Editing Channel service unit

{{short description|Telecommunications equipment}}
{{no footnotes|date=January 2013}}

In [[telecommunications]], a '''channel service unit''' ('''CSU''') is a [[telephone line|line]] [[Bridging (networking)|bridging]] device for use with [[T-carrier]], which
*is used to perform [[loopback]] testing;
*may perform [[bit stuffing]];
*may also provide a [[framing (telecommunication)|framing]] and formatting pattern compatible with the [[telecommunications network|network]];
*provides a barrier for electrical interference from either side of the unit; and
*is the last signal regeneration point, on the loop side, coming from the [[telephone exchange|central office]], before the regenerated signal reaches a [[multiplexer]] or [[data terminal equipment]] (DTE).

== Common varieties ==
CSUs can be categorized by the class of service they support ([[Digital Signal 1|DS1]], [[Digital Signal 3|DS3]], DDS, etc.) and by the capabilities within that class. 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.

== Common practice ==
CSUs are required by [[PSTN]] providers at digital interfaces that terminate in a [[Data service unit|DSU]] on the customer side. They are not used when the service terminates in a modem, such as the [[Digital subscriber line|DSL]] family of service. The maintenance capabilities of the CSU provide important guidance as to whether the provider needs to dispatch a repairman to the customer location.

== See also ==
* [[CSU/DSU]]

==References==
{{FS1037C}}

[[Category:Telecommunications equipment]]