Telstra roadside cabinet housing a RIM and CMUX

A digital loop carrier (DLC) is a system which uses

digital Digital usually refers to something using discrete digits, often binary digits. Technology and computing Hardware *Digital electronics, electronic circuits which operate using digital signals ** Digital camera, which captures and stores digital ...

transmission to extend the range of the

local loop In telephony, the local loop (also referred to as the local tail, subscriber line, or in the aggregate as the last mile) is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the commo ...

farther than would be possible using only

twisted pair Twisted pair cabling is a type of wiring used for communications in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted b ...

copper wires. A DLC digitizes and multiplexes the individual signals carried by the local loops onto a single datastream on the DLC segment.

Reasons for using DLCs

Subscriber Loop Carrier systems address a number of problems: *Electrical constraints on long loops. *Insufficient available cable pairs. *Cable route congestion (inability to add cable due to lack of space, particularly in urban street, bridge, and building conduit) *Construction challenges (in areas of difficult terrain) when limited cable pairs are already available *Expense due to cable cost and the associated labour-intensive installation work (especially to solve the specific problems listed above) Long loops, such as those terminating at more than 18,000 feet (5.49 kilometres) from the central office, pose electrical challenges. When the subscriber goes

off-hook In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...

, a cable pair behaves like a single loop inductance coil with a -48 V dc

potential Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social sciences to indicate things that are in a state where they are able to change in ways ranging from the simple r ...

and an

Electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movi ...

of between 20–50 mA dc. Electric current values vary with cable length and gauge. A minimum current of around 20 mA dc is required to convey terminal signalling information to the network. There is also a minimum power level required to provide adequate volume for the voice signal. A variety of schemes were implemented before DLC technology to offset the impedance long loops offered to signalling and volume levels. They included the following: *Use heavy-gauge conductors – Up to 19

gauge Gauge ( or ) may refer to: Measurement * Gauge (instrument), any of a variety of measuring instruments * Gauge (firearms) * Wire gauge, a measure of the size of a wire ** American wire gauge, a common measure of nonferrous wire diameter, es ...

(approximately the gauge of pencil lead), which is costly and bulky. The heavy-gauge cables yielded far fewer pairs per cable and led to early congestion in cable routes, especially in bridge crossings and other areas of limited space. *Increase battery

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...

– This violation of operating standards could pose a safety hazard. *Add

amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost t ...

s to power the voice signal on long loops. This requires volumes of auxiliary equipment, many cross wiring points and extensive record-keeping. *Add signal regeneration and signal extension equipment – The comments regarding amplifiers apply here as well. *Add

loading coil A loading coil or load coil is an inductor that is inserted into an electronic circuit to increase its inductance. The term originated in the 19th century for inductors used to prevent signal distortion in long-distance telegraph transmission c ...

s to reduce the attenuation of voice signals over long loops. These have detrimental effect to new transmission technologies using the local loop, like

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 ...

, and must be removed. DLC eliminates the need for these remedies by extending closer to the customer the

line card A line card or digital line card is a modular electronic circuit designed to fit on a separate printed circuit board (PCB) and interface with a telecommunications access network. A line card typically interfaces the twisted pair cable of a plain ...

which digitises the voice signal for use by the

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 ...

. Once the voice signal is digitised, it is easily manipulated and is no longer subject to the vagaries of the

analog Analog or analogue may refer to: Computing and electronics * Analog signal, in which information is encoded in a continuous variable ** Analog device, an apparatus that operates on analog signals *** Analog electronics, circuits which use analog ...

loop caused by distance, impedance,

attenuation In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at var ...

and

noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...

. The DLC solution was dubbed " pair gain" (from the days when DLC was deployed to recover copper pairs in the loop plant environment).

Configuration

In a typical configuration, DLC remote terminals are installed in new neighbourhoods or buildings as a means of reducing the labour and complexity of installing individual local loops from the customer to the central office (CO). A fibre optic cable or several copper pairs for the whole system from the CO to the DLC remote terminal replace the individual pair previously needed for each loop. DLC remote terminals are typically stored in

Serving Area Interface The serving area interface or service area interface (SAI) is an outdoor enclosure or metal box that allows access to telecommunications wiring. Alternate names *Access point (AP) *Cabinet (cab) *B-box (breakout box) *Cross box *Cross-connect b ...

s–metal cabinets alongside or near roadways that overlie communications rights-of-ways. With the growth in popularity of

digital subscriber line 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 ...

(DSL) and the benefits provided by shorter metallic loops used with DLC systems, digital loop carriers are sometimes integrated with

digital subscriber line access multiplexer A digital subscriber line access multiplexer (DSLAM, often pronounced ''DEE-slam'') is a network device, often located in telephone exchanges, that connects multiple customer digital subscriber line (DSL) interfaces to a high-speed digital co ...

s (DSLAM), both systems then taking advantage of the digital transmission link from the DLC to the CO. Fibre in the loop (FITL) systems are functionally equivalent to DLC. FITL accomplishes the same two primary functions DLC was intended for: pair gain and the elimination of electrical constraints due to long metallic loops. FITL architectures vary from simply deploying fibre feeder plants (between central office and remote terminal site) to " fibre to the curb" and, ultimately " fibre to the home" where an optical network unit (ONU) is located at each home. See also

Remote concentrator In modern telephony a remote concentrator, remote concentrator unit (RCU), or remote line concentrator (RLC) is a concentrator at the lowest level in the telephone switch hierarchy. Subscribers' analogue telephone/PSTN lines are terminated on co ...

References

Local loop