A SIP server utilizes SIP to manage telephone calls. SIP or Session Initiation Protocol is application layer signaling protocol that defines initiation, modification and termination of interactive, multimedia communication sessions between users. SIP makes use of elements called proxy servers to help route requests to the user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide features to users. SIP also provides a registration function that allows users to upload their current locations for use by proxy servers. As such SIP requires proxy and registrar network elements to work as a practical service. Although two SIP endpoints can communicate without any intervening SIP infrastructure (which is why the protocol is described as peer-to-peer), this approach is impractical for a public service. A SIP Server integrates proxy and registrar into one server and hence the term "SIP Server".

SIP is fast becoming the de facto standard of VoIP in the place of H.323.

Check out the links provided here for more information about SIP servers.

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