H.323 and Associated ProtocolsAsim Karim, [email protected] is an ITU-T recommendation for multimedia conferencing over packet-based networks such as LANs and the Internet. H.323 isbroad and comprehensive in its scope yet flexible and practical in its applicability. This report describes the components, protocols, andprocedures in H.323; the challenges to the widespread adoption of H.323; the outlook for H.323 as a standard for multimediaconferencing; and key H.323 products and services.See Also: Voice Over IP (Lecture by Prof. Jain) | Internet Telephony | Voice Over IP: Protocols and Standards | Voice Over IP:Products, Services, and Issues | Multimedia Networking Products | Voice Over IP: References | Books on Voice Over IPOther Reports on Recent Advances in NetworkingBack to Raj Jain's Home PageTable of Contents1. Introduction● 2. Scope of H.323● 3. Why is H.323 Important?● 4. H.323 Architecture: Components4.1 Terminal❍ 4.2 Gateway❍ 4.3 Gatekeeper❍ 4.4 Multipoint Control Unit (MCU)❍ ● 5. H.323 Architecture: Protocols and Procedures5.1 H.323 Terminal Protocol Stack❍ 5.2 Audio Codecs (G.711, G.722, G.723.1, G.728, G.729)❍ 5.3 Video Codecs (H.261, H.263)❍ 5.4 Data Conferencing (T.120)❍ 5.5 Control and Signaling Mechanisms (H.225.0, H.245. H.225.0 RAS)❍ 5.6 Call Setup and Tear Down❍ 5.7 Supplementary Services (H.450.x)❍ 5.8 Security (H.235)❍ ● 6. Challenges6.1 Interoperability and Implementation Issues❍ 6.2 Lack of Value Added Services❍ 6.3 Quality and Security Issues❍ ● 7. Future Prospects7.1 H.323 or SIP?❍ 7.2 Trends in the Enterprise Market❍ 7.3 Trends in the Home and Small Business Markets❍ 7.4 Future Developments in H.323❍ ● 8. Products and Services● H.323 and Associated Protocolshttp://www.cis.ohio-state.edu/~jain/cis788-99/h323/index.html (1 of 19) [2/7/2000 10:40:28 AM]8.1 MCUs❍ 8.2 ITSPs❍ 9. Summary● References● List of Acronyms● 1. IntroductionRecommendation H.323 is a set of protocols for voice, video, and data conferencing over packet-based networks such as the Internet.The current recommendation, known as version 2.0, was ratified by the Study Group 16 (SG16) of the Telecommunications Sector ofthe International Telecommunication Union (ITU-T). The H.323 protocol stack is designed to operate above the transport layer of theunderlying network. As such, H.323 can be used on top of any packet-based network transport like Ethernet, TCP/UDP/IP, ATM, andFrame Relay to provide real-time multimedia communication. H.323 uses the Internet Protocol (IP) for inter-network conferencing.H.323 is one of several videoconferencing recommendations issued by ITU-T. The other recommendations in the series include H.310for conferencing over broadband ISDN (B-ISDN), H.320 for conferencing over narrowband ISDN, H.321 for conferencing over ATM,H.322 for conferencing over LANs that provide a guaranteed quality of service, and H.324 for conferencing over public switchedtelephone networks (PSTN). The H.323 standard is designed to allow clients on H.323 networks to communicate with clients on theother videoconferencing networks.This report provides a detailed overview of the components, protocols, and procedures defined in H.323. The technical challengesfacing the widespread adoption of H.323, the current extensions and developments in H.323, and the future prospects of H.323 as avideoconferencing standard are also discussed.Back to Table of Contents2. Scope of H.323H.323 is a broad and flexible recommendation. As a minimum, H.323 specifies protocols for real-time point-to-point audiocommunication between two terminals on a packet-based network that do not provide a guaranteed quality of service. The scope ofH.323, however, is much broader and encompasses inter-network multipoint conferencing among terminals that support not only audiobut also video and data communication.The scope of Recommendation H.323 can be summarized in the following broad categories:Point-to-point and multipoint conferencing support: H.323 conferences may be set up between two or more clients withoutany specialized multipoint control software or hardware. However, when a multipoint control unit (MCU) is used H.323supports a flexible topology for multipoint conferences. A multipoint conference may be centralized where new participants canjoin all the others in the conference. This is the so-called hub-and-spoke topology. Or, a multipoint conference may bedecentralized where new participants can elect to join one or more participants in the conference but not all. This approach willproduce a flexible tree topology.● Inter-network interoperability: H.323 clients are interoperable with switched-circuit network (SCN) conferencing clients suchas those based on Recommendations H.320 (ISDN), H.321 (ATM), and H.324 (PSTN/Wireless).● Heterogeneous client capabilities: A H.323 client must support audio communication; video and data support is optional. Thisheterogeneity and flexibility does not make the clients incompatible. During call set-up capabilities are exchanged andcommunication established based on the lowest common denominator.● Audio and video codecs: H.323 specifies a required audio and video codec. However, there is no restriction on the use of othercodecs and two clients can agree on any codec which is supported by both of them.● Management and accounting support: H.323 calls can be restricted on a network based on the number of calls already inprogress, bandwidth limitations, or time restrictions. Using these policies the network manager can manage H.323 traffic.Further, H.323 also provides accounting facilities that can be used for billing purposes.● Security: H.323 provides authentication, integrity, privacy, and non-repudiation support.● Supplementary services: Recommendation H.323 recognizes the huge potential for applications based on IP telephony andmultimedia. It provides a basic framework for development of such services. In version 2.0 of H.323, two services -- call transferand call forwarding -- have been specified.● H.323 and Associated Protocolshttp://www.cis.ohio-state.edu/~jain/cis788-99/h323/index.html (2 of 19) [2/7/2000 10:40:28 AM]Back to Table of Contents3. Why is H.323 Important?Trend: The explosive growth of the Internet and the almost universal deployment of corporate LANs have made packet-basednetworks ubiquitous. It is therefore