H.323 Technology Guide

Overview

H.323 is a standard approved by the International Telecommunication Union (ITU) in order to promote compatibility (components, protocols, procedures) of real-time point to point and multipoint audio, video, and data communication over packet-based networks, such as the Internet.

H.323 is part of a larger body of standards (H.32x) that addresses multimedia communications over a variety of networks, as following:

• H.310 - for multimedia communications over B-ISDN (Broadband Integrated Services Digital Network)
• H.320 - for multimedia communications over narrowband ISDN
• H.321 - for multimedia communications over ATM
• H.322 - for multimedia communications over LAN's
• H.324 - for multimedia communications over PSTN

As a minimum, H.323 specifies protocols for real-time point-to-point audio communication between two terminals on a packet-based network that do not provide a guaranteed quality of service. The scope of H.323 is much broader and encompasses inter-network multipoint conferencing among terminals that support not only audio, but also video and data communication.

H.323 Architecture

In a general H.323 implementation four logical entities or components are required: terminal, gateways (GW), gatekeepers(GK) and multipoint control units (MCU):

H.323 Terminal
A terminal, or a client, is an endpoint where H.323 data streams and signaling originate and terminate. It may be a multimedia PC with a H.323 compliant stack or a standalone device such as a USB (universal serial bus) IP telephone, which provides for real-time, two-way communications with another H.323 Terminal, Gateway or MCU. This communication between endpoints consists of speech only, speech and data, speech and video, or speech, data and video.

Gateway (GW)
A Gateway is an optional component in an H.323-enabled network. When communication is required between different networks (e.g. between an IP-based network and PSTN) a gateway is needed at the interface. A H.323 Gateway is an H.323 endpoint that provides for real-time, two-way communications between terminals belonging to networks with different protocol stacks. For example, it is possible for an H.323 terminal to set up conference with terminals based on H.320 or H.324 through an appropriate gateway. A gateway provides data format translation, control-signaling translation, audio and video codec translation, and call setup and termination functionality on both sides of the network. Depending on the type of network to which translation is required a gateway may support H.310, H.320, H.321, H.322, or H.324 endpoints.

Gatekeeper (GK)
A Gatekeeper is a very useful, but optional, component of an H.323-enabled network. The gatekeeper provides address translation and controls access to the network resources for H.323 terminals, GW's and MCU's.

An endpoint does register itself at a GK. All H.323 endpoints registered to a single GK build an H.323 zone. A gatekeeper provides several services to all endpoints in its zone. These services include:


• Address translation - a GK translates H.323 aliases into call signaling IP addresses (especially useful for endpoints with dynamic IP addresses). A gatekeeper maintains a database for translation between aliases (such as international phone numbers) and network addresses
  
  
• Admission and access control of endpoints - this control can be based on bandwidth availability, limitations on the number of simultaneous H.323 calls, or the registration privileges of endpoints
  
  
• Bandwidth management - Network administrators can manage bandwidth by specifying limitations on the number of simultaneous calls and by limiting authorization of specific terminals to place calls at specified times
  
  
• Routing capability - A GK can route all calls originating or terminating in its zone. Thus, accounting information of calls can be maintained for billing and security purposes. A gatekeeper can re-route a call to an appropriate gateway, based on bandwidth availability. Re-routing can be used to develop advanced services such as mobile addressing, call forwarding, and voice mail diversion

Multipoint Control Unit (MCU)
MCU is an optional component of an H.323-enabled network and it is taking care of establishing multipoint conferences. It consists of:


• A mandatory Multipoint Controller (MC) - used for call signaling and conference control.
  
  
• An optional Multipoint Processor (MP) - used switching/mixing of media stream, and sometimes real-time trans-coding of the received audio/video streams

Although the MCU is a separate logical unit, it may be combined into a terminal, gateway or gatekeeper.

The MCU is required in a centralized multipoint conference where each terminal establishes a point-to-point connection with the MCU. The MCU determines the capabilities of each terminal and sends each a mixed media stream. In the decentralized model of multipoint conferencing, a MC ensures communication compatibility but the media streams are multicast and the mixing is performed at each terminal.

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