3Com MSR 50 Series Configuration Manual page 2369

Registrar
A registrar records the location information of UAs for proxy servers to retrieve. In
some simple applications, the registrar and the associated proxy server are usually
co-located.
Functionality and Functions
Features of SIP
SIP supports five facets of establishing and terminating multimedia
communications:
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Features
The following are the features delivered by SIP:
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Locating users or called SIP endpoints, the most powerful function of SIP. For
this purpose, SIP can use the registration information of SIP endpoints on the
registrar. In addition, it can enhance its user location service by using the
location server provided by domain name server (DNS) or lightweight directory
access protocol (LDAP).
Identifying user availability, making sure whether a called endpoint can
participate in a session. SIP supports multiple address description and
addressing styles, including username@hostname and called number@PSTN
gateway address and telephone number (such as Tel: 01012345678). Thus, a
SIP caller can identify whether a callee is attached to a PSTN network by callee's
address, and then initiate and set up the call to the callee through the gateway
connected to the PSTN.
Identifying user capabilities, or the media and media parameters to be used by
a called endpoint. In a message exchange process, each SIP endpoint carries
such information in transmitted messages so that all other participants can
learn about its capabilities.
Setting up a session, or session parameters, at both callee and caller sides. Two
parties can select the appropriate capabilities for session setup through
negotiation about media and media parameters to be used.
Managing sessions by modifying session parameters or terminating sessions.
Open standards. It can accommodate new functions, products, and services
introduced by different service providers.
Flexible configurations. It accommodates a wide range of dialup, wire, and
wireless devices, allows highly flexible configurations, and can work with other
systems.
Scalable system. The system allows expansion as enterprises grow.
Support to remote users. With SIP, an enterprise network can extend to all its
users, wherever they are.
Competitive advantage potentials. More SIP-based services are emerging.
Consistent communication method. Management becomes easier as the result
of consistency in dialup mode and system access method used by branches,
SOHOs, and traveling personnel.
Quick launch. The system can be updated quickly to accommodate new
branches and personnel, as well as changes resulted from job rotation or
relocation.
Introduction to SIP 2369