1、 International Telecommunication Union ITU-T H.243TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (10/2005) SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMSInfrastructure of audiovisual services Communication procedures Procedures for establishing communication between three or more audiovisual terminal
2、s using digital channels up to 1920 kbit/s ITU-T Recommendation H.243 ITU-T H-SERIES RECOMMENDATIONS AUDIOVISUAL AND MULTIMEDIA SYSTEMS CHARACTERISTICS OF VISUAL TELEPHONE SYSTEMS H.100H.199 INFRASTRUCTURE OF AUDIOVISUAL SERVICES General H.200H.219 Transmission multiplexing and synchronization H.220
3、H.229 Systems aspects H.230H.239 Communication procedures H.240H.259 Coding of moving video H.260H.279 Related systems aspects H.280H.299 Systems and terminal equipment for audiovisual services H.300H.349 Directory services architecture for audiovisual and multimedia services H.350H.359 Quality of s
4、ervice architecture for audiovisual and multimedia services H.360H.369 Supplementary services for multimedia H.450H.499 MOBILITY AND COLLABORATION PROCEDURES Overview of Mobility and Collaboration, definitions, protocols and procedures H.500H.509 Mobility for H-Series multimedia systems and services
5、 H.510H.519 Mobile multimedia collaboration applications and services H.520H.529 Security for mobile multimedia systems and services H.530H.539 Security for mobile multimedia collaboration applications and services H.540H.549 Mobility interworking procedures H.550H.559Mobile multimedia collaboration
6、 inter-working procedures H.560H.569 BROADBAND AND TRIPLE-PLAY MULTIMEDIA SERVICES Broadband multimedia services over VDSL H.610H.619 For further details, please refer to the list of ITU-T Recommendations. ITU-T Rec. H.243 (10/2005) i ITU-T Recommendation H.243 Procedures for establishing communicat
7、ion between three or more audiovisual terminals using digital channels up to 1920 kbit/s Summary This Recommendation concerns the system operation for a conference call between three or more audiovisual terminals conforming to ITU-T Rec. H.320. This revised version of ITU-T Rec. H.243 introduces a n
8、umber of enhancements and clarifications to the previous version, primarily the description on the usage of Unicode characters in H.320 systems. Source ITU-T Recommendation H.243 was approved on 7 October 2005 by ITU-T Study Group 16 (2005-2008) under the ITU-T Recommendation A.8 procedure. ii ITU-T
9、 Rec. H.243 (10/2005) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and
10、 tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Re
11、commendations on these topics. The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with ISO and IEC. NOTE In this Reco
12、mmendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory provisions (to ensure e.g. interopera
13、bility or applicability) and compliance with the Recommendation is achieved when all of these mandatory provisions are met. The words “shall“ or some other obligatory language such as “must“ and the negative equivalents are used to express requirements. The use of such words does not suggest that co
14、mpliance with the Recommendation is required of any party. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, valid
15、ity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, ITU had received notice of intellectual property, protected by patents, which may be required
16、to implement this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. ITU 2006 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without
17、the prior written permission of ITU. ITU-T Rec. H.243 (10/2005) iii CONTENTS Page 1 Scope 1 2 References. 2 3 Conventions 3 4 Capabilities and “selected“ communication modes 3 5 Initialization procedures for establishing communication between standard terminals and an MCU 5 5.1 First terminal added
18、to conference . 5 5.2 Second terminal added to conference. 6 5.3 Third terminal added to conference 7 5.4 Fourth and subsequent calls added to conference 7 5.5 Extension to multiple channels. 7 5.6 Other standard terminals. 8 5.7 MCU-MCU interconnections . 8 5.8 Closure of conference. 10 5.9 User-in
19、itiated connection establishment. 10 6 Video switching and mixing. 11 6.1 Video switching procedure. 11 6.2 Video mixing 12 6.3 Automatic switching and visualization-forcing 20 7 Numbering of terminals 23 7.1 Numbering method. 23 7.2 Terminal-MCU interconnection . 24 7.3 MCU interconnection . 26 7.4
