ZyXEL Communications MSC1000G User Manual page 601

16.1.3.7 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.
16.1.3.8 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.
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 406 SIP Proxy Server
A
16.1.3.9 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.
16.1.3.10 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.
Management Switch Card User's Guide
B
1
2
C
Chapter 16 VoIP
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