AT&T MERLIN LEGEND Release 3.1 System Manager's Manual page 367

About Telecommunications
Coaxial Cable. Consists of a number of one-way voice circuits. Two such
cables make a two-way pair, with each cable carrying the transmission in
one direction. Its high frequencies and copper grounding decrease
crosstalk. Used since 1946 for long-distance transmission, coaxial cable
is now being replaced by optical fiber.
Microwave Radio. Used to carry conversations across and between
continents, microwave radio was the backbone of the telephone system
until the advent of optical fiber. Because the microwave radio beam
follows a straight path, towers need to be located about every 26 miles to
allow for the curvature of the earth. Thus, it's very costly to reach remote
telephone users, and can't be used across great distances or oceans.
This problem was solved by the launching in 1962 of a "tower" very high
in the sky: Telstar I , the first communications satellite.
Optical Fiber. A long, hair-thin strand of high-quality glass surrounded
by a sheath of glass with different characteristics, clad in a plastic outer
sheath, and using a laser as the light source. First described in 1887 by a
British physicist, the idea of using light as a transmission medium was
implemented in 1880 by Bell. The first transoceanic undersea optical
cable was completed by AT&T in 1988.
Optical fiber is flexible, inexpensive, and not prone to electrical and
electromagnetic interference; there is no crosstalk, and it's well-suited to
carrying digital signals.
Sometimes these media carry only one signal, while other times they carry many
signals combined together (multiplexed). They also vary greatly in the number
of signals they can carry.
Some of the equipment used in transmission facilities is located at different
points along the transmission media to amplify, and sometimes regenerate, the
transmitted signals. Other transmission equipment is used where transmission
facilities connect to switching systems.
The communications paths provided by transmission facilities can be classified
into three broad categories:
Lines. A facility that connects a customer's telephone station equipment
and a switching system.
Trunks. A facility that connects two switching systems.
Channels. A facility dedicated to a specific customer to provide special
services.
As noted earlier in this guide, the differentiation between the terms line and
trunk has blurred and the two terms are now often used interchangeably.
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System Manager's Guide