ITU-T Recommendation H.323

Title: Packet-based multimedia communication systems

Date of adoption: 02/1998

Summary

This Recommendation describes terminals and other entities that provide multimedia communications services over Packet Based Networks (PBN) which may not provide a guaranteed Quality of Service. H.323 entities may provide real-time audio, video and/or data communications. Support for audio is mandatory, while data and video are optional, but if supported, the ability to use a specified common mode of operation is required, so that all terminals supporting that media type can interwork.

The packet based network over which H.323 entities communicate may be a point-to-point connection, a single network segment, or an internetwork having multiple segments with complex topologies.

H.323 entities may be used in point-to-point, multipoint, or broadcast (as described in Recommendation H.332) configurations. They may interwork with H.310 terminals on B-ISDN, H.320 terminals on N-ISDN, H.321 terminals on B-ISDN, H.322 terminals on Guaranteed Quality of Service LANs, H.324 terminals on GSTN and wireless networks, V.70 terminals on GSTN, and voice terminals on GSTN or ISDN through the use of Gateways.

H.323 entities may be integrated into personal computers or implemented in stand-alone devices such as videotelephones.

Products claiming compliance with Version 1 of H.323 shall comply with all of the mandatory requirements of H.323 (1996) which references Recommendations H.225.0 (1996) and H.245 (1996). Version 1 products can be identified by H.225.0 messages containing a protocolIdentifier = {itu-t (0) recommendation (0) h (8) 2250 version (0) 1} and H.245 messages containing a protocolIdentifier = {itu-t (0) recommendation (0) h (8) 245 version (0) 2}. Products claiming compliance with Version 2 of H.323 shall comply with all of the mandatory requirements of this Recommendation, H.323 (1998), which references Recommendations H.225.0 (1998) and H.245 (1998). Version 2 products can be identified by H.225.0 messages containing a protocolIdentifier = {itu-t (0) recommendation (0) h (8) 2250 version (0) 2} and H.245 messages containing a protocolIdentifier = {itu-t (0) recommendation (0) h (8) 245 version (0) 3}.

Note that the title of H.323 (1996) was "Visual telephone systems and equipment for local area networks which provide a non-guaranteed quality of service". The title has been changed in this version to be consistent with its expanded scope.

