Session Border Controller

A session border controller (SBC) is a piece of network equipment or a collection of functions that control real-time session traffic at the signaling, call control, and packet layers as they cross a notional packet-to-packet network border between networks or between network segments. SBCs are critical to the deployment of VoIP networks, because they address the inability of real-time session traffic to cross network address translation (NAT) device or firewall boundaries.
[Source: Heavy Reading – VOIP Session Border Controls]