User`s guide

Chapter 12 Voice
P-2602HWUDL-D User’s Guide
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1 A sends a SIP INVITE request to B. This message is an invitation for B to participate in
a SIP telephone call.
2 B sends a response indicating that the telephone is ringing.
3 B sends an OK response after the call is answered.
4 A then sends an ACK message to acknowledge that B has answered the call.
5 Now A and B exchange voice media (talk).
6 After talking, A hangs up and sends a BYE request.
7 B replies with an OK response confirming receipt of the BYE request and the call is
terminated.
12.2.6 SIP Call Progression Through Proxy Servers
Usually, the SIP UAC sets up a phonecall 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 138 SIP Call Through Proxy Servers
6. BYE
7. OK
Table 53 SIP Call Progression (continued)
A B
UA 1
UA 2
PROXY 1
PROXY 2
SIP
SIP
SIP
SIP & RTP