1.3.1. Call Setup

Assuming a slow start connection procedure, establishing a call between two endpoints requires two TCP connections between the end points:

• One for the call setup
• One for capability exchange and call control

Note: H.225.0 version 2 also supports a fast start connection procedure where only one TCP connection is required. See ITU-T Recommendation H.225.0 for more information.

The caller at endpoint A connects to the callee at endpoint B on a well-known port, port 1720, and sends the call Setup message as defined in the H.225.0 specification. The Setup message includes:

• Message type, in this case, Setup
• Bearer capability, which indicates the type of call, for example, audio only
• Called party number and address
• Calling party number and address
• Protocol Data Unit (PDU), which includes an identifier that indicates which version of H.225.0 should be used and other information.

When endpoint B receives the Setup message, it responds with one of the following messages:

• Release Complete
• Alerting
• Connect
• Call Proceeding

In this case, endpoint B responds with the Alerting message. Endpoint A must receive the Alerting message before its setup timer expires. After sending this message, the user at endpoint B must either accept or refuse the call with a predefined time period. When the user at endpoint B picks up the call, a Connect message is sent to endpoint A and the next phase of the call scenario, Capability Exchange, can begin.

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