1、INTERNATIONAL TELECOMMUNICATION UNIONITU-TH.231TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU(07/97)SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMSInfrastructure of audiovisual services Systems aspectsMultipoint control units for audiovisual systemsusing digital channels up to 1920 kbit/sITU-T Recommend
2、ation H.231(Previously CCITT Recommendation)ITU-T H-SERIES RECOMMENDATIONSAUDIOVISUAL AND MULTIMEDIA SYSTEMSFor further details, please refer to ITU-T List of Recommendations.Characteristics of transmission channels used for other than telephone purposes H.10H.19Use of telephone-type circuits for vo
3、ice-frequency telegraphy H.20H.29Telephone circuits or cables used for various types of telegraph transmission or simultaneoustransmissionH.30H.39Telephone-type circuits used for facsimile telegraphy H.40H.49Characteristics of data signals H.50H.99CHARACTERISTICS OF VISUAL TELEPHONE SYSTEMS H.100H.1
4、99INFRASTRUCTURE OF AUDIOVISUAL SERVICES H.200H.399General H.200H.219Transmission multiplexing and synchronization H.220H.229Systems aspects H.230H.239Communication procedures H.240H.259Coding of moving video H.260H.279Related systems aspects H.280H.299Systems and terminal equipment for audiovisual
5、services H.300H.399Recommendation H.231 (07/97) iITU-T RECOMMENDATION H.231MULTIPOINT CONTROL UNITS FOR AUDIOVISUAL SYSTEMSUSING DIGITAL CHANNELS UP TO 1920 kbit/sSummaryThis Recommendation specifies the means by which three or more audiovisual terminals conforming to Recommenda-tion H.320 may commu
6、nicate simultaneously over constant bit rate digital paths, such communication being designated a“multipoint call“.SourceITU-T Recommendation H.231 was revised by ITU-T Study Group 16 (1997-2000) and was approved under theWTSC Resolution No. 1 procedure on the 10th of July 1997.ii Recommendation H.2
7、31 (07/97)FOREWORDITU (International Telecommunication Union) is the United Nations Specialized Agency in the field of telecommuni-cations. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of the ITU. The ITU-T isresponsible for studying technical, operating and tariff q
8、uestions and issuing Recommendations on them with a view tostandardizing telecommunications on a worldwide basis.The World Telecommunication Standardization Conference (WTSC), which meets every four years, establishes thetopics for study by the ITU-T Study Groups which, in their turn, produce Recomm
9、endations on these topics.The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down inWTSC Resolution No. 1.In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on acollaborative basis with ISO and IEC.
10、NOTEIn this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunicationadministration and a recognized operating agency.INTELLECTUAL PROPERTY RIGHTSThe ITU draws attention to the possibility that the practice or implementation of this Recommendation m
11、ay involve theuse of a claimed Intellectual Property Right. The ITU takes no position concerning the evidence, validity or applicabilityof claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendationdevelopment process.As of the date of approval of th
12、is Recommendation, the ITU had not received notice of intellectual property, protected bypatents, which may be required to implement this Recommendation. However, implementors are cautioned that this maynot represent the latest information and are therefore strongly urged to consult the TSB patent d
13、atabase. ITU 1998All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Recommendation H.231 (07/97) iiiCONTENTSPage1 Scope. 12 References. 1
14、3 Definitions. 24 Abbreviations 35 Functional representation 45.1 General representation. 45.2 Description of functional units. 46 Multipoint configurations 87 Multipoint control unit characteristics and classification 87.1 Conformance . 87.2 Classification of MCUs . 97.3 Summary of MCU BAS capabili
15、ties 97.4 Other attributes 108 “Listed“ multipoint control units. 109 Terminal requirements and options. 11Recommendation H.231 (07/97) 1Recommendation H.231Recommendation H.231 (07/97)MULTIPOINT CONTROL UNITS FOR AUDIOVISUAL SYSTEMSUSING DIGITAL CHANNELS UP TO 1920 kbit/s(revised in 1997)1 ScopeThi
16、s Recommendation specifies the means by which three or more audiovisual terminals conforming to Recommenda-tions H.320, H.221, H.242, H.261, H.262 and H.263 may communicate simultaneously over constant bit rate digitalpaths, such communication being designated a “multipoint call“. The terminals shou
17、ld not need any modification formultipoint working, though some optional enhancements may require additional software; in particular, considerablyenhanced functionality is attained via the use of T.120 protocols in the MLP channel. The interconnection is effected byone or more Multipoint Control Uni
