(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.