ETSI I-ETS 300 837 ed.1 (1998-01)

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.HGLMVNHPublic Switched Telephone Network (PSTN); Terminals for low bit-rate multimedia communication33.160.60WHOHNRQIHUHQFHMultimedia systems and teleconferencing equipment33.040.35Telefonska omrežjaTelephone networksICS:Ta slovenski standard je istoveten z:I-ETS 300 837 Edition 1SIST I-ETS 300 837 E1:2003en01-december-2003SIST I-ETS 300 837 E1:2003SLOVENSKI
STANDARD
INTERIMEUROPEAN I-ETS 300 837TELECOMMUNICATION January 1998STANDARDSource: MTA Reference: DI/MTA-004033ICS:33.020Key words:Coding, multimedia, PSTNPublic Switched Telephone Network (PSTN);Terminals for low bit-rate multimedia communicationETSIEuropean Telecommunications Standards InstituteETSI SecretariatPostal address: F-06921 Sophia Antipolis CEDEX - FRANCEOffice address: 650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCEX.400: c=fr, a=atlas, p=etsi, s=secretariat - Internet: secretariat@etsi.frTel.: +33 4 92 94 42 00 - Fax: +33 4 93 65 47 16Copyright Notification: No part may be reproduced except as authorized by written permission. The copyright and theforegoing restriction extend to reproduction in all media.© European Telecommunications Standards Institute 1998. All rights reserved.SIST I-ETS 300 837 E1:2003

Page 2I-ETS 300 837: January 1998Whilst every care has been taken in the preparation and publication of this document, errors in content,typographical or otherwise, may occur. If you have comments concerning its accuracy, please write to"ETSI Editing and Committee Support Dept." at the address shown on the title page.SIST I-ETS 300 837 E1:2003

Page 3I-ETS 300 837: January 1998ContentsForeword.7Introduction.71Scope.91.1Block diagram and functional elements.91.2System elements outside the scope of this I-ETS.91.3Functional elements covered by this I-ETS.102Normative references.113Definitions and abbreviations.123.1Definitions.123.2Abbreviations.133.3Conventions.144Functional requirements.144.1Required elements.144.2Information streams.154.3Modems.154.4Multiplexing.154.4.1Logical channel numbers.154.4.2Multiplex table entries.164.4.3Flow control.164.4.4Error control.164.4.5Adaptation layers.164.5Control channel.164.5.1Capabilities exchange.174.5.2Logical channel signalling.184.5.3Mode preferences.194.5.4Interface to multiplex.194.5.5Timer and counter values and protocol errors.194.6Video channels.194.6.1Interface to multiplex.204.7Audio channels.214.7.1Delay compensation.214.7.2Maximum delay jitter.224.7.3Interface to multiplexer.224.8Data channels.224.8.1Data protocols.234.8.1.1Buffered ITU-T Recommendation V.14.244.8.1.2LAPM / ITU-T Recommendation V.42.244.8.1.3HDLC frame tunnelling.254.8.1.4Transparent data.254.8.2Data applications.254.8.2.1ITU-T Recommendation T.120 multimediateleconferencing applications.264.8.2.2ITU-T Recommendation T.84 (SPIFF) point-to-point stillimage transfer cutting across application borders.264.8.2.3ITU-T Recommendation T.434 point-to-point telematic filetransfer cutting across application borders.264.8.2.4ITU-T Recommendation H.224 real-time control protocol,for ITU-T Recommendation H.281 far-end camera control.274.8.2.5Network link layer.274.8.2.6External data ports and unspecified user data.275Terminal procedures.27SIST I-ETS 300 837 E1:2003

Page 4I-ETS 300 837: January 19985.1Phase A - call set-up of voiceband channel.285.2Phase B - initial analogue telephony communication.285.2.1ITU-T Recommendation V.8 [10] procedure.285.2.2ITU-T Recommendation V.8bis procedure.285.3Phase C - establishment of digital communication, modem training.285.3.1ITU-T Recommendation V.8 procedure.285.3.2ITU-T Recommendation V.8bis procedure.295.4Phase D - initialization.295.4.1Exchange of video by mutual agreement.295.5Phase E - communication.305.5.1Rate changes and retrains.305.5.2Involuntary disconnection.305.6Phase F - end of session.305.7Phase G - change of communication mode and call clearing.306Interoperation with other terminals.316.1Speech only terminals.316.2ETS 300 145 multimedia telephone terminals attached to the ISDN.316.3Multimedia telephone terminals over mobile radio.317Optional enhancements.317.1Data facilities.317.2Encryption.327.2.1EncryptionSE messages.327.2.2EIV channel.327.2.3Encryption procedure.327.2.4Encryption initialization vectors.337.2.5Error recovery.337.3Multilink.348Multipoint considerations.348.1Establishment of common mode.348.2Multipoint rate matching.348.3Multipoint lip synchronization.348.4Multipoint encryption.348.5Cascaded MCU operation.359Maintenance.359.1Loopbacks for maintenance purposes.359.1.1Normal mode.359.1.2System loopback.359.1.3Media loopback.359.1.4Logical channel loopback.35Annex A (normative):Protocol stack for control channel.37A.1General.37A.2SRP mode.38A.2.1SRP command frames.38A.2.2SRP response frames.38A.2.3SRP procedure at transmitter.38A.2.4SRP procedure at receiver.39A.3LAPM / ITU-T Recommendation V.42 mode.39Annex B (normative):HDLC frame structure transparency for asynchronous transmission.40Annex C (informative):Bit and octet order.41Annex D (informative):ITU-T Recommendation V.8bis codepoints.42Annex E (informative):Bibliography.43SIST I-ETS 300 837 E1:2003