18、ts (MCU) which may reside in the network or at one of the terminals.This Recommendation covers both mandatory requirements and optional enhancements for multipoint working. Theoptional enhancements include: numbering of terminals; chair control; data broadcast; video mixing; cascading.2 ReferencesTh
19、e following ITU-T Recommendations and other references contain provisions which, through reference in this text,constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. AllRecommendations and other references are subject to revision; all users of t
20、his Recommendation are thereforeencouraged to investigate the possibility of applying the most recent edition of the Recommendations and otherreferences listed below. A list of the currently valid ITU-T Recommendations is regularly published. CCITT Recommendation G.711 (1988), Pulse Code Modulation
21、(PCM) of voice frequencies. CCITT Recommendation G.722 (1988), 7 kHz Audio-coding within 64 kbit/s. CCITT Recommendation G.728 (1992), Coding of speech at 16 kbit/s using low-delay code excited linearprediction. ITU-T Recommendation H.221 (1997), Frame structure for a 64 to 1920 kbit/s channel in au
22、diovisualteleservices. ITU-T Recommendation H.230 (1997), Frame synchronous control and indication signals for audiovisualsystems. ITU-T Recommendation H.233 (1995), Confidentiality system for audiovisual services. ITU-T Recommendation H.234 (1994), Encryption key management and authentication syste
23、m for audiovisualservices. ITU-T Recommendation H.242 (1997), System for establishing communication between audiovisual terminalsusing digital channels up to 2 Mbit/s. ITU-T Recommendation H.243 (1997), Procedures for establishing communication between three or moreaudiovisual terminals using digita
24、l channels up to 1920 kbit/s .2 Recommendation H.231 (07/97) ITU-T Recommendation H.320 (1997), Narrow-band visual telephone systems and terminal equipment . ITU-T Recommendation T.122 (1993), Multipoint communication service for audiographics and audiovisualconferencing service definition. ITU-T Re
25、commendation T.123 (1996), Network specific data protocol stacks for multimedia conferencingTeleconference Applications. ITU-T Recommendation T.124 (1995), Generic conference control. ITU-T Recommendation T.125 (1994), Multipoint communication service protocol specification. ITU-T Recommendation H.2
26、24 (1995), A real time control protocol for simplex applications using theH.221 LSD/HSD/MLP channels. ITU-T Recommendation H.261 (1993), Video codec for audiovisual systems at p 64 kbit/s. ITU-T Recommendation H.262 (1995), Generic coding of moving pictures and associated audioinformation: Video. IT
27、U-T Recommendation H.263 (1996), Video coding for low bit rate communication.3 DefinitionsThis Recommendation defines the following terms:3.1 adding and dropping terminals from a conference: Refers to connection to the APU, VPU and DPU(see 5.2), not to network (dis-)connection which is outside the s
28、cope of this Recommendation.3.2 chair-control terminal: An enhanced terminal possessing a token conveying a certain measure of authorityover the operation of the MCU; the token may be assigned by rearrangement, by an operator, or by protocol during thecall. The person controlling need not be the act
29、ual chairperson of the meeting.3.3 chair-control port: That port of the MCU serving the terminal to which chair-control has been assigned (thisport is not physically different from the others).3.4 directly-connected terminal: Where reference is made to “directly“ connected terminals, this should not
30、 betaken to mean that the terminal in question is collocated with the MCU, but rather that it is not connected to adifferent MCU.3.5 local MCU: That to which the terminal in question is directly connected.3.6 multipoint control unit (MCU): A multi-port device, by means of which two or more audiovisu
31、al terminalsmay intercommunicate in a conference call; a “listed MCU“ has the properties defined in clause 8. Master and SlaveMCUs: A “master MCU“ has been assigned a superior controlling function in a call where two or more MCUs areinterconnected, while the other MCUs in that call are termed “slave
32、 MCUs“.NOTE The physical realization of an MCU may be such that two or more independent conferences may be set up within the sameunit; logically, however, there is no relationship between these conferences; the text herein refers to an MCU only as a logical entitypertinent to the particular call of
33、concern.3.7 primary and secondary ports: While all ports of an MCU may be physically the same, distinctions may bemade by the internal software, on the basis of declared terminal capabilities, such that the ports are not all treated on anequal basis. In general, a multipoint call will involve two or
34、 more terminals intercommunicating on an equal basis, at theirhighest common capability; the MCU would designate as “primary“ those ports to which these terminals are connected,and for simplicity these terminals can be referred to as “primary terminals“ for the purposes of this one call. The selecti
