X.25
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 |
Website | https://www.itu.int/rec/T-REC-X.25/ |
X.25 is an
The protocol suite is designed as three conceptual layers, which correspond closely to the lower three layers of the seven-layer OSI Reference Model, although it was developed several years before the OSI model (1984).[3][4] It also supports functionality not found in the OSI network layer.[5][6] 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 was a popular with
History
The
How the CCITT standardized virtual circuits
The CCITT appointed a special Rapporteur on packet switching,
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).[16] A document submitted by France “with the active support of a number of European administrations” served as “main basis for discussion in this meeting”. It was then “agreed that two types of services should be considered, a ‘datagram’ service and a ‘virtual call’ service”.[16]: p3
At the third meeting, focus had moved from whether there should be packet-mode networks to whether there could be “a standard for the interface between the network and the computers”.[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. Meetings started between the Canadian DATAPAC and the French TRANSPAC, continued with the startup Telenet of the USA, and continued with the BPO of the UK.[8]: p39 [17]: 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. They prepared contributions to be submitted to SG VII in their name by administrations having voting right in CCITT. One contribution was a X.2x interface specification, the first version of what will become X.25).[18][19]
The fourth Rapporteur meeting, in May 1975 in Geneva, had 45 participants and 27 new documents. The Rapporteur asked whether packet switching recommendations should be issued "with a view to making international interworking possible”, "the French administration answered in the affirmative and Canada strongly supported the French proposal". No firm conclusion was however obtained yet.[20]
The fifth Rapporteur meeting, in September 1975 in Geneva, had about 60 participants. After discussions on the proposed virtual circuit interface, numerous issues were left unresolved.[8]: p40 Concerning datagrams, ‘It was proposed by Larry Roberts of the US delegation and supported by representatives from France and Canada respectively that the datagram classification be changed from “E” to “A”’, i.e., from essential "to be available internationally” to additional that "may be available in certain countries and internationally”.[21] The Rapporteur's last report expressed doubts “that a standard would be ready for adoption by SG VII”.[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. After intense work during it, all issues had been dealt with. For an approval by the full study group, a challenge remained: copies of the updated X.25 draft had to be available in two languages. To get them in due time, Anton Rybzynski 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. COM VII then reviewed distributed copies, and unanimously approved them for submission to the forthcoming CCITT plenary. [17] At this plenary of September 1976, the X.25 recommendation and the other 10 of SG VII were unanimously approved.[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
Worldwide public data networks
Publicly accessible X.25 networks, commonly called
Beginning in the early 1990s, in North America, use of X.25 networks (predominated by Telenet and Tymnet)
X.25 networks are still in use throughout the world. A variant called
X.25 is still used in the aeronautical business (especially in Asia) even though a transition to modern protocols like
France was one of the last remaining countries where commercial end-user service 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.[25] As planned, service was terminated 30 June 2012. There were 800,000 terminals in operation at the time.[26] An X.25 service was still purchasable from BT in the United Kingdom in 2019.[27]
Architecture
The general concept of the X.25 was to create a universal and global
The X.25 specification defines only the interface between a subscriber (DTE) and an X.25 network (DCE).
X.25 originally defined three basic protocol levels or architectural layers. In the original specifications these were referred to as levels and also had a level number, whereas all ITU-T X.25 recommendations and ISO 8208 standards released after 1984 refer to them as layers.[29] 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 telephony concept of establishing reliable circuits through a shared network, but using software to create "
For a brief period, the specification also included a connectionless datagram service, but this was dropped in the next revision. 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. The data is carried in an extended call request packet and the response is carried in an extended field of the call reject packet, with a connection never being fully established.
Closely related to the X.25 protocol are the protocols to connect asynchronous devices (such as dumb terminals and printers) to an X.25 network:
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 reliable itself. The X.25 packet layer provides the virtual call 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 cannot recover from data transmission errors. All but the earliest versions of X.25 include facilities[30] which provide for OSI network layer Addressing (NSAP addressing, see below).[31]
User device support
X.25 was developed in the era of
Having connected to the PAD, the dumb-terminal user tells the PAD which host to connect to, by giving a phone-number-like address in the
For the purpose of flow-control, a
While the PAD function defined by
Error control
Error recovery procedures at the packet layer assume that the data link layer is responsible for retransmitting data received in error. Packet layer error handling focuses on resynchronizing the information flow in calls, as 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
X.25 supports two types of
VC may be established using X.121 addresses. The X.121 address consists of a three-digit data country code (DCC) plus a network digit, together forming the four-digit data network identification code (DNIC), followed by the national terminal number (NTN) of at most ten digits. Note the use of a single network digit, seemingly allowing for only 10 network carriers per country, but some countries are assigned more than one DCC to avoid this limitation. Networks often used fewer than the full NTN digits for routing, 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 routing on the subscribers networks.
