Session Border Controllers (SBCs) are increasingly necessary in modern Voice Over IP environments due to provider requirements and customer needs. These are typically grouped into three primary areas:

  • Security
  • Routing
  • Media conversion (transcoding)
  • Protocol normalization

Let’s look at each of these in turn.

The term “security” encompasses many concepts, but it primarily boils down to preventing unwanted traffic from entering the voice network, and ensuring that that which does enter the network is handled in a safe manner. This is generally accomplished using admission control (only allowing white-listed addresses to be admitted) and various routing safeguards, which further classify the call and determine if it should be handled by the SBC.

Routing refers to the SBC’s ability to examine source and destination of the proposed call and determine where it should be presented on the voice network. Most SBCs allow for routing of calls based on the source IP address, source telephone number, destination IP address, and destination telephone number. Using this identifying information, a call can be routed precisely to the correctly destination endpoint.

Media conversion (transcoding) is an important function of an SBC. It allows two different voice networks, supporting different voice codec protocols, to pass calls to each other. For example, without transcoding, a voice network that supports only the G.711 voice codec would not be able to interoperate with a voice network supporting only the G.729 voice codec. With transcoding, the SBC provides a conversion path between the two voice networks allowing them to pass calls end-to-end.

Protocol normalization refers to the process by which an SBC can ensure two disparate VoIP networks can interoperate with one another. This primarily has to do with the different manners in which the Session Initiation Protocol (SIP) is implemented. While it is a standard, different manufacturers implement it differently. Therefore, there are times a different vendor’s equipment, which is technically SIP compliant, will not be able to complete calls with another vendor’s equipment. An SBC can enable this interoperability by normalizing the SIP messages passed between the different vendors’ products.

This is a very simplistic view of the value a Session Border Controller brings to VoIP networks. There are several other benefits (protocol conversion, attack mitigation, et al.). However, SBCs are becoming an integral part in modern Voice Over IP networks.

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