ZyXEL Communications NVG2053, User Manual

Page: 188 / 200

Chapter 17 Voice
NVG2053 User’s Guide
188
SIP Call Progression
The following figure displays the basic steps in the setup and tear down of a SIP
call. A calls B.
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.
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.
Table 64 SIP Call Progression
A B
1. INVITE
2. Ringing
3. OK
4. ACK
5.Dialogue (voice
traffic)
6. BYE
7. OK