ITU-T Recommendation H.323 pertains to the H.323 packet-based multimedia communications systems. It describes an infrastructure of terminals, common control components, services, and protocols that are used for multimedia (voice, video, and data) communications.
An H.323 gateway is an optional type of endpoint that provides interoperability between H.323 endpoints and endpoints located on a Switched Circuit Network (SCN), such as the public switched telephone network (PSTN) or an enterprise voice network. Ideally, the gateway is transparent to both the H.323 endpoint and the SCN-based endpoint.
Recommendation H.323 describes an infrastructure of terminals, common control components, services, and protocols that are used for multimedia (voice, video, and data) communications. Figure 5-1 illustrates the elements of an H.323 terminal and highlights the protocol infrastructure of an H.323 endpoint.
H.323 was originally created to provide a mechanism for transporting multimedia applications over LANs. Although numerous vendors still use H.323 for videoconferencing applications, it has rapidly evolved to address the growing needs of Voice over IP (VoIP) networks. H.323 is currently the most widely used VoIP signaling and call control protocol, with international and domestic carriers relying on it to handle billions of minutes of use each year.
H.323 is considered an "umbrella protocol" because it defines all aspects of call transmission, from call establishment to capabilities exchange to network resource availability. H.323 defines these protocols:
■ H.245 for capabilities exchange
■ H.225.0 for registration, admission, and status (RAS) control for call routing
H.323 is based on the Integrated Services Digital Network (ISDN) Q.931 protocol, which allows H.323 to easily interoperate with legacy voice networks, such as the PSTN or Signaling System 7 (SS7). In addition to providing support for call setup, H.225.0 provides a message transport mechanism for the H.245 control function and the RAS signaling function. Following is a description of these functions:
■ Call-signaling function: The call-signaling function uses a call-signaling channel that allows an endpoint to create connections with other endpoints. The call-signaling function defines call setup procedures, based on the call setup procedures for ISDN (ITU-T Recommendation Q.931). The call-signaling function uses messages formatted according to H.225.0.
■ H.245 control function: The H.245 control function uses a control channel to transport control messages between endpoints or between an endpoint and a common control component, such as a gatekeeper or multipoint controller (MC). The control channel used by the H.245 control function is separate from the call-signaling channel.
The H.245 control function is responsible for these functions:
■ Logical channel signaling: Opens and closes a channel that carries the media stream
■ Capabilities exchange: Negotiates audio, video, and coder-decoder (codec) capability between endpoints
■ Master or responder determination: Determines which endpoint is master and which is responder; used to resolve conflicts during the call
■ Mode request: Requests a change in mode, or capability, of the media stream
■ Timer and counter values: Establishes values for timers and counters and agreement of those values by the endpoints
■ RAS signaling function: The RAS signaling function uses a separate signaling channel (RAS channel) to perform registration, admissions, bandwidth changes, status, and disengage procedures between endpoints and a gatekeeper. The RAS signaling function uses messages formatted according to H.225.0.
A typical implementation of H.323 goes beyond the original LAN context of H.323. Figure 5-2 illustrates a specific application of H.323 on an IP internetwork. Notice that real-time aspects of H.323 rely on User Datagram Protocol (UDP). Both the session-oriented control procedures and the data media type of H.323 use TCP.
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