VoIP & SIP
SBCAlso known as Session Border Controller
Definition
A session border controller (SBC) is a SIP-aware network element placed at the border between networks, such as a company PBX and a carrier. It controls, secures and normalises SIP signalling and RTP media, providing topology hiding, NAT traversal, protocol fixes, encryption, call admission control and protection against attacks and toll fraud.
What an SBC does
Most SBCs work as a back-to-back user agent (B2BUA): they terminate each call on one side and originate a new one on the other, so they can rewrite anything in between.
- Security: blocks SIP scanners, rate-limits registrations, enforces IP allow-lists and terminates TLS and SRTP.
- Topology hiding: removes internal IP addresses and Via/Record-Route headers from messages leaving the network.
- [NAT traversal](/glossary/nat-traversal): anchors media and fixes private addresses in SDP.
- Interoperability: rewrites headers, number formats and DTMF modes that differ between vendors.
- Transcoding: converts between codecs, for example Opus to G.711.
- Admission control: limits concurrent calls per trunk and routes around failed peers.
Enterprise vs carrier SBCs
Sizing an SBC is mostly about concurrent sessions, registrations and whether media is transcoded, because transcoding consumes far more CPU than simply relaying packets.
Enterprise SBCs sit between a company’s PBX and its SIP trunk provider. Carrier SBCs sit at the edge of a service provider’s network, in front of a softswitch, facing customers and interconnect partners. Software SBCs such as Kamailio or OpenSIPS-based designs are common alongside hardware appliances.
SBCs with mobile gateways
GSMCalls connects to PBXs, SBCs and softswitches as a standard SIP trunk, so an existing SBC can front the gateway trunk exactly as it fronts any carrier. Configure the SBC with the trunk’s transport, codecs and authentication method; see the platform for supported options.
Frequently asked questions
Do I need an SBC for a SIP trunk?
Not always. Many PBXs connect directly. An SBC becomes valuable when you need security at the edge, multiple trunks, interop fixes or NAT handling you cannot do on the PBX.
Is an SBC a firewall?
It complements one. A firewall filters packets; an SBC understands SIP and RTP sessions and can act on individual calls.
What is topology hiding?
Removing internal addresses and routing headers from SIP messages so external parties cannot map your network.