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H.323 - technical definition

The ITU-T Recommendation for multimedia communications over packet networks.The recommendation addresses service over local area networks (LANs), but extends to the Internet and other IPbased networks. H.323 is not linked to any specific hardware device or operating system (OS) and, therefore, can be deployed in a wide variety of devices, including PCs, telephone sets, cable modems, and set-top boxes. H.323 supports multicast communications, thereby avoiding the requirement for specialized multipoint control units (MCUs) in a network where routers assume the responsibility for packet replicating.Version 2 1998) provides a means for encryption, includes mechanisms for call transfer and call forward, supports URL-style addresses, and provides the ability for endpoints to set quality of service (QoS) levels through Resource Reservation Protocol (RSVP). The four major components specified for H.323 include terminals, gateways, gatekeepers, and MCUs.

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An ITU standard for real-time voice and videoconferencing over packet networks, including LANs, WANs and the Internet. Although H.323 is a very comprehensive standard that supports voice, video, data, application sharing and whiteboarding, the parts relating to audio protocols have been widely used for IP telephony applications.

H.323 defines various elements including media gateways for conversion to packets, gatekeepers for call control and multipoint control units (MCUs) for conferencing. Following are the various protocols referenced under the H.323 umbrella. See videoconferencing, codec, H.225, H.245, IP telephony, gatekeeper, SIP and MGCP/MEGACO.

   Standard   Function
   G.711      Audio codec
   G.722      Audio codec
   G.723      Audio codec
   G.728      Audio codec
   G.729      Audio codec

   H.261      Video codec
   H.263      Video codec
   H.264      Video codec

   H.225      Call control
   H.245      Control messages
   Q.931      Connection messages
   RAS        Control channel

   T.120      Data collaboration

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