1、 INTERNATIONAL TELECOMMUNICATION UNION ITU-T H.320TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (03/2004) SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMSInfrastructure of audiovisual services Systems and terminal equipment for audiovisual services Narrow-band visual telephone systems and terminal equ
2、ipment ITU-T Recommendation H.320 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.220H.229 Systems aspects H.230H.239 Com
3、munication 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 service architecture for audiovisual
4、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 H.510H.519 Mobile multimedia collab
5、oration 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.559 Mobile multimedia collaboration inter-working procedures H.560H.56
6、9 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.320 (03/2004) i ITU-T Recommendation H.320 Narrow-band visual telephone systems and terminal equipment Summary This
7、Recommendation specifies technical requirements for narrow-band visual telephone systems and terminal equipment, typically for videoconferencing and videophone services. It describes a generic system configuration consisting of a number of elements which are specified by respective ITU-T Recommendat
8、ions, definition of communication modes and terminal types, call control arrangements, terminal aspects and interworking requirements. This revised version of H.320 introduces a number of enhancements and clarifications to the previous version, primarily the description on the usage of ITU-T Recs G.
9、722.1, H.239, H.241, H.264, and ISO/IEC 14496-3 in H.320 systems. Source ITU-T Recommendation H.320 was approved on 15 March 2004 by ITU-T Study Group 16 (2001-2004) under the ITU-T Recommendation A.8 procedure. ii ITU-T Rec. H.320 (03/2004) FOREWORD The International Telecommunication Union (ITU) i
10、s 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 tariff questions and issuing Recommendations on them with a view to standardizi
11、ng 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 Recommendations on these topics. The approval of ITU-T Recommendations is covered
12、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 Recommendation, the expression “Administration“ is used for conciseness to indicate
13、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. interoperability or applicability) and compliance with the Recommendation is achieved when
14、 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 compliance with the Recommendation is required of any party. INTELLECTUAL PROPERTY
15、 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, validity or applicability of claimed Intellectual Property Rights, whether asserted b
16、y 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 property, protected by patents, which may be required to implement this Recommendation. However, implementors are cautioned that t
17、his may not represent the latest information and are therefore strongly urged to consult the TSB patent database. ITU 2004 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. ITU-T Rec. H.320 (03/2004) iii CONTENTS
18、 Page 1 Scope 1 2 Definitions 1 3 System description 2 3.1 Block diagram and identification of elements 2 3.2 Signals 3 3.3 Bit rate options and infrastructure 3 3.4 Call control arrangements. 6 3.5 Optional enhancements 11 4 Terminal requirements 11 4.1 Environments 11 4.2 Audio and video arrangeme
19、nts. 12 4.3 Delay compensation in the audio path 13 4.4 Control and Indications (C for the time being, therefore, it is convenient to treat all the text in a single Recommendation. The service requirements for visual telephone services are presented in ITU-T Recs F.720 for videotelephony and F.702 f
20、or videoconference; video and audio coding systems and other technical aspects common to audiovisual services are covered in other Recommendations in the H.200/F.700-series. 2 Definitions This Recommendation defines the following terms: 2.1 bit-rate allocation signal (BAS): eight bits positioned wit
21、hin the frame structure of ITU-T Rec. H.221 to transmit, e.g., commands, control and indication signals, capabilities. 2.2 control and indication (C see also ITU-T Rec. H.230. 2.3 data port: Input/output gate for the user data transmitted within service channel or sub-channels according to ITU-T Rec
22、. H.221. 2.4 human-machine interface (HMI): Human-machine interface between user and terminal/system which consists of a physical section (electro-acoustic, electro-optic transducer, keys, etc.) and a logical section dealing with functional operation states. 2.5 in-band signalling: Signalling via BA
23、S of the H.221 frame structure. 2.6 lip synchronization: Operation to provide the feeling that the speaking motion of the displayed person is synchronized with that persons voice. Alternatively, the minimization of the relative delay between the visual display of a person speaking and the audio of t
24、he voice of the person speaking. The objective is to achieve a natural relationship between the visual image and the aural message for the viewer/listener. 2.7 multipoint control unit (MCU): A piece of equipment located in a node of the network or in a terminal which receives several channels from a
25、ccess ports and, according to certain criteria, processes audiovisual signals and distributes them to the connected channels. 2.8 narrow-band: Bit rates ranging from 64 kbit/s to 1920 kbit/s. This channel capacity may be provided as a single B/H0/H11/H12-channel or multiple B/H0-channels in ISDN. 2.