35、onof an appropriate common level for primary communication is described in Recommendation H.243.One or more additional terminals may take part in the multipoint call, even though they do not have sufficient capabilityto communicate on an equal basis with primary terminals; these may be designated “s
36、econdary terminals“, communi-cating with the others only by such compatible signals as can be made available (e.g. speech only), the MCU havingdesignated the appropriate port accordingly. Note that if this provision were not made, then the addition of a telephonyterminal to a videophone conference w
37、ould cause all picture transmission to be discontinued.Recommendation H.231 (07/97) 3It is not mandatory that an MCU be able to handle secondary terminals; in such a case, the MCU may either:a) disconnect a terminal not declaring capabilities adequate to participate as a primary; orb) lower its defi
38、nition of “primary“ to include the said terminal.The provider of the MCU should make clear which procedure is followed.4 AbbreviationsThis Recommendation uses the following abbreviations:APU Audio Processing UnitBAS Bit-rate Allocation SignalCIF Common Intermediate FormatCPU Control Processing UnitD
39、PU Data Processing UnitECS Encryption Control SignalFAS Frame Alignment SignalFAW Frame Alignment WordGOB Group of BlocksH-MLP High speed Multi-Layer ProtocolHSD High Speed DataLD-CELP Low Delay Code Excited Linear PredictionLSB Least Significant BitLSD Low Speed DataMB H.261/H.262/H.263 MacroblockM
40、BE Multi-Byte ExtensionMCC Multipoint Command ConferenceMCN Multipoint Command NegatingMCS Multipoint Command Data Symmetrical Data-TransmissionMF MultiFrameMLP Multi-Layer ProtocolMPI Minimum Picture IntervalMSB Most Significant BitNS Non-StandardPCM Pulse Code ModulationQCIF Quarter Common Interme
41、diate FormatSBE Single Byte ExtensionSC Service ChannelSCM Selected Communications ModeSMF Sub-MultiFrameSP Still PictureVCF Video Command “Freeze Picture Request“VCU Video Command “Fast Update Request“VPU Video Processing Unit4 Recommendation H.231 (07/97)5 Functional representation5.1 General repr
42、esentationA multipoint call may be represented as in Figure 1, wherein are shown a number of terminals T, not necessarilyidentical, linked individually into a network by symmetrical bidirectional digital connections, not necessarily all of thesame capacity. There is no particular limit set by the sy
43、stem to the number N of terminals connected in the call, though inpractice, depending on implementation, the difficulties and cost will rise as N increases, while performance tends to fall.T1518800-95/d01T1TnT4T3T2NetworkFigure 1/H.231 Representation of a multipoint callFIGURE 1/H.231.D01 = 3 CMIn t
44、he representation of Figure 1, the network need only be described by the signal flows at its ports, and theirinterdependencies. The hardware realization need not be of concern: there may be a single MCU at one location;alternatively the functions may be distributed to two or more locations, but in p
45、ractical terms we then refer to a series ofsingle MCUs linked together. In this Recommendation, the text applies in general to both single-location anddistributed MCUs, and the linking of MCUs is only treated specifically where there is a particular need to do so.The MCU is represented in more detai
46、l in Figure 2.Each port of the MCU has a network interface unit, with associated call control if appropriate; on the MCU side of thenetwork interface unit, the signal flows are contained in one or more bidirectional channels normally of equal capacity,according to the transfer rates listed in Annex
47、A/H.221. The incoming flow is passed to the demultiplexer, which extractsthe several types of information (audio, video, data, and control) and passes them to their respective processors. Theprocessors are controlled in such a way that an appropriate output from each is made available for transmissi
48、on to everyterminal; the latter are brought together in the multiplexer to be combined into the outgoing channels.The “call-control“ and “call-control processor“ units are outside the scope of this Recommendation (see RecommendationH.200/AV.440); the other units are described in the following subcla
49、uses.5.2 Description of functional units5.2.1 Network interface unitA physical entity that converts to/from line code the signals of 64 kbit/s (56 kbit/s) or multiples thereof which enter thedemultiplexer and leave the multiplexer. Although in Figure 2 such a unit is shown for each multiplexer/demultiplexerpair, in practice a network interface might support a number of logical ports (see below).Recommendation H.231 (07/97) 5T1518810-95/d02call controlNetwork interfaceDemultiplexerMultiplexerCall controlprocessorAudio Processi
