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X.21, sometimes referred to as X21, interface is a specification for differential communications introduced in the mid 1970’s by the ITU-T. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications between carriers and customer’s equipment. This includes specifications for DTE/DCE physical interface elements, alignment of call control characters and error checking, elements of the call control phase for circuit switching services, and test loops.
When X.21 is used with V.11, it provides synchronous data transmission at rates from 100 kbit/s to 10 Mbit/s. There is also a variant of X.21 which 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 (your 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 DATEX-L circuit switched networks during the 1980s.
References
Article based on X.21 at FOLDOC, and X.21 Pinouts, used with permission.
