X.25 is an
ITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
standard protocol suite for
packet switchedA packet-switched network is a digital communications network that groups all transmitted data, irrespective of content, type, or structure into suitably sized blocks, called packets...
wide area networkA wide area network is a telecommunication network that covers a broad area . Business and government entities utilize WANs to relay data among employees, clients, buyers, and suppliers from various geographical locations...
(WAN) communication. An X.25 WAN consists of
packet-switching exchangePacket switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...
(PSE) nodes as the networking hardware, and
leased lineA leased line is a service contract between a provider and a customer, whereby the provider agrees to deliver a symmetric telecommunications line connecting two or more locations in exchange for a monthly rent . It is sometimes known as a 'Private Circuit' or 'Data Line' in the UK or as CDN in Italy...
s,
Plain old telephone servicePlain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
connections or ISDN connections as physical links. X.25 is a family of protocols that was popular during the 1980s with telecommunications companies and in
financial transactionA financial transaction is an event or condition under the contract between a buyer and a seller to exchange an asset for payment. It involves a change in the status of the finances of two or more businesses or individuals.-History:...
systems such as
automated teller machineAn automated teller machine or automatic teller machine, also known as a Cashpoint , cash machine or sometimes a hole in the wall in British English, is a computerised telecommunications device that provides the clients of a financial institution with access to financial transactions in a public...
s. X.25 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.
While X.25 has been, to a large extent, replaced by less complex protocols, especially the
Internet protocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP), the service is still used and available in niche and legacy applications.
History
X.25 is one of the oldest packet-switched services available. It was developed before the
OSI Reference ModelThe Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
. The protocol suite is designed as three conceptual layers, which correspond closely to the lower three layers of the seven-layer
OSI modelThe Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
. It also supports functionality not found in the OSI
Network LayerThe network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
.
X.25 was developed in the
ITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
(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 telecommunication systems. These books were published every fourth year with different-colored covers. The X.25 specification is only part of the larger set of X-Series specifications on
public data networkA public data network is a network established and operated by a telecommunications administration, or a recognized private operating agency, for the specific purpose of providing data transmission services for the public....
s.
The
Public data networkA public data network is a network established and operated by a telecommunications administration, or a recognized private operating agency, for the specific purpose of providing data transmission services for the public....
was the common name given to the international collection of X.25 providers. Their combined network had large global coverage during the 1980s and into the 1990s.
Publicly-accessible X.25 networks (
CompuserveCompuServe was the first major commercial online service in the United States. It dominated the field during the 1980s and remained a major player through the mid-1990s, when it was sidelined by the rise of services such as AOL with monthly subscriptions rather than hourly rates...
,
TymnetTymnet was an international data communications network headquartered in San Jose, California that used virtual call packet switched technology and X.25, SNA/SDLC, ASCII and BSC interfaces to connect host computers at thousands of large companies, educational institutions, and government agencies....
,
EuronetEuronet may refer to:* Euronat* EuroNet , a telecommunications network in Europe utilising X.25....
,
PSSIn the United Kingdom, Packet Switch Stream was an X.25-based packet-switched network, provided by the British Post Office Telecommunications and then British Telecom starting in 1980...
,
DatapacDATAPAC was Canada's packet switched X.25-equivalent data network. Operated first by Trans-Canada Telephone System, then Telecom Canada, then the Stentor Alliance, it finally reverted to Bell Canada when the Stentor Alliance was dissolved.-Use:...
, Datanet 1 and
TelenetTelenet was a commercial packet switched network which went into service in 1974. It was the first packet-switched network service that was available to the general public. Various commercial and government interests paid monthly fees for dedicated lines connecting their computers and local...
) were set up in most countries during the 1970s and 80s, to lower the cost of accessing various online services.
Beginning in the early 1990s in North America, use of X.25 networks (predominated by
TelenetTelenet was a commercial packet switched network which went into service in 1974. It was the first packet-switched network service that was available to the general public. Various commercial and government interests paid monthly fees for dedicated lines connecting their computers and local...
and
TymnetTymnet was an international data communications network headquartered in San Jose, California that used virtual call packet switched technology and X.25, SNA/SDLC, ASCII and BSC interfaces to connect host computers at thousands of large companies, educational institutions, and government agencies....
