ZyXEL Communications LTE5366 Series User Manual page 127

Chapter 18 Voice over 3G
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 1122334455@VoIP-provider.com, then "VoIP-provider.com" is the SIP service domain.
SIP Registration
Each LTE5366 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 LTE5366). 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 LTE5366 attempts to register all enabled subscriber ports when it is switched on. When you enable a
subscriber port that was previously disabled, the LTE5366 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 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.
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