H3C XE 200 IP, Operation Manual

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Operation Manual – Process Server Configuration
H3C XE 200/2000 IP PBX
Chapter 4 SIP Overview
4-2
username, session ID, network type, address type, and address elements in the origin
field.
II. User agent
A user agent (UA), or SIP endpoint, is a SIP-supported multimedia session endpoint,
which can be a phone, a gateway, or a router.
III. User agent client
A user agent client (UAC) is a device that creates a new session request. It can be a
calling SIP endpoint or a proxy server forwarding a request to a called endpoint for
example.
IV. User agent server
A user agent server (UAS) is a device that generates response to a SIP request. It can
be a called SIP endpoint or a proxy server receiving a request from a calling endpoint
for example.
V. Proxy server
A proxy server is a device that forwards session requests to a called UA on behalf of a
calling UA (SIP endpoint) and responds to the calling UA on behalf of the called UA.
When the proxy server receives a request from a calling UA, it first requests its registrar
for information on callee location and call policies of caller and callee. If the location
information of the callee is available and the caller is allowed to make the call, the proxy
server then forwards the request to the callee.
VI. Redirect server
A redirect server is a device that directs a calling UA to contact an alternate location for
calling the called UA.
When receiving a request from a calling UA, the redirect server searches for the
location information of the called UA and returns information on a location. 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.
VII. LS
A LS is a device that provides UA information to proxy and redirect servers; it retains UA
information received by a registrar. Normally, LS and registrar are co-located.
VIII. Registrar
A registrar records location information of UAs for proxy servers to retrieve. In some
simple applications, the registrar and the associated proxy server are usually
co-located.