Source : Free On-Line Dictionary of Computing

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)