PVCs are permanently established in the network and therefore do not require the use of addresses for call setup. PVCs are identified at the subscriber interface by their logical 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 maximum of 4095 logical channels on which it is allowed to establish virtual calls and permanent virtual circuits,[36] although networks are not expected to support a full 4095 virtual circuits.[37] For identifying the channel to which a packet is associated, each packet contains a 12 bit logical channel identifier made up of an 8-bit logical channel number and a 4-bit logical channel group number.[36] Logical channel identifiers remain assigned to a virtual circuit for the duration of the connection.[36] Logical channel identifiers identify a specific logical channel between the DTE (subscriber appliance) and the DCE (network), and only has local significance on the link between the subscriber and the network. The other end of the connection at the remote DTE is likely to have assigned a different logical channel identifier. The range of possible logical channels is split into 4 groups: channels assigned to permanent virtual circuits, assigned to incoming virtual calls, two-way (incoming or outgoing) virtual calls, and outgoing virtual calls.[38] (Directions refer to the direction of virtual call initiation as viewed by the DTE – they all carry data in both directions.)[39] The ranges allowed a subscriber to be configured to handle significantly differing numbers of calls in each direction while reserving some channels for calls in one direction. All international networks are required to implement support for permanent virtual circuits, two-way logical channels and one-way logical channels outgoing; one-way logical channels incoming is an additional optional facility.[40] DTE-DCE interfaces are not required to support more than one logical channel.[38] Logical channel identifier zero will not be assigned to a permanent virtual circuit or virtual call.[41] The logical channel identifier of zero is used for packets which don't relate to a specific virtual circuit (e.g. packet layer restart, registration, and diagnostic packets).
Billing
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.[42] 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 segment was 64 bytes of data (rounded up, with no carry-over between packets),[43] charged to the caller[44] (or callee in the case of reverse charged calls, where supported).[45] Calls invoking the Fast Select facility (allowing 128 bytes of data in call request, call confirmation and call clearing phases)[46] would generally attract an extra charge, as might use of some of the other X.25 facilities. 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.
X.25 packet 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 | |
RNR | RNR | X | X | ||
REJ | REJ | 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 |
X.25 details
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. The maximal length may be different at the two ends of the virtual 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 set of user facilities defined and described in ITU-T Recommendation X.2.[47] The X.2 user facilities fall into five categories:
- Essential facilities;
- Additional facilities;
- Conditional facilities;
- Mandatory facilities; and,
- Optional facilities.
X.25 also provides X.25 and ITU-T specified DTE optional user facilities defined and described in ITU-T Recommendation X.7.[48] The X.7 optional user facilities fall into four categories of user 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 public data networks (PDN).[49] The ISO/IEC versions address additional features for private networks (e.g. local area networks (LAN) use) while maintaining compatibility with the CCITT/ITU-T specifications.[50]
The user facilities and other features supported by each version of X.25 and ISO/IEC 8208 have varied from edition to edition.[51] Several major protocol versions of X.25 exist:[52]
- 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[53]
- ITU-T Recommendation X.25 (1996) Grey Book[54]
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 conformance testing, which included testing for strict adherence and enforcement of their network specific options. (Network operators were particularly concerned about the possibility of a badly behaving or misconfigured DTE appliance taking out parts of the network and affecting other subscribers.) Therefore, subscriber's DTE appliances have to be configured to match the specification of the particular network to which they are connecting. Most of these were sufficiently different to prevent interworking if the subscriber didn't configure their appliance correctly or the appliance manufacturer didn't include specific support for that network. In spite of protocol conformance testing, this often lead to interworking problems when initially attaching an appliance to a network.
In addition to the CCITT/ITU-T versions of the protocol, four editions of ISO/IEC 8208 exist:[51]
- 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)
Legacy
The X.25 protocol had a lot of overhead to deal with loss (circuits back then ran over poor grade cabling and had a lot of single-bit errors to deal with). As circuits got more and more reliable, the overhead was no longer needed, and less expensive Frame Relay took over. Frame Relay has its technical base in X.25, but does not attempt to correct errors.
The world-wide public data networks based on X.25 helped grow IP as a protocol riding on top.
X.25 was also available in niche applications such as Retronet that allow vintage computers to use the Internet.
