SBC stands for Session Border Controller.
An SBC is used in the context of VoIP networks to offer many functionalities:
- NAT traversal
- Policy enforcement
SBCs were originally created as a temporary solution for NAT traversal and since then grew with functionality, encompassing things that exist in gateways for example.
While SBCs are not necessary in WebRTC, there are use cases and deployments of WebRTC where SBCs may be required.