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]