) Digital loop request (LCD)<br />
(H<br />
( )<br />
— — —<br />
c) Audio/vi<strong>de</strong>o loop requ est (LCA/LCV)<br />
(H<br />
Co<strong>de</strong>e<br />
PKUJRH 2/H.32C)<br />
Loop back<br />
/- -N<br />
á i Neiwork<br />
* V ^<br />
Po achieve error free and uncomplicaied uiilixaiinn o!'lerminal equipm<strong>en</strong>l and sf.rvice froni Ihe tusers s<strong>la</strong>ndpoini, human<br />
actor re<strong>la</strong>led aspecls have lo he sludied and reaomm<strong>en</strong><strong>de</strong>d. These. a.specls <strong>de</strong>al wilh ihe. I'low of informalion heiwe<strong>en</strong> user<br />
ind lerminal/nelwork. Thi.s informalion c'.an he divi<strong>de</strong>d inlo a phyfiical seclion and a logical s<strong>en</strong>iinn of Ihe MM1.<br />
7.1<br />
7.2<br />
Physical sectíon<br />
- Figures and properlitfs of Iransducers (ciuniera, microphone, ele.).<br />
- Signáis parlicu<strong>la</strong>rly re<strong>la</strong>led lo ihe .service, keys, piciogram.s.<br />
Lógica] section<br />
- Procedures, e.g. for cali e.s<strong>la</strong>hli.shm<strong>en</strong>l/release, durin^ communicalion phase.<br />
- Consisl<strong>en</strong>cy helwe<strong>en</strong> Ihe MM Is o i' visual lelephone and lermlnals of olher leleservícK.s.<br />
12 Recorara<strong>en</strong>dation H.320 (03/93)
BIBLIOGRAFÍA<br />
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