1、 International Telecommunication Union ITU-T T.120TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (01/2007) SERIES T: TERMINALS FOR TELEMATIC SERVICES Data protocols for multimedia conferencing ITU-T Recommendation T.120 ITU-T Rec. T.120 (01/2007) i ITU-T Recommendation T.120 Data protocols for mult
2、imedia conferencing Summary The T.120-series Recommendations collectively define a multipoint data communication service for use in multimedia conferencing environments. The purpose of ITU-T Recommendation T.120 is to provide an introduction and guide to the T.120 series. ITU-T Recommendation T.120
3、defines the T.120 architectural model and shows the interrelationships between the constituent Recommendations. Each Recommendation in the series is outlined and the requirements for T.120 compliance are specified. Annex A specifies T.120 channel and token allocations. Annex B specifies MCS domain p
4、arameters. Annex C specifies lightweight profiles for the T.120 architecture. This revised version of ITU-T Recommendation T.120 introduces a number of clarifications to the previous version. Source ITU-T Recommendation T.120 was approved on 13 January 2007 by ITU-T Study Group 16 (2005-2008) under
5、the ITU-T Recommendation A.8 procedure. ii ITU-T Rec. T.120 (01/2007) 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 r
6、esponsible for studying technical, operating and 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 t
7、he ITU-T study groups which, in turn, produce Recommendations 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 collabo
8、rative basis with ISO and IEC. NOTE In this Recommendation, 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
9、 mandatory provisions (to ensure e.g. interoperability 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
10、. The use of such words does not suggest that compliance 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
11、takes no position concerning the evidence, validity 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 not received notice of intellectual pr
12、operty, protected by patents, which may be required to implement this Recommendation. However, implementers are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2007 All rights reserv
13、ed. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. ITU-T Rec. T.120 (01/2007) iii CONTENTS Page 1 Scope 1 2 References. 1 3 Abbreviations and acronyms 1 4 Overview 2 5 Introduction to multipoint multimedia communication 3 6 The T.
14、120 system model 5 6.1 User applications 6 6.2 Application protocols . 7 6.3 Node controller. 7 6.4 Communications infrastructure 7 6.5 Networks. 8 7 T.120 infrastructure Recommendations . 8 7.1 Protocol stacks for audiographic and audiovisual conferencing ITU-T T.123. 8 7.2 Multipoint communication
15、 service (MCS) ITU-T T.122, ITU-T T.125. 9 7.3 Generic conference control (GCC) ITU-T T.124 . 10 8 Application protocol Recommendations 10 8.1 The generic application template (GAT) ITU-T T.121 10 8.2 Multipoint still image and annotation protocol (MSIA) ITU-T T.126. 11 8.3 Multipoint binary file tr
16、ansfer (MBFT) ITU-T T.127 11 8.4 T.120 extensions for audiovisual control For further study 12 8.5 Proprietary extensions to standardized protocols. 12 8.6 Non-standard application protocols 12 9 T.120 compliance . 12 Annex A T.120 channel and token allocations . 14 A.1 Static channels 14 A.2 Static
17、 tokens 14 A.3 Standard application protocol session identifiers. 14 Annex B MCS domain parameters . 15 Annex C Lightweight profiles for the T.120 architecture. 17 C.1 Lite T.123 profile 18 C.2 Lite T.122/T.125 profile . 18 C.3 Lite T.124 profile 24 C.4 Lite T.121 profile 37 iv ITU-T Rec. T.120 (01/
18、2007) ITU-T Rec. T.120 (01/2007) 1 ITU-T Recommendation T.120 Data protocols for multimedia conferencing 1 Scope This Recommendation introduces a suite of standards, collectively referred to as the T.120 series. This Recommendation describes the T.120 system model that provides an architecture for m
19、ultipoint data communication in a multimedia conferencing environment. It provides an introduction and functional description of the Recommendations that go to make up the T.120 infrastructure. In addition, it provides an overview to other Recommendations in the series that provide standardized appl
20、ication protocol functionality. This Recommendation defines the criteria for compliance when the T.120 data protocols are used in a conferencing or group-working environment. This Recommendation only covers completed work contained in approved Recommendations. When new Recommendations are approved,
21、supporting text will be generated for inclusion in this Recommendation at the next publication date. 2 References The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publicatio
22、n, 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 applying the most recent edition of the Recommendations and other references listed below. A list of the cur
23、rently 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 T.121 ITU-T Recommendation T.121 (1996), Generic application template. ITU-T T.122 ITU-T Recommendation
24、 T.122 (1998), Multipoint communication service Service definition. ITU-T T.123 ITU-T Recommendation T.123 (2007), Network-specific data protocol stacks for multimedia conferencing. ITU-T T.124 ITU-T Recommendation T.124 (2007), Generic conference control. ITU-T T.125 ITU-T Recommendation T.125 (199
25、8), Multipoint communication service protocol specification. ITU-T T.126 ITU-T Recommendation T.126 (1997), Multipoint still image and annotation protocol. ITU-T T.127 ITU-T Recommendation T.127 (1995), Multipoint binary file transfer protocol. 3 Abbreviations and acronyms This Recommendation uses t
26、he following abbreviations and acronyms: APE Application Protocol Entity ARM Application Resource Manager ASE Application Service Element B-ISDN Broadband Integrated Services Digital Network 2 ITU-T Rec. T.120 (01/2007) CSDN Circuit Switched Data Network GAT Generic Application Template GCC Generic
27、Conference Control ISDN Integrated Services Digital Network LAN Local Area Network MBFT Multipoint Binary File Transfer MCS Multipoint Communication Service MCU Multipoint Control Unit MSIA Multipoint Still Image and Annotation PDU Protocol Data Unit PSDN Packet Switched Data Network PSTN Public Swi
28、tched Telephone Network QoS Quality of Service SI Still Image (SI is a commonly used abbreviation for MSIA) 4 Overview The T.120-series Recommendations collectively define a multipoint communication service for use in multimedia conferencing environments. The purpose of this Recommendation is to pro
