H.323 is a standard for audio and video conferencing established by the International Telecommunication Union (ITU).

What is H.323?

A standard for audio and video conferencing established by the International Telecommunication Union (ITU). H.323 is a comprehensive standard for multimedia communication among computers, terminals, network devices, and network services running on connectionless networks that do not support intrinsic quality of service (QoS) functions, such as Internet Protocol-based networks and the Internet.

This standard is widely accepted by manufacturers of Voice over IP (VoIP) and Internet phone software and devices. Any hardware or software system that supports the H.323 standard can communicate with any other H.323 system. H.323 is designed to make multimedia communication over the Internet as standardized as telephone communication.

For example, with telephone communication, you can purchase telephone equipment from any vendor and plug it into your phone system, and the equipment works. The idea is to make Internet communication appliances just as easy to use.

ITU-T H.323 Recommendation Summary

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 Rec. ITU-T 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.

How it works

H.323 defines a protocol stack that performs functions related to establishing and maintaining real-time multimedia communication that involve transmission of audio, video, and data. H.323 does not represent one standard, but rather an umbrella of other ITU standards. The H.323 protocol stack includes more than two dozen G-level, H-level, and T-level protocols from the ITU. H.323 is designed to run over Internet Protocol (IP) and includes specifications for the following components:

• H.323 Conference Manager, which manages the establishment of multimedia conferencing connections
• The H.245 protocol, which controls communication with data terminal equipment (DTE)
• The H.225 protocol, which performs synchronization and framing mechanisms for multimedia data streams
• The T.120 protocol, which provides support for real-time, multipoint data communication

H.323 also specifies standard codecs for audio (G.723) and for video (H.263) that enable H.323 products to send and receive voice and video images. Specifically, H.323 defines standards for data codecs for audio transmission rates of 14.4 Kbps or faster and video transmission rates of 28.8 Kbps or faster. H.323 recognizes the T.120 protocol for data conferencing and provides a set of standards-based conferencing features for audio and video conferencing.

H.323 Network Elements

H.323 defines four types of components, which, when internetworked, deliver point-to-multipoint as well as point-to-point multimedia communication services. The components are as follows:

• Terminals: Terminals are the most fundamental elements in any H.323 system. Terminals are the Local area network (LAN) client endpoints that deliver bi-directional, real-time multimedia communications. The H.323 terminal can be a computer or device that runs an H.323 stack and multimedia applications.
• Gateways: Used to connect two distinct networks, the H.323 gateway provides connectivity between H.323 and non-H.323 networks. This distinct network connectivity is established by translating protocols intended for call setup and release, transforming media formats in between various networks and switching details between networks that are interconnected by the gateway. Gateways are also used in order to enable videoconferencing devices based on H.320 and H.324 to communicate with H.323 systems.
• Gatekeepers: Considered the most vital H.323 component, the gatekeeper serves as the core point for every call inside its zone, while providing registered H.323 endpoints with call control services. In H.323 networks, gatekeepers are optional. However, if they are available in the network, the endpoints must definitely use their services.
• Multipoint Control Units (MCU): A multipoint control unit (MCU) is responsible for managing multipoint conferences and is composed of two logical entities referred to as the Multipoint Controller (MC) and the Multipoint Processor (MP). MCU deliver support for conferencing three or more H.323 endpoints or terminals. Each terminal participating in a conference sets up a MCU connection.

H.323 Gateway Configuration

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