) began being replaced with
Frame RelayFrame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...
service offered by national telephone companies. and thanks to the huge growth of the
InternetThe Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
most systems that used X.25 in the past now make use of TCP/IP, but for some older systems that need X.25 for communications it is possible to route X.25 data over TCP/IP networks
X.25 networks are still in use throughout the world. A variant called
AX.25AX.25 is a data link layer protocol derived from the X.25 protocol suite and designed for use by amateur radio operators. It is used extensively on amateur packet radio networks....
is also used widely by
amateurAmateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
packet radioPacket radio is a form of packet switching technology used to transmit digital data via radio or wireless communications links. It uses the same concepts of data transmission via Datagram that are fundamental to communications via the Internet, as opposed to the older techniques used by dedicated...
.
RacalRacal Electronics plc was once the third-largest British electronics firm. Listed on the London Stock Exchange and once a constituent of the FTSE 100 Index, Racal was a diversified company, offering products including: as voice and data recorders; point of sale terminals; laboratory instruments;...
Paknet, now known as Widanet, is still in operation in many regions of the world, running on an X.25 protocol base. In some countries, like the
NetherlandsThe Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
or
GermanyGermany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
, it is possible to use a stripped version of X.25 via the D-channel of an ISDN-2 (or
ISDN BRIBasic Rate Interface is an Integrated Services Digital Network configuration intended primarily for use in subscriber lines similar to those that have long been used for plain old telephone service...
) connection for low volume applications such as point-of-sale terminals; but, the future of this service in the Netherlands is uncertain.
Additionally X.25 is still under heavy use in the aeronautical business (especially in the Asian region) even though a transition to modern protocols like
X.400X.400 is a suite of ITU-T Recommendations that define standards for Data Communication Networks for Message Handling Systems — more commonly known as "email"....
is without option as X.25 hardware becomes increasingly rare and costly. As recently as March 2006, the National Airspace Data Interchange Network has used X.25 to interconnect remote airfields with Air Route Traffic Control Centers.
France is one of the only countries that still has a commercial end-user service known as
MinitelThe Minitel is a Videotex online service accessible through the telephone lines, and is considered one of the world's most successful pre-World Wide Web online services. It was launched in France in 1982 by the PTT...
which is based on
VideotexVideotex was one of the earliest implementations of an "end-user information system". From the late 1970s to mid-1980s, it was used to deliver information to a user in computer-like format, typically to be displayed on a television.In a strict definition, videotex refers to systems that provide...
which in turn runs on X.25. In 2002
MinitelThe Minitel is a Videotex online service accessible through the telephone lines, and is considered one of the world's most successful pre-World Wide Web online services. It was launched in France in 1982 by the PTT...
had about 9 million users, and in 2011 it still accounts for about 2 million users in France though France Télécom has announced it will completely shut down the service by 30 June 2012.
Architecture
The general concept of X.25 was to create a universal and global
packet-switched networkA packet-switched network is a digital communications network that groups all transmitted data, irrespective of content, type, or structure into suitably sized blocks, called packets...
. 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.
The X.25 specification defines only the interface between a subscriber (DTE) and an X.25 network (DCE).
X.75X.75 is an International Telecommunication Union standard specifying the interface for interconnecting two X.25 networks. X.75 is almost identical to X.25...
, a very similar protocol 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 network implementations used something very similar to X.25 or
X.75X.75 is an International Telecommunication Union standard specifying the interface for interconnecting two X.25 networks. X.75 is almost identical to X.25...
internally, but others used quite different protocols internally. The ISO equivalent protocol 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 additional networks such as ISO 8802 LLC2 (ISO LAN) and the OSI data link layer.
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. The layer numbers were dropped to avoid confusion with the OSI Model layers.
- 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
X.21 is an interface specification for differential communications introduced in the mid 1970s by the ITU-T. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications between carriers and customers' equipment...
