The H.323 standard is a cornerstone technology for the transmission of real-time audio, video, and data communications over packet-based networks. It specifies the components, protocols, and procedures providing multimedia communication over packet-based networks (see Figure 1). Packet-based networks include IP–based (including the Internet) or Internet packet exchange (IPX)–based local-area networks (LANs), enterprise networks (ENs), metropolitan-area networks (MANs), and wide-area networks (WANs). H.323 can be applied in a variety of mechanisms—audio only (IP telephony); audio and video (videotelephony); audio and data; and audio, video and data. H.323 can also be applied to multipoint-multimedia communications. H.323 provides myriad services and, therefore, can be applied in a wide variety of areas—consumer, business, and entertainment applications.

Figure 1. H.323 Terminals on a Packet Network

H.323 Versions

The H.323 standard is specified by the ITU–T Study Group 16. Version 1 of the H.323 recommendation—visual telephone systems and equipment for LANs that provide a nonguaranteed quality of service (QoS)—was accepted in October 1996. It was, as the name suggests, heavily weighted towards multimedia communications in a LAN environment. Version 1 of the H.323 standard does not provide guaranteed QoS.

The emergence of voice-over–IP (VoIP) applications and IP telephony has paved the way for a revision of the H.323 specification. The absence of a standard for voice over IP resulted in products that were incompatible. With the development of VoIP, new requirements emerged, such as providing communication between a PC–based phone and a phone on a traditional switched circuit network (SCN). Such requirements forced the need for a standard for IP telephony. Version 2 of H.323—packet-based multimedia communications systems—was defined to accommodate these additional requirements and was accepted in January 1998.

New features are being added to the H.323 standard, which will evolve to Version 3 shortly. The features being added include fax-over-packet networks, gatekeeper-gatekeeper communications, and fast-connection mechanisms.

H.323 in Relation to Other Standards of the H.32x Family

The H.323 standard is part of the H.32x family of recommendations specified by ITU–T. The other recommendations of the family specify multimedia communication services over different networks:

• H.324 over SCN
• H.320 over integrated services digital networks (ISDN)
• H.321 and H.310 over broadband integrated services digital networks (B–ISDN)
• H.322 over LANs that provide guaranteed QoS

One of the primary goals in the development of the H.323 standard was interoperability with other multimedia-services networks. This interoperability is achieved through the use of a gateway. A gateway performs any network or signaling translation required for interoperability. Gateways are explained in detail in Topic 6.