A packet-switching protocol for wide area network (WAN) connectivity that uses a public data network (PDN) that parallels the voice network of the Public Switched Telephone Network (PSTN). The current X.25 standard supports synchronous, full-duplex communication at speeds up to 2 Mbps over two pairs of wires, but most implementations are 64-Kbps connections via a standard DS0 link.

X.25 was developed by common carriers in the early 1970s and approved in 1976 by the CCITT, the precursor of the International Telecommunication Union (ITU), and was designed as a global standard for a packet-switching network. X.25 was originally designed to connect remote character-based terminals to mainframe hosts. The original X.25 standard operated only at 19.2 Kbps, but this was generally sufficient for character-based communication between mainframes and terminals.

Because X.25 was designed when analog telephone transmission over copper wire was the norm, X.25 packets have a relatively large overhead of error-correction information, resulting in comparatively low overall bandwidth. Newer WAN technologies such as frame relay, Integrated Services Digital Network (ISDN), and T-carrier services are now generally preferred over X.25. However, X.25 networks still have applications in areas such as credit card verification, automatic teller machine transactions, and other dedicated business and financial uses.

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