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. Businesses *Digital bank, a form of financial institution *Digital Equipment Corporation (DEC) or Digital, a computer company *Digital Research (DR or DRI), a software ...

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 co ...

further than would be possible using only twisted pair 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 ...

, 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 flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge c ...

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 (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 (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

– 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 is a two-port electronic circuit that uses electric power from a power su ...

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 coils 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 di ...

, and must be removed. DLC eliminates the need for these remedies by extending closer to the customer the line card which digitises the voice signal for use by the

PSTN The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists ...

. Once the voice signal is digitised, it is easily manipulated and is no longer subject to the vagaries of the analog loop caused by distance, impedance,

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

and

noise Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...

. 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 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 achieves 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

References

Local loop