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Page 7I-ETS 300 837: January 1998ForewordThis Interim European Telecommunication Standard (I-ETS) has been produced by the MultimediaTerminals and Applications (MTA) Technical Committee of the European Telecommunications StandardsInstitute (ETSI).An ETSI standard may be given I-ETS status either because it is regarded as a provisional solution aheadof a more advanced standard, or because it is immature and requires a "trial period". The life of an I-ETSis limited to three years after which it can be converted into an ETS, have it's life extended for a furthertwo years, be replaced by a new version, or be withdrawn.Announcement dateDate of adoption of this I-ETS:2 January 1998Date of latest announcement of this I-ETS (doa):30 April 1998IntroductionThis I-ETS describes terminals for low bit-rate multimedia communication, utilizingITU-T Recommendation V.34 [15] modems connected to the Public Switched Telephone Network(PSTN). The terminals described in this I-ETS may carry real-time voice, data, and video, or anycombination, including videotelephony.Terminals described in this I-ETS may be integrated into personal computers or implemented instand-alone devices such as videotelephones. Support for each medium type (voice, data, video) isoptional, but if supported, the ability to use a specified common mode of operation is required, so that allterminals supporting that medium type can interwork. This I-ETS allows more than one channel of eachtype to be in use. ITU-T Recommendations relating to this I-ETS are: H.223 [1], H.245 [2], H.263 [4] andG.723.1 [3].This I-ETS makes use of the logical channel signalling procedures of ITU-T Recommendation H.245 [2],in which the content of each logical channel is described when the channel is opened. Procedures areprovided for expression of receiver and transmitter capabilities, so transmissions are limited to whatreceivers can decode, and so that receivers may request a particular desired mode from transmitters.Since the procedures of ITU-T Recommendation H.245 [2] are also planned for use by ITU-TRecommendation H.310 (see annex E) for ATM networks, and ITU-T Recommendation H.323 (seeannex E) for non-guaranteed bandwidth LANs, interworking with these systems should be straightforward.Terminals described in this I-ETS may be used in multipoint configurations through Multipoint Control Unit(MCUs), and may interwork with ETS 300 145 [22] terminals on the Integrated Services Digital Network(ISDN), as well as with terminals on wireless networks.SIST I-ETS 300 837 E1:2003

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Page 9I-ETS 300 837: January 19981ScopeThis Interim European Telecommunication Standard (I-ETS) covers the technical requirements for verylow bit-rate multimedia telephone terminals connected to the Public Switched Telephone Network (PSTN).Interworking with visual telephone systems on the Integrated Services Digital Network (ISDN) (describedin ETS 300 145 [22]) and on mobile radio networks (known as the
ITU-T Recommendation H.324 [26]/Mseries of Recommendations) are also covered.Terminals described in this I-ETS provide real-time video, audio, or data, or any combination, between twomultimedia telephone terminals over a PSTN voice band network connection. Communication may beeither one-way or two-way. Multipoint communication using a separate Multipoint Control Unit (MCU)among more than two terminals described in this I-ETS is possible. MCUs and other non-terminal devicesare not bound by the requirements in this I-ETS, but they should comply where practical.The multimedia telephone terminals defined in this I-ETS can be integrated into PCs or workstations, orbe stand-alone units.Control protocolITU-T Rec.H.245 [2]Scope of this I-ETSSystem controlUser dataapplicationsAudio I/O equipmentVideo I/O equipmentVideo