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

channel service unit

    (CSU) A type of interface used to connect a
   terminal or computer to a digital medium in the same way
   that a modem is used for connection to an analogue medium.

   A CSU is provided by the communication carrier to customers
   who wish to use their own equipment to retime and regenerate
   the incoming signals.  The customer must supply all of the
   transmit logic, receive logic and timing recovery in order to
   use the CSU, whereas a digital service unit DSU performs
   these functions.

   (1995-01-30)