1.4. H.323 Call Scenario Via a Gateway

While the call scenario described in Section 1.3, "Basic H.323 Call Scenario" is useful for explaining the fundamentals of an H.323 call, is not a realistic call scenario. The IP addresses of both endpoints were defined to be known. Most Internet Service Providers (ISPs) allocate IP addresses to subscribers dynamically. This section describes the fundamentals of a more realistic example that involves a gateway.

A gateway provides a bridge between different technologies for example, an H.323 gateway (or IP gateway) provides a bridge between an IP network and the PSTN. Figure 3 shows a configuration that uses a gateway. User A is at a terminal, while user B is by a phone connected to the PSTN.

Figure 3. Basic H.323 Network with a Gateway

Figure 3 also shows a gatekeeper. The gatekeeper provides network services such as Registration, Admission and Status (RAS) and address mapping. When a gatekeeper is present, all endpoints managed by the gatekeeper must register with the gatekeeper at startup. The gatekeeper tracks which endpoints are accepting calls. The gatekeeper can perform other functions also, such as redirecting calls. For example, if a user does not answer the phone, the gatekeeper may redirect the call to an answering machine.

The call scenario in this example involves the following phases:

• Establishing Contact with the Gatekeeper
• Requesting Permission to Call
• Call Signaling and Data Exchange
• Call Termination

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