Simple definition
A session border controller secures and manages SIP signaling and media at VoIP network boundaries.
Purpose and use
An SBC, Session Border Controller, controls VoIP communication at domain boundaries, such as between a PBX and a provider.
How does it work?
Depending on the product, it can terminate dialogs, adapt signaling, enforce access policy and relay or process media.
Example and verification
For a call crossing an SBC, examine both legs: identifiers, addresses and parameters may be rewritten between segments.
Points to watch
Its presence does not automatically guarantee security or compatibility with every codec. Verify actual media and encryption configuration.
In short
An SBC provides an interconnection and control point whose rules must match intended uses and authorized partners.