ZyXEL Communications MSC1024G Series User Manual page 540

Chapter 18 VoIP
18.1.3.1 SIP Registration
Each SIP-enabled VoIP line card in the IES is an individual SIP User Agent (UA). To provide
voice service, it has an 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; see
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
VOP). 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 lifespan value is sent from the SIP server to the
UAC when the UAC first registers. The UAC 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 VOP attempts to register all enabled subscriber ports when it is switched on. When you
enable a subscriber port that was previously disabled, the VOP attempts to register the port
immediately.
18.1.3.2 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").
18.1.3.3 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.
18.1.3.4 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 (johndoe@your-ITSP.com for example) or numbers
like a telephone number (1122334455@VoIP-provider.com for example).
18.1.3.5 SIP Service Domain
The SIP service domain of the VoIP service provider (the company that lets you make phone
calls over the Internet) 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.
18.1.3.6 SIP Servers and Clients
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.
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Section 18.1.3.3 on page 540) are represented by the UA, and knows the IP
MSC1000G/1024G/1224G Series User's Guide