Standard protocol suite for mailboat switched wide area network ( WAN ) communication

Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for terminals operating in the packet mode and connected to public data networks by dedicated circuit
Status In force
Year started 1976
Latest version (10/96)
October 1996
Organization ITU-T
Committee Study Group VII
Domain networking

X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks ( WAN ). It was in the first place 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. [ 1 ] [ 2 ] This makes it one of the oldest packet-switching communication protocols available ; it was developed several years before IPv4 ( 1981 ) and the OSI Reference Model ( 1984 ). [ 3 ] The protocol cortege is designed as three conceptual layers, which correspond closely to the lower three layers of the seven-layer OSI model. [ 4 ] It besides supports functionality not found in the OSI network level. [ 5 ] [ 6 ]

Networks using X.25 were popular during the late 1970s and 1980s with telecommunications companies and in fiscal transaction systems such as automatize teller machines. An X.25 WAN consists of packet-switching exchange ( PSE ) nodes as the network hardware, and leased lines, plain previous telephone service connections, or ISDN connections as forcible links. however, most users have moved to Internet Protocol ( IP ) systems rather. X.25 was used up to 2015 ( e.g. by the credit menu payment industry ) [ 7 ] and is calm used by air travel, purchasable from telecommunication companies. [ 8 ] X.25 was besides available in niche applications such as Retronet that leave vintage computers to use the Internet .


The CCITT ( late ITU-T ) Study Group VII began developing a criterion for packet-switched data communication in the mid-1970s based upon a number of emerging data network projects. Participants in the design of X.25 included engineers from Canada, France, Japan, the UK, and the USA representing a desegregate of national PTTs ( France, Japan, UK ) and secret operators ( Canada, USA ). In finical, the work of Rémi Després, contributed importantly to the standard. A few minor changes, which complemented the proposed stipulation, were accommodated to enable Larry Roberts to join the agreement. [ 9 ] [ 10 ] [ 11 ] Various updates and additions were worked into the standard, finally 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 depart of the larger set of X-Series. [ 12 ] [ 13 ] Publicly accessible X.25 networks, normally called populace data networks, were set up in most countries during the late 1970s and 1980s to lower the cost of accessing versatile on-line services. Examples include Iberpac, TRANSPAC, Compuserve, Tymnet, Telenet, Euronet, PSS, Datapac, Datanet 1 and AUSTPAC arsenic well as the International Packet Switched Service. Their combine net had large ball-shaped coverage during the 1980s and into the 1990s. [ 14 ] Beginning in the early 1990s, in North America, use of X.25 networks ( predominated by Telenet and Tymnet ) [ 14 ] started to be replaced by Frame Relay services offered by national telephone companies. [ 15 ] Most systems that required X.25 nowadays use TCP/IP, however it is possible to transport X.25 over TCP/IP when necessity. [ 16 ] X.25 networks are placid in use throughout the world. A variant called AX.25 is used widely by amateur packet radio. Racal Paknet, now known as Widanet, remains in operation in many regions of the earth, running on an X.25 protocol base. In some countries, like the Netherlands or Germany, it is possible to use a strip interpretation 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. X.25 is silent used in the aeronautical business ( particularly in Asia ) even though a conversion to modern protocols like X.400 is without option as X.25 hardware becomes increasingly rare and costly. [ clarification needed ] As recently as March 2006, the United States National Airspace Data Interchange Network has used X.25 to interconnect distant airfields with vent route traffic dominance centers. France was one of the last stay countries where commercial end-user serve 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 accounted for about 2 million users in France when France Télécom announced it would shut down the service by 30 June 2012. [ 17 ] As planned, military service was terminated 30 June 2012. There were 800,000 terminals in operation at the prison term. [ 18 ]


