ZyXEL Communications P-2602HWLNI, User Manual, Page: 170 / 496

Chapter 11 Voice
P-2602HWLNI User’s Guide
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11.2.1.2 SIP Service Domain
The SIP service domain of the VoIP service provider is the domain name in a SIP URI. For
example, if the SIP address is
1122334455@VoIP-provider.com , then “VoIP-provider.com” is
the SIP service domain.
11.2.2 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.
11.2.2.1 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 97 SIP User Agent
11.2.2.2 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.