26、9 out-band signalling: Signalling via a channel not being part of the B/H0/H11/H12-channel (due to I.400-series Recommendations). 2.10 visual telephone services: A group of audiovisual services including videophone defined in ITU-T Rec. F.721 and videoconferencing defined in ITU-T Rec. F.702. 2 ITU-
27、T Rec. H.320 (03/2004) 3 System description 3.1 Block diagram and identification of elements A generic visual telephone system is shown in Figure 1. It consists of terminal equipment, network, Multipoint Control Unit (MCU) and other system operation entities. Figure 1/H.320 Visual telephone system A
28、 configuration of the terminal equipment consisting of several functional units is also shown in Figure 1. Video I/O equipment includes cameras, monitors and video processing units to provide functions such as split-screen scheme. Audio I/O equipment includes microphones, loudspeakers and audio proc
29、essing units to provide such functions as acoustic echo cancellation (see ITU-T Rec. G.167). Telematic equipment includes visual aids such as electronic blackboard, text conversation facility and still picture transceiver to enhance basic visual telephone communication. The system control unit carri
30、es out such functions as network access through end-to-network signalling and end-to-end control to establish common mode of operation and signalling for proper operation of the terminal through end-to-end signalling. The video codec carries out redundancy reduction coding and decoding for video sig
31、nals, while audio codec does the same thing for audio signals. The delay in the audio path compensates video codec delay to maintain lip synchronization. The mux/dmux unit multiplexes transmitting video, audio, data and control signals into a single bit stream and demultiplexes a received bit stream
32、 into constituent multimedia signals. Network interface makes necessary adaptation between the network and the terminal according to the user-network interface requirements defined in the I.400-series Recommendations (see Note). NOTE For leased line networks, the network interface is defined in ITU-
33、T Rec. G.703 for bit rates in the range of 64 kbit/s to 2048 kbit/s. An alternative interface is defined in ITU-T Rec. X.21. For n H0channels, timeslot allocation is given in clause 5/G.704 for the G.703 interface. It is stressed that interworking towards ISDN requires synchronous operation of the l
34、eased line network. ITU-T Rec. H.320 (03/2004) 3 3.2 Signals Visual telephone signals are classified into video, audio, data and control as follows: Audio signals are continuous traffic and require real-time transmission. NOTE In order to reduce the average bit rate of audio signals, voice activatio
35、n can be introduced (in which case the audio signals are no longer continuous). Video signals are also continuous traffic; the bit rate allocated to video signals should be as high as possible, in order to maximize the quality within the available channel capacity. Data signals include still picture
36、s, facsimile and documents, or other facilities such as text conversation; this signal may occur only occasionally as required and may temporarily displace all or part of the audiovisual signal content. It should be noted that data signals are associated only with optional enhancements to the basic
37、visual telephone system; therefore, the opening of a path to carry such signals is preceded by negotiation between the terminals. Control signals are some system control signals by definition. The path for the terminal-to-network control signals is provided in the D-channel, while the path for the t
