1.3.3. Call Initiation

Once the capability setup is agreed, endpoint A and B must set up the voice channels over which the voice data (media stream) will be exchanged.

To open a logical channel at endpoint B, endpoint A sends an H.245 OpenLogicalChannel message to endpoint B. This message specifies the type of data being sent, for example, the codec that will be used. For voice data, the message also includes the port number that endpoint B should use to send RTCP receiver reports. When endpoint B is ready to receive data, it sends an OpenLogicalChannelAck message to endpoint A. This message contains the port number on which endpoint A is to send RTP data and the port number on which endpoint A should send RTCP data.

Endpoint B repeats the process above to establish on which ports endpoint A will send both RTP data and RTCP reports. Once these ports have been identified, the next phase of the call scenario, Data Exchange, can begin.

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