codecITU-T Recs.H.263/H.261 [4,5]Audio codecITU-T Rec.G.723.1 [3]Data protocolsITU-T Rec. V.14[11], LAPM, etc.Receivepath delayModemITU-T Rec.V.34/V.8[15, 10]Multiplex/DemultiplexITU-T Rec.H.223 [1]ModemcontrolITU-T Rec.V.25ter[12]NetworkMCUSRP/LAPMproceduresOptionalITU-TRecs.V.34/V.8bis[15, 29]Figure 1: Block diagram of generic multimedia videophone system1.1Block diagram and functional elementsA generic multimedia videophone system is shown in figure 1. It consists of terminal equipment, a PSTNmodem, PSTN network, MCU and other system operational entities. Implementations complying with thisI-ETS are not required to have each functional element.NOTE:Functions for interworking with a normal telephone are not depicted. The support ofsuch a function is described in subclause 6.1.1.2System elements outside the scope of this I-ETSThe following system elements, covered by other (I-)ETSs and ITU-T Recommendations, are not subjectto standardization, or are for other reasons outside the scope of this I-ETS:-video input/output equipment including cameras and monitors, their control and selection, videoprocessing to improve compression or provide split-screen functions;NOTE:In this diagram, Rec. means Recommendation.SIST I-ETS 300 837 E1:2003

Page 10I-ETS 300 837: January 1998-audio input/output equipment including handset or other types of acoustic cabinets, microphone(s)and loudspeaker(s), telephone instrument or equivalent, attached audio devices providing voiceactivation sensing, multiple microphone mixers, acoustic echo cancellation;NOTE 1:The audio characteristics of handset telephones supporting the analogue telephonyservices on the PSTN are normally subject to type approval by national regulatoryauthorities. I-ETS 300 677 [24] contains information about relevant national values anda proposed harmonized value for the audio characteristics of a handset telephonewhen attached to the PSTN.NOTE 2:There are no requirements in this I-ETS for the audio characteristics of a terminalcommunicating in a multimedia mode. Until such requirements are available,manufacturers are recommended to use I-ETS 300 245-2 [23] as a guide.-data application equipment such as computers, non-standardized data application protocols,telematic visual aids such as electronic whiteboards, etc.;-PSTN network interface supporting appropriate signalling, ringing functions and networktermination. Human user system control, user interface and operation.1.3Functional elements covered by this I-ETSThe scope of this I-ETS is indicated by the elements within the dashed line of figure 1, which include:-the Video codec (according to ITU-T Recommendations H.263 [4] or H.261 [5]) carries outredundancy reduction coding and decoding for video streams;-the Audio codec (according to ITU-T Recommendation G.723.1 [3]) encodes the audio signal fromthe microphone for transmission, and decodes the audio code which is output to the speaker.Optional delay in the receiving audio path compensates for the video delay, so as to maintain audioand video synchronization;-the Data Protocols support data applications such as electronic whiteboards, still image transfer, fileexchange, database access, audiographics conferencing, remote device control, network protocols,etc. Standardized data applications include ITU-T Recommendation T.120 [17] for real-timeaudiographics conferencing, ITU-T Recommendation T.84 [16] simple point-point still image filetransfer, ITU-T Recommendation T.434 [18] simple point-point file transfer, ITU-TRecommendations H.224 [8]/H.281 [9] far-end camera control, ISO/IEC TR9577 [25] networkprotocols including Point-to-Point Protocol (PPP), Internet Protocol (IP) and transport of user datausing buffered ITU-T Recommendations V.14 [11] or V.42 [13]. Other applications and protocolsmay also be used via ITU-T Recommendation H.245 [2] negotiation;-the Control Protocol (ITU-T Recommendation H.245 [2]) provides end-to-end signalling for properoperation of the terminal described in this I-ETS, and signals all other end-to-end system functionsincluding reversion to analogue speech-only telephony mode. It provides for capability exchange,signalling of channels;-the Multiplex Protocol (ITU-T Recommendation H.223 [1]) multiplexes transmitted video, audio,data and control streams into a single bit stream, and demultiplexes a received bit stream intovarious multimedia streams. In addition, it performs logical framing, sequence numbering, errordetection, and error correction by means of retransmission, as appropriate to each media type;-the Modem (ITU-T Recommendation V.34 [15]) converts the ITU-T Recommendation H.223 [1]synchronous multiplexed bit stream into an analogue signal that can be transmitted over the PSTN,and converts the received analogue signal into a synchronous bit stream that is sent to theMultiplex/Demultiplex protocol unit. ITU-T Recommendation V.25ter [12] is used to providecontrol/sensing of the modem/network interface, when the modem with network signalling andITU-T Recommendation V.8 [10]/V.8bis (see annex E) functional elements is a separate physicalitem.SIST I-ETS 300 837 E1:2003

