The use of an IP network, in particular the Internet, to carry verbal conversations. This has been a major growth area in recent years, with several companies offering services that not only allow conversations between their members but also connect to the global telephone network. Indeed, many traditional telephone providers now use VoIP internally. Implementing conversations in real time imposes special constraints because they are particularly sensitive to any delay or lost data. For example, the scope for using buffering to assemble an incoming message from data packets that might arrive out of order, be delayed, be lost and require retransmission, etc., is severely limited. There are now (2019) quite a few standards used to implement VoIP but the most significant are ITU’s H323, which was one of the first and the IETF’s SIP, which has found widespread use.

https://web.archive.org/web/20090318200924/http://iec.org/online/tutorials/h323/index.asp H323 information site

http://tools.ietf.org/html/rfc3261 The SIP specification