X.25 is an ITU-T standard protocol suite for wide area networks using leased lines, the phone or ISDN system as the networking hardware. It defines one of many standards implemented for the physical layer, data link layer, and network layer of the OSI Reference Model or the equivalent Network Access Layer of the DoD protocol model. Packet switched network was the common name given to the international collection of X.25 providers, typically the various national telephone companies. Their combined network had large global coverage during the 1980s and into the '90s, and it is still in use mainly in transaction systems. The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications on behalf of the International Telecommunication Union (ITU) and is based in Geneva, Switzerland. ... A protocol stack is a particular software implementation of a computer networking protocol suite. ... A wide area network or WAN is a computer network covering a broad geographical area. ... Plain old telephone service, or POTS, are the services available from analogue telephones prior to the introduction of electronic telephone exchanges into the public switched telephone network. ... ISDN is also short for isosorbide dinitrate Integrated Services Digital Network (ISDN) is a type of circuit switched telephone network system, designed to allow digital (as opposed to analog) transmission of voice and data over ordinary telephone copper wires, resulting in better quality and higher speeds, than available with analog... This article does not cite any references or sources. ... The data link layer is layer two of the seven-layer OSI model as well as of the five-layer TCP/IP reference model. ... The network layer is level three of the seven level OSI model. ... The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model for short) is a layered, abstract description for communications and computer network protocol design, developed as part of Open Systems Interconnection (OSI) initiative. ... The TCP/IP model or Internet reference model, sometimes called the DoD model (DoD, Department of Defense), ARPANET reference model, is a layered abstract description for communications and computer network protocol design. ... // A Packet Switched Network, or PSN, refers to the packet switched networks that existed before Internet. ...

History

X.25 was developed in the ITU (formerly CCITT) Study Group VII based upon a number of emerging data network projects. Various updates and additions were worked into the standard, eventually recorded in the ITU series of technical books describing the telecoms systems. These books were published every fourth year with different colored covers.

Architecture

A Televideo terminal model 925 made around 1982

The general concept of X.25 was to create a universal and global packet-switched network on what was then the bit-error prone analog phone system. Much of the X.25 system is a description of the rigorous error correction needed to achieve this, a system known as LAP-B. The X.25 model was based on the concept of establishing "virtual calls" through the network, with "data terminal equipment" (DTE) providing endpoints to users, which looked like point-to-point connections. The X.25 specification defines only the interface between a subscriber (DTE) and an X.25 network (DCE). It does not specify how the network operates internally—many X.25 network implementations used something very similar to X.25 internally, but others used quite different protocols internally. X.75, a very similar protocol to X.25, defines the interface between two X.25 networks (i.e. two DCEs). ISO 8208 is the same as X.25, but additionally includes provision for two DTEs to be directly connected to each other with no X.25 network. Download high resolution version (1232x1059, 265 KB) A Televideo 925 computer terminal. ... Download high resolution version (1232x1059, 265 KB) A Televideo 925 computer terminal. ... An analog or analogue signal is any continuously variable signal. ... In computer science and information theory, error correction consists of using methods to detect and/or correct errors in the transmission or storage of data by the use of some amount of redundant data and (in the case of transmission) the selective retransmission of incorrect segments of the data. ... LAPB state Diagram LAPB (Link Access Protocol, Balanced) is a data link layer protocol in the X.25 protocol stack. ... In telecommunication, a virtual call capability, sometimes called a virtual call facility, is a service feature in which: a call set-up procedure and a call disengagement procedure determine the period of communication between two data terminal equipments (DTEs) in which user data are transferred by the network in the... DTE is an abbreviation for Data Terminal Equipment, and refers to an end instrument that converts user information into signals for transmission, or reconverts the received signals into user information. ... This article does not cite any references or sources. ... X.75 is an International Telecommunication Union (ITU) standard specifying the interface between DCE units in a network. ...

