X.25

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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 used especially during the 1980s by 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[1] and finalized in a publication known as The Orange Book in 1976.[2]

X.25 is today to a large extent replaced by less complex protocols, especially the Internet protocol (IP) although some telephone operators offer X.25-based communication via the signaling (D) channel of ISDN lines.

Contents

History

X.25 is one of the oldest packet-switched services available. It was developed before the OSI Reference Model.[3] The protocol suite is designed as three conceptual layers, which correspond closely to the lower three layers of the seven-layer OSI model.[4] It also supports functionality not found in the OSI Network Layer.[5][6]

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