User`s guide

Chapter 12 Voice
P-2602HWUDL-D User’s Guide
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12.2.1.2 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 [e-mail]
, then “VoIP-provider.com” is
the SIP service domain.
12.2.1.3 SIP Registration
Each ZyXEL Device is an individual SIP User Agent (UA). To provide voice service, it has a
public IP address for SIP and RTP protocols to communicate with other servers.
A SIP user agent has to register with the SIP registrar and must provide information about the
users it represents, as well as its current IP address (for the routing of incoming SIP requests).
After successful registration, the SIP server knows that the users (identified by their dedicated
SIP URIs) are represented by the UA, and knows the IP address to which the SIP requests and
responses should be sent.
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.
12.2.1.4 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 RFC3261, "SIP: Session Initiation Protocol").
12.2.2 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.
12.2.2.1 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.