X.21 (sometimes referred to as X21) is an interface specification for differential communications introduced in the mid-1970s by the

ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...

. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications between carriers and customers' equipment. This includes specifications for DTE/ DCE physical interface elements, alignment of

call control In telephony, call control refers to the software within a telephone switch that supplies its central function. Call control decodes addressing information and routes telephone calls from one end point to another. It also creates the features that c ...

characters and

error checking In information theory and coding theory with applications in computer science and telecommunication, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communica ...

, elements of the call control phase for

circuit switching Circuit switching is a method of implementing a telecommunications network in which two network nodes establish a dedicated communications channel ( circuit) through the network before the nodes may communicate. The circuit guarantees the full b ...

services, and test loops. When X.21 is used with V.11, it provides synchronous data transmission at rates from 600 bit/s to 10 Mbit/s. There is also a variant of X.21 that is only used in select legacy applications, “circuit switched X.21”. X.21 normally is found on a 15-pin D-sub connector and is capable of running full-duplex data transmissions. The Signal Element Timing, or clock, is provided by the carrier (the telephone company), and is responsible for correct clocking of the data. X.21 was primarily used in Europe and Japan, for example in the Scandinavian DATEX and

German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ger ...

circuit-switched networks during the 1980s.

References

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used with permission.''

External links

An X.21 brief Tutorial and overviewX.21 pinout and some explanations
Physical layer protocols ITU-T recommendations ITU-T X Series Recommendations {{compu-network-stub