Channel Service Unit (CSU)

A line bridging device for use with T-carriers that:

Is used to perform loopback testing - a method of performing transmission tests of access lines from the serving switching center, which usually does not require the assistance of personnel at the served terminal, OR a method of testing between stations (not necessarily adjacent) wherein two lines are used, with the testing being done at one station and the two lines interconnected at the distant station.

May perform bit stuffing - the insertion of noninformation bits into data. Bit stuffing is used for various purposes, such as for bringing bit streams that do not necessarily have the same or rationally related bit rates up to a common rate, or to fill buffers or frames. Bit stuffing may be used to synchronize several channels before multiplexity or to rate-match to single channels to each other.

May also provide a framing and formatting pattern compatible with the network.
Is the last signal regeneration point, on the loop side, coming from the CO (Central Office), before the regenerated signal reaches a multiplexer or data terminal equipment.

CSUs can be categorized by the class of service they support (DDS, DS1, DS3, etc.) and by the capabilities within that class. For example, basic DS1 (T1) CSUs support loopback of each interface and will produce an alarm indication signal (a signal transmitted by a system that is part of a concatenated telecommunications system to let the receiver know that some remote part of the end-to-end link has failed at a logical or physical level, even if the system it is directly connected to is still working) to the provider's network interface device 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 (Public Switched Telephone Networks) providers at digital interfaces that terminate in a DSU (Data Service Unit) on the customer side. They are not used when the service terminates in a modem, such as the DSL (Digital Subscriber Line) family of services. The maintenance capabilities of the CSU provide important guidance as to whether the provider needs to dispatch a repair technician to the customer location.