R2511-HP MSR Router Series Voice Configuration Guide(V5)

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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 receiving 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 located on the same device as
two logical components or located 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.
SIP functions and features
Functions
SIP supports the following facets of establishing and terminating multimedia communications:
• 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 enhance its user
location service by using other location services provided by the domain name server (DNS) and
lightweight directory access protocol (LDAP).
• Determining user availability, making sure whether a called endpoint can participate in a session.
SIP supports multiple address description and addressing styles, including SIP-URI (for example, SIP:
123456@172.18.24.11), Tel-URL (for example, Tel: [phone]), and SIPS-URI (SIPS:
123456@172.18.24.11). 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.
• Determining user capabilities, that is, the media type and media parameters of a called endpoint.
In a message exchange process, each SIP endpoint sends 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 type and media
parameters to be used.
• Managing sessions by modifying session parameters or terminating sessions.
Features
SIP delivers the following features:
• Open standards—Can accommodate new functions, products, and services introduced by different
service providers.
• Flexible configuration—Accommodates a wide range of dialup, wire, and wireless devices, allows
highly flexible configurations, and can work with other systems.