x.21

from The Free On-line Dictionary of Computing (8 July 2008)

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.

   (2000-02-28)