ZyXEL Communications ADSL VoIP IAD with 802.11g Wireless 2602HW Series - Manual (Page 133)

Prestige 2602HW Series User’s Guide
Chapter 9 Introduction to VoIP 133
9.2.3.1 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 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 server to receive the call.
Figure 47 SIP User Agent Server
9.2.3.2 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.