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SIP-Session Initiation Protocol is the real-time communication protocol for VOIP-Voice over IP. SIP has been expanded to support video and instant-messaging applications. SIP uses a text-based programming language designed to perform basic call-control tasks, such as session call set up and tear down and signaling for features such as call hold, caller ID, conferencing and call transferring. However with SIP, the intelligence for call setup and features resides on the SIP device or UA-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 with SS7-Signaling System 7 in PSTN-Public Switched Telephone Network telephone switching and H.323 or Media Gateway Control Protocol in IP telephony providing call control/routing. For more go to www.sipforum.org
Some SIP servers/PBX systems may use a special type of server called a B2BUA-Back-to-Back User Agent. Like a central server or PBX acts as a central signaling connection and supervision system (or mimic) to “act like” a receiver to one end and the caller on the other), B2BUA’s may be able to provide gateway functions to connect calls to the PSTN-Public Switched Telephone network or convert to other protocols such as Cisco’s Skinny, H.323 or proprietary call signaling.
While B2BUA is a migration strategy for some PBX systems. In many cases SIP servers based on the evolving standard may not be able to provide all the features even though comply with IETF-RFC-Request For Comments. This is especially important for a multi-vendor SIP environment. In addition, the “open” programming capability of SIP makes it very possible that the way a feature like automatic callback is programmed for one customer could mean group call in another SIP system.
The devices in a SIP network generally consist of:
UA-User Agent is a logical (not necessarily physical) function that initiates or responds to SIP communications. A UA can be either a client or server that maintains a “session” status in the SIP communications.
UAC-User Agent Client is also a logical and physical function that initiates or responds to SIP communications.
UAS-User Agent Server is a logical and physical function that initiates or responds to SIP communications.
Proxy Server acts like (hence the term proxy) as an intermediate server that is responsible for forwarding SIP requests to the destination UAS-User Agent Server on behalf of the UAC-User Agent Client.
Redirect Server is an optional SIP component that does not route SIP messages. Redirect Server returns a redirect (via 3xx class address change in routing) to UA-User Agent or UAC-User Agent Client for direct routing (SIP is designed for end-to-end signaling without intervention by a server).
Registrar Server is a user agent server that accepts SIP REGISTER requests and updates.
B2BUA-Back-to-Back User Agent acts like a central PBX connection server processing incoming and outgoing messaging between other user agent servers. The B2BUA maintains “answer supervision” (overseeing) the dialog state and participates in all dialog communications.