Digital loop carrier
Encyclopedia
A digital loop carrier (DLC) is a system which uses digital
transmission to extend the range of the local loop
farther 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.
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
, a cable pair behaves like a single loop
inductance coil
with a -48 V dc potential
and an Electric current
of between 20–50 mA
dc. Electrical 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:
DLC eliminates the need for these remedies by extending out closer to the customer the line card
which digitises the voice signal for use by the PSTN. 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
and noise
.
The DLC solution was dubbed "pair gain
" (from the days when DLC was deployed to recover copper pairs in the loop plant environment).
s–metal cabinets alongside or near roadways that overlie communications rights-of-ways.
With the growth in popularity of digital subscriber line
(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
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
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
transmission to extend the range of the local loop
Local loop
In telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network...
farther than would be possible using only twisted pair
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
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
Off-hook
In telephony, the term off-hook has the following meanings:# The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece , which hung...
, a cable pair behaves like a single loop
inductance coil
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
with a -48 V dc potential
Electric potential
In classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...
and an Electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
of between 20–50 mA
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
dc. Electrical 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
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
long loops offered to signalling and volume levels. They included the following:
- Use heavy-gauge conductors – Up to 19 gaugeAmerican wire gaugeAmerican wire gauge , also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States and Canada for the diameters of round, solid, nonferrous, electrically conducting wire...
(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 voltageVoltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
– This violation of operating standards could pose a safety hazard. - Add amplifierAmplifierGenerally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s to power the voice signal on long loops. This however, requires volumes of auxiliary equipment, a myriad number of cross wiring points, and extensive record-keeping. - Add signal regeneration and signal extension equipment – The comments regarding amplifiers apply here as well.
- Add loading coilLoading coilIn electronics, a loading coil or load coil is a coil that does not provide coupling to any other circuit, but is inserted in a circuit to increase its inductance. The need was discovered by Oliver Heaviside in studying the disappointing slow speed of the Transatlantic telegraph cable...
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, and must be removed.
DLC eliminates the need for these remedies by extending out closer to the customer the line card
Line card
A line card or Digital Line Card is a modular electronic circuit on a printed circuit board, the electronic circuits on the card interfacing the telecommunication lines coming from the subscribers to the rest of the telecommunications access network.A line card commonly interfaces the twisted pair...
which digitises the voice signal for use by the PSTN. 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
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
, attenuation
Attenuation
In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...
and noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
.
The DLC solution was dubbed "pair gain
Pair gain
In telephony, pair gain is a method of transmitting multiple POTS signals over the twisted pairs traditionally used for a single traditional subscriber line in telephone systems. Pair gain has the effect of creating additional subscriber lines...
" (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 InterfaceServing area interface
The serving area interface or service area interface often called B-box, cross-connect box, cross box, ped , or access point is an outdoor telecommunications cabinet usually mounted on the ground on cable right-of-ways, but sometimes also on telephone poles...
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
Digital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
(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
Digital subscriber line access multiplexer
A digital subscriber line access multiplexer is a network device, located in the telephone exchanges of the telecommunications operators. It connects multiple customer digital subscriber line interfaces to a high-speed digital communications channel using multiplexing techniques...
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
Remote concentrator
In modern telephony a remote concentrator, Remote Concentrator Unit , or Remote Line Concentrator is the lowest level in the telephone switch hierarchy.Subscribers' analogue telephone/PSTN lines are terminated on concentrators...