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):

• 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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