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lecommendation H.242
[ Introduction
SYSTEM FOR ESTABLISHING COMMÜNICATION
BETWEEN AUDIOVISUAL TERMINALS
ÜSING DIGITAL CHANNELS ÜP TO 2 Mbit/s
(Genova, 1990; revised at Helsinki, 1993)
^his Recommendation should be associated with Recommendations G.725 (System aspects for the use of the 7 kHz
udio codee within 64 kbit/s), H.221 (Frame structure for 64 to 1920 kbít/s channels in audiovisual teleservices)
nd H.230 (Frame-synchronous control and indication signáis for audiovisual systems).
L number of applications utilizing narrow (3 kHz) and wideband (7 kHz) speech together with video and/or data have
icen identified, including high quality telephony, audio and videoconferencing (with or without various kinds of
elematic aids), audíographic conferencing and so on. More applications will undoubtedly emerge in the future.
'o províde these services, a scheme ís recommended ín which a channel accommodates speech, and optionally video
nd/or data at several rates, in a number of dífferent modes. Signalling procedures are required to establish a
ompatible mode upon cali set-up, to switch between modes during a cali and to allow for cali transfer.
lome services wíll require only a single channel, which could according to the procedures in this Recommendation be
! (64 kbit/s), H0 (384 kbit/s), Hn (1536 kbit/s) or H12 (1920 kbit/s). Other services will require the establishment of
tvo or more connections providíng B or HQ channels: in such cases the first established is called hereafter the initial
hannel while the others are called addítíonal channels, Unless otherwise specified, all references to frame alignment
ignal (FAS), bit rate allocation signal (BAS) and service channel (SC) refer to the initial channel or, in the case of a
ígher-order channel, to the time-slot No. 1 of this channel.
Lll audio and audiovisual termináis using G.722 audio coding and/or G.711 speech coding or other standardizad audio
odings at lower bit rales should be compatible to permit connection between any two termináis. This implies that a
ommon mode of operation has to be established for the cali. The initial mode might be the only one used during a cali
r, altematively, switching to another mode can occur as needed depending on the capabilities of the termináis. Thus,
or these termináis an in-channel procedure for dynamic mode switching is required,
'he followíng clauses develop these considerations and describe recommended in-channel procedures.
Terminal capabilities
'he procedures in this Recommendation are mtended to ensure that only those signáis are transmitted which can be
;ceived and appropriately treated by the remote terminal, without ambiguity. This requíres that the capabilities of each
¡rminal to receive and decode be known to the other terminal. Some capabilities are defined with a hierarchical
Eructure; a terminal with capability valué N is then also capable of all lower valúes. Where there Ís no hierarchy, then
¡vo or more codes of the same type may have to be transmitted in successive frames.
'he following subclauses define audio, vídeo, transfer rate, and data rate capabilities of a terminal. It Ís not necessary
lat a terminal understand or store all íncoming capabilities. Those which are not understood, or which cannot be used
secause the terminal has no means to transmít corresponding information), can be ignored.