(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.