ZyXEL Communications ZyXEL ZyWALL 35 - Manual (Page 626)

ZyWALL 35 User’s Guide
624 Appendix J SIP Passthrough
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 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.
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.
Figure 394 SIP User Agent Server
SIP Proxy Server
A SIP proxy server receives requests from clients and forwards them to another server.
In the following example, you want to use client device A to call someone who is using client
device C.
4. ACK
5.Dialogue (voice traffic)
6. BYE
7. OK
Table 242 SIP Call Progression (continued)
A B