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H.323 Multimedia Conferencing Standard
Expanded version: contains additional text not in the book

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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)."

H.323 is part of the larger set of ITU H.32x recommendations. See "G Series ITU Recommendations" and "H Series ITU Recommendations."

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.

  • H.323 terminals These are devices such as desktop PCs with LAN or modem connections that run H.323-compatible software applications.
  • H.323 gateways Gateways connect different systems and devices that are not normally interoperable. A H.323 gateway connects IP-based H.323 terminals with the switched telephone network, for example. Gateways perform the appropriate mapping of call signals and control protocols between systems.
  • H.323 gatekeepers Gatekeepers are systems that manage a group of H.323 terminals and gateways within a "zone." A zone can be thought of as a management area consisting of terminals, gateways, and MCUs within an organization. There may be multiple zones within an organization. Gatekeepers provide address translation functions between H.323 addresses and IP addresses. They also provide supervisory functions (admitting or rejecting users), bandwidth allocation, and call signaling functions.
  • MCU (multipoint control unit) Provides multipoint conferencing support for multiple H.323 terminals. A multipoint conference is one that takes place between three or more terminals. Broadcast conferences are also possible where one terminal transmits to many receiving 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:

Audio applications

Video applications

Terminal control and management

Data Applications





Terminal to Gateway signalling (RAS)


Call signalling


Control channel




Unreliable Transport (UDP)

Reliable Transport (TCP)


Network Layer (IP)

Link Layer (IEEE 802.3)

Physical Layer (IEEE 802.3)

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)

  • RTCP (Real-time Transport Control Protocol) This is an IETF-defined protocol that RTP uses to control and synchronize streaming audio and video. It provides feedback information to the source that can be used to adapt the flow to changing network conditions.

  • H.225 RAS (Registration, Admission, Status) Terminals and gatekeepers communicate with the protocol. It is used to exchange signal about call registrations, admissions, and terminations. This protocol runs over UDP.

  • H.225 Call Signaling This protocol is used to establish connections between two terminals. It is a reliable protocol that runs over TCP.

  • H.245 Control Signaling This protocol is used between two terminals to exchange control messages such as flow control and channel management commands.

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.

  • ITU-T Recommendation H.225.0 (1998): "Call Signaling Protocols and Media Stream Packetization for Packet Based Multimedia Communications Systems." This defines the signaling for connection establishment called RAS signaling (registration, admission control, and status signaling). A RAS channel is set up and messages are exchanged that determine whether a user session will be allowed. ISDN-based Q.931 call control signaling is also specified.
  • ITU-T Recommendation H.235 (02/98): "Security and encryption for H-Series (H.323 and other H.245-based) multimedia terminals".
  • ITU-T Recommendation H.245 (1998): "Control Protocol for Multimedia Communication". This specifies messages between endpoints and how specific operations are negotiated.

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.
All rights reserved under Pan American and International copyright conventions.