7.1 Introduction

The ITU-T Recommendation H.323 is a standard that describes systems and equipment for use in providing multimedia communications over packet-based networks. In this context, multimedia communications is generally considered to consist of audio or audiovisual communications for conversational applications. This may also include a data or graphic communications component that is intended to enhance the utility of the audiovisual communications by providing additional tools for collaboration.

While the original development of H.323 was directed toward corporate LANs, the scope has been broadened to any packet-based network such as enterprise area networks, metropolitan area networks, intra-networks, and internetworks (including the Internet). They also include dial-up connections or point-to-point connections, over general switched telephone network (GSTN) or ISDN, that use an underlying packet-based transport such as point-to-point protocol (PPP). These networks can consist of a single network segment, or they can have complex topologies which incorporate many network segments interconnected by other communications links.

The ITU is the United Nations' Specialized Agency in the field of telecommunications. The ITU-T is a permanent organ of the ITU and is responsible for studying technical, operating, and tariff questions and issuing recommendations on them with a view to standardizing telecommunications on a worldwide basis.

Prior to the development of H.323, the ITU-T had developed other audiovisual communications recommendations including:

• H.320 for operation over N-ISDN networks;
• H-321 for using H.320 equipment on B-ISDN (ATM);
• H.322 for using H.320 equipment on guaranteed quality of service LANs (Iso-Ethernet);
• H.324 for operation of the analog telephone network...