Voice over Internet Protocol
Voice over Internet Protocol is a family of standards that permits carrying voice telephony not over decdicated telephony networks, but over Internet Protocol networks that handle both voice and data.
Voice Digitizing
Real-time transport
Call control
Session Initiation Protocol
The Session Initiation Protocol (SIP) 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)