The pair 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 a fax machine or dial-up internet connection.
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.
Analog pair gain came into disfavor in the 21st century, as it is detrimental to high speed dial-up modem connections, does not support 56k and is incompatible with digital subscriber line (DSL) systems.
Where digital 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 as ADSL are unavailable due to infrastructure issues.