TABLE OF CONTENTS

    Page
1 Scope 1
2 Normative references 2
3 Definitions 4
4 Symbols and abbreviations 9
5 Conventions 12
6 System description 12
6.1 Information streams 12
6.2 Terminal characteristics 13
6.2.1 Terminal elements outside the scope of this Recommendation 14
6.2.2 Terminal elements within the scope of this Recommendation 14
6.2.3 Packet based network interface 14
6.2.4 Video codec 15
6.2.5 Audio codec 15
6.2.6 Receive path delay 17
6.2.7 Data channel 17
6.2.8 H.245 control function 19
6.2.9 RAS signalling function 23
6.2.10 Call signalling function 24
6.2.11 H.225.0 layer 24
6.3 Gateway characteristics 24
6.4 Gatekeeper characteristics 27
6.5 Multipoint controller characteristics 28
6.6 Multipoint processor characteristics 29
6.7 Multipoint control unit characteristics 30
6.8 Multipoint capability 30
6.8.1 Centralized multipoint capability 30
6.8.2 Decentralized multipoint capability 31
6.8.3 Hybrid multipoint – Centralized audio 31
6.8.4 Hybrid multipoint – Centralized video 31
6.8.5 Establishment of common mode 32
6.8.6 Multipoint rate matching 32
6.8.7 Multipoint lip synchronization 32
6.8.8 Multipoint encryption 33
6.8.9 Cascading multipoint control units 33
7 Call signalling 33
7.1 Addresses 33
7.1.1 Network address 33
7.1.2 TSAP identifier 33
7.1.3 Alias address 33
7.2 Registration, Admission and Status (RAS) channel 34
7.2.1 Gatekeeper discovery 34
7.2.2 Endpoint registration 35
7.2.3 Endpoint location 37
7.2.4 Admissions, bandwidth change, status and disengage 37
7.2.5 Access tokens 38
7.3 Call signalling channel 38
7.3.1 Call signalling channel routing 38
7.3.2 Control channel routing 39
7.4 Call reference value 40
7.5 Call ID 41
7.6 Conference ID and Conference Goal 41
8 Call signalling procedures 41
8.1 Phase A – Call setup 41
8.1.1 Basic call setup – Neither endpoint registered 42
8.1.2 Both endpoints registered to the same Gatekeeper 42
8.1.3 Only calling endpoint has Gatekeeper 44
8.1.4 Only called endpoint has Gatekeeper 45
8.1.5 Both endpoints registered to different Gatekeepers 47
8.1.6 Optional Called Endpoint Signalling 51
8.1.7 Fast Connect Procedure 53
8.1.8 Call setup via gateways 55
8.1.9 Call setup with an MCU 56
8.1.10 Call forwarding 57
8.1.11 Broadcast call setup 57
8.1.12 Overlapped Sending 57
8.1.13 Call setup to conference alias 57
8.2 Phase B – Initial communication and capability exchange 59
8.2.1 Encapsulation of H.245 messages within Q.931 messages 59
8.2.2 Tunneling through intermediate signalling entities 60
8.2.3 Switching to a separate H.245 connection 60
8.3 Phase C – Establishment of audiovisual communication 61
8.3.1 Mode changes 61
8.3.2 Exchange of video by mutual agreement 61
8.3.3 Media stream address distribution 62
8.3.4 Correlation of media streams in multipoint conferences 62
8.3.5 Communication Mode Command Procedures 62
8.4 Phase D – Call services 63
8.4.1 Bandwidth changes 63
8.4.2 Status 65
8.4.3 Ad hoc conference expansion 66
8.4.4 Supplementary services 74
8.4.5 Multipoint cascading 74
8.4.6 Third party initiated pause and re-routing 75
8.5 Phase E – Call termination 75
8.5.1 Call clearing without a Gatekeeper 76
8.5.2 Call clearing with a Gatekeeper 76
8.5.3 Call clearing by Gatekeeper 76
8.6 Protocol failure handling 77
9 Interoperation with other terminal types 78
9.1 Speech-only terminals 78
9.2 Visual telephone terminals over the ISDN (H.320) 78
9.3 Visual telephone terminals over GSTN (H.324) 78
9.4 Visual telephone terminals over mobile radio (H.324/M) 79
9.5 Visual telephone terminals over ATM (H.321 and H.310 RAST) 79
9.6 Visual telephone terminals over guaranteed quality of service LANs (H.322) 79
9.7 Simultaneous voice and data terminals over GSTN (V.70) 80
9.8 T.120 terminals on the packet based network 80
10 Optional enhancements 80
10.1 Encryption 80
10.2 Multipoint operation 80
10.2.1 H.243 Control and Indication 80
11 Maintenance 80
11.1 Loopbacks for maintenance purposes 80
11.2 Monitoring methods 81
Annex A H.245 messages used by H.323 endpoints 82
Annex B Procedures for layered video codecs 87
B.1 Scope 87
B.2 Introduction 87
B.3 Scalability methods 88
B.4 Call establishment 88
B.5 Use of RTP sessions and codec layers 88
B.5.1 Associate base to audio for lip synchronization 88
B.5.2 Enhancement layer dependency 89
B.6 Possible layering models 90
B.6.1 Multiple logical channels and RTP sessions for a layered stream 90
B.6.2 Impact of one layer per logical channel and per RTP session 90
B.7 Impact on multipoint conferences 91
B.7.1 MC Impartial model 91
B.7.2 MC Decision model 91
B.7.3 Multipoint conference containing endpoints on different bandwidths 91
B.8 Use of network QOS for layered video streams 93
Annex C H.323 on ATM 94
C.1 Introduction 94
C.2 Scope 94
C.2.1 Point-to-point conferencing 94
C.2.2 MCU-based multipoint 94
C.2.3 H.323 interoperability with endpoints using IP 94
C.3 Architecture 94
C.3.1 Overview of system 95
C.3.2 Interoperation with other ITU-T H-Series endpoints 95
C.3.3 H.225.0 on IP over ATM 95
C.3.4 H.245 on TCP/IP over ATM 95
C.3.5 Addressing for A/V streams 96
C.3.6 Transport Capabilities added to Terminal Capability Set 96
C.3.7 Elements of ATM signalling 96
C.3.8 A/V streams on RTP on AAL5 96
C.3.9 QOS considerations (Optional) 97
C.4 Protocol section 99
C.4.1 ATM signalling information elements 99
C.4.2 H.245 Usage 101
C.4.3 RTP usage 102
C.4.4 Interoperation with H.323 on IP 102
Appendix I Sample MC to Terminal Communication Mode Command 103
I.1 Sample conference Scenario A 103
I.2 CommunicationModeTable sent to all Endpoints 103
I.3 Sample conference Scenario B 104
I.4 CommunicationModeTable sent to all Endpoints 104
Appendix II Transport level resource reservation procedures 105
II.1 Introduction 105
II.2 QOS Support for H.323 106
II.3 RSVP background 107
II.4 The H.245 capability exchange phase 108
II.5 Open logical channel and setting up reservations 109
II.6 Close logical channel and tearing down reservations 111
II.7 Resource reservation for multicast H.323 logical channels 111
Appendix III Gatekeeper based user location 112
III.1 Introduction 112
III.2 Signalling 112