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Collaboration, Voice & AV · Intermediate

Back-to-Back User Agent (B2BUA)

A SIP element that terminates one call leg as a UAS and originates a separate leg as a UAC, controlling both.

Definition

A B2BUA sits in the middle of a call and maintains two independent dialogs, which lets it modify headers, normalize messages, hide topology, interwork protocols and enforce policy. CUCM, CUBE and most session border controllers are B2BUAs. Because each leg is independent, issues such as DTMF or codec mismatches can be interworked between legs.

Example

CUBE receives an INVITE from CUCM on one dial peer and creates a new INVITE to the carrier on another dial peer.

Exam tip

Choose B2BUA when a question involves topology hiding, header manipulation or protocol interworking between two legs.

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