V.21

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

V.21

   <protocol, standard> An {ITU-T} {modem} {protocol} for 300
   {bps} 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)