Channel service unit

In telecommunication, a channel service unit (CSU) is a line bridging device that (a) is used to perform loop-back testing, (b) may perform bit stuffing, (c) may also provide a framing and formatting pattern compatible with the network, and (d) is the last signal regeneration[?] point, on the loop side, coming from the central office, before the regenerated signal reaches a multiplexer or data terminal equipment (DTE).

Source: from Federal Standard 1037C

See also: CSU/DSU[?]