Now that it has bandwidth allocated, the PC can send a Q.931 SETUP message over the TCP connection. This message specifies the number of the telephone being called (or the IP address and port, if a computer is being called). The gatekeeper responds with a Q.931 CALL PROCEEDING message to acknowledge correct receipt of the request. The gatekeeper then forwards the SETUP message to the gateway.
The gateway, which is half computer, half telephone switch, then makes an ordinary telephone call to the desired (ordinary) telephone. The end office to which the telephone is attached rings the called telephone and also sends back a Q.931 ALERT message to tell the calling PC that ringing has begun. When the person at the other end picks up the telephone, the end office sends back a Q.931 CONNECT message to signal the PC that it has a that ringing has begun.
When either party hangs up, the Q.931 call signaling channel is used to tear down the connection.
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