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. This is typically used as an expedient way to solve subscriber line shortage problems by using existing wiring, instead of installing new wires from the central office to the customer premises. The term was invented in the middle 20th century by analogy with earlier use of gain to extend telephone local loops far from the telephone exchange.

A pair gain system consists of concentrators or multiplexers which combine the separate signals into a single signal which is transmitted through the existing copper cable pair. The signals are then separated into individual subscriber lines at the customer premises. The pair gain unit which performs the multiplexing can simply 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 a fax machine or dial-up internet connection. A larger analog pair gain system made by Anaconda 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 in serving area interfaces or metal cabinets typically resembling small apartment-sized refrigerators alongside or near roadways that overlie communications rights-of-way.

DACS (Digital Access Carrier System) is a form of pair gain used in the United Kingdom. It uses a form of time division multiple access called ISDN.

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