Source : Free On-Line Dictionary of Computing

channel service unit/data service unit
     
         (CSU/DSU, or "..digital..") A
        device that performs both the {channel service unit} (CSU) and
        {data service unit} (DSU) functions.  The Channel Service Unit
        (CSU) is used to terminate a {DS1} or {DS0} (56/64 kb/s)
        digital circuit.  It peforms {line conditioning}, protection,
        {loop-back} and timing functions.  The Data Service Unit (DSU)
        terminates the data circuit to the {Data Terminal Equipment}
        (DTE) and converts the customer's data stream into a bi-polar
        format for transmission.
     
        (2001-10-19)