In telecommunications, a channel service unit (CSU) is a line bridging device for use with T-carrier, which CSUs can be categorized by the class of service they support (DS1, DS3, DDS, etc.)
For example, basic DS1 (T1) CSUs support loopback of each interface and will produce alarm indication signal to the provider's network interface device (NID) in the event of loss of signal from the customer-premises equipment (CPE).
More advanced units will include internal monitors of the performance of the carrier in both directions and may have test pattern generation and monitor capabilities.
CSUs are required by PSTN providers at digital interfaces that terminate in a DSU on the customer side.
This article incorporates public domain material from Federal Standard 1037C.