The general concept of the X.25 was to create a universal and global packet-switched network. much of the X.25 system is a description of the rigorous error correction needed to achieve this, ampere well as more efficient communion of capital-intensive physical resources. The X.25 specification defines only the interface between a subscriber ( DTE ) and an X.25 network ( DCE ). X.75, a protocol very like to X.25, defines the interface between two X.25 networks to allow connections to traverse two or more networks. X.25 does not specify how the network operates internally – many X.25 net implementations used something very similar to X.25 or X.75 internally, but others used quite different protocols internally. The ISO protocol equivalent to X.25, ISO 8208, is compatible with X.25, but additionally includes provision for two X.25 DTEs to be directly connected to each other with no network in between. By separating the Packet-Layer Protocol, ISO 8208 permits operation over extra networks such as ISO 8802 LLC2 ( ISO LAN ) and the OSI data link layer. [ 19 ] X.25 primitively defined three basic protocol levels or architectural layers. In the original specifications these were referred to as levels and besides had a level total, whereas all ITU-T X.25 recommendations and ISO 8208 standards released after 1984 refer to them as layers. [ 20 ] The layer numbers were dropped to avoid confusion with the OSI Model layers. [ 1 ]

  • Physical layer: This layer specifies the physical, electrical, functional and procedural characteristics to control the physical link between a DTE and a DCE. Common implementations use X.21, EIA-232, EIA-449 or other serial protocols.
  • Data link layer: The data link layer consists of the link access procedure for data interchange on the link between a DTE and a DCE. In its implementation, the Link Access Procedure, Balanced (LAPB) is a data link protocol that manages a communication session and controls the packet framing. It is a bit-oriented protocol that provides error correction and orderly delivery.
  • Packet layer: This layer defined a packet-layer protocol for exchanging control and user data packets to form a packet-switching network based on virtual calls, according to the Packet Layer Protocol.

The X.25 model was based on the traditional telephone concept of establishing reliable circuits through a shared net, but using software to create “ virtual calls “ through the network. These calls interconnect “ data terminal equipment ” ( DTE ) providing endpoints to users, which looked like point-to-point connections. Each end point can establish many distinguish virtual calls to different endpoints. For a brief menstruation, the specification besides included a connectionless datagram servicing, but this was dropped in the following revision. The “ fast choose with restricted response adeptness ” is intermediate between wide call establishment and connectionless communication. It is wide used in query-response transaction applications involving a single request and reaction limited to 128 bytes of data carried each room. The datum is carried in an extend address request mailboat and the reply is carried in an cover field of the call disapprove package, with a association never being in full established. close related to the X.25 protocol are the protocols to connect asynchronous devices ( such as dense terminals and printers ) to an X.25 network : X.3, X.28 and X.29. This functionality was performed using a packet assembler/disassembler or PAD ( besides known as a triple-X device, referring to the three protocols used ) .

relation to the OSI Reference Model.

Although X.25 predates the OSI Reference Model ( OSIRM ), the physical layer of the OSI model corresponds to the X.25 physical layer, the data link layer to the X.25 data link layer, and the network layer to the X.25 packet layer. [ 13 ] The X.25 data link layer, LAPB, provides a reliable data path across a data link ( or multiple parallel data links, multilink ) which may not be authentic itself. The X.25 packet layer provides the virtual shout mechanisms, running over X.25 LAPB. The packet layer includes mechanisms to maintain virtual calls and to signal data errors in the event that the data link layer can not recover from data transmittance errors. All but the earliest versions of X.25 include facilities [ 21 ] which provide for OSI net layer Addressing ( NSAP address, see below ). [ 22 ]

User device patronize.

