System Components
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SIPxNano IP-PBX Getting Started Guide
A • Session Initiation Protocol (SIP)
System Components
Components in a SIP system send and respond to the messages that set up, establish, and terminate sessions.
Note
SIP components are abstractions. They do not have a one-to-one correspon-
dence to specific VoIP devices.
Generally,
•
User agents are the peers in the VoIP peer-to-peer communication model.
•
Proxy servers monitor sessions and provide services.
•
Redirect, Registry, and Location servers support user mobility by tracking the location of, and redirecting
messages to, session targets.
Table 11. SIP System Components
Component
Description
UAC (User Agent Client)
An application that initiates a request and sends it
in a SIP message.
UAS (User Agent Server)
An application that uses a SIP message to respond
to a request; accepts, redirects, or refuses sessions.
Proxy server
An intermediary program that accepts a SIP mes-
sage, optionally performs services, and then passes
the message on. Acts as both a UAC and a UAS.
Redirect server
An intermediary program that accepts a SIP mes-
sage and then returns a response to the sender. A
Redirect server may or may not perform services.
Registry server
A server that accepts SIP messages from, and reg-
isters the current location of, a user agent. Main-
tains a database of addresses for user agents.
Location server
A server that provides information to Redirect
and Proxy servers about the possible locations of a
session target.