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Intelephony,pair gain is the transmission of multipleplain old telephone service (POTS) channels over thetwisted pairlocal loop traditionally used for a singlesubscriber line intelephone systems. Pair gain has the effect of creating additional subscriber lines. This is typically used as an expedient way to solve subscriber line shortages at a location by using existing wiring, instead of installing new wires from thecentral office to the customer premises. The term was invented in the middle 20th century by analogy with earlier use ofgain to extend telephone local loops far from thetelephone exchange.
Apair gain system consists ofconcentrators ormultiplexers which combine the individual channels into a single signal that is transmitted through the existing copper cable pair. The signals are then separated into individual subscriber lines at the customer premises. Thepair gain unit which performs the multiplexing can provide a second telephone connection over a single subscriber line (called an Analog Multi-Line Carrier or AML) in circumstances where a customer wants to add a new phone line for afax machine ordial-up internet connection. A larger analog pair gain system made byAnaconda in the 1960s provided seven lines. Some pair gain units can expand the number of subscriber lines available over a single copper pair to as many as sixty. Large pair gain units are stored inserving area interfaces or metalcabinets typically resembling small apartment-sizedrefrigerators alongside or near roadways that overlie communications rights-of-way.
DACS (Digital Access Carrier System) is a form of pair gain used in theUnited Kingdom. It uses a form oftime-division multiple access calledISDN.
Analog pair gain came into disfavor in the 21st century, as it is detrimental to high speed dial-up modem connections, does not support56k and is incompatible withdigital subscriber line (DSL) systems. 20th centurySubscriber Loop Carrier systems had similar problems. More recent digital pair gain systems, however, restore 56k and DSL capabilities by performing the functions of aDSLAM at the pair gain device.
More recently, the termpair gain has been used to refer toany multiplex/demultiplex unit used between thecentral office and end users, not just equipment used with copper twisted pair.
Wheredigital loop carrier (called Remote Integrated Multiplexer in Australia) is installed,broadband Internet subscribers may be disappointed when informed that although initial line checks were successful, high speed connections such asADSL are unavailable due to infrastructure issues. The problem is that pair gain lines cannot carry high speed data.[1]
