Session border controllers: what an SBC does

An SBC is a controlled boundary for real-time communications. It can separate address spaces, enforce policy, normalise signalling, anchor media, protect the PBX, and provide interoperability between different networks.

Common SBC functions

• SIP topology hiding and message validation
• Registration and authentication policy
• NAT traversal and RTP anchoring
• Number and header normalisation
• Codec negotiation and transcoding
• Rate limiting, fraud controls, and access lists
• Failover and route selection
• Call admission control and quality statistics

Design questions

Place the SBC where it can see the relevant signalling and media, define which side owns number formatting, document trusted peers, and decide whether media should remain local or be anchored centrally. A poorly placed SBC can add delay, hide useful evidence, or become a single point of failure.

Related: SIP trunking, RTP ranges, and NAT troubleshooting.