Page 11I-ETS 300 837: January 19982Normative referencesThis I-ETS incorporates by dated and undated reference, provisions from other publications. Thesenormative references are cited at the appropriate places in the text and the publications are listedhereafter. For dated references, subsequent amendments to or revisions of any of these publicationsapply to this I-ETS only when incorporated in it by amendment or revision. For undated references thelatest edition of the publication referred to applies.[1]ITU-T Recommendation H.223 (1995): "Multiplexing protocol for low bit ratemultimedia communication".[2]ITU-T Recommendation H.245 (1995): "Control protocol for multimediacommunication".[3]ITU-T Recommendation G.723.1 (1995): "Dual rate speech coder for multimediacommunications transmitting at 5.3 and 6.3 kbit/s".[4]ITU-T Recommendation H.263 (1995): "Video coding for low bit ratecommunication".[5]ITU-T Recommendation H.261 (1993): "Video codec for audiovisual services atp x 64 kbit/s".[6]ITU-T Recommendation H.233 (1994): "Confidentiality system for audiovisualservices".[7]ITU-T Recommendation H.234 (1994): "Encryption key management andauthentication system for audiovisual services".[8]ITU-T Recommendation H.224 (1994): "A real time control protocol for simplexapplications using the H.221 LSD/HSD/MLP channels".[9]ITU-T Recommendation H.281 (1994): "A far end camera control protocol forvideoconferences using H.224".[10]ITU-T Recommendation V.8 (1994): "Procedures for starting sessions of datatransmission over the general switched telephone network".[11]ITU-T Recommendation V.14 (1993): "Transmission of start-stop charactersover synchronous bearer channels".[12]ITU-T Recommendation V.25ter (1995): "Serial asynchronous automatic diallingand control".[13]ITU-T Recommendation V.42 (1993): "Error-correcting procedures for DCEsusing asynchronous-to-synchronous conversion".[14]ITU-T Recommendation V.42bis (1990): "Data compression procedures for datacircuit terminating equipment (DCE) using error correction procedures".[15]ITU-T Recommendation V.34 (1994): "A modem operating at data signallingrates of up to 33 600 bit/s for use on the general switched telephone networkand on leased point-to-point 2-wire telephone-type circuits".[16]ITU-T Recommendation T.84/ISO/IEC 10918-3 (1996): "Information Technology- Digital compression and coding of continuous-tone still images
Extensions".[17]ITU-T Recommendation T.120 (1996): "Data protocols for multimediaconferencing".[18]ITU-T Recommendation T.434 (1992): "Binary file transfer format for thetelematic services".SIST I-ETS 300 837 E1:2003

Page 12I-ETS 300 837: January 1998[19]ISO/IEC 3309 (1991): "Information Technology - Telecommunications andinformation exchange between systems - High-level data link control (HDLC)procedures - Frame structure".[20]ITU-T Recommendation G.711 (1988): "Pulse code modulation (PCM) of voicefrequencies".[21]ETS 300 144: "Integrated Services Digital Network (ISDN); Audiovisual services;Frame structure for a 64 kbit/s to 1 920 kbit/s channel and associated syntax forinband signalling".[22]ETS 300 145 (1994): "Integrated Services Digital Network (ISDN); Audiovisualservices; Videotelephone systems and terminal equipment operating on one ortwo 64 kbit/s channels".[23]I-ETS 300 245-2 (1994): "Integrated Services Digital Network (ISDN); Technicalcharacteristics of telephony terminals; Part 2: PCM A-law handset telephony".[24]I-ETS 300 677 (1996): "Public Switched Telephone Network (PSTN);Requirements for handset telephony".[25]ISO/IEC TR9577 (1990): "Information Technology - Protocol identification in thenetwork layer".[26]ITU-T Recommendation H.324 (1996): "Terminal for low bit rate MultimediaCommunication".[27]ITU-T Recommendation V.24 (1993): "List of definitions for interchange circuitsbetween data terminal equipment (DTE) and data circuit-terminating equipment(DCE)".