Editing X.25 (section)

==Architecture==
The general concept of the X.25 was to create a universal and global [[packet switched network|packet-switched network]]. 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.75]], a protocol very similar 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{{snd}} many X.25 network 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 additional networks such as ISO 8802 LLC2 (ISO LAN) and the OSI data link layer.<ref>ISO 8208:2000</ref>

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''.<ref>ISO 8208, Annex B.</ref> The layer numbers were dropped to avoid confusion with the OSI Model layers.<ref name=draft1976/>

* 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 [[LAPB|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 "[[Virtual call capability|virtual calls]]" through the network. These calls interconnect [[Data terminal equipment|"data terminal equipment" (DTE)]] providing endpoints to users, which looked like [[Point-to-point (telecommunications)|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.3]], [[X.28]] and [[X.29]]. This functionality was performed using a [[packet assembler/disassembler]] 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 model|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''.<ref name="Friend 1988 230"/> 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<ref>[http://www.itu.int/rec/T-REC-X.25-199610-I/en/ ITU-T Recommendation X.25], G.3.2 Called address extension facility, pp. 141–142.</ref> which provide for OSI [[network layer]] Addressing (NSAP addressing, see below).<ref>[http://www.itu.int/rec/T-REC-X.223/en/ ITU-T Recommendation X.223], Appendix II.</ref>

===User device support===
[[Image:Televideo925Terminal.jpg|thumb|A [[Televideo]] terminal model 925 made around 1982]]
X.25 was developed in the era of [[computer terminal]]s connecting to host computers, although it also can be used for communications between computers. Instead of dialing directly “into” the host computer{{snd}} 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{{snd}} 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 assembler/disassembler|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 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 call capability|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 switching|packet switched]] 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.28]] and [[X.29]] 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.