X.21

X.21

(communications, standard)

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 switched services, data transfer at up to 2 Mbps, and test loops. 64 kbps is the most commonly used transfer rate.

X.21

An ITU standard protocol for a circuit switching network.