ZyXEL Communications EX5501-B0 User Manual page 257

A sends a SIP INVITE request to B. This message is an invitation for B to participate in a SIP telephone call.
1
B sends a response indicating that the telephone is ringing.
2
B sends an OK response after the call is answered.
3
A then sends an ACK message to acknowledge that B has answered the call.
4
Now A and B exchange voice media (talk).
5
After talking, A hangs up and sends a BYE request.
6
B replies with an OK response confirming receipt of the BYE request and the call is terminated.
7
SIP Call Progression Through Proxy Servers
Usually, the SIP UAC sets up a phone call by sending a request to the SIP proxy server. Then, the proxy
server looks up the destination to which the call should be forwarded (according to the URI requested
by the SIP UAC). The request may be forwarded to more than one proxy server before arriving at its
destination.
The response to the request goes to all the proxy servers through which the request passed, in reverse
sequence. Once the session is set up, session traffic is sent between the UAs directly, bypassing all the
proxy servers in between.
The following figure shows the SIP and session traffic flow between the user agents (UA 1 and UA 2) and
the proxy servers (this example shows two proxy servers, PROXY 1 and PROXY 2).
Figure 162 SIP Call Through Proxy Servers
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Chapter 22 VoIP
EX5501-B0 / AX7501-B0 / PX7501-B0 User's Guide
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