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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 is a family of protocols that was popular during the 1980s with telecommunications companies and in financial transaction systems such as automated teller machines. X.25 was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts〔CCITT, Study Group VII, ''Draft Recommendation X-25'', March 1976〕 and finalized in a publication known as ''The Orange Book'' in 1976.〔(History of X.25, CCITT Plenary Assemblies and Book Colors )〕 While X.25 has, to a large extent, been replaced by less complex protocols, especially the Internet protocol (IP), the service is still used and available in niche and legacy applications.〔Foregenix (2010) (X.25 within the Payment Card Industry )〕 ==History== 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.〔(ITU-T Recommendation X.28 ).〕〔(ITU-T Recommendation X.3 ).〕 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〔(X-Series recommendations )〕 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)〔 began being replaced with Frame Relay service offered by national telephone companies.〔.〕 Most systems that required X.25 now utilize TCP/IP, however it is possible to transport X.25 over IP when necessary.〔(Running X.25 over tcpip using Cisco routers ), 1 February 2001, visited 4 April 2011〕 X.25 networks are still in use throughout the world. A variant called AX.25 is also used widely by amateur packet radio. Racal Paknet, now known as Widanet, is still in operation in many regions of the world, running on an X.25 protocol base. In some countries, like the Netherlands or Germany, it is possible to use a stripped version of X.25 via the D-channel of an ISDN-2 (or ISDN BRI) connection for low volume applications such as point-of-sale terminals; but, the future of this service in the Netherlands is uncertain. Additionally X.25 is still under heavy use in the aeronautical business (especially in the Asian region) even though a transition to modern protocols like X.400 is without option as X.25 hardware becomes increasingly rare and costly. As recently as March 2006, the United States National Airspace Data Interchange Network has used X.25 to interconnect remote airfields with Air Route Traffic Control Centers. France was one of last remaining countries where commercial end-user service based on X.25 operated. Known as Minitel it was based on Videotex, itself running on X.25. In 2002 Minitel had about 9 million users, and in 2011 it still accounted for about 2 million users in France when France Télécom announced it would completely shut down the service by 30 June 2012.〔 〕 As planned, service was terminated 30 June 2012. There were 800 000 terminals still in operation at the time.〔()〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「X.25」の詳細全文を読む スポンサード リンク
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