The H.323 protocol suite allows dissimilar communication devices to communicate with each other. H.323 (which is implemented primarily at versions 4 and 5 as of the time of this writing) is a sometimes Byzantine international protocol published by the ITU that supports interoperability between differing vendor implementations of telephony and multimedia products across IP-based networks. H.323 entities provide for real-time audio, video, and/or data communications. Support for audio is mandatory; support for data and video is optional.

The H.323 specification defines four different H.323 entities as the functional units of a complete H.323 network (see Figure 3.9). These components of an H.323 system include endpoints (terminals), gateways, gatekeepers, and multipoint control units (MCUs).

Figure 3.9: H.323 Entities

Endpoints (telephones, softphones, IVRs, voice mail, video cameras, etc.) are typically devices that end-users interact with. MS Netmeeting is an example of an H.323 endpoint. Endpoints provide voice-only and/or multimedia such as video and real-time application collaboration.

Gateways handle signaling and media transport, and are optional components. Gateways typically serve as the interface to other types of networks such as ISDN, PSTN, or other H.323 systems. You can think of a gateway as providing translation functions. For example, an H.323 gateway will handle conversion of H.323 to SIP or H.323 to ISUP (ISUP (ISDN User Part) defines the interexchange signaling procedures for the trunk call control). Another way to think of this is that a gateway provides the interface between a packet-based network (e.g., a VoIP network) and a...