SBC: definition and a practical example
Short answer: An SBC (Session Border Controller) is the device or software at the edge of a VoIP network that controls, protects and adapts the SIP signaling and audio going in and out.
In Portuguese: SBC
What is an SBC?
An SBC (Session Border Controller) is the device that sits on the border between two VoIP networks, usually between the company’s PBX and the carrier’s SIP trunk. Every call coming in or going out passes through it, both the SIP signaling and the RTP audio. It can be a physical appliance, a virtual machine or software on the telephony server itself.
The most common functions of an SBC are:
- Topology hiding: replaces internal IPs with a public address, so the carrier cannot see the company’s network.
- NAT traversal: fixes addresses in SIP and SDP, the job a SIP ALG tries to do and often does badly.
- Security: blocks scans, fraud attempts and message floods.
- Normalization: adjusts headers so equipment from different vendors can understand each other.
- Transcoding: converts between codecs when the two ends have none in common.
- Admission control: limits how many simultaneous calls get through.
Because it sits in the middle of everything, the SBC is also a place where a call can fail: a wrong rule, an exhausted license or an expired certificate blocks traffic.
In practice
An INVITE leaves the PBX and passes through the SBC before reaching the carrier:
| Field | Leaving the PBX | Leaving the SBC |
|---|---|---|
| IP in Contact | 10.0.0.5 | 203.0.113.10 |
| Media IP in SDP | 10.0.0.5 | 203.0.113.10 |
| RTP port | 12000 | 40000 |
| Codecs offered | G.711, G.729 | G.711 |
The carrier only sees the public address and the codec the SBC let through. If port 40000 is blocked on the firewall, the call is answered and stays silent.
How Monitor uses this
There is nothing to install on the SBC. Monitor calls the number from outside, as any customer would, so the test call passes through the SBC exactly like a real call. See SIP trunk monitoring.
Frequently asked questions
Do I need an SBC to have a SIP trunk?
Not always. Many IP PBXs talk directly to the carrier. An SBC becomes worthwhile when the company needs to hide its internal network, control how many calls come in, connect equipment that speaks different SIP dialects or transcode between codecs. Some carriers also put their own SBC in front of the trunk.
Can an SBC cause calls with no audio?
Yes. If the SBC rewrites the SDP with a wrong IP or port, or if the firewall in front of it blocks the RTP port range, the call is answered and the audio does not get through in one or both directions. The signaling shows 200 OK, so only a test that checks the audio reveals the problem.