[28]ITU-T Recommendation T.123 (1994): "Network specific data protocol stacksfor multimedia conferencing".3Definitions and abbreviations3.1DefinitionsFor the purposes of this I-ETS, the definitions of ITU-T Recommendations H.223 [1] clause 3 andH.245 [2] clause 3 apply, in addition to the following:Adaptation Layer - Service Data Unit (AL-SDU): The logical unit of information exchanged between theITU-T Recommendation H.223 [1] multiplexer and the audio codec, video codec, or data protocol.channel: A unidirectional link between two end-points.codec: A coder/decoder, used to convert audio or video signals to/from digital format.connection: A bi-directional link between two end-points.control channel: Dedicated LCN 0 carrying system control protocol perITU-T Recommendation H.245 [2].data: Information streams other than control, audio and video, carried in a logical data channel(see ITU-T Recommendation H.223 [1]).in-band signalling: Control signals sent within a specific logical channel other than the control channel,carrying information applicable only to that logical channel.interworking adapter: A device connected to terminals or MCUs working according to two or moreRecommendations, which translates the content of one or more logical channels to allow interoperationbetween otherwise incompatible equipment.SIST I-ETS 300 837 E1:2003

Page 13I-ETS 300 837: January 1998LAPM/V.42: The link access procedures that are used in ITU-T Recommendation V.42 [13].lip synchronization: Operation to provide the impression that speaking motion of the displayed person issynchronized with the voice sounds.logical channel: One of several logically distinct channels carried over a single bit-stream.medium: audio, video, or data.multilink: The use of more than one physical connection to obtain a larger aggregate bit-rate.multipoint: The simultaneous interconnection of three or more terminals to allow communication amongseveral sites through the use of multipoint control units (bridges) which centrally direct the flow ofinformation.MUltipleX-Protocol Data Unit (MUX-PDU): The logical unit of information exchanged between the ITU-TRecommendation H.223 [1] multiplex layer and the underlying physical layer. It is a packet framed byHigh-level Data Link Control (HDLC) flags and using HDLC zero-bit insertion for transparency.non-segmentable: The ITU-T Recommendation H.223 [1] mode of operation in which AL-SDUs need tobe sent as consecutive octets in a single MUX-PDU (see Recommendation H.223 [1]).segmentable: The ITU-T Recommendation H.223 [1] mode of operation in which AL-SDUs may be sentin separate multiplex slots carried on one or more MUX-PDUs (see ITU-T Recommendation H.223 [1]).support: The ability to operate in a given mode, however a requirement to "support" a mode does notmean that the mode needs actually to be used at all times. Unless prohibited, other modes may be usedby mutual negotiation.videophone: A terminal capable of sending and receiving audio and video information simultaneously.3.2AbbreviationsFor the purposes of this I-ETS, the following abbreviations apply:ALAdaptation Layer (see ITU-T Recommendation H.223 [1])AL-SDUAdaptation Layer-Service Data UnitATMAsynchronous Transfer ModeBASBit-rate Allocation SignalBCHBose, Chaudhuri, HocquenghamCCCommunications CapabilitiesCIFCommon Intermediate FormatCMCall Menu signalCRCCyclic Redundancy CheckC/RCommand/Response bitDCEData Circuit terminating EquipmentDLCIData Link Connection IdentifierDTEData Terminal EquipmentEAAddress field ExtensionECSEncryption Control SignalEIVEncryption Initialization VectorFASFrame Aligment SignalFCSFrame Check SequenceHDLCHigh-level Data Link Control (see ISO/IEC 3309 [19])IETFInternet Engineering Task ForceIPInternet ProtocolISDNIntegrated Services Digital NetworkIVInitialization VectorJBIGJoint Bi-level Image experts GroupJFIFJPEG File Interchange FormatSIST I-ETS 300 837 E1:2003

