User's Manual

Chapter 21 Voice 182
SIP
The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol that handles
the setting up, altering and tearing down of voice and multimedia sessions over the Internet.
SIP signaling is separate from the media for which it handles sessions. The media that is exchanged
during the session can use a different path from that of the signaling. SIP handles telephone calls
and can interface with traditional circuit-switched telephone networks.
SIP Identities
A SIP account uses an identity (sometimes referred to as a SIP address). A complete SIP identity is
called a SIP URI (Uniform Resource Identifier). A SIP account's URI identifies the SIP account in a way
similar to the way an e-mail address identifies an e-mail account. The format of a SIP identity is SIP-
Number@SIP-Service-Domain.
SIP Number
The SIP number is the part of the SIP URI that comes before the “@” symbol. A SIP number can use
letters like in an e-mail address ([e-mail] for example) or numbers like a telephone
number ([e-mail] for example).
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.
SIP Registration
Each Router 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 Router). 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 Router attempts to register all enabled subscriber ports when it is switched on. When you
enable a subscriber port that was previously disabled, the Router attempts to register the port
immediately.