, 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
Link Access Procedure, Balanced implements the data link layer as defined in the X.25 protocol suite. LAPB is a bit-oriented protocol derived from HDLC that ensures that frames are error free and in the right sequence. LAPB is specified in and ISO/IEC 7776...
(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
Packet Layer Protocol or PLP is the Network Layer protocol for the X.25 protocol suite. PLP manages the packet exchanges between DTE devices across VCs...
.
The X.25 model was based on the traditional telephony concept of establishing reliable circuits through a shared network, but using software to create "
virtual callsIn 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 in which user data are transferred by the...
" through the network. These calls interconnect
"data terminal equipment" (DTE)Data Terminal Equipment is an end instrument that converts user information into signals or reconverts received signals. These can also be called tail circuits. A DTE device communicates with the data circuit-terminating equipment...
providing endpoints to users, which looked like point-to-point connections. Each endpoint can establish many separate virtual calls to different endpoints.
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:
X.3X.3 is an ITU-T standard indicating what functions are to be performed by a Packet Assembler/Disassembler when connecting character-mode data terminal equipment , such as a computer terminal, to a packet switched network such as an X.25 network, and specifying the parameters that control this...
,
X.28X.28 is an ITU-T standard specifying the interface between asynchronous character-mode data terminal equipment , such as computer terminals, and a Packet Assembler/Disassembler that connects the DTE to a packet switched network such as an X.25 network.-External links:*...
and
X.29X.29 is the ITU-T standard, approved by ISO, specifying procedures for the exchange of control information and user data between a Packet Assembler/Disassembler , and a remote packet-mode data terminal equipment or another PAD...
. This functionality was performed using a
Packet Assembler/DisassemblerA packet assembler/disassembler, abbreviated PAD is a communications device which provides multiple asynchronous terminal connectivity to an X.25 network or host computer. It collects data from a group of terminals and places the data into X.25 packets...
or PAD (also 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 ModelThe Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
(OSIRM), the
Physical LayerThe physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
of the OSI model corresponds to the X.25
physical layer, the
Data Link LayerThe data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
to the X.25
data link layer, and the
Network LayerThe network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
to the X.25
packet layer. The X.25
data link layer,
LAPBLink Access Procedure, Balanced implements the data link layer as defined in the X.25 protocol suite. LAPB is a bit-oriented protocol derived from HDLC that ensures that frames are error free and in the right sequence. LAPB is specified in and ISO/IEC 7776...
, 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
LAPBLink Access Procedure, Balanced implements the data link layer as defined in the X.25 protocol suite. LAPB is a bit-oriented protocol derived from HDLC that ensures that frames are error free and in the right sequence. LAPB is specified in and ISO/IEC 7776...
. 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 which provide for OSI
network layerThe network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
Addressing (NSAP addressing, see below).
User device support
X.25 was developed in the era of dumb terminals connecting to host computers, although it also can be used for communications between 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/DisassemblyA packet assembler/disassembler, abbreviated PAD is a communications device which provides multiple asynchronous terminal connectivity to an X.25 network or host computer. It collects data from a group of terminals and places the data into X.25 packets...
facility), a gateway device connecting modems and serial lines to the X.25 link as defined by the
X.29X.29 is the ITU-T standard, approved by ISO, specifying procedures for the exchange of control information and user data between a Packet Assembler/Disassembler , and a remote packet-mode data terminal equipment or another PAD...
and
X.3X.3 is an ITU-T standard indicating what functions are to be performed by a Packet Assembler/Disassembler when connecting character-mode data terminal equipment , such as a computer terminal, to a packet switched network such as an X.25 network, and specifying the parameters that control this...
standards.
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.121X.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 , connecting two network user addresses...
address format (or by giving a host name, if the service provider allows for names that map to
X.121X.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 , connecting two network user addresses...
addresses). The PAD then places an X.25 call to the host, establishing a virtual call. Note that X.25 provides for virtual calls, so
appears to be a circuit switched network, even though in fact the data itself is
packet switchedPacket switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...
internally, similar to the way TCP provides connections 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.
For the purpose of flow-control, a sliding window protocol is used with the default window size of 2. The acknowledgements 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 acknowledgement. When D=1, it means that the acknowledgement 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 acknowledged or even received the data.