38、erminal-to-terminal control signals is provided in BAS or service channel only when necessary by the mechanism defined in ITU-T Rec. H.221. 3.3 Bit rate options and infrastructure 3.3.1 Communication modes of visual telephone Communication modes of visual telephone are defined in Table 1 according t
39、o their channel configuration and coding. A particular communication mode of operation is established according to the H.242 procedures. Table 1/H.320 Communication modes of visual telephone ISDN interface Coding Visual telephone mode (Suffix n corresponds to audio coding) Channel rate kbit/s ISDN c
40、hannelBasic Primary rate Audio Video Notes an64 B 1, 2 bn128 2B 3 cn192 3B 3, 4 dn256 4B n Rec. 3, 4 en320 5B 0 G.711 Rec. H.261 3, 4 fn384 6B Applicable 1 G.722 Rec. H.262 3, 4 gn384 H02 G.728 Rec. H.263 hn768 2H03 G.723.1 (Note 6) 3 in1152 3H0(Note 5) 3 jn1536 4H04 G.729 3 kn1536 H11ln1920 5H03 mn
41、1920 H12Not applicable4 ITU-T Rec. H.320 (03/2004) Table 1/H.320 Communication modes of visual telephone ISDN interface Coding Visual telephone mode (Suffix n corresponds to audio coding) Channel rate kbit/s ISDN channelBasic Primary rate Audio Video Notes NOTE 1 If a visual telephone interworks wit
42、h a wideband speech terminal, G.722 audio (mode a1) may be used instead of G.711 audio (mode a0). NOTE 2 If two terminals connect at this rate and run G.711 (mode a0) or G.722 (mode a1) and both have video capability, H.261, H.262 or H.263 may be used. It should be noted, however, that the video per
43、formance is limited due to the very low bit rate available for this purpose. NOTE 3 For multiple channels of B/H0, all channels are synchronized at the terminal according to 2.7/H.221. For bit rates higher than 64 kbit/s, channel aggregation of multiple B-channels may be used as specified in ITU-T R
44、ec. H.244 or ISO/IEC 13871. NOTE 4 This mode is applicable to the ISDN basic interface if multiple basic accesses are used. NOTE 5 For G.723.1 audio, a channel shall be allocated as specified in ITU-T Rec. H.221. The audio data and their packet formats are described in ITU-T Recs G.723.1 and H.223.
45、Use of bits not required for G.723.1 audio coding is specified in ITU-T Rec. H.221. NOTE 6 The hierarchy of H.261, H.262 and H.263 is defined in Annex A. 3.3.2 Terminal types of visual telephone The terminal type is categorized according to the type of communication channels with which the terminal
46、can communicate; mxB (type X with parameter a-f), n H0 (type Y with parameter 1-5; see Note), H11/H12(type Z with parameter -) or their combinations. Table 2 lists terminal types of visual telephone and communication modes which each terminal type is required to support. NOTE Type Y terminals shall
47、have the SM-comp or 6B-H0compatibility mode defined in ITU-T Rec. H.221 for interworking of evolving networks (see 3.3.2.2). Table 2/H.320 Visual telephone terminal types Modes to be supported Terminal type ISDN channel Mandatory Recommended Optional XaB a0 all other anXb2B a0, b0 all other an,bnX X
48、c3B a0, b0, c0 all other an, bn, cn(Note 1) Xd4B a0, b0, c0, d0 all other an, bn, cn, dnXe5B a0, b0, c0, d0, e0 all other an, bn, cn, dn, enXf6B a0, b0, c0, d0, e0, f0 all other an, bn, cn, dn, en, fnITU-T Rec. H.320 (03/2004) 5 Table 2/H.320 Visual telephone terminal types Modes to be supported Ter
49、minal type ISDN channel Mandatory Recommended Optional Y1H0g0g1all other gnY Y22H0g0, h0 1, h1all other gn, hn(Note 2) Y33H0g0, h0, i0g1, h1, i1all other gn, hn, inY44H0g0, h0, i0, j0 1, h1, i1, j1all other gn, hn, in, jnY55H0g0, h0, i0, j0, l0g1, h1, i1, j1, l1all other gn, hn, in, jn, lnZH11 k0 k1 all other kn Z ZH12 m0 m1 all other mn NOTE 1 If a visual telephone interworks with a wideband speech terminal, it should support G.722 audio (mode a1) in addition to G.711 audio (mode a0). NOTE 2 Terminal of th
