Chapter 22 Voice
EMG6726/8726-B10A User’s Guide
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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
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.
1 The client device (A in the figure) sends a call invitation to the SIP proxy server (B).
2 The SIP proxy server forwards the call invitation to C.
Figure 149 SIP Proxy Server
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