Televideo terminal model 925 made around 1982 X.25 was developed in the era of computer terminals connecting to host computers, although it besides can be used for communications between computers. rather of dialing directly “ into ” the server calculator – which would require the host to have its own pool of modems and phone lines, and require non-local callers to make long-distance calls – the host could have an X.25 association to a network service provider. nowadays 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. Having connected to the PAD, the dumb-terminal user tells the PAD which server to connect to, by giving a phone-number-like address in the X.121 address format ( or by giving a master of ceremonies name, if the service supplier allows for names that map to X.121 addresses ). The PAD then places an X.25 bid to the server, establishing a virtual call. note that X.25 provides for virtual calls, sol appears to be a circumference switched network, flush though in fact the data itself is package switched internally, alike to the way TCP provides connections even though the underlying data is package switched. Two X.25 hosts could, of course, call one another directly ; no PAD is involved in this case. In theory, it does n’t matter whether the X.25 caller and X.25 address are both connected to the lapp mailman, but in drill it was not always possible to make calls from one carrier to another. For the determination of flow-control, a skid window protocol is used with the nonpayment window size of 2. The acknowledgements may have either local anesthetic or end to end significance. A D spot ( Data Delivery bit ) in each data packet indicates if the transmitter requires end to end acknowledgment. When D=1, it means that the recognition has end to end meaning and must take place lone after the distant DTE has acknowledged receipt of the data. When D=0, the network is permitted ( but not required ) to acknowledge before the outback DTE has acknowledged or even received the data. While the PAD serve defined by X.28 and X.29 specifically supported asynchronous character terminals, PAD equivalents were developed to support a wide-eyed range of proprietorship intelligent communications devices, such as those for IBM System Network Architecture ( SNA ) .

mistake control.

Error convalescence procedures at the mailboat layer assume that the data link layer is creditworthy for retransmitting data received in mistake. Packet layer error handling focuses on resynchronizing the information menstruate in calls, ampere well as clearing calls that have gone into unrecoverable states :

  • Level 3 Reset packets, which re-initializes the flow on a virtual call (but does not break the virtual call).
  • Restart packet, which clears down all virtual calls on the data link and resets all permanent virtual circuits on the data link.

Addressing and virtual circuits.

An X.25 modem once used to connect to the german Datex-P net X.25 supports two types of virtual circuits ; virtual calls ( VC ) and permanent virtual circuits ( PVC ). virtual calls are established on an as-needed basis. For exemplar, a VC is established when a call is placed and torn down after the call is complete. VCs are established through a name establishment and clear procedure. On the other hand, permanent wave virtual circuits are preconfigured into the network. [ 23 ] PVCs are rarely tear toss off and frankincense provide a give connection between end points. VC may be established using X.121 addresses. The X.121 cover consists of a three-digit data country code ( DCC ) plus a network digit, together forming the four-digit datum network identification code ( DNIC ), followed by the national terminal count ( NTN ) of at most ten digits. Note the use of a individual network digit, apparently allowing for merely 10 network carriers per country, but some countries are assigned more than one DCC to avoid this limitation. Networks much used fewer than the wax NTN digits for spread-eagle, and made the spare digits available to the subscriber ( sometimes called the sub-address ) where they could be used to identify applications or for further route on the subscribers networks. NSAP addressing facility was added in the X.25 ( 1984 ) revision of the stipulation, and this enabled X.25 to better meet the requirements of OSI Connection Oriented Network Service ( CONS ). [ 24 ] Public X.25 networks were not required to make use of NSAP cover, but, to support OSI CONS, were required to carry the NSAP addresses and other ITU-T specified DTE facilities transparently from DTE to DTE. [ 25 ] Later revisions allowed multiple addresses in addition to X.121 addresses to be carried on the same DTE-DCE interface : Telex addressing ( F.69 ), PSTN addressing ( E.163 ), ISDN addressing ( E.164 ), Internet Protocol addresses ( IANA ICP ), and local anesthetic IEEE 802.2 MAC addresses. [ 26 ] PVCs are permanently established in the net and therefore do not require the use of addresses for call frame-up. PVCs are identified at the subscriber interface by their legitimate channel identifier ( see below ). however, in practice not many of the national X.25 networks supported PVCs. One DTE-DCE interface to an X.25 network has a utmost of 4095 logical channels on which it is allowed to establish virtual calls and permanent virtual circuits, [ 27 ] although networks are not expected to support a fully 4095 virtual circuits. [ 28 ] For identifying the transmit to which a packet is associated, each package contains a 12 bit coherent channel identifier made up of an 8-bit coherent duct number and a 4-bit logical channel group act. [ 27 ] Logical channel identifiers remain impute to a virtual circuit for the duration of the joining. [ 27 ] Logical channel identifiers identify a specific coherent channel between the DTE ( subscriber appliance ) and the DCE ( network ), and only has local significance on the connection between the subscriber and the network. The other end of the connection at the remote control DTE is probable to have assigned a different coherent channel identifier. The range of possible coherent channels is split into 4 groups : channels assigned to permanent virtual circuits, assigned to incoming virtual calls, bipartite ( incoming or outgoing ) virtual calls, and outgoing virtual calls. [ 29 ] ( Directions refer to the focus of virtual call initiation as viewed by the DTE – they all carry data in both directions. ) [ 30 ] The ranges allowed a subscriber to be configured to handle significantly differing numbers of calls in each focus while reserving some channels for calls in one focus. All International networks are required to implement patronize for permanent virtual circuits, bipartisan logical channels and one-way logical channels outgoing ; one-way coherent channels incoming is an extra optional facility. [ 31 ] DTE-DCE interfaces are not required to support more than one coherent channel. [ 29 ] Logical impart identifier zero will not be assigned to a permanent virtual racing circuit or virtual address. [ 32 ] The logical distribution channel identifier of nothing is used for packets which do n’t relate to a specific virtual circuit ( e.g. mailboat level restart, registration, and diagnostic packets ) .


