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X.25 is the name given to a suite of protocols used for packet-switched wide area network communication. Defined by the International Telegraph and Telephone Consultative Committee in 1976, X.25 had the original purpose of carrying voice signals over analog telephone lines.
X.25 is the oldest packet-switching technique available and was commonly used before the Open System Interconnection (OSI) reference model became standard. Originally developed for use in the 1970s and used widely in the 1980s, X.25 has since fallen out of favor, having been replaced by less complex protocols such as Internet Protocol. Today, it is mostly relegated to ATMs and credit card verification networks.
X.25 protocols work at the physical, data link and network layers of the network. Each X.25 packet contains 128 bytes of data. The protocols themselves cover such tasks as packet assembly at the source, delivery, disassembly at destination, error-checking and retransmission in case of errors.
X.25 devices fall into three common categories: