 With the advent of sophisticated electronics in the early part of the 1990s, telephone service providers developed methods to share a single copper line to transmit multiple telephone calls simultaneously.Various pair gain methods using time-division multiplexing and frequency-division multiplexing prevented interference between simultaneous calls.A distant suburb may have a subscriber loop carrier or digital loop carrier system in which a remote concentrator is located near the subscribers to connect multiple local subscriber loops to one common line to a central office exchange.