Digital loop carrier

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. 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, 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. 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 – This violation of operating standards could pose a safety hazard. Add amplifiers to power the voice signal on long loops. This requires volumes of auxiliary equipment, many cross wiring points and extensive record-keeping.