1.3.2. Capability Exchange

Call control and capability exchange messages, as defined in the H.245 standard are sent on a second TCP connection. Endpoint A opens this connection on a dynamically allocated port at the endpoint B after receiving the address in one of the following H.225.0 messages:

• Alerting
• Call Proceeding
• Connect

This connection remains active for the entire duration of the call. The control channel is unique for each call between endpoints so that several different media streams can be present.

An H.245 TerminalCapabilitySet message that includes information about the codecs supported by that endpoint is sent from one endpoint to the other. Both endpoints send this message and wait for a reply which can be one of the following messages:

• TerminalCapabilitySetAck - Accept the remote endpoints capability
• TerminalCapabilitySetReject - Reject the remote endpoints capability

The two endpoints continue to exchange these messages until a capability set that is supported by both endpoints is agreed. When this occurs, the next phase of the call scenario, Call Initiation, can begin.

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