After receiving the ACF message from the gatekeeper, the calling endpoint sends call signaling to
set up a call. In a direct call signaling for example, the calling endpoint first sends a Setup message
to the called endpoint requesting for a connection.
After receiving the Setup message, the called endpoint replies with a Call Proceeding message.
However, it might not send this message.
Then the called endpoint might send an Alerting message to the calling endpoint, indicating its status
(for example, ringing). The endpoint might not send this message.
If the called endpoint accepts the call, it must send a Connect message.
After the calling endpoint receives the connect message, an H.245 control signaling channel is
established to control the media session between the endpoints. First, the two endpoints exchange
information on their capabilities, such as media format.
Opening/closing logical channels
The two endpoints open one or more logical channels between them for transporting media streams.
(The logical channels are specified by IP address plus port number.)
These channels are closed when the communication is over.
Finally, either endpoint in communication can release resources by sending a Release Complete
The endpoints each send a Disconnect_Request (DRQ) to their own gatekeepers, which will confirm
or reject the request. The gatekeepers, however, might send a DRQ to the endpoints. In this case,
the endpoints can only confirm the request.