X.25 was developed in the era of dumb terminals connecting to host computers. Instead of dialing directly “into” the host computer — 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 connection to a network service provider. 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 ITU-T X.29 and X.3 standards. It has been suggested that this article or section be merged into Data terminal. ... This article needs to be cleaned up to conform to a higher standard of quality. ...

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 X.121 address format (or by giving a host name, if the service provider allows for names that map to X.121 addresses). The PAD then places an X.25 call to the host, establishing a virtual circuit. Note that X.25 provides for virtual circuits, so appears to be a circuit switched network, even though in fact the data itself is packet switched internally, similar to the way TCP provides virtual circuits even though the underlying data is packet switched. Two X.25 hosts could, of course, call one another directly; no PAD is involved in this case. In theory, it doesn't matter whether the X.25 caller and X.25 destination are both connected to the same carrier, but in practice it was not always possible to make calls from one carrier to another. X.121 is the ITU-T address format of the X.25 protocol suite used as part of call setup to establish a switched virtual circuit between Public Data Networks (PDNs), connecting two network user addresses (NUAs). ... A virtual circuit (VC) is a communications arrangement in which data from a source user may be passed to a destination user over more than one real communications circuit during a single period of communication, but the switching is hidden from the users. ... In telecommunication, the term circuit switching has the following meanings: 1. ... In computer networking and telecommunications, packet switching is a communications paradigm in which packets (messages or fragments of messages) are individually routed between nodes, with no previously established communication path. ...

For the purpose of flow-control, a sliding window protocol is used with the default window size of 2. The acknowledgments may have either local or end to end significance. A D bit (Data Delivery bit) in each data packet indicates if the sender requires end to end acknowledgment. When D=1, it means that the acknowledgment has end to end significance and must take place only after the remote DTE has acknowledged receipt of the data. When D=0, the network is permitted (but not required) to acknowledge before the remote DTE has acknowledge or even received the data. In transmit flow control, sliding window is a variable-duration window that allows a sender to transmit a specified number of data units before an acknowledgement is received or before a specified event occurs. ...

Error recovery procedures are such:

• Level 3 Reset packets, which re-initializes the flow on a virtual circuit (but does not break the virtual circuit)
• Restart packet, which clears down all switched virtual circuits 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 network.

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. NSAP addressing was added in the X.25(1984) revision of the specification, and this enabled X.25 to better meet the requirements of OSI Layer 3. Public X.25 networks didn't make use of NSAP addressing, but some carried it transparently. Image File history File linksMetadata Download high-resolution version (1424x706, 259 KB) Siemens DAG 64, a device used to connect to the german Datex-P X.25 network. ... Image File history File linksMetadata Download high-resolution version (1424x706, 259 KB) Siemens DAG 64, a device used to connect to the german Datex-P X.25 network. ... A Network Service Access Point (NSAP) addresses, defined in ISO/IEC 8348, are identifying labels for network endpoints used in OSI networking. ...

For much of its history X.25 was used for permanent virtual circuits (PVCs) to connect two host computers in a dedicated link. This was common for applications such as banking, where distant branch offices could be connected to central hosts for a cost that was considerably lower than a permanent long distance telephone call. X.25 was typically billed as a flat monthly service fee depending on link speed, and then a price-per-packet on top of this. Link speeds varied, typically from 2400bit/s up to 2Mbits/s, although speeds above 64kbits/s were uncommon in the public networks. A virtual circuit (VC) is a communications arrangement in which data from a source user may be passed to a destination user over more than one real communications circuit during a single period of communication, but the switching is hidden from the users. ...

Publicly-accessible X.25 networks (Compuserve, Tymnet, Euronet, PSS, and Telenet) were set up in most countries during the 1970s and 80s, to lower the cost of accessing various online services, in which the user would first interact with the network interface to set up the connection. Known as switched virtual circuits (SVC) or "virtual calls" in public data networks (PDN), this use of X.25 disappeared from most places fairly rapidly as long distance charges fell in the 1990s and today's Internet started to emerge. CompuServe, (in full, CompuServe Information Services, or CIS), was the first major commercial online service in the United States. ... Italic textTymnet was an international data communications network headquartered in San Jose, CA that utilized circuit switched technology and used X.25, SNA/SDLC, BSC interfaces to connect host computers (servers)at thousands of large companies, educational institutions, and government agencies. ... Euronat (also known as EuroNet and Euro-Nat) is/was an effort by Jean-Marie Le Pen of Front National to gather all the Euronationalist parties of Europe. ... In the UK, an X.25-based packet-switched network, provided by British Telecom. ... Telenet is the largest provider of broadband cable services in Belgium. ... An online service provider is an entity which provides a service online. ...

