Channel Service Unit is a device that converts a digital data frame from the communications technology used on a local area network (LAN) into a frame appropriate to a wide-area network (WAN) and vice versa. The DSU is a device that performs protective and diagnostic functions for a telecommunications line. Typically, the two devices are integrated into a single unit. It is actually a complicate and expensive modem. As an example of using the CSU/DSU, if you have leased a digital line (perhaps a T1, T3 or fractional T1/T3 line) for your business from a phone company or an ISP, you need a CSU/DSU at your end and the phone company or ISP need to have a CSU/DSU at its end.

The Channel Service Unit (CSU) receives and transmits signals from and to the WAN line and provides a barrier for electrical interference from either side of the unit. The CSU can also echo loopback signals from the phone company for testing purposes. The Data Service Unit (DSU) manages line control, and converts input and output between RS-232C, RS-449, or V.35 frames from the LAN and the time-division multiplexed (TDM) DSX frames on the T-1 line. The DSU manages timing errors and signal regeneration. The DSU provides a modem-like interface between the computer as Data Terminal Equipment (DTE) and the CSU.

CSU/DSUs are made as separate products or are sometimes part of a T-1 WAN card. A CSU/DSU's Data Terminal Equipment interface is usually compatible with the V.xx and RS-232C or similar serial interface.

Applications of a CSU/DSU

Related Terms: T1, T3, Modem