While the PAD function defined by
X.28X.28 is an ITU-T standard specifying the interface between asynchronous character-mode data terminal equipment , such as computer terminals, and a Packet Assembler/Disassembler that connects the DTE to a packet switched network such as an X.25 network.-External links:*...
and
X.29X.29 is the ITU-T standard, approved by ISO, specifying procedures for the exchange of control information and user data between a Packet Assembler/Disassembler , and a remote packet-mode data terminal equipment or another PAD...
specifically supported asynchronous character terminals, PAD equivalents were developed to support a wide range of proprietary intelligent communications devices, such as those for IBM System Network Architecture (SNA).
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
virtual circuitIn telecommunications and computer networks, a virtual circuit , synonymous with virtual connection and virtual channel, is a connection oriented communication service that is delivered by means of packet mode communication...
s, Virtual Calls (VC) and Permanent Virtual Circuits (PVC). Virtual Calls are established on an as-needed basis. For example, a VC is established when a call is placed and torn down after the call is complete. VCs are established through a call establishment and clearing procedure. On the other hand, Permanent Virtual Circuits are preconfigured into the network. PVCs are seldom torn down and thus provide a dedicated connection between end points. Virtual Calls were also commonly referred to as Switched Virtual Circuits (SVC).
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.
NSAP addressA Network Service Access Point address , defined in ISO/IEC 8348, is an identifying label for a Service Access Point used in OSI networking....
ing facility was added in the X.25(1984) revision of the specification, and this enabled X.25 to better meet the requirements of
OSIOpen Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...
Connection Oriented Network Service (CONS)CONS, Connection-Oriented Network Service, is one of the two OSI network layer protocols, the other being CLNS...
. Public X.25 networks were not required to make use of NSAP addressing, but, to support OSI CONS, were required to carry the NSAP addresses and other ITU-T specified DTE facilities transparently from DTE to DTE. Later revisions allowed multiple addresses in addition to X.121 addresses to be carried on the same DTE-DCE interface: Telex addressing (
F.69Telex may refer to:* Telex , , a communications network** Teleprinter, the device used on the above network* Telex , a Belgian pop group...
),
PSTNThe public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
addressing (E.163),
ISDNIntegrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...
addressing (
E.164E.164 is an ITU-T recommendation that defines the international public telecommunication numbering plan used in the PSTN and some other data networks. It also defines the format of telephone numbers. E.164 numbers can have a maximum of fifteen digits and are usually written with a + prefix...
),
Internet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
addresses (IANA ICP), and local
IEEE 802.2IEEE 802.2 is the IEEE 802 standard defining Logical Link Control , which is the upper portion of the data link layer of the OSI Model. The LLC sublayer presents a uniform interface to the user of the data link service, usually the network layer...
MACThe media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
addresses.
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, although networks are not expected to support a full 4095 virtual circuits. 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. Logical channel identifiers remain assigned to a virtual circuit for the duration of the connection. Logical channel identifiers identify a specific logical channel between the
DTEData Terminal Equipment is an end instrument that converts user information into signals or reconverts received signals. These can also be called tail circuits. A DTE device communicates with the data circuit-terminating equipment...
(subscriber appliance) and the
DCEA data circuit-terminating equipment is a device that sits between the data terminal equipment and a data transmission circuit. It is also called data communications equipment and data carrier equipment...
(network), and only has local significance on the link between the subscriber and the network. The other end of the connection at the remote
DTEData Terminal Equipment is an end instrument that converts user information into signals or reconverts received signals. These can also be called tail circuits. A DTE device communicates with the data circuit-terminating equipment...
