22.1 Introduction

H.323 in its broad sense is an umbrella recommendation by ITU-T that sets standards for multimedia communications over packet-based local area networks such as Ethernet, fast Ethernet, Token Ring, and other technologies that do not provide guaranteed QoS.

The series of ITU-T recommendations under the H.323 umbrella can be divided into the following four categories:

H.323 architecture and guideline

• H.323. Describes overall architecture, operations, and procedures of H.323 systems (ITU-T 2000c)

• H.221. Provides guidelines for implementing the H.323 series recommendations (ITU-T 2002a)

Control and call signaling protocols

• H.225.0. Specifies messages for call control including signaling, registration, and admissions, and packetization/synchronization of media streams (ITU-T 2000a)

• H.245. Specifies messages for opening and closing channels for media streams, and other commands, requests, and indications (ITU-T 2001)

Services supported on H.323 systems

• H.450.x. Define procedures and protocols for providing telephone-like services on H.323 systems (ITU-T 1998a)

• H.235. Defines a security framework used to provide authentication, encryption, and integrity for H.323 systems (ITU-T 2000d)

• H.332. Provides large-scale or loosely coupled conferencing service using H.323 systems (ITU-T 1998b)

Audio and video codec for H.323 systems

• H.261. Defines video codec for audiovisual services at P 64 Kbps (ITU-T 1993b)

• H.263. Specifies a new video codec for video over POTS (ITU-T 1998c)

• G.711. Defines Audio codec: 3.1 KHz at 48, 56, and 64 Kbps, i.e., the normal telephony codec (ITU-T 1988b)

• G.722. Defines audio codec: 7 KHz at 48, 56, and 64 Kbps (ITU-T 1988a)

• G.723.1.