See CSU/DSU

A piece of hardware that you use to translate the digital data frames of a T1 line into a 10BaseT connection where Internet connectivity is concerned....

Data Service Unit/Channel Service Unit. The devices used to access digital data channels are called DSU/CSUs (Data Service Unit/Channel Service Units). At the customer's end of the telephone connection, these devices perform much the same function for digital circuits that modems provide for analog connections. For example, DSU/CSUs take data from terminals and computers, encode it, and transmit it down the link. At the receive end, another DSU/CSU equalizes the received signal, filters it, and decodes it for interpretation by the end-user.