Pair gain

From Free net encyclopedia

In telephony, pair gain is a method of transmitting multiple POTS signals over a single traditional subscriber line used in telephone systems, in effect 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.

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 pair. The signals are then separated into individual subscriber lines at the customer premises. The pair gain unit which performs the multiplexing can be as simple as providing two telephone connections over a single subscriber line (called an Analog Multi-Line Carrier) in circumstances where a customer wants to add a new phone line for a fax machine or dial-up internet connection. Some pair gain units can expand the number of subscriber lines available over a single copper pair to as many as thirty-four. Large pair gain units are stored in metal cabinets typically resembling small apartment-sized refrigerators alongside or near roadways that overlie communications rights-of-ways.

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

Pair gain has come under some scrutiny in recent years, as it is detrimental to 56k dial-up modem connections, and is incompatible with Digital Subscriber Line (DSL) systems.