X.21 (sometimes referred to as X21) is an interface specification for Differential signaling, 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. This includes specifications for Data terminal equipment, DTE/Data circuit-terminating equipment, DCE physical interface elements, alignment of call control characters and error control, error checking, elements of the call control phase for circuit switched, circuit switching services, and test loops.
When X.21 is used with ITU-T V-Series Recommendations, 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 Germany, German circuit-switched networks during the 1980s.
References
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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
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