ZyXEL Communications 2002 Series - Manual (Page 40)

Prestige 2002 Series User’s Guide
40 Chapter 5 Introduction to VoIP
5.2.2 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.
5.2.3 SIP Servers
SIP is a client-server protocol. A SIP client is an application program or device that sends SIP
requests. A SIP server responds to the SIP requests.
When you use SIP to make a VoIP call, it originates at a client and terminates at a server. A
SIP client could be a computer or a SIP phone. One device can act as both a SIP client and a
SIP server.
5.2.3.1 SIP User Agent Server
A SIP user agent server can make and receive VoIP telephone calls. This means that SIP can
be used for peer-to-peer communications even though it is a client-server protocol. In the
following figure, either A or B can act as a SIP user agent client to initiate a call. A and B can
also both act as a SIP user agent server to receive the call.
Table 9 SIP Call Progression
A B
1. INVITE
2. Ringing
3. OK
4. ACK
5.Dialogue (voice traffic)
6. BYE
7. OK