 Session Initiation Protocol (SIP)
SIP is the real-time communication protocol for Voice over IP (VoIP), and it has been expanded to support video and instant-messaging applications. SIP performs basic call-control tasks, such as session set up and tear down and signaling for features such as hold, caller ID and call transferring. Its functions are similar to Signaling System 7 (SS7) in standard telephony and H.323 or Media Gateway Control Protocol in IP telephony.
With SIP, most of the intelligence for call setup and features resides on the SIP device or user agent, such as an IP phone or a PC with voice or instant-messaging software. In contrast, traditional telephony or H.323-based telephony uses a model of intelligent, centralized phone switches with dumb phones.
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