20、 Terminal identity information 27 8 Mode switching and data broadcast procedures . 29 8.1 General mode switching. 29 8.2 Mode switching for data distribution in multipoint conferences 32 9 Chair-control procedure using BAS codes . 39 9.1 General . 39 9.2 Assignment, release and withdrawal of chair-c
21、ontrol token 40 9.3 Information available to the chair-control terminal 41 9.4 Video selection . 41 9.5 Terminal dropping by chair-control . 42 9.6 Withdrawal of data tokens by chair-control . 42 9.7 Request for floor. 42 9.8 Dropping the entire conference 42 10 BAS sequencing 43 11 Capability excha
22、nge during a call. 43 iv ITU-T Rec. H.243 (10/2005) Page 12 Procedure for loop detection at an MCU 43 13 Exceptional procedures. 44 13.1 A terminal connected does not indicate capability for the SCM 44 13.2 Contention resolution principle 44 14 MCU loopback procedures. 45 15 Interactions with T.120
23、control. 46 15.1 Chair-control interactions. 46 15.2 Interactions with passwording 46 15.3 Interactions with TIX/TIA 46 15.4 Interactions with SCM management 47 15.5 Rate matching in a cascade. 47 Appendix I C logically, however, there is no relationship between these conferences; the text herein re
24、fers to an MCU only as a logical entity pertinent to the particular call of concern. This Recommendation concerns only the flow of signals along the fixed digital paths, which may be at 64 kbit/s (56 kbit/s in certain networks) or multiples thereof up to 1920 kbit/s. The flow consists of a multiplex
25、 of audio, video, control and indication signals, and optional user data as defined in ITU-T Rec. H.221 which must be handled by the MCU in a way which is satisfactory to the users. The signal multiplex on each path is fully in accordance with ITU-T Rec. H.221: the BAS commands define explicitly how
26、 the demultiplexer at the end of each link shall operate. Likewise, the basic procedures for initialization and mode switching are fully in accordance with those defined in ITU-T Rec. H.242 for point-to-point working. However, the composition of the multiplexed signal transmitted by each terminal an
27、d by the MCU is determined by terminal procedures and multipoint system procedures, as follows: a) Terminal procedures are defined in service-specific system Recommendations, such as ITU-T Rec. H.320 for visual telephony. b) Multipoint system procedures are defined in this Recommendation and are not
28、 of themselves service-specific. c) ITU-T Rec. T.120: by making use of the T series, MCU and terminal procedures may be greatly enhanced, offering far more sophisticated specific applications to the user. Such enhancement is outside the scope of this Recommendation, although interactions with specif
29、ic T.120 methods are described here. For definitions of terms used in this Recommendation, see ITU-T Rec. H.231; for definitions of SBE and MBE symbols, see ITU-T Rec. H.230. 2 ITU-T Rec. H.243 (10/2005) 2 References The following ITU-T Recommendations and other references contain provisions which,
30、through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision; users of this Recommendation are therefore encouraged to investigate the possibility of apply
31、ing the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. The reference to a document within this Recommendation does not give it, as a stand-alone document, the status of a Recommendation. ITU-T
32、Recommendation G.711 (1988), Pulse code modulation (PCM) of voice frequencies. ITU-T Recommendation G.722 (1988), 7 kHz audio-coding within 64 kbit/s. ITU-T Recommendation G.728 (1992), Coding of speech at 16 kbit/s using low-delay code excited linear prediction. ITU-T Recommendation H.221 (2004), F
33、rame structure for a 64 to 1920 kbit/s channel in audiovisual teleservices. ITU-T Recommendation H.224 (2005), A real time control protocol for simplex applications using the H.221 LSD/HSD/MLP channels. ITU-T Recommendation H.230 (2004), Frame-synchronous control and indication signals for audiovisu
34、al systems. ITU-T Recommendation H.231 (1997), Multipoint control units for audiovisual systems using digital channels up to 1920 kbit/s. ITU-T Recommendation H.233 (2002), Confidentiality system for audiovisual services. ITU-T Recommendation H.242 (2004), System for establishing communication betwe
35、en audiovisual terminals using digital channels up to 2 Mbit/s. ITU-T Recommendation H.261 (1993), Video codec for audiovisual systems at p 64 kbit/s. ITU-T Recommendation H.262 (2000) | ISO/IEC 13818-2:2000, Information technology Generic coding of moving pictures and associated audio information:
