telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...

s, a channel service unit (CSU) is a

line Line most often refers to: * Line (geometry), object with zero thickness and curvature that stretches to infinity * Telephone line, a single-user circuit on a telephone communication system Line, lines, The Line, or LINE may also refer to: Arts ...

bridging device for use with

T-carrier The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls. The first version, the Transmission System 1 (T1), was introduced in 1962 in the Bell System ...

, which *is used to perform

loopback Loopback (also written loop-back) is the routing of electronic signals or digital data streams back to their source without intentional processing or modification. It is primarily a means of testing the communications infrastructure. There are m ...

testing; *may perform

bit stuffing In data transmission and telecommunication, bit stuffing (also known—uncommonly—as positive justification) is the insertion of non-information bits into data. Stuffed bits should not be confused with overhead bits. Bit stuffing is used for ...

; *may also provide a framing and formatting pattern compatible with the

network Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...

; *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, before the regenerated signal reaches a

multiplexer In electronics, a multiplexer (or mux; spelled sometimes as multiplexor), also known as a data selector, is a device that selects between several analog or digital input signals and forwards the selected input to a single output line. The sel ...

data terminal equipment Data terminal equipment (DTE) is an end instrument that converts user information into signals or reconverts received signals. These can also be called tail circuits. A DTE device communicates with the data circuit-terminating equipment (DCE) ...

(DTE).

Common varieties

CSUs can be categorized by the class of service they support ( DS1, 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 Alarm indication signal (AIS) (also called “all ones” because of the data and framing pattern) is a signal transmitted by an intermediate element of a multi-node transport circuit that is part of a concatenated telecommunications system to ale ...

to the provider's

network interface device upTwo simple NIDs, carrying six lines each, on the outside of a building upA German copper phone line termination box called '' Abschlusspunkt LinienTechnik'' (APL, " Demarcation point") In telecommunications, a network interface device (NID; ...

(NID) in the event of loss of signal from the

customer-premises equipment In telecommunications, a customer-premises equipment or customer-provided equipment (CPE) is any terminal and associated equipment located at a subscriber's premises and connected with a carrier's telecommunication circuit at the demarcation poi ...

(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 The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local teleph ...

providers at digital interfaces that terminate in a DSU on the customer side. They are not used when the service terminates in a modem, such as the

DSL Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric dig ...

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.

References

{{FS1037C Telecommunications equipment

Common varieties

Common practice

See also

References