H.323 - The Basics

The H.323 architecture includes the following elements:

- Gateways to link LAN-based H.323 endpoints and endpoints in the PSTN and other networks. These translate protocols, convert media formats and transfer information.

- Gatekeepers to translate addresses, allocate LAN bandwidth and provide other control and management functions. Gatekeepers are the brains of an H.323 network and act like SIP servers.

- Multipoint control units, which mix and distribute conference media streams for three or more H.323 terminals

The gatekeepers, gateways and MCUs are logically separate components of the H.323 architecture but can be implemented as a single physical device.

H.323 uses a number of protocols to set up a call. First of all, a supported client queries an H.323 gatekeeper for the address of a new user. The gatekeeper retrieves the address and forwards it to the client, which then establishes a session with the new client using H.225. Once the session is established, another H.323 protocol, H.245, negotiates the available features of each client. Because H.323 must first establish a session before it negotiates the features and functions of that session, call setup can take a long time. The amount of delay will depend upon the type of network.