X.21

X.21 is a digital signaling interface recommended by ITU-T that 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, data transfer at up to 2 Mbit/s, and test loops. 64 kbit/s is the most commonly used transfer rate.

Article based on X.21 (http://foldoc.doc.ic.ac.uk/foldoc/foldoc.cgi?query=X.21) at FOLDOC (http://www.foldoc.org), used with permission.

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