ZyXEL Communications EX5501-B0 User Manual page 254

Registration is initiated by the User Agent Client (UAC) running in the VoIP gateway (the Zyxel Device).
The gateway must be configured with information letting it know where to send the REGISTER message,
as well as the relevant user and authorization data.
A SIP registration has a limited lifespan. The User Agent Client must renew its registration within this
lifespan. If it does not do so, the registration data will be deleted from the SIP registrar's database and
the connection broken.
The Zyxel Device attempts to register all enabled subscriber ports when it is switched on. When you
enable a subscriber port that was previously disabled, the Zyxel Device attempts to register the port
immediately.
Authorization Requirements
SIP registrations (and subsequent SIP requests) require 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 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 SIP user agent to
receive the call.
Figure 159 SIP User Agent
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.
The client device (A in the figure) sends a call invitation to the SIP proxy server (B).
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The SIP proxy server forwards the call invitation to C.
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Chapter 22 VoIP
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