CSU/DSU - Telecommunications
A CSU/DSU manages digital transmission, monitors signals for problems, and responds to Central Office commands. It performs many of the functions that modems do, but it does not have to convert digital signals to/from analog, since the end device and the underlying transmission facility are both digital.
Specifically, a CSU/DSU is the device that actually transmits the data across the telephone company line with electrical pulses of +5 and -5 Volts. It also handles framing of the signal with extra bits used to group a series of pulses into a larger structure. Often, this is called ESF, or Extended Super Frame, in the case of T1 lines. The CSU/DSU can perform testing, such as loop-back tests, from either side of the line using the messaging link, and finally, the CSU/DSU either synchronizes its timing to the line or can generate timing on the circuit so that each end can communicate effectively.