ZyXEL Communications VMG892-B10A User Manual page 256

Chapter 21 Voice
SIP Call Progression
The following figure displays the basic steps in the setup and tear down of a SIP call. A calls B.
Table 119 SIP Call Progression
A
1. INVITE
4. ACK
6. BYE
A sends a SIP INVITE request to B. This message is an invitation for B to participate in a SIP
1
telephone call.
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.
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5.Dialogue (voice traffic)
B
2. Ringing
3. OK
7. OK
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