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Call connection
Introduction to SIP
The Session Initiation Protocol (SIP) is an application layer control protocol that can establish, modify,
and terminate multimedia sessions such as IP phone calls, multimedia session and multimedia
conferences. It is the core component in the multimedia data and control architecture of the IETF
(RFC 3261).
SIP is responsible for signaling control in IP networks and communication with soft switch platforms.
The intent is to build a next generation value-added service platform to deliver better value-added
services to telecom carriers, banks, and financial organizations.
SIP is used for initiating sessions. It sets up and terminates a multimedia session involving a group of
participants and dynamically adjusts and modifies session characteristics such as required session
bandwidth, media type (voice, video, or data), media encoding/decoding format, and
multicast/unicast. SIP is based on text encoding, and it is constructed by taking the mature protocol
HTTP as a model. Easy to extend and implement, it is suitable for implementing Internet-based
multimedia conference systems.
Terminology
Multimedia session
According to RFC 2327, a multimedia session is a set of multimedia senders and receivers and the
data streams flowing from senders to receivers. A multimedia conference is an example of a
multimedia session.
A session is identified by a set of username, session ID, network type, address type, and address.
User agent
A user agent (UA), or a SIP endpoint, is a SIP-enabled multimedia session endpoint. Usually, a
SIP-enabled router serves as a SIP UA.
There are two types of UAs: user agent client (UAC) and user agent server (UAS). To make a call, a
SIP endpoint needs to process the SIP request as a UAS and initiate the SIP request as a UAC.
A UAC is a device that initiates a session request. It can be a calling SIP endpoint or a proxy server
forwarding a request to a called endpoint for example.
A UAS is a device that generates a 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.
Proxy server
A proxy server is a device that forwards session requests to a called UA on behalf of a calling UA (a
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 location server for
information on called UA location and call policies of calling UA and called UA. If the location
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
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