The Free On-line Dictionary of Computing (30 December 2018):

data service unit
digital 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)