network protocols handbook.pdf

170Protocols GuideWAN - Broadband Access ProtocolsProtocol NameISDN: Integrated ServicesDigital NetworkProtocol DescriptionIntegrated Services Digital Network (ISDN) is a system withdigitized phone connections. For decades, telephony has usedpurely analogue connections. ISDN is the first protocol to definea digital communications line that allows for the transmission ofvoice, data, video and graphics, at high speeds, over standardcommunication lines. The various media are simultaneously carriedby bearer channels (B channels) occupying a bandwidth of64 kbits per second (some switches limit bandwidth to 56 kb/s).A defined data channel (D channel) handles signaling at 16 kb/sor 64 kb/s, depending on the service type. ISDN is not restrictedto public telephone networks alone; it may be transmitted viapacket switched networks, telex, CATV networks, etc. There aretwo basic types of ISDN service:• Basic Rate Interface (BRI) - consists of two 64 kb/s Bchannels and one 16 kb/s D channel for a total of 144kb/s. This basic service is intended to meet the needsof most individual users. The U interface provided bythe telco for BRI is a 2-wire, 160 kb/s digital connection.Echo cancellation is used to reduce noise, and data encodingschemes (2B1Q in North America, 4B3T in Europe)permit this relatively high data rate over ordinarysingle-pair local loops.• Primary Rate Interface (PRI) is intended for users withgreater capacity requirements. Typically the channelstructure is 23 B channels plus one 64 kb/s D channelfor a total of 1536 kb/s. In Europe, PRI consists of 30 Bchannels plus one 64 kb/s D channel for a total of 1984kb/s. It is also possible to support multiple PRI lines withone 64 kb/s D channel using Non-Facility AssociatedSignaling (NFAS).The CCITT (now ITU-T) study group responsible for ISDN firstpublished a set of ISDN recommendations in 1984. Prior to thispublication, various geographical areas had developed differentversions of ISDN. The use of nation-specific information elementsis enabled by using the Codeset mechanism which allowsdifferent areas to use their own information elements within thedata frames. A common nation-specific ISDN variant is NationalISDN by Bellcore, used in the USA. It has four network-specificmessage types. It does not have any single octet informationelements. Other changes are the addition of the SEGMENT,FACILITY and REGISTER message types and the SegmentedMessage and Extended Facility information elements. Also,some meanings of field values have changed and some newaccepted field values have been added.Due to its limitation of bandwidth and services, this traditionalISDN is called narrowband ISDN, in contrast to the BISDN(Broadband ISDN).Protocol StructureBelow is the general structure of the ISDN frame:8 7 6 5 4 3 2 1Protocol discriminator0 0 0 0 Length of reference call valueFlagCall reference value0 Message typeOther information elements as required• Protocol discriminator - The protocol used to encodethe remainder of the Layer.• Length of call reference value - Defines the length ofthe next field. The Call reference may be one or twooctets long depending on the size of the value beingencoded.• Flag - Set to zero for messages sent by the party thatallocated the call reference value; otherwise set toone.• Call reference value - An arbitrary value that is allocatedfor the duration of the specific session, whichidentifies the call between the device maintaining thecall and the ISDN switch.• Message type - Defines the primary purpose of theframe. The message type may be one octet or twooctets (for network specific messages). When thereis more than one octet, the first octet is coded aseight zeros. A complete list of message types is givenin ISDN Message Types below.• ISDN Information Elements - There are two typesof information elements: single octet and variablelength.• Single octet information elements - The single octetinformation element appears as follows:8 7 6 5 4 3 2 11 Information element identifier Information element• Variable length information elements - The followingis the format for the variable length information element:8 7 6 5 4 3 2 10 Information element identifierLength of information elementsInformation elements (multiple bytes)• The information element identifier identifies thechosen element and is unique only within the givenCodeset. The length of the information element informsthe receiver as to the amount of the following