What is a Session Border Controller?

A session border controller (SBC) is the computer system that controls Voice over Internet Protocol (VoIP), video conferencing and other common personal media connections. The SBC has a great deal of influence, as it governs everything from call quality to security and monitors communication processes for regulatory systems. The control exerted by the session border controller system has caused significant controversy between various telecommunication watchdog groups.

The term ‘session border controller’ is made up of three fundamental concepts in Internet-based communication. A session refers to the actual communication process from when the first user dials until the last user disconnects. The border, in this case, refers to any point where one network ends and another begins. Lastly, a controller is an overseer program that monitors a session and makes changes to the system based on the call’s requirements.

In the past, the session border controller system was only responsible for negotiating borders between non-trusting networks. As Internet communication became more common, that began to change. Now, the SBC governs most aspects of communication sessions, both in terms of time and network usage.

Negotiating between systems is still one of the session border controller’s main duties. As communication moves over networks, it needs to comply with that network’s standards. The SBC finds out what those specifications are and routs calls appropriately. This allows the session to avoid problematic systems or work out arrangements in advance.

Security and quality are two other prominent areas of a session border controller’s job. While the session is active, the controller is constantly looking for intrusions in or out of the call. They also protect the systems connected to the session, whether they are participants or carriers, from service attacks and malicious activity. In addition, the SBC monitors call quality and takes steps to rectify quality problems as they occur.

An SBC is also part of the communication regulatory system. A session border controller needs to prioritize emergency calls, just like a standard phone system. They also need to have monitoring capabilities, should law enforcement require such information. Lastly, they monitor the call's length and resource usage for potential billing.

The all-encompassing nature of the session border controller has caused its share of problems. The main one relates to the parties that should be involved in Internet-based communication. The two camps sit on opposite sides of the issue, with very little common ground. One side believes that sessions should only be between the participants in that session. The other side believes that service providers need to keep involved in sessions in order to protect their bandwidth and resource usage.