ZyXEL Communications MSC1024G Series User Manual page 541

18.1.3.7 SIP User Agent
A SIP user agent can make and receive VoIP telephone calls. A User Agent Client (UAC) is a
logical entity that initiates a SIP request, and a User Agent Server (UAS) is a logical entity that
creates a response to a SIP request. These logical entities last only as long as the duration of
the transaction they have initiated or responded to. So, a piece of software can act as the UAC
in one call and the UAS in another.
18.1.3.8 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 a telephone connected to client device A to call
someone who is using a telephone connected to client device C. In this example, A is the UAC
and C is the UAS.
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 373 SIP Proxy Server
A
18.1.3.9 SIP Registrar Server
A SIP registrar server (also known as a register server) maintains a database of SIP identity-to-
IP address (or domain name) mapping. The registrar server checks your user name and
password when you register.
18.1.3.10 SIP Call Progression
Usually, the SIP UAC sets up a phone call by sending a request to the SIP proxy server. Then,
the proxy server looks up the destination to which the call should be forwarded (according to
the URI requested by the SIP UAC). The request may be forwarded to more than one proxy
server before arriving at its destination.
MSC1000G/1024G/1224G Series User's Guide
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Chapter 18 VoIP
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