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Chapter 23 Voice
VMG8825-B Series User’s Guide
245
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 ([email protected] for example) or numbers like a telephone number
([email protected] 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
, then “VoIP-provider.com” is the SIP service domain.
SIP Registration
Each VMG 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 VMG). 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 VMG attempts to register all enabled subscriber ports when it is switched on. When you enable a
subscriber port that was previously disabled, the VMG 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.