Page 14I-ETS 300 837: January 1998JPEGJoint Photographic Experts GroupJMJoint Menu signalLANLocal Area NetworkLAPMLink Access Procedures for Modems (see ITU-T Recommendation V.42 [13])LCNLogical Channel Number (see ITU-T Recommendation H.223 [1])LSBLeast Significant BitMCUMultipoint Control UnitMUXMUltipleXMSBMost Significant BitNLPIDNetwork Layer Protocol IDentifier (see ISO/IEC TR9577 [25])PDUProtocol Data UnitPPPPoint-to-Point ProtocolPSTNPublic Switched Telephone NetworkQCIFQuarter CIFSDUService Data Unit (see ITU-T Recommendation H.223 [1])SESession Exchange (see ITU-T Recommendation H.233 [6])SPIFFStill Picture Interchange File FormatSQCIFSub QCIFSRPSimple Retransmission Protocol (see annex A)XIDeXchange IDentification3.3ConventionsReferences in this I-ETS to specific ITU-T Recommendation H.245 [2] message structures are presentedin this typeface.4Functional requirements4.1Required elementsImplementations of this I-ETS are not required to have each functional element, except for the ITU-TRecommendation V.34 [15] modem, ITU-T Recommendation H.223 [1] multiplex, and ITU-TRecommendation H.245 [2] system control protocol, all of which shall be supported by all terminalscomplying with this I-ETS.Terminals complying with this I-ETS and offering audio communication shall support the ITU-TRecommendation G.723.1 [3] audio codec. Likewise, the same terminals offering video communicationshall support the ITU-T Recommendations H.263 [4] and H.261 [5] video codecs. Terminals complyingwith this I-ETS and offering real-time audiographic conferencing should support the ITU-TRecommendation T.120 [17] protocol suite. In addition, other video and audio codecs, and other dataprotocols, may optionally be used via negotiation over the ITU-T Recommendation H.245 [2] controlchannel.If a modem external to a terminal compliant with this I-ETS is used, terminal/modem control shall conformto ITU-T Recommendation V.25ter [12].The presence of optional facilities is signalled via the ITU-T Recommendation H.245 [2] control channel. Ifboth ends support an optional facility, and choose to make use of it, the opening of a path to carry suchinformation streams is negotiated according to the procedures of ITU-T recommendation H.245 [2].NOTE:This I-ETS does not specify a particular implementation. Any implementation thatprovides the required functionality, and that conforms to the bit-stream formatultimately described by this I-ETS, is considered compliant.SIST I-ETS 300 837 E1:2003

Page 15I-ETS 300 837: January 19984.2Information streamsMultimedia information streams are classified into video, audio, data, and control as follows:-video streams are continuous traffic carrying moving colour pictures. When used, the bit-rateavailable for video streams may vary according to the needs of the audio and data channels;-audio streams are real-time, but may optionally be delayed in the receiver processing path tomaintain synchronization with the video streams. In order to reduce the average bit-rate of audiostreams, voice activation may be provided;-data streams may represent still pictures, facsimile, documents, computer files, computerapplication data, undefined user data, and other data streams;-control streams pass control commands and indications between remote counterparts.Terminal-to-modem control conforms to ITU-T Recommendation V.25ter [12] for terminals usingexternal modems connected by a separate physical interface. Terminal-to-terminal control isaccording to ITU-T Recommendation H.245 [2].4.3ModemsModems used as terminals compliant with this I-ETS shall operate in full duplex, synchronous mode andconform to ITU-T Recommendations V.34 [15] and V.8 [10]. Support of ITU-T Recommendation V.8bis(see annex E) is optional. The output of the ITU-T Recommendation H.223 [1] multiplexer shall be applieddirectly to the ITU-T Recommendation V.34 [15] synchronous data pump. When an external, non-integrated ITU-T Recommendation V.34 [15] modem is used, control between the modem and theterminal shall be via ITU-T Recommendation V.25ter [12]. In such cases the physical interface isimplementation specific.The use of the optional ITU-T Recommendation V.34 [15] auxiliary channel is reserved for further study.4.4MultiplexingLogical channels of video, audio, data or control information may be transmitted, after the channels havebeen established according to the procedures of ITU-T Recommendation H.245 [2]. Logical channels areunidirectional, and are independent in each direction of transmission. Any number of logical channels ofeach medium type may be transmitted, except for the ITU-T Recommendation H.245 [2] control channelof which there shall be one. The multiplexing method used to transmit these logical channels shallconform to ITU-T Recommendation H.223 [1]. The optional exclusive-OR procedure in subclause 6.4.2 ofITU-T Recommendation H.223 [1] shall not be used by terminals compliant with this I-ETS.The ITU-T Recommendation H.223 [1] multiplexer consists of a multiplex layer, which mixes the variouslogical channels into a single bit-stream, and an adaptation layer, which handles error control andsequence numbering, as appropriate to each information