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Back-to-back user agent

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A back-to-back user agent (B2BUA) is a logical Session Initiation Protocol (SIP) network element.[1] It resides between both end points of a phone call or communications session and divides the communication session into two call legs and mediates all SIP signaling between both ends of the call, from call establishment to termination. Each call is tracked from beginning to end, allowing the operators of the B2BUA to offer value-added features to the call.

In the originating call leg the B2BUA acts as a user agent server (UAS) and processes the request as a user agent client (UAC) to the destination end, handling the signaling between end points back-to-back. A B2BUA maintains complete state for the calls it handles. Each side of a B2BUA operates as a standard SIP network element as specified in RFC 3261, the official SIP specification.

A B2BUA may provide the following functions:

  • call management (billing, automatic call disconnection, call transfer, etc.)
  • network interworking (perhaps with protocol adaptation)
  • hiding of network internals (private addresses, network topology, etc.)

Often, B2BUAs are implemented in media gateways to also bridge the media streams for full control over the session.

A Signaling gateway, part of a Session Border Controller, is an examples of a B2BUA.

See also

References

  1. ^ RFC 3261, SIP: Session Initiation Protocol, IETF, The Internet Society (2002)
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