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:
“The telephone gives us the happiness of being together yet safely apart.”
—Mason Cooley (b. 1927)
“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)
“It is possible that the telephone has been responsible for more business inefficiency than any other agency except laudanum.... In the old days when you wanted to get in touch with a man you wrote a note, sprinkled it with sand, and gave it to a man on horseback. It probably was delivered within half an hour, depending on how big a lunch the horse had had. But in these busy days of rush-rush-rush, it is sometimes a week before you can catch your man on the telephone.”
—Robert Benchley (1889–1945)