Source : Free On-Line Dictionary of Computing

data service unit
     
         (DSU or "data service unit") A device used in
        digital transmission for connecting a CSU (Channel Service
        Unit) to {Data Terminal Equipment} (a terminal or computer),
        in the same way that a {modem} is used for connection to an
        analogue medium.
     
        A DSU provides a standard interface to a user's terminal which
        is compatible with {modems} and handles such functions as
        signal translation, regeneration, reformatting, and timing.
        The transmitting portion of the DSU processeses the customers'
        signal into bipolar pulses suitable for transmission over the
        digital facility.  The receiving portion of the DSU is used
        both to extract timing information and to regenerate mark and
        space information from the received {bipolar} signal.
     
        (1995-01-30)