B2BUA
Back to Back User Agent

A B2BUA is logically positioned between the IMS (IP Multimedia Subsystem) and external networks. It handles all SIP (Session Initiation Protocol) signalling, including session attempts, subscriptions, instant messaging, etc, as well as including signalling where the flows may be forward without B2BUA intervention. The originating Serving CSCF (Call Session Control Function) should route signalling through the B2BUA when the destination network is outside the home IMS. Unfortunately, the originating S-CSCF does not currently have functionality to decide when the call is destined for a 3GPP IMS network or a non 3GPP IMS network. As a consequence, the only solution is for the originating S-CSCF to statically insert the B2BUA for all the signalling that is leaving the home IMS.