Editing X.21 (section)

{{Short description|ITU-T telecommunications interface specification}}
{{other uses|X21 (disambiguation)}}
[[File:Backpanel of InterCOM 98 INTELLIGENT COMMUNICATION ADAPTER PC98 6751B (15986540312).jpg|thumb|X.21 adaptor for a Japanese [[NEC PC-98]].]]{{OSI model|expanded=physical}}
'''X.21''', also referred to as X21, is an interface specification for [[Differential signaling|differential]] communications introduced in the mid-1970s by the [[CCITT]], now ITU-T. X.21 was the first [[Digital signal|digital signaling interface]] developed.<ref>{{Cite journal |last=Tanenbaum |first=Andrew S. |date=1 December 1981 |title=Network Protocols |journal=ACM Computing Surveys |volume=13 |issue=4 |pages=453–489 |doi=10.1145/356859.356864 |via=ACM}}</ref> The connector specification is defined by the ISO document 4903.<ref>{{Cite journal |date=1 October 1989 |title=ISO4903 15-pole DTE/DCE interface connector and contact number assignments |url=https://cdn.standards.iteh.ai/samples/10923/349d6f02e89c4384bacacc84eb9522ad/ISO-4903-1989.pdf |journal=ISO International Standard |edition=2}}</ref>

At the time, most physical layer protocols such as [[RS-232-C]] and [[RS-449]] use [[analog signal]]ing. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications. 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.

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]] {{ill|Datex-L|de}} circuit-switched networks during the 1980s.

A variant of X.21 can be found in select legacy applications as “circuit switched X.21”.