Session Border Controller

A Session Border Controller (SBC) is a device used in some VoIP networks. SBCs are put into the signalling path between calling and called party. The SBC acts as if it was the called VoIP phone and places a second call to the called party. The effect of this behaviour is that not only the signalling traffic, but also the media traffic (voice, video etc) crosses the SBC. Without an SBC, the media traffic travels directly between the VoIP phones. Private SBCs are used along with firewalls to enable VoIP calls to and from a protected enterprise network. Public VoIP service providers use SBCs to allow the use of VoIP protocols from private networks with internet connections using NAT.

Additionally, some SBCs can also allow VoIP calls to be set up between two phones using different VoIP signalling protocols ( SIP, H.323, Megaco/MGCP, etc...) as well as perfoming transcoding of the media stream when different codecs are in use. Many SBCs also provide firewalls features for VoIP traffic (denial of service protection, call filtering, bandwidth management, etc...).

In contrast to conventional phone systems, the OSI layers of a VoIP-based network need not to be operated by a single company. A VoIP user may purchase her internet access from one internet service provider and her VoIP service from a second company. Some telecom companies use SBCs to revert this characteristic in order to retain their current business model when offering public VoIP services.

The concept of the SBC is controversial.

• SBCs can extend the length of the media path (the way of media packets through the network) significantly. A long media path is undesirable, as it increases the delay of voice packets (especially if the SBC implements transcoding) and the probability of packet loss. Both effects deteriorate the voice/video quality.
• SBCs break the end-to-end transparency. VoIP phones can't use new protocol features unless they are understood by the SBC. End-to-End encryption can't be used if the SBC does not have the key.
• NAT traversal can be done without SBCs if the VoIP phones support protocols like STUN, TURN or Universal plug-and-play.

SBCs may be required by regulatory bodies to enforce lawful interception (eg. CALEA in the US).