What is SIP?
It is created in such a way so the SIP telephony clients can use UDP and TCP port 5060 to connect to the SIP servers and other SIP endpoints. The main purpose of SIP is to set up and tear down video or voice calls. In cases where initiation of the session is required, however, SIP is being used as well. Events Subscription, event notification and Terminal mobility are included there. Many SIP related RFCs in reality define the behavior for this solutions. All video/voice communications are made over separated session protocols, similar to RTP.
SIPs main task is to provide a call setup and signaling protocol or IP communications, which support a mega set of the call processing features and functions available in the PSTN (Public Switched Telephone Network). The protocol itself, is defining the focus of signaling and the call setup. Nevertheless it was designed to be able to initiate a building of featured elements of network called User Agents and Proxy Servers. Basically, those features that allow making traditional telephone operations: ringing, dialing, hearing busy signals or ring tones.
SIP powered telephone networks also implement many advanced features of call processing available in SS7 (Signaling System 7), however those two protocols are different. SIP is a P2P (peer to peer) protocol, with very complex architecture and dumb endpoints of central network; due to this it requires really simple core network with distributed intelligence to the network edge. SIP features are used comparing to traditional SS7 ones, which are implemented in the network.
SIP is described as if it has IP roots, rather than traditional telephone industry, but other VoIP signaling protocols exist. The H.323 VoIP protocol is traditionally more relevant to the ITU, SIP has governed and standardized by the IETF. Those two companies endorsed the protocols in fashion.
SIP is similar to HTTP and shares its principles as well. "404 not found" codes, structured request response can be found in both protocols. However SIP is simpler than H.323. Although some do counter that, of course SIP has an aim to be simple, but at its current state it is as complex as H.323.
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