3Com MSR 50 Series Configuration Manual page 2371

Figure 686 Message exchange for a UA to register with a Registrar
User agent
Call setup
SIP operates in Client/Server model and sets up calls through UA-proxy server
communication.
n
A SIP endpoint is called UA when it acts as both UAC and UAS. Normally, a SIP
endpoint plays both roles.
Figure 687 Network diagram for call setup involving a proxy server
User agent
(Router A)
Telephone A
In the above 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 dials 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 alerts
Telephone B 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).
IP network
Register
100 Trying
200 OK
Proxy server
Host A
Introduction to SIP
Registrar
User agent
(Router B)
Internet
Host B
2371
PBX
Telephone B