1.4.3. Call Signaling and Data Exchange

Endpoint A can now sends the Setup message to the gateway. Since the destination phone is connected to an analog line (the PSTN), the gateway goes off-hook and dials the phone number using dual tone multifrequency (DTMF) digits. The gatekeeper therefore is converting the H.225.0 signaling into the signaling present on the PSTN. Depending on the location of the gateway, the number dialed may need to be converted. For example, if the gateway is located in Europe, then the international dial prefix will be removed.

As soon as the gateway is notified by the PSTN that the phone at endpoint B is ringing, it sends the H.225.0 Alerting message as a response to endpoint A. As soon as the phone is picked up at endpoint B, the H.225.0 Connect message is sent to endpoint A. As part of the Connect message, a transport address that allows endpoint A to negotiate codecs and media streams with endpoint B is sent.

The H.225.0 and H.245 signaling used to negotiate capability, initiate and call, and exchange data are the same as that described in the basic H.323 call scenario. See Section 1.3.2, "Capability Exchange", Section 1.3.3, "Call Initiation", and Section 1.3.4, "Data Exchange" for more information.

In this example the destination phone is analog, therefore, it requires the gateway to detect the ring, busy and connect conditions so it can respond appropriately.

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