# Channel service unit

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In [telecommunications](/source/Telecommunications), a **channel service unit** (**CSU**) is a [line](/source/Telephone_line) [bridging](/source/Bridging_(networking)) device for use with [T-carrier](/source/T-carrier), which

- is used to perform [loopback](/source/Loopback) testing;
- may perform [bit stuffing](/source/Bit_stuffing);
- may also provide a [framing](/source/Framing_(telecommunication)) and formatting pattern compatible with the [network](/source/Telecommunications_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 [central office](/source/Telephone_exchange), before the regenerated signal reaches a [multiplexer](/source/Multiplexer) or [data terminal equipment](/source/Data_terminal_equipment) (DTE).

## Common varieties

CSUs can be categorized by the class of service they support ([DS1](/source/Digital_Signal_1), [DS3](/source/Digital_Signal_3), 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](/source/Alarm_indication_signal) to the provider's [network interface device](/source/Network_interface_device) (NID) in the event of loss of signal from the [customer-premises equipment](/source/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](/source/PSTN) providers at digital interfaces that terminate in a [DSU](/source/Data_service_unit) on the customer side. They are not used when the service terminates in a modem, such as the [DSL](/source/Digital_subscriber_line) 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](/source/CSU/DSU)

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Adapted from the Wikipedia article [Channel service unit](https://en.wikipedia.org/wiki/Channel_service_unit) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Channel_service_unit?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.