A session border controller (SBC) is a network device that sits between an IP PBX and the public switched telephone network (PSTN). It provides a number of functions, including:
- Security: The SBC acts as a firewall, protecting the IP PBX from unauthorized access.
- NAT traversal: The SBC allows the IP PBX to communicate with devices on the PSTN, even if the IP PBX is behind a firewall or NAT device.
- Call routing: The SBC can route calls between the IP PBX and the PSTN, based on a variety of criteria, such as caller ID, dialed number, and time of day.
- Media transcoding: The SBC can convert media streams between different formats, such as between G.711 and G.729.
SBCs are an essential component of any IP PBX system. They provide a number of important functions that help to ensure the security, reliability, and performance of the system.