In public networks, X.25 was typically billed as a flat monthly service fee depending on connect speed, and then a price-per-segment on top of this. [ 33 ] Link speeds varied, typically from 2400 bit/s up to 2 Mbit/s, although speeds above 64 kbit/s were uncommon in the public networks. A section was 64 bytes of data ( rounded up, with no transfer between packets ), [ 34 ] charged to the caller [ 35 ] ( or callee in the case of reverse charged calls, where supported ). [ 36 ] Calls invoking the Fast Select adeptness ( allowing 128 bytes of data in call request, call confirmation and call clearing phases ) [ 37 ] would generally attract an extra bang, as might use of some of the early X.25 facilities. PVCs would have a monthly rental cathexis and a lower price-per-segment than VCs, making them cheaper only where bombastic volumes of data are passed .

X.25 package types.

Packet Type DCE → DTE DTE → DCE Service VC PVC
Calling the Super Setup Incoming Call Call Request X
Call Connected Gaming Call Accepted Regaining X
Clear Indication Request Clear Request Indication X
Clear Confirmation City Clear Confirmation City X
Data and Interrupt or Currput Data Data X X
Interrupt Interrupt X X
Interrupt Confirmation Interrupt Confirmation X X
Flow Control and Reset RR RR X X
Reset Indication Reset Request X X
Reset Confirmation Reset Confirmation X X
Restart Restart Indication Restart Request X
Restart Confirmation Restart Confirmation X
Diagnostic Diagnostic X
Registration Registration Confirmation Registration Request X
Restart Confirmation Restart Confirmation X
Diagnostic Diagnostic X
Registration Registration Confirmation Registration Request X
Restart Confirmation Restart Confirmation X
Diagnostic Diagnostic X
Registration Registration Confirmation Registration Request X
Restart Confirmation Restart Confirmation X
Diagnostic Diagnostic X
Registration Registration Confirmation Registration Request X

X.25 details.

The net may allow the excerpt of the maximal length in range 16 to 4096 octets ( 2n values entirely ) per virtual lap by negotiation as part of the call setup procedure. The maximal length may be different at the two ends of the virtual racing circuit .

  • Data terminal equipment constructs control packets which are encapsulated into data packets. The packets are sent to the data circuit-terminating equipment, using LAPB Protocol.
  • Data circuit-terminating equipment strips the layer-2 headers in order to encapsulate packets to the internal network protocol.

X.25 facilities.

