Monday, 09 October 2006 H.323 can be used with the global PSTN, a narrowband ISDN (N-ISDN) network (which runs at or below 1.5 or 2.0 Mb/s), or a broadband ISDN B-ISDN) network employing ATM (which runs above 1.5 or 2.0 Mb/s). Even a telephone, or speech terminal, can engage in an H.323 conference, but only with audio capabilities. More functions are found in V.70 terminals, which support both digitized voice and data over a “regular” telephone circuit, and H.324 terminals (H.324 terminals can carry real-time voice, data, and video or any combination, including videotelephony, over V.34 modems running at a maximum speed of 33.6 kb/s). Usually, an H.324 terminal is just a PC with some special software.

When H.323 is used with narrowband ISDN, ISDN telephones or H.320 terminals can be used. H.320 describes a generic terminal arrangement for narrowband ISDN visual telephone systems and terminal equipment. These are usually used for videoconferencing and videophone services. If there is a LAN attached to the ISDN that guarantees certain quality of service (QoS) parameters, then an H.322 terminal can be used. An H.322 terminal is a terminal attached to a network in which the transmission path includes one or more LANs, and each of the LANs is configured to provide a guaranteed quality of service (QoS) equal to that of N-ISDN. H.322 was more or less intended to run on an extension to Ethernet called isochronous Ethernet, or isoethernet, which added a number of 64-kb/s channels to the basic Ethernet architecture. However, for a number of reasons, isoethernet never caught on in the industry.

Finally, B-ISDN networks based on asynchronous transfer mode (ATM) can employ full video/audio H.321 terminals. H.321 applies the H.320 terminal concept to B-ISDN. B-ISDN also can employ a configuration called an H.310 terminal operating in H.321 mode. H.310 terminals are sort of “super” audio/visual terminals that can take full advantage of B-ISDN and ATM networks in terms of services and signaling.

In summary, H.320 is used to define four different types of terminals (user devices). There are H.321 terminals for B-ISDN and ATM, H.322 terminals for QoS LANs, H.323 terminals for conferencing and more, and H.324 terminals (for 33.6-kb/s dial-up connections). When used for IP telephony, H.323 covers VoIP calls made between H.323 terminals or between an H.323 terminal and an H.323 gateway. This does not rule out the use of VoIP for other members of the H.320 family; this just focuses VoIP on the H.323 terminal type.

 

 

source- http://www.voip-brands.com/technical/h.323-architecture.html