A session border control is like a firewall specifically designed for VoIP. It is a hardware device or a software application that governs call admission to a network at the border of said network.

A session, in this context, means a call, but an SBC also governs other real-time communication, such as video or data. and is the same session in session initiation protocol (SIP). The border refers to the border where the private network meets the public Internet. As part of controlling the border, the SBC can filter calls, manage bandwidth, and protect against malware, such as viruses.

An SBC is different from a traditional firewall. A firewall looks at packets and either grants them admission to the private network by opening up what’s called a “port” for data to go through, or it denies access and deletes the packets. VoIP traffic requires more open ports, and that leaves the network vulnerable to attack. The SBC monitors the traffic to make sure that VoIP and other important traffic gets through, but malware does not.

Session Border Controls perform the following functions:
– Connectivity: NAT Transversal and Ipv4 to Ipv6
– Call tracking, and possible toll collection
– Fault Tolerance to ensure 99.999 availability
– Media bridging: Voice and Video calls and faxes
– Regulatory: Prioritizing emergency calls and performing legal wiretaps
– QoS: Resource allocation and call admission
– Security: Protecting against DDOS and SPIT (unauthorized traffic)

There are several different places an SBC may be placed:
– At the border between a provider and the customer. This is the most common place for an SBC.
– On the border of two different providers that have a peering agreement, called a Network to Network Interface.
– Within a provider that offers virtual private networks. The SBC routes calls within each VPN
– Within a private network. If two high-bandwidth sites are linked by a low-bandwidth backbone, the SBC ensures the backbone is not overwhelmed by VoIP traffic

Session border controllers grow with the size of the business. The hardware for an SBC made to handle a dozen calls can be around $2000, but enterprise-grade SBCs made to handle thousands of calls, can cost tens of thousands of dollars. It should come as no surprise that the market for SBCs is a hotly contested one, with major names in hardware putting out white papers on the importance of session border controllers. The SBC is the guard who watches the watchmen.

Additional Reading
What are E-SBCs?
IPv6 Migration Plan: 5 Facts to Know
VoIP Uptime: What Does 99.99% Really Mean?