X.21

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.

(2000-02-28)