H3C Technologies H3C MSR 50 - Manual (Page 666)

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information of the called UA is available and the calling UA is allowed to make the call, the proxy server
then forwards the request to the called UA.
Redirect server
A redirect server sends a new connection address to a requesting client.
For example, when it receives a request from a calling UA, the redirect server searches for the location
information of the called UA and returns the location information to the UA. This location can be that of
the called UA or another proxy server, to which the UA can initiate the session request again. The
subsequent procedure is the same as that for calling a called UA directly or for calling a proxy server.
Location server
A location server is a device that provides UA information to proxy and redirect servers. It retains UA
information received by a registrar. The location server and registrar can locate on the same server as
two logical components or locate on different devices.
Registrar
A registrar receives UAs' registrations. The registration information (for example, the local telephone
number) is usually stored on the location server for future retrieval. The location server and the registrar
are both logical components and are usually co-located.
Functions and features of SIP
Functions
SIP supports five basic functions:
• Locating 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 improve its user location
service by using other location services provided by the DNS and LDAP.
• Determining user availability—Makes sure whether a called endpoint can participate in a session.
SIP supports multiple address description and addressing styles, SIP-URI (for example, SIP:
[email protected]), Tel-URL (for example, Tel: +1312000), and SIPS-URI (SIPS:
[email protected]). Therefore, 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.
• Determining user capabilities—Determines the media type and media parameters of a called
endpoint. In a message exchange process, each SIP endpoint sends such information in 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 type and media
parameters to be used.
• Managing sessions—Manages sessions by modifying session parameters or terminating sessions.
Features
The following are the features delivered by SIP:
• Open standards. It can accommodate new functions, products, and services introduced by different
service providers.
• Flexible configuration. It accommodates a wide range of dialup, wire, and wireless devices, allows
highly flexible configurations, and can work with other systems.