Source : Free On-Line Dictionary of Computing

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)