Source : Free On-Line Dictionary of Computing

V.21
     
         An {ITU-T} {modem} {protocol} for 300 bits per
        second two-wire {full-duplex} communications using {Frequency
        Shift Keying} {modulation}.
     
        All modern modems support both V.21 and the close variant,
        Bell 103.
     
        (1996-07-22)