Voice over Internet Protocol

Voice over Internet Protocol (VoIP) is a general term for a family of transmission technologies for delivery of voice communications over IP networks such as the Internet or other packet-switched networks. Other terms frequently encountered and synonymous with VoIP are IP telephony, Internet telephony, voice over broadband (VoBB), broadband telephony, and broadband phone.
VoIP systems usually interface with the traditional public switched telephone network (PSTN) to allow for transparent phone communications worldwide.
VoIP systems employ session control protocols to control the set-up and tear-down of calls as well as audio codecs which encode speech allowing transmission over an IP network as digital audio via an audio stream. Codec use is varied between different implementations of VoIP (and often a range of codecs are used); some implementations rely on narrowband and compressed speech, while others support high fidelity stereo codecs.

VoIP can be a benefit for reducing communication and infrastructure costs. Examples include:
Routing phone calls over existing data networks to avoid the need for separate voice and data networks
Conference calling, IVR, call forwarding, automatic redial, and caller ID features that traditional telecommunication companies (telcos) normally charge extra for are available for free from open source VoIP implementations such as Asterisk.
Resource links:

• An Introduction to Voice over IP
• Voice over IP
• Voice over IP discussion note
• Voice-over-IP Security
• Voice over Internet Protocol
• A brief guide to Voice over Internet Protocol
• Voice over IP: what it is, why people want it, and where it is going