User's Manual

Chapter 21 Voice 183
Authorization Requirements
SIP registrations (and subsequent SIP requests) r
equire a username and password for
authorization. These credentials are validated via a challenge / response system using the HTTP
digest mechanism (as detailed in RFC 3261, "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 origin
ates 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 113 SI
P User Agent
SIP Proxy Server
A SIP proxy server receives requests from clie
nts 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 T
he client device (A in the figure) sends a call invitation to the SIP proxy server (B).