Voice over Internet Protocol

Voice over Internet Protocol is a family of standards that permits carrying voice telephony not over dedicated telephony networks, but over Internet Protocol networks that handle both voice and data.

Session Initiation Protocol

The Session Initiation Protocol (SIP), a modern version of session-layer network protocols, is key to deployed VoIP, where SIP may need to traverse a firewall-like function. Conventional firewalls make assumptions about port numbers, but SIP uses a dynamic range. SIP is the dominant protocol found inside the local multimedia border, although it rapidly is becoming the outside standard.

Session Border Controllers

A specialized class of security gateways called Session Border Controllers (SBC) deal with this problem, which are again controlled violations of the end-to-end principle. They terminate the SIP session coming from "inside", and create a new session to the outside. They may have firewalling or other security capabilities optimized for a session layer protocol.

Transcoding

Between those two session termination points, depending on the particular SBC, quite a number of things can happen. There can be deep packet inspection for security or accounting. If the particular codec being used to convert analog voice to digitized [[[packet]]s on the inside is different than the one expected from the outside (e.g., high-bandwidth G.711 versus low-bandwidth G.729A), the SBC can convert -- "transcode" -- although it is always advisable to avoid transcoding. Transcoding adds delay and may decrease quality.

Security

Encrypted voice is a problem unless the SBC is trusted to encrypt, examine plaintext, and encrypt in a new cryptosystem.

Lightweight call processing

An intelligent SBC, in the right topology, can considerably speed the processing of calls in the same part of the IP network)