stream. The multiplex layer transfers logicalchannel information in packets called MUltipleXer-Protocol Data Units (MUX-PDUs), delimited by HDLCflags and using HDLC zero-bit insertion for transparency. Each MUX-PDU contains a one-byte headerfollowed by a variable number of information field bytes. The header byte includes a multiplexing code,which specifies, by reference to a multiplex table, the mapping of the information field bytes to variouslogical channels. Each MUX-PDU may contain a different multiplex code, and therefore a different mix oflogical channels.I-ETS compliant terminals shall signal their ITU-T Recommendation H.223 [1] capabilities via theH223Capability message.4.4.1Logical channel numbersEach logical channel is identified by a Logical Channel Number (LCN), in the range 0 to 65 535, whichserves merely to associate logical channels with the corresponding entries in the ITU-TRecommendation H.223 [1] multiplex table. LCNs are selected arbitrarily by the transmitter, except thatlogical channel 0 shall be permanently assigned to the ITU-T Recommendation H.245 [2] control channel.SIST I-ETS 300 837 E1:2003

Page 16I-ETS 300 837: January 19984.4.2Multiplex table entriesMultiplex table entries are independent in each direction of transmission, and are sent from transmitters toreceivers using the MultiplexEntrySend request message. Multiplex table entry 0 shall not sent, but shallbe permanently assigned to logical channel 0, used for the control channel. Multiplex table entry 0 shalltherefore be used for initial capability exchanges and transmission of initial multiplex table entries.4.4.3Flow controlTerminals compliant with this I-ETS shall respond to the FlowControlCommand message, whichcommands a limit to the overall bit rate of one or more logical channels, or the entire multiplexer.When one or more logical channels are limited by the FlowControlCommand, other less restricted logicalchannels may increase their transmission rate. The limit applies to the content of the logical channel at theinput to the multiplexer layer, before flags or zero-bit insertion are applied.When the entire ITU-T Recommendation H.223 [1] multiplexer is limited by the FlowControlCommand, orwhen the terminal has no information to send, the terminal shall send HDLC flags in place of logicalchannel information. The limit applies to the entire multiplex output, including opening flags, header bytes,and inserted zero bits, but not including idle flags.4.4.4Error controlThe multiplex layer of ITU-T Recommendation H.223 [1] does not perform error control, except for aCyclic Redundancy Check (CRC) on the header byte. Error control for each logical channel is handledseparately by the adaptation layers of ITU-T Recommendation H.223 [1], which may use a variety of errorcontrol techniques, including but not limited to error detection and retransmission.4.4.5Adaptation layersITU-T Recommendation H.223 [1] defines three Adaptation Layers (AL), AL1, AL2, and AL3. AL1 isintended primarily for variable-rate framed information, including unframed bytes treated as a single frameof indefinite length. AL2 is intended primarily for digital audio, and includes an 8-bit CRC and optionalsequence numbers. AL3 is intended primarily for digital video and includes provision for retransmission.The logical unit of information exchanged between the ITU-T Recommendation H.223 [1] multiplexer andthe audio codec, video codec, data protocol, or control protocol is called an Adaptation Layer-Service DataUnit (AL-SDU).Logical channels carried by the ITU-T Recommendation H.223 [1] multiplexer may be of either"segmentable" or "non-segmentable" type, as defined in ITU-T Recommendation H.223 [1], and signalledby ITU-T Recommendation H.245 [2] when each channel is opened. AL-SDUs of segmentable logicalchannels may be segmented by the ITU-T Recommendation H.223 [1] multiplexer. AL-SDUs of non-segmentable logical channels are not segmented by the ITU-T Recommendation H.223 [1] multiplexer.Generally, segmentable channels should be used for variable bit-rate information streams such as control,video, and data, while non-segmentable channels should be used for constant bit-rate streams such asaudio.Receivers shall signal their capability to process various adaptation layers and channel types according toITU-T Recommendation H.245 [2]. Transmitters shall signal which adaptation layers, options, and channeltype are used for each logical channel when opening the channel, according to ITU-TRecommendation H.245 [2].4.5Control channelThe control channel carries end-to-end control messages governing operation of the multimediavideophone system, including capabilities exchange, opening and closing of logical channels, modepreference requests, multiplex table entry transmission, flow control messages, and general commandsand indications.SIST I-ETS 300 837 E1:2003

