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:
“Women who marry early are often overly enamored of the kind of man who looks great in wedding pictures and passes the maid of honor his telephone number.”
—Anna Quindlen (b. 1952)
“Men will not give up their privilege of helplessness without a struggle. The average man has a carefully cultivated ignorance about household matters—from what to do with the crumbs to the grocer’s telephone number—a sort of cheerful inefficiency which protects him better than the reputation for having a violent temper.”
—Crystal Eastman (1881–1928)
“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)