See also
- History of the Internet
- OSI protocol suite
- Packet switched networks – are networks, including X.25, that have protocols using "packets"
- Protocol Wars
- XOT – is an "X.25 Over TCP" protocol, i.e. with X.25 encapsulation on TCP/IP networks
- X.PC
References
- ^ a b CCITT, Study Group VII, Draft Recommendation X-25, March 1976
- ^ History of X.25, CCITT Plenary Assemblies and Book Colors
- ^ (Friend et al. 1988, p. 242)
- ^ (Friend et al. 1988, p. 243)
- ^ ITU-T Recommendation X.28.
- ^ ITU-T Recommendation X.3.
- ^ Foregenix (February 2012). "X.25 within the Payment Card Industry" (PDF). Archived from the original (PDF) on 4 March 2016. Retrieved 25 May 2016.
- ^ a b c d e f g Sirbu, Marvin A; Zwimpfer, Laurence E (March 1985). "Standards Setting for Computer Communication: The Case of X.25". IEEE.
- ISSN 1558-1896.
- S2CID 23243636.
- ^ "Short History of Study Group ... 7". www.itu.int. Retrieved 4 February 2020.
- ^ X-Series recommendations
- ^ a b (Friend et al. 1988, p. 230)
- ^ "European cooperation in the field of scientific and technical research (COST), 1971-".
- ^ Bothner-By, Halvor (July 1974). "REPORT OF THE RAPPORTEUR'S GROUP ON POINT C -".
- ^ a b Rapporteur group on packet switching. "Report of Meeting in Oslo (15 - 16 AUGUST 1974)". CCITT.
- ^ S2CID 23639680.
- ^ CCITT Rapporteur on packet switching (26 March 1975). "Proposals for Recommendations" (PDF).
- S2CID 23243636.
- ^ INWG#99. "Report of the Meeting in Geneva (28 May - 6 June 1975) (Extracts)".
{{cite web}}
: CS1 maint: numeric names: authors list (link) - ^ Pouzin, Louis (October 1975). "Meeting of the CCITT Rapporteur group on packet switching – (Geneva, 16 – 19 September 1975)". p. 6 in Annex 4.
- ^ a b (Schatt 1991, p. 200).
- ^ (Schatt 1991, p. 207).
- ^ "Running X.25 over TCP/IP on Cisco routers". 1 February 2001. Archived from the original on 21 January 2012.
- ^ (in French) Presse, Agence France (21 July 2011). "Le Minitel disparaîtra en juin 2012" [Minitel will disappear in June 2012]. Le Figaro (in French).
- ^ (in French)[1]
- ^ "BT price list: Section 13:BT IP Networking". BT. Retrieved 30 May 2019.
- ^ ISO 8208:2000
- ^ ISO 8208, Annex B.
- ^ ITU-T Recommendation X.25, G.3.2 Called address extension facility, pp. 141–142.
- ^ ITU-T Recommendation X.223, Appendix II.
- ^ ITU-T Recommendation X.7 (04/2004), pp. 17–18.
- ^ ITU-T Recommendation X.223.
- ^ ITU-T Recommendation X.25 (10/96), Annex G, p. 140.
- ^ ITU-T Recommendation X.213, Annex A.
- ^ a b c ITU-T Recommendation X.25 (10/96), p. 45.
- ^ ITU-T Recommendation X.283 (12/97), p. 42.
- ^ a b ITU-T Recommendation X.25 (10/96), Annex A, pp. 119–120.
- ^ ISO/IEC 8208:2000, Fourth Edition, p. 61.
- ^ ITU-T Recommendation X.2 (03/2000), p. 4.
- ^ ISO/IEC 8208:2000, Fourth Edition, 3.7.1, p. 7.
- ^ ITU-T Recommendation D.11 (03/91), p. 2.
- ^ ITU-T Recommendation D.12 (11/88), p. 1.
- ^ ITU-T Recommendation X.7 (04/2004), p. 42.
- ^ ITU-T Recommendation D.11 (03/91), p. 3.
- ^ ITU-T Recommendation X.7 (04/2004), p. 38.
- ^ ITU-T Recommendation X.2
- ^ ITU-T Recommendation X.7
- ^ ITU-T Recommendation X.25 (10/96), Summary, p. v.
- ^ ISO/IEC 8208:2000, Fourth Edition, Section 1: Scope, p. 1.
- ^ a b ISO/IEC 8208:2000, Fourth Edition, Annex C.
- ^ ITU-T Recommendation X.25.
- ^ ITU-T Recommendation X.25 (1993) White Book
- ^ ITU-T Recommendation X.25 (1996) Grey Book
Further reading
- 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.