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Session Initiation Protocol

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PCQ Bureau
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Designed by IETF (Internet Engineering Task Force), SIP (Session Initiation Protocol) is used for making sessions on an IP network. These sessions can support data varying from a two-way phone call to a collaborative multimedia session. It is a general-purpose protocol and sort of resembles HTTP and SMTP. It simply initiates, terminates and modifies sessions. Thus, SIP can integrate or use various protocols for a specific purpose as voice-enriched e-commerce, instant messaging, etc. 

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How it works



SIP works on a client-server approach. It has four functions: establishing connection with the user, determining compatibility features for various sessions, creating a management mechanism and changing features of a session in real time. For this, SIP uses three entities: User Agents, SIP Intermediate Servers (Proxy and/or Redirect servers) and SIP Register.

User Agent, Intermediate Server

and Register are the three components that make up SIP’s architecture 

The User Agents work as clients for sending requests and act as servers while responding to them. A User Agent can also connect to other User Agents directly or through a SIP Proxy Server and/or a SIP Redirect Server. 

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The SIP Proxy and Redirect servers are also known as SIP Intermediate Servers and are used to forward requests from various User Agents on a network. These requests can be forwarded to a User Agent or another SIP Intermediate Server.



SIP Register receives registration information from User Agents and saves it in a location service using a Non-SIP protocol and informs the User Agent. 

Why SIP?



VoIP tools prefer SIP because of compatibility with different network architectures and scenarios. SIP allows telephony to take the shape of a Web application and, hence, can be integrated into other Internet services. The service providers can easily apply convergence of voice and multimedia services by using the SIP toolkit.

SIP has been designed for the reusability of existing protocols. The Session Description Protocol helps SIP by conveying the session information and describing the media type. SIP functions can also be bound to other protocols and applications like Web browsers.

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The registration of a

User Agent involves storing the registration information on a location service that uses a Non-SIP protocol 

Unlike the H.323 protocol, which is actually a suite of protocols, SIP does not try to do everything by itself. Instead, it integrates various protocols to get the job done and controls these protocols for sessions. This proves that SIP was designed to be a modular

component that works well with existing protocols for accomplishing tasks.

Varun Sharma

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