X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN).

It 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.

A few minor changes, which complemented the proposed specification, were accommodated to enable Larry Roberts to join the agreement.

[9][10][11] Various updates and additions were worked into the standard, eventually recorded in the ITU series of technical books describing the telecommunication systems.

The CCITT appointed a special Rapporteur on packet switching, Halvor Bothner-By, who held an initial meeting in January 1974.

This resulted in a question, to be answered by study group (SG) VII for the next CCITT plenary in 1976, which was “Should the packet-node of operation be provided on public data networks and, if so, how should it be implemented?”.

[14] The second Rapporteur meeting, hosted in Oslo by the Norwegian Telecommunications Administration in November 1974, gathered 24 participants, including representatives of other international organizations (ISO, IFIP, ECMA).

[15] A document submitted by France “with the active support of a number of European administrations” served as “main basis for discussion in this meeting”.

[8]: p39 Starting in January 1975, several bilateral and multilateral meetings took place between network operators having commitments for a packet switching service, in view to draft a common interface specification.

[8]: p39 [16]: p44 In March 1975, Halvor Bothner-By produced a list of recommendations to be created, or simply updated, for a packet switching standard to become possible.

It was used as a framework at a drafting meeting in Ottawa between engineers of the four operators wishing to have a standard as soon as possible in the USA, Canada, France, UK and Japan.

[8]: p40 At the last meeting of the full SG VII before the CCITT plenary of September 1976, the available draft X.25 raised numerous clarification questions and/or and technical objections.

SG VII's chairman Vern MacDonald appointed an editor and provided a meeting facility for the weekend.

To get them in due time, Tony Rybczynski of DATAPAC and Paul Guinaudeau of TRANSPAC spent a full night to handwrite all negotiated amendments, and to assemble them with paste and scissors into clean documents.

[8]: p40 As requested by the USA, an optional datagram service was added to the revised X.25 of 1980, together with an alignment of its link layer, now called LAPB, with a recent evolution of HDLC in ISO.

Examples include Iberpac, TRANSPAC, Compuserve, Tymnet, Telenet, Euronet, PSS, Datapac, Datanet 1 and AUSTPAC as well as the International Packet Switched Service.

[21] Beginning in the early 1990s, in North America, use of X.25 networks (predominated by Telenet and Tymnet)[21] started to be replaced by Frame Relay services offered by national telephone companies.

[clarification needed] 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.

Much of the X.25 system is a description of the rigorous error correction needed to achieve this, as well as more efficient sharing of capital-intensive physical resources.

These calls interconnect "data terminal equipment" (DTE) providing endpoints to users, which looked like point-to-point connections.

The "fast select with restricted response facility" is intermediate between full call establishment and connectionless communication.

It is widely used in query-response transaction applications involving a single request and response limited to 128 bytes of data carried each way.

Now dumb-terminal users could dial into the network's local “PAD” (packet assembly/disassembly facility), a gateway device connecting modems and serial lines to the X.25 link as defined by the X.29 and X.3 standards.

NSAP addressing facility was added in the X.25(1984) revision of the specification, and this enabled X.25 to better meet the requirements of OSI Connection Oriented Network Service (CONS).

In public networks, X.25 was typically billed as a flat monthly service fee depending on link speed, and then a price-per-segment on top of this.

PVCs would have a monthly rental charge and a lower price-per-segment than VCs, making them cheaper only where large volumes of data are passed.

The network may allow the selection of the maximal length in range 16 to 4096 octets (2n values only) per virtual circuit by negotiation as part of the call setup procedure.

[50] Several major protocol versions of X.25 exist:[51] The X.25 Recommendation allows many options for each network to choose when deciding which features to support and how certain operations are performed.

In spite of protocol conformance testing, this often lead to interworking problems when initially attaching an appliance to a network.

The world-wide public data networks based on X.25 helped grow IP as a protocol riding on top.