ZyXEL Communications VMG8825-B Series User Manual page 246

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 148 SIP User Agent
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
Figure 149 SIP Proxy Server
SIP Redirect Server
A SIP redirect server accepts SIP requests, translates the destination address to an IP address and sends
the translated IP address back to the device that sent the request. Then the client device that originally
sent the request can send requests to the IP address that it received back from the redirect server.
Redirect servers do not initiate SIP requests.
Chapter 23 Voice
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