H3C Technologies H3C MSR 50 - Manual (Page 668)

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registrar sends back a response notifying receipt of the request, and a 200 OK (SUCCESS) message if
the registration is accepted. The following figure shows the message exchange.
Figure 672 Message exchange for a UA to register with a Registrar
Call setup
SIP operates in the Client/Server mode and sets up calls through communication between UA and proxy
server.
Figure 673 Network diagram
In the previous figure, Telephone A wants to call Telephone B, and Router A and Router B work as SIP
endpoints (UAs).
The following is the procedure for connecting a call from Telephone A to Telephone B:
1. Telephone A sends the number of Telephone B.
2. Upon receipt of the call, Router A sends a session request (INVITE) to the proxy server.
3. The proxy server consults its database for information corresponding to the number of Telephone
B. If such information is available, it forwards the request to Router B.
4. Router B, after receiving the request, responds to the proxy server and makes Telephone B ring if
Telephone B is available.
5. The proxy server forwards the response to Router A. The response discussed here includes two
provisional response messages (100 Trying and 180 Ringing) and one success response (200
OK).
Figure 674 shows the complete call setup procedure.