ZyXEL Communications P-2612HW Series User Manual page 211

Table 64 SIP Call Progression (continued)
A
4. ACK
6. BYE
A sends a SIP INVITE request to B. This message is an invitation for B to
1
participate in a SIP 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
7
terminated.
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.
P-2612HW Series User's Guide
5.Dialogue (voice
traffic)
B
3. OK
7. OK
Chapter 10 Voice
211