Page 17I-ETS 300 837: January 1998There shall be exactly one control channel in each direction within systems complying with this I-ETS,which shall use the messages and procedures of ITU-T Recommendation H.245 [2]. The control channelshall be carried on logical channel 0. The control channel shall be considered to be permanently openfrom the establishment of digital communication until the termination of digital communication; the normalprocedures for opening and closing logical channels shall not apply to the control channel.General commands and indications shall be chosen from the message set contained in ITU-TRecommendation H.245 [2]. In addition other command and indication signals may be sent which havebeen specifically defined to be transferred in-band within video, audio or data streams (see theappropriate Recommendation to determine if such signals have been defined).ITU-T Recommendation H.245 [2] messages fall into four categories: Request, Response, Command,and Indication. Request messages require a specific action by the receiver, including an immediateresponse. Response messages respond to a corresponding request. Command messages require aspecific action, but do not require a response. Indication messages are informative only, and do notrequire any action or response. Terminals compliant with this I-ETS shall respond to all supportedcommands and requests as specified in ITU-T Recommendation H.245 [2], and shall transmit accurateindications reflecting the state of the terminal.NOTE 1:All control channel messages are sent over a link layer protocol which acknowledgescorrect receipt. This acknowledgment is distinct from the response messages, whichconvey content beyond that of correct receipt of the message.Terminals compliant with this I-ETS shall be capable of parsing all MultimediaSystemControlPDUmessages, and shall send and receive all messages needed to implement required functions and thoseoptional functions which are supported by the terminal. All messages and procedures of ITU-TRecommendation H.245 [2] related to needed functions compliant with this I-ETS are required, except forthose explicitly described as optional, or which are related to defined optional capabilities the terminaldoes not support. Terminals compliant with this I-ETS shall send the FunctionNotSupported message inresponse to unrecognized request, response, or command messages.A control channel indication, UserInputIndication, is available for transport of user input alphanumericcharacters from a keypad or keyboard, equivalent to the DTMF signals used in analogue telephony. Thismay be used to manually operate remote equipment such as voice mail or video mail systems, menu-driven information services, etc. Terminals compliant with this I-ETS shall support the transmission of userinput characters "0-9", "*", and "#". Transmission of other characters is optional.NOTE 2:If the encryption procedures of this I-ETS are in use, the control channel will not beencrypted. Users are therefore cautioned regarding the carriage of user data in thecontrol channel, the use of non-standard messages and the confidentiality risk fromtraffic analysis of the control channel.4.5.1Capabilities exchangeCapabilities exchange shall follow the procedures of ITU-T Recommendation H.245 [2], which provides forseparate receive and transmit capabilities, as well as a system by which the terminal may describe itsability to operate in various combinations of modes simultaneously.Receive capabilities describe the terminal's ability to receive and process incoming information streams.Transmitters shall limit the content of their transmitted information to that which the receiver has indicatedit is capable of receiving. The absence of a receive capability indicates that the terminal cannot receive(i.e. it is a transmitter only).Transmit capabilities describe the terminal's ability to transmit information streams. Transmit capabilitiesserve to offer receivers a choice of possible modes of operation, so that the receiver may request themode which it prefers to receive. The absence of a transmit capability indicates that the t
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