What is VoIP ? :
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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.
Internet telephony refers to communications services — voice, facsimile, and/or voice-messaging applications — that are transported via the Internet, rather than the public switched telephone network (PSTN). The basic steps involved in originating an Internet telephone call are conversion of the analog voice signal to digital format and compression/translation of the signal into Internet protocol (IP) packets for transmission over the Internet; the process is reversed at the receiving end.[1]
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
1974 — The Institute of Electrical and Electronic Engineers (IEEE) published a paper titled "A Protocol for Packet Network Interconnection."[2]
1981 — IPv4 is described in RFC 791.
1985 — The National Science Foundation commissions the creation of NSFNET.[3]
1995 — VocalTec releases the first commercial Internet phone software.[4][5]
1996 —
ITU-T begins development of standards for the transmission and signaling of voice communications over Internet Protocol networks with the H.323 standard.[6]
US telecommunication companies petition the US Congress to ban Internet phone technology.[7]
1997 — Level 3 began development of its first softswitch, a term they coined in 1998.[8]
1999 —
The Session Initiation Protocol (SIP) specification RFC 2543 is released.[9]
Mark Spencer of Digium develops the first open source Private branch exchange (PBX) software (Asterisk).[10]
2004 — Commercial VoIP service providers proliferate.[11]
2005 — OpenSER (later Kamailio and OpenSIPS) SIP proxy server is forked from the SIP Express Router.
2006 — FreeSWITCH open source software is released

VoIP technologies and implementations
Voice-over-IP has been implemented in various ways using both proprietary and open protocols and standards. Examples of technologies used to implement Voice over Internet Protocol include:
H.323
IP Multimedia Subsystem (IMS)
Media Gateway Control Protocol (MGCP)
Session Initiation Protocol (SIP)
Real-time Transport Protocol (RTP)
The Session Initiation Protocol has gained wide-spread VoIP market penetration, while H.323 deployments are increasingly limited to carrying existing long-haul network traffic.[citation needed]
A notable proprietary implementation is the Skype network. Other examples of specific implementations and a comparison between them are available in Comparison of VoIP software.
Articl From wikipedia