A data Terminal Equipment is allowed to establish up to 4095 virtual circuits, which can be both permanent and virtual. For this purpose each packet has 12 bit virtual circuit number

X.25 (or OSI) protocol suite

A number of systems were developed to directly use the underlying packet nature of X.25, back when it appeared that X.25 would become the single universal networking system. Many of these were "private" applications, but the X.400 e-mail system was also based on X.25 as a transmission layer. The basic idea was to develop a universal set of standards for "Open Systems Interconnection (OSI)", however, industry developments eventually led down the path to Internet. OSI is primarily used as an abbreviation and has many meanings: OSI Pharmaceuticals, an American pharmaceutical company OSI Restaurant Partners, the restaurant and entertainment group that includes Outback Steakhouse OSI Systems a company that manufactures security scanners and medical equipment based in California Olomouc Summer Institute, an educational program for... In computing, X.400 is an OSI standard developed by the ITU-T (at the time the CCITT) in cooperation with ISO for the exchange of messages, both between people and between software applications. ... This article or section does not adequately cite its references or sources. ...

Examples of protocols in the X.25 protocol suite are:

7. Application layer: FTAM, X.400, X.500, DAP The application layer is the seventh level of the seven-layer OSI model. ... FTAM, an ISO 8571 standard, is an OSI Application layer protocol for File Transfer Access and Management. ... In computing, X.400 is an OSI standard developed by the ITU-T (at the time the CCITT) in cooperation with ISO for the exchange of messages, both between people and between software applications. ... X.500 is the set of ITU-T computer networking standards covering electronic directory services such as white pages, Knowbot and whois. ... Directory Access Protocol (DAP) is a computer networking standard promulgated by ITU-T and ISO in 1988 for accessing an X.500 directory service. ...

6. Presentation layer: ISO 8823, X.226 The presentation layer is the sixth level of the seven layer OSI model. ...

5. Session layer: ISO 8327, X.225 The session layer is level five of the seven level OSI model. ...

4. Transport layer: TP0, TP1, TP2, TP3, TP4 In computing and telecommunications, the transport layer is layer four of the seven layer OSI model. ...

3. Network layer: X.25 PLP, CLNP The network layer is level three of the seven level OSI model. ... PLP abbreviates Packet-Layer Protocol. ... CLNS is an abbreviation of Connectionless Network Service. ...

2. Data link layer: LAPB, IEEE802.2 - IEEE802.5 The data link layer is layer two of the seven-layer OSI model as well as of the five-layer TCP/IP reference model. ... LAPB state Diagram LAPB (Link Access Protocol, Balanced) is a data link layer protocol in the X.25 protocol stack. ... IEEE 802 refers to a family of IEEE standards about local area networks and metropolitan area networks. ...

1. Physical layer: X.21bis, EIA/TIA-232, EIA/TIA-449, EIA-530, IEEE802.3 - IEEE802.5 This article does not cite any references or sources. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... In telecommunications, RS-232 is a standard for serial binary data interconnection between a DTE (Data terminal equipment) and a DCE (Data communication equipment). ... The RS-449 specifiecs the functional and mechanical characteristics of the interface between data terminal equipment and data communications equipment. ... EIA-530 is a balanced serial interface standard that generally uses a 25 pin connector. ... IEEE 802 refers to a family of IEEE standards about local area networks and metropolitan area networks. ...

