ZyXEL Communications FMG3024-D10A Series User Manual page 196

Chapter 18 VoIP
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
A SIP user agent 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 to receive the call.
Figure 99 SIP User Agent
A
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.
The client device (A in the figure) sends a call invitation to the SIP proxy server B.
1
The SIP proxy server forwards the call invitation to C.
2
196
B
FMG3024-D10A / FMG3025-D10A Series User's Guide