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H.323 Multimedia Conferencing Standard
Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
H.323 is part of a family of ITU-T recommendations that specify multimedia communications services such as real-time audio, video, and data over a variety of communication services, including multipoint links where multiple users participate in the same exchange (such as a videoconference). The ITU calls H.323 a recommendation for a "visual telephone system" that works over LANs. It does not guarantee high quality of service due to the packet-based, but QoS can be obtained by relying on other means as discussed under "QoS (Quality of Service)."
Note: The IETF's SIP (Session Initiation Protocol) competes with H.323. While H.323 is considered a traditional telecom standard oriented toward the old phone system, SIP and its family of protocol (RTP, RTCP, SDP) are oriented toward more versatile Internet multimedia. See " Multimedia" and "SIP (Session Initiation Protocol)."
An important feature of H.323 is that it defines multimedia communications over packet-switched networks, regardless of the underlying physical topology. That includes voice and videoconferencing in point-to-point and multipoint configurations using desktop computers or audiovisual devices. Audio is a required component while video and data are optional. Since packet-switched networks are not good at supporting real-time audio and video, H.323 addresses problems related to packet delay and packet loss on LANs, corporate intranets, and the Internet.
An H.323 environment consists of H.323 terminals, gateways to the public telephone network, gatekeepers (management functions), and multipoint control units as described below and pictured in Figure 1.
Figure 1: The H.323 voice/video over IP environment
Keep in mind that H.323 specifies the protocols that are required to support multimedia functions rather than the design of multimedia devices such as H.323 terminals.
The H.323 protocol stack is pictured in Table 1 (yellow area). The G.7xx components are audio codecs. G.711 (audio coding at 64 kbits/sec) is required, but more efficient protocols are available. The H.26x components are video codecs. The standard is H.261. Audio and video components sit on top of RTP (Real-time Transport Protocol), an IETF protocol, which itself uses UDP. The T.1xx protocols enable real-time exchange of data (i.e., whiteboard applications, for example). The terminal control protocols are outlined below:
Table 1: The H.323 protocol stack (in yellow) running over Internet protocols and Ethernet (Source: Queensland University of Technology-- See Web site on related entries page)
Calls may be initiated directly between terminals or through the gatekeeper. The procedure is that the terminal asks the gatekeeper for permission to make a call. The gatekeeper either admits or rejects the call. If the call is allowed, the calling terminal sends a "setup" message to a target terminal, which responds with a "call proceeding" message. The called terminal gets approval from its gatekeeper to accept the call. If approved, the called terminal alerts its user that a call incoming. If the user picks up the call, the called terminal sends a "contact" message to the calling terminal.
As mentioned, zones include a gatekeeper that controls the devices within a zone and their access to resources. Zones may encompass an entire service providers network, or the service provider may divide its network into several zones. An important zone feature is that service providers use zones as a basis for charging for resource usage, where services within a zone are cheaper than those used outside a zone.
H.323 is part of a suite of protocols that include the following protocols among others.
As mentioned, two protocols are available for managing multimedia across IP networks: H.323 and the IETF's SIP. MCI/Worldcom has been pushing SIP while the H.323 is pushed by the traditional telecom industry. Many believe that SIP is a superior protocol in terms of simplicity, scalability, and the ability to expand it. However, the ITU has the ability to push its standards over the IETF.
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.