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What Is a Session Border Controller?
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Most people who make or take business calls have never heard of a session border controller. Yet in many organizations, this piece of technology quietly sits at the edge of the phone network, protecting it, connecting it to the outside world, and making sure calls work properly.
The Definition
A session border controller (SBC) is a device or software that sits at the border between a business's internal phone network and external networks, such as a phone provider or the internet. It manages and protects the voice and video sessions, meaning calls, that pass through that border.
A useful way to think about it is as a combination of a security guard, a translator, and a traffic controller for calls.
Why SBCs Exist
With internet-based calling, calls are carried as data using SIP (session initiation protocol) to set them up and other protocols to carry the audio. This brings flexibility, but also challenges:
- Security threats. Phone systems connected to the internet can be targeted by attackers.
- Compatibility problems. Different phone systems and providers sometimes implement SIP slightly differently, causing calls to fail or features to break.
- Network complications. Firewalls and network address translation can interfere with call audio.
- Quality control. Businesses need to manage call quality and capacity.
SBCs were developed to handle these challenges at the point where calls enter and leave a network.
What an SBC Does
Security
The SBC acts as a specialized firewall for voice traffic. It can:
- Block unauthorized access to the phone system.
- Protect against attacks, such as attempts to overwhelm the system with traffic.
- Help prevent toll fraud, where criminals try to use a business's phone system to make expensive calls.
- Encrypt calls, using technologies such as TLS for signaling and SRTP for audio, so calls can't easily be intercepted.
Hiding the internal network
The SBC can hide details of a business's internal phone network from the outside world, a function known as topology hiding. Outside parties see the SBC, not the systems behind it, which reduces the information available to attackers.
Solving network address problems
Many networks use network address translation (NAT), which can stop call audio from reaching its destination properly, causing one-way audio or no audio. SBCs handle this so calls connect correctly.
Interoperability
Different phone systems and providers don't always "speak" SIP in exactly the same way. The SBC can adjust and translate SIP messages, a process sometimes called SIP normalization, so systems from different vendors can work together.
Media handling
The SBC can manage the audio itself, including converting between different audio formats, or codecs, when two systems don't support the same one. This is called transcoding.
Call routing
The SBC can decide where calls should go, for example sending calls to different providers based on destination, cost, or availability, and switching to a backup provider if one fails.
Managing capacity
The SBC can limit how many calls happen at once, a function called call admission control, to protect network quality and prevent overload.
Monitoring quality
SBCs can measure call quality, tracking factors such as delay, jitter, and packet loss, which helps diagnose problems.
Supporting recording
In many call centers, the SBC sends a copy of call audio to recording systems, often using a standard called SIPREC.
Supporting regulatory requirements
SBCs can help with requirements such as routing emergency calls correctly.
Types of SBC
Hardware SBCs are physical devices installed in a data center or office.
Virtual SBCs are software running on servers or virtual machines.
Cloud SBCs are run by providers as part of a hosted service.
SBCs are also often described by where they're used:
Enterprise SBCs protect a business's own network where it connects to providers.
Carrier SBCs are used by phone providers at the edges of their networks, where they connect with customers and other providers.
Who Needs an SBC?
Businesses that connect their own phone systems to SIP trunks or the internet often use SBCs, particularly larger organizations and call centers with high call volumes and strict security needs.
Businesses using fully cloud-based phone systems may not need their own SBC, because their provider runs SBCs within its own network. Even then, SBCs are still at work, just managed by someone else.
Why It Matters for Call Centers
For call centers, the SBC contributes to:
- Security, protecting customer calls and preventing fraud.
- Reliability, keeping calls flowing and switching to backups when needed.
- Call quality, managing capacity and diagnosing problems.
- Compatibility, allowing different systems and providers to work together.
- Compliance, supporting encryption and call recording.
The Gatekeeper of Business Calls
The session border controller is one of the less visible parts of business calling, but it plays a vital role. By guarding the edge of the phone network, translating between different systems, and managing the flow of calls, it helps make sure business calls are secure, reliable, and clear. When everything works, no one notices it. That's exactly the point.