Obsolescence

With the widespread introduction of "perfect" quality digital phone services and error correction in modems, the overhead of X.25 was no longer worthwhile. The result was called Frame relay, essentially the X.25 protocol with the error correction systems removed, and somewhat better throughput as a result. The concept of virtual circuits is still used within ATM to allow for traffic engineering and network multiplexing. In the context of computer networking, frame relay (also found written as frame-relay) consists of an efficient data transmission technique used to send digital information quickly and cheaply in a relay of frames to one or many destinations from one or many end-points. ... Asynchronous Transfer Mode (ATM) is a cell relay, Circuit switching network and data link layer protocol which encodes data traffic into small (53 bytes; 48 bytes of data and 5 bytes of header information) fixed-sized cells. ... In telecommunications, multiplexing (also muxing or MUXing) is the combining of two or more information channels onto a common transmission medium using hardware called a multiplexer or (MUX). ...

X.25 Today

X.25 networks are still in use throughout the world, although in dramatic decline, being largely supplanted by newer layer 2 technologies such as frame relay, ISDN, ATM, ADSL, POS, and the ubiquitous layer 3 Internet Protocol. X.25 however remains one of the only available reliable links in many portions of the developing world, where access to a PDN may be the most reliable and low cost way to access the Internet. A variant called AX.25 is also used widely by amateur packet radio, though there has been some movement in recent years to replace it with TCP/IP. RACAL Paknet or Widanet as it is otherwise branded is still in operation in many regions of the world, running on an X.25 protocol base. Used as a secure wireless low rate data transfer platform, Paknet is commonly used for GPS Tracking and POS solutions currently. The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model for short) is a layered, abstract description for communications and computer network protocol design, developed as part of Open Systems Interconnection (OSI) initiative. ... In the context of computer networking, frame relay (also found written as frame-relay) consists of an efficient data transmission technique used to send digital information quickly and cheaply in a relay of frames to one or many destinations from one or many end-points. ... // Integrated Services Digital Network (ISDN) is a circuit-switched telephone network system, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better quality and higher speeds than that is available with the PSTN system. ... Asynchronous Transfer Mode (ATM) is a cell relay, Circuit switching network and data link layer protocol which encodes data traffic into small (53 bytes; 48 bytes of data and 5 bytes of header information) fixed-sized cells. ... Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. ... Packet over SONET/SDH, abbreviated POS, is communications protocol for transmitting packets over SDH or SONET, which are both circuit switched protocols. ... The Internet Protocol (IP) is a data-oriented protocol used for communicating data across a packet-switched internetwork. ... For the Jamaican reggae band, see Third World (band). ... AX.25 is a data link layer protocol derived from the X.25 protocol suite and designed for use by amateur radio operators. ... Amateur radio station with modern solid-state transceiver featuring LCD display and DSP capabilities Amateur radio, often called Ham radio, is a hobby enjoyed by about six million people[1] throughout the world. ... Packet radio is a form of digital data transmission used in amateur radio to construct wireless computer networks. ... The Internet protocol suite is the set of communications protocols that implement the protocol stack on which the Internet runs. ...

X.25 packet types

X.25 details

The minimum data field length the network must support is 128 octets per packet. However 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. In computer technology and networking, an octet is a group of 8 bits. ...

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

DTE is an abbreviation for Data Terminal Equipment, and refers to an end instrument that converts user information into signals for transmission, or reconverts the received signals into user information. ... LAPB state Diagram LAPB (Link Access Protocol, Balanced) is a data link layer protocol in the X.25 protocol stack. ...

Related Technologies

• DATAPAC - Canadian variant of X.25 offered by Bell Canada

DATAPAC is Canadas packet switched X.25-equivalent data network. ... Bell Canada Enterprises (TSX: BCE, NYSE: BCE), legally BCE Inc. ...

External links

• Cisco X.25 Reference
• An X.25 Networking Guide with comparisons to TCP/IP
• X.25 - Directory & Informational Resource

Bibliography

Computer Communications, lecture notes by Prof. Chaim Zieglier PhD, Brooklyn College Brooklyn College is a senior college of the City University of New York, located in Brooklyn, New York. ...