X.25 provides a stage set of drug user facilities defined and described in ITU-T Recommendation X.2. [ 38 ] The X.2 drug user facilities fall into five categories :

  • Essential facilities;
  • Additional facilities;
  • Conditional facilities;
  • Mandatory facilities; and,
  • Optional facilities.

X.25 besides provides X.25 and ITU-T specified DTE optional drug user facilities defined and described in ITU-T Recommendation X.7. [ 39 ] The X.7 optional drug user facilities fall into four categories of exploiter facilities that require :

  • Subscription only;
  • Subscription followed by dynamic invocation;
  • Subscription or dynamic invocation; and,
  • Dynamic invocation only.

X.25 protocol versions.

The CCITT/ITU-T versions of the protocol specifications are for populace data networks ( PDN ). [ 40 ] The ISO/IEC versions address extra features for private networks ( e.g. local area networks ( LAN ) use ) while maintaining compatibility with the CCITT/ITU-T specifications. [ 41 ] The user facilities and other features supported by each translation of X.25 and ISO/IEC 8208 have varied from edition to edition. [ 42 ] respective major protocol versions of X.25 exist : [ 43 ]

  • CCITT Recommendation X.25 (1976) Orange Book
  • CCITT Recommendation X.25 (1980) Yellow Book
  • CCITT Recommendation X.25 (1984) Red Book
  • CCITT Recommendation X.25 (1988) Blue Book
  • ITU-T Recommendation X.25 (1993) White Book[44]
  • ITU-T Recommendation X.25 (1996) Grey Book[45]

The X.25 Recommendation allows many options for each network to choose when deciding which features to support and how certain operations are performed. This means each network needs to publish its own document giving the specification of its X.25 implementation, and most networks required DTE appliance manufacturers to undertake protocol conformity quiz, which included testing for stern attachment and enforcement of their network specific options. ( network operators were peculiarly concerned about the possibility of a badly behave or misconfigured DTE appliance taking out parts of the network and affecting other subscribers. ) consequently, subscriber ‘s DTE appliances have to be configured to match the specification of the especial network to which they are connecting. Most of these were sufficiently different to prevent interworking if the subscriber did n’t configure their appliance correctly or the appliance manufacturer did n’t include specific support for that network. In hurt of protocol conformity test, this often lead to interworking problems when initially attaching an appliance to a network. In accession to the CCITT/ITU-T versions of the protocol, four editions of ISO/IEC 8208 exist : [ 42 ]

  • ISO/IEC 8208:1987, First Edition, compatible with X.25 (1980) and (1984)
  • ISO/IEC 8208:1990, Second Edition, compatible with 1st Ed. and X.25 (1988)
  • ISO/IEC 8208:1995, Third Edition, compatible with 2nd Ed. and X.25 (1993)
  • ISO/IEC 8208:2000, Fourth Edition, compatible with 3rd Ed. and X.25 (1996)

See besides.


far read.

  • Computer Communications, lecture notes by Prof. Chaim Ziegler PhD, Brooklyn College
  • Motorola Codex (1992). The Basics Book of X.25 Packet Switching. The Basics Book Series (2nd ed.). Reading, MA: Addison-Wesley. ISBN 0-201-56369-X.
  • Deasington, Richard (1985). X.25 Explained. Computer Communications and Networking (2nd ed.). Chichester UK: Ellis Horwood. ISBN 978-0-85312-626-3.
  • Friend, George E.; Fike, John L.; Baker, H. Charles; Bellamy, John C. (1988). Understanding Data Communications (2nd ed.). Indianapolis: Howard W. Sams & Company. ISBN 0-672-27270-9.
  • Pooch, Udo W.; William H. Greene; Gary G. Moss (1983). Telecommunications and Networking. Boston: Little, Brown and Company. ISBN 0-316-71498-4.
  • Schatt, Stan (1991). Linking LANs: A Micro Manager’s Guide. McGraw-Hill. ISBN 0-8306-3755-9.
  • Thorpe, Nicolas M.; Ross, Derek (1992). X.25 Made Easy. Prentice Hall. ISBN 0-13-972183-5.

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