Telephone
For long distance telephone connections, 20th century telephone companies used L-carrier and similar co-axial cable systems carrying thousands of voice circuits multiplexed in multiple stages by channel banks.
For shorter distances, cheaper balanced pair cables were used for various systems including Bell System K- and N-Carrier. Those cables didn't allow such large bandwidths, so only 12 voice channels (Double Sideband) and later 24 (Single Sideband) were multiplexed into four wires, one pair for each direction with repeaters every several miles, approximately 10 km. See 12-channel carrier system. By the end of the 20th Century, FDM voice circuits had become rare. Modern telephone systems employ digital transmission, in which time-division multiplexing (TDM) is used instead of FDM.
Since the late 20th century Digital Subscriber Lines have used a Discrete multitone (DMT) system to divide their spectrum into frequency channels.
The concept corresponding to frequency-division multiplexing in the optical domain is known as wavelength division multiplexing.
Read more about this topic: Frequency-division Multiplexing
Famous quotes containing the word telephone:
“Language is as real, as tangible, in our lives as streets, pipelines, telephone switchboards, microwaves, radioactivity, cloning laboratories, nuclear power stations.”
—Adrienne Rich (b. 1929)
“The press and politicians. A delicate relationship. Too close, and danger ensues. Too far apart and democracy itself cannot function without the essential exchange of information. Creative leaks, a discreet lunch, interchange in the Lobby, the art of the unattributable telephone call, late at night.”
—Howard Brenton (b. 1942)
“But even in a telephone booth
evil can seep out of the receiver
and we must cover it with a mattress,
and then tear it from its roots
and bury it,
bury it.”
—Anne Sexton (1928–1974)