29、vide an introduction and guide for the T.120 series, showing the interrelationships between the constituent Recommendations and to define the requirements for compliance to this Recommendation for conferencing. This Recommendation provides facilities to establish and manage interactive communication
30、s (conferences) involving two or more participants on and between a variety of different networks. It provides a comprehensive data communication service for those participants, which is independent of the underlying network. Within a conference it allows communications to be established between any
31、 combination of conference participants. This Recommendation also provides support for applications and their associated protocols, defining start-up mechanisms and capability exchange procedures, etc. This Recommendation makes provisions to ensure interoperability of commonly required functionality
32、 such as file transfer, still image exchange and shared whiteboards through the definition of standardized application protocols. The T.120 protocols provide a means of telecommunicating many forms of Data/Telematic information between two or more multimedia terminals and of managing such communicat
33、ion. They provide a multipoint data communication service that has a particular application in multimedia conferencing. The T.120 protocols are suitable for use on many types of network: PSTN, ISDN, CSDN, PSDN, B-ISDN, LANs. They provide the capability for seamless interworking of applications betwe
34、en terminals connected to different networks. The T.120 protocols provide: support for conference establishment among a group of network nodes (such as conference terminals and MCUs); mechanisms to identify the participating nodes and a comprehensive roster and capability exchange mechanism; ITU-T R
35、ec. T.120 (01/2007) 3 flexible management of communication between any combination of these elements. The T.120 protocols can handle one or more simultaneous conferences. A terminal may participate in more than one of these if authorized to do so. The convenor of a conference may control the partici
36、pation in that conference and the information which flows in that conference. In a conference that allows conductorship, the convenor may delegate part or all authority to the conductor. If a conference enters conducted mode, application protocols that are “conductor aware“ modify their behaviour, a
37、s specified by their protocol for this mode of operation. This Recommendation imposes few inherent constraints on the configuration of connections between conference nodes (terminals and MCUs): but they must be arranged in a hierarchy, with a single node at the top of a tree. Nodes may be all connec
38、ted to one star-point, or connected one to two others in a chain, or a chain of star-points, and so on, as long as it is clear for each connection which direction is upward and there are no loops. The top node should be present from the beginning of a conference, since any change in the top can be d
39、isruptive. No constraint is placed on the rate or volume of information transmitted within the various media; the T.120 protocols have the capability to organize different rates of information flow, within the constraints imposed by the type of network and connections established thereon. They allow
40、 relative priorities to be set by the applications using the T.120 protocols. The structure of the T.120 protocols is described in clause 6. Not all of the T.120 protocol provisions are mandatory: T.123, T.122/125, and T.124 are mandatory for conferencing and group working environments. The remainde
41、r are conditional: where functionality covered by the standards is provided, the standard protocols of the T.120 series must be implemented (see clause 9 for T.120 compliance requirements). This ensures that it is always possible to achieve a basic level of interworking, and does not prohibit custom
42、ized enhancements and negotiation of proprietary modes if (and only if) all participating elements are able to support such modes. 5 Introduction to multipoint multimedia communication Traditionally, telephony services have been constrained to point-to-point operation. In order to support group acti
43、vities such as meetings, conferences etc., involving physically separated participants, there is a requirement to join together more than two locations. The term “multipoint communication“ simply describes the interconnection of multiple terminals. Normally, a special network element, known as a Mul
44、tipoint Control Unit (MCU), or more simply a bridge, is required in order to provide this function. A conference typically refers to a group of geographically dispersed nodes that are electronically joined together and that are capable of exchanging audiographic and audiovisual information across va
45、rious communication networks. Conference participants may have access to various types of media handling capabilities such as audio only (telephony), audio and data, audio and video, or audio, video and data. The T.120 series of Recommendations define the component which is used to provide both a da
46、ta communications service, and a management service for any other media services present. T.120 protocols provide the infrastructure required to provide data services for many types of conferencing and group working, making it suitable for a diverse range of application areas. It is expected to find
47、 use in videotelephony and audiographic conferencing as well as other forms of multipoint multimedia communication. This Recommendation regards point-to-point connections as the simplest form (a degenerate case) of a multipoint connection. Both forms of connection are supported by the T.120 protocol
48、s. Terminals with multiple communication ports (each with an appropriate T.120 transport stack) can act as T.120 data bridges and allow multipoint connections to be established involving three or 4 ITU-T Rec. T.120 (01/2007) more nodes. Figure 1 b) shows a four site conference with multiport termina
49、ls acting as data bridges. MCUs are nodes that do not normally support terminal functionality. They act as bridging nodes, bridging data and other media streams present in the connections. Figure 1 c) shows an example of how three MCUs may be connected to bridge a group of terminals. Figure 1 Examples of multipoint conference configurations showing various connection topologies and node types ITU-T Rec. T.120 (01/2007) 5 Figure 2 is an example of a conference that involves terminals of diverse capability, on multiple different networ
