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 at FOLDOC, used with permission.