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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
client to initiate a call. A and B can also both act as a SIP user agent server to
receive the call.
Figure 160 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, client device A calls someone who is using client
device C.
1
2
The client device (A in the figure) sends a call invitation to the SIP proxy
server (B).
The SIP proxy server forwards the call invitation to C.
Nortel Business Secure Router 252 Configuration — Advanced
Figure
160, either A or B can act as a SIP user agent
Appendix L SIP 335
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