X.25

X.25 is an ITU-T standard protocol suite for packet-switched wide area network (WAN) communication. An X.25 WAN consists of packet-switching exchange (PSE) nodes as the networking hardware, and leased lines, plain old telephone service connections, or ISDN connections as physical links. X.25 was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts and finalized in a publication known as The Orange Book in 1976. X.25 networks were popular during the 1980s with telecommunications companies and in financial transaction systems such as automated teller machines. However, most users have moved to Internet Protocol (IP) systems instead. X.25 was used up to 2015 (e.g. by the credit card payment industry) and is still used by aviation, purchasable from telecoms companies. X.25 was available in niche application such as Retronet that allows vintage computers to use the internet. X.25 is one of the oldest packet-switched services available. It was developed before the OSI Reference Model. The protocol suite is designed as three conceptual layers, which correspond closely to the lower three layers of the seven-layer OSI model. It also supports functionality not found in the OSI network layer. X.25 was developed in the ITU-T (formerly CCITT) Study Group VII based upon a number of emerging data network projects. Various updates and additions were worked into the standard, eventually recorded in the ITU series of technical books describing the telecommunication systems. These books were published every fourth year with different-colored covers. The X.25 specification is only part of the larger set of X-Series specifications on public data networks. The public data network was the common name given to the international collection of X.25 providers. Their combined network had large global coverage during the 1980s and into the 1990s. Publicly accessible X.25 networks (Compuserve, Tymnet, Euronet, PSS, Datapac, Datanet 1 and Telenet) were set up in most countries during the 1970s and 1980s, to lower the cost of accessing various online services. Beginning in the early 1990s, in North America, use of X.25 networks (predominated by Telenet and Tymnet) started to be replaced by Frame Relay services offered by national telephone companies. Most systems that required X.25 now use TCP/IP, however it is possible to transport X.25 over TCP/IP when necessary.