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Because digital signaling on the telephone network is different than the digital signaling used by computer equipment, special devices are required to connect the two types of transmission facilities together. The CSU/DSU is part of the hardware you need to connect digital computer equipment to telephone company digital transmission lines (T1 and T3 lines).

CSUs (channel service units) and DSUs (data service units) are actually two separate devices, as described next, but they are used in conjunction and often combined into the same box.

• Channel service unit    This inexpensive device connects with the digital communication line and provides a termination for the digital signal. The CSU provides various loop-back tests on the line and keeps the line connected if the other communication equipment attached to it fails. It also provides synchronization (clocking) with similar devices.


• Data service unit    This device, sometimes called a digital service unit, is the hardware component you need to transmit digital data over the hardware channel. The device converts signals from bridges, routers, and multiplexers into the bipolar digital signals used on the telephone company digital lines and ensures that voltage levels are correct.