36、Video. ITU-T Recommendation H.263 (2005), Video coding for low bit rate communication. ITU-T Recommendation H.320 (2004), Narrow-band visual telephone systems and terminal equipment. ITU-T Recommendation H.321 (1998), Adaptation of H.320 visual telephone terminals to B-ISDN environments. ITU-T Recom
37、mendation Q.931 (1998), ISDN user-network interface layer 3 specification for basic call control. ITU-T Recommendation T.120 (1996), Data protocols for multimedia conferencing. ITU-T Recommendation T.122 (1998), Multipoint communication service Service definition. ITU-T Recommendation T.123 (1999),
38、Network-specific data protocol stacks for multimedia conferencing. ITU-T Recommendation T.124 (1998), Generic Conference Control. ITU-T Recommendation T.125 (1998), Multipoint communication service protocol specification. ITU-T Rec. H.243 (10/2005) 3 ISO 639-1: 2002, Codes for the representation of
39、names of languages Part 1: Alpha-2 code. ISO/IEC 10646 (2003), Information technology Universal Multiple-Octet Coded Character Set (UCS). 3 Conventions In this Recommendation, the word “shall“ is used to indicate a mandatory requirement, while the word “should“ is used to indicate an option or a sug
40、gestion. 4 Capabilities and “selected“ communication modes The MCU shall send appropriate capabilities, according to the type of communication intended. Table 2/H.231 lists the MCU types classified as “Listed“, showing the capabilities which each will be able to declare if the terminals connected ha
41、ve appropriate capabilities and the modes it will be able to transmit. For each conference call, a “Selected Communication Mode (SCM)“ is identified in the MCU. During the call, the MCU strives to maintain this SCM as that transmitted bidirectionally between itself and all terminals, and between its
42、elf and other MCUs. The exceptions are those channels where mode-0 forcing (ITU-T Rec. H.242) has been applied or, in some cases, where data is temporarily being transmitted. Table 1 gives some examples of SCM which could be specified for the listed MCUs of ITU-T Rec. H.231. The SCM may include one
43、or more data channels. Table 1/H.243 Examples of selected communication modes MCU Type (according to Table 2/H.231) Transfer rate A B(d) C C(d) D D(d) Code 64/56 kbit/s a8+v a1 A-law,0U a6+d6400 a2 -law,0U a7+d8000 a3 A-law,0F 2 64/56 kbit/s a3/4+v a8+v+d6400 a4 -law,0F a7+v a7+v+d8000 a5 Rec. G.722
44、, m1 a6 Rec. G.722, m2 128 kbit/s a8+v a7 Rec. G.722, m3 a7+v a8 Rec. G.728 v H.261-ON 384 kbit/s a6+v a6+v d6400 LSD a6+v+d64k d8000 LSD d64k HSD r restrict The following sample methods may be used to determine the SCM; other methods are feasible: 1) The SCM may be fixed as a permanent feature of t
45、he MCU as manufactured. 2) The MCU may provide for several possible values of SCM, and one of these is specified by the service provider or at the time of booking the call. 3) The SCM is selected automatically within the MCU according to the capabilities of the terminals connected. 4 ITU-T Rec. H.24
46、3 (10/2005) NOTE 1 The algorithm for this selection is outside the scope of this Recommendation; for example, the SCM is set at the value transmitted by the first terminal to access the MCU; or the highest common mode of all primary terminals is selected; or the SCM is set at the value transmitted b
47、y the chair-control terminal, if any. 4) The SCM is set by utilizing procedures effectuated using T.120-series protocols. In the cases 2, 3 and 4 it is also possible that the SCM may change during the course of the call. It is not mandatory that the MCU have all of these methods available but it sho
48、uld be clear to the user and service provider what method is in force. NOTE 2 Where there are several modes common to the connected terminals (e.g., both 16 kbit/s and wideband audio), implementers should document what method is used to determine which one is selected. It should be noted that the po
49、wer of the MCU to control the mode transmitted by a terminal is limited to a determination of the rates of audio, video and any data paths; it cannot set the video mode CIF or QCIF except by limiting the SCM to QCIF via capability changes, nor the audio mode where there is a choice at one bit rate (for example: ITU-T Recs G.711 or G.722 at 56 kbit/s). Specifically, if the MCU sends the BAS command for ITU-T Rec. G.711 at 56 kbit/s, the terminal may respond with either ITU-T Rec. G.711 at 56
