ZyXEL P-2612HNU-F3 User Guide - Page 246
SIP Servers, SIP User Agent, SIP Proxy Server
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Chapter 16 VoIP response system using the HTTP digest mechanism (as detailed in RFC3261, "SIP: Session Initiation Protocol"). 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 104 SIP User Agent A B 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. 246 P-2612HNU(L)-FxF User's Guide