A digital communications device that combines the functions of both a Channel Service Unit (CSU) and a Data Service Unit (DSU). These devices lie between the telephone company network and the customer network at the demarcation point and are the local interfaces between the data terminal equipment (DTE) at the customer premises and the telco’s digital communications line (such as a T1 line).
CSU/DSUs essentially function as the digital counterpart to analog modems. They are typically external units that look similar to an external modem, but they can also come in sizes that can be mounted in a rack. Unlike analog modems, CSU/DSUs do not perform signal conversion because the signal at both ends is already digital. CSU/DSUs package digital data into a format suitable for the particular digital transmission line they are servicing, and buffer and rate-adapt digital signals going to and from the telephone company network. CSU/DSUs ensure that data frames are properly formed and timed for the telephone company network and provide a protective barrier to electrical disturbances that can harm customer premises equipment (CPE).
Digital lines usually terminate at customer premises with four-wire connections having various connector types, including RJ-45, four-screw terminal blocks, and M-block connectors (used for V.35 interfaces). The four-wire connection is joined to the appropriate connector on the CSU/DSU. The CSU/DSU typically adjusts itself to the line speed of the digital data service (DDS) line using an autosensing feature. The customer’s CSU/DSU then connects directly to the customer’s router, and from there connects to the customer’s network.
At the other end of the DDS line at the central office (CO), the telco has a similar CSU that interfaces with a multiplexer to feed into the carrier’s backbone network.
When purchasing CSU/DSUs, consider first the traffic requirements of your wide area network (WAN) link and make sure they support the full range of data rates for the DDS lines you plan to use (56 K, 64 K, or T1 speed).