(Digital (or Data) Service Unit/Channel Service Unit) A pair of communications devices that connect an inhouse line to an external digital circuit (T1, DDS, etc.). It is similar to a modem, but connects a digital circuit rather than an analog one. The CSU terminates the external line at the customer's premises. It also provides diagnostics and allows for remote testing. If the customer's communications devices are T1 ready and have the proper interface, then the CSU is not required, only the DSU. The DSU does the actual transmission and receiving of the signal and provides buffering and flow control. The DSU and CSU are often in the same unit. The DSU may also be built into the multiplexor, commonly used to combine digital signals for high-speed lines.