A DSU provides a standard interface to a user's terminal which

is compatible with modems and handles such functions as

signal translation, regeneration, reformatting, and timing.

The transmitting portion of the DSU processeses the customers'

signal into bipolar pulses suitable for transmission over the

digital facility. The receiving portion of the DSU is used

both to extract timing information and to regenerate mark and

space information from the received bipolar signal.