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. (Directions refer to the direction of virtual call initiation as viewed by the DTE—they all carry data in both directions.) 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. DTE-DCE interfaces are not required to support more than one logical channel. Logical channel identifier zero will not be assigned to a permanent virtual circuit or virtual call. 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. Link speeds varied, typically from 2400bit/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), charged to the caller (or callee in the case of reverse charged calls, where supported). Calls invoking the
Fast Select facility (allowing 128 bytes of data in call request, call confirmation and call clearing phases) 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 |
| Call setup and Clearing |
Incoming Call |
Call Request |
|
X |
|
|
Call Connected |
Call Accepted |
|
X |
|
|
Clear Indication |
Clear Request |
|
X |
|
|
Clear Confirmation |
Clear Confirmation |
|
X |
|
| Data and Interrupt |
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 (2
n 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
Data Terminal Equipment is an end instrument that converts user information into signals or reconverts received signals. These can also be called tail circuits. A DTE device communicates with the data circuit-terminating equipment...
constructs control packets which are encapsulated into data packets. The packets are sent to the data circuit-terminating equipment, using LAPBLink Access Procedure, Balanced implements the data link layer as defined in the X.25 protocol suite. LAPB is a bit-oriented protocol derived from HDLC that ensures that frames are error free and in the right sequence. LAPB is specified in and ISO/IEC 7776...
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. 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. 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 NetworkA public data network is a network established and operated by a telecommunications administration, or a recognized private operating agency, for the specific purpose of providing data transmission services for the public....
s (PDN). The ISO/IEC versions address additional features for private networks (e.g.
Local Area NetworkA local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
s (LAN) use) while maintaining compatibility with the CCITT/ITU-T specifications.
The user facilities and other features supported by each version of X.25 and ISO/IEC 8208 have varied from edition to edition. Several major protocol versions of X.25 exist:
- 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
- ITU-T Recommendation X.25 (1996) Grey Book
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. This is in stark contrast to the
Robustness PrincipleIn computing, the robustness principle is a general design guideline for software:The principle is also known as Postel's law, after Internet pioneer Jon Postel, who wrote in an early specification of the Transmission Control Protocol that:...
employed in the
Internet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
family.
Public networks were adopters of the earlier protocol versions, but reluctant to upgrade fearing subscriber compatibility issues and struggling to justify the expense. Most public networks ended up running something roughly on a parity with X.25 (1980) with some parts of X.25 (1984). Private networks started using X.25 later and were more likely to take upgrades, and many of those operated something nearer to X.25 (1984) with a few X.25 (1988) features. By about 1990, X.25 development by all the major network switch vendors had ceased, and there were no significant implementations of the 1993 and 1996 protocol versions.
In addition to the CCITT/ITU-T versions of the protocol, four editions of ISO/IEC 8208 exist:
- 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 also
- OSI protocol suite
- Packet switched network
A packet-switched network is a digital communications network that groups all transmitted data, irrespective of content, type, or structure into suitably sized blocks, called packets...
- protocols related to X.25
- Frame Relay
Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...
- has its technical base in X.25 packet-switching technology, but does not attempt to correct errors.
- DATAPAC
DATAPAC was Canada's packet switched X.25-equivalent data network. Operated first by Trans-Canada Telephone System, then Telecom Canada, then the Stentor Alliance, it finally reverted to Bell Canada when the Stentor Alliance was dissolved.-Use:...
- Canadian variant of X.25 offered by Bell CanadaBell Canada is a major Canadian telecommunications company. Including its subsidiaries such as Bell Aliant, Northwestel, Télébec, and NorthernTel, it is the incumbent local exchange carrier for telephone and DSL Internet services in most of Canada east of Manitoba and in the northern territories,...
- TRANSPAC - The French variant of X.25 public data network
- AUSTPAC
AUSTPAC was a public X.25 network operated by Telstra. Started by Telecom Australia in 1982, AUSTPAC was Australia's first public packet-switched data network, supporting applications such as online betting, financial applications and remote terminal access to academic institutions, some of which...
- An Australian public X.25 network
- XOT
XOT is a protocol developed by Cisco Systems that enables X.25 packets to be encapsulated and routed through TCP/IP connections instead of LAPB links....
- X.25 Over TCP : X25 encapsulation on TCPIP networks.
Further reading
- Computer Communications, lecture notes by Prof. Chaim Ziegler PhD, Brooklyn College
Brooklyn College is a senior college of the City University of New York, located in Brooklyn, New York, United States.Established in 1930 by the New York City Board of Higher Education, the College had its beginnings as the Downtown Brooklyn branches of Hunter College and the City College of New...
External links