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MERLIN LEGEND Communications System Release 6.0
System Manager’s Guide
555-660-118
Issue 1
February 1998
About Telecommunications
Page B-4Transmission Facilities
B
A problem encountered, however, with many wire pairs running parallel to
each other is
crosstalk
, that is, the leaking of the electric signal from one
pair to another so that you can hear noise or intelligible speech.
■ Coaxial Cable. Consists of a number of one-way voice circuits. Two such
cables make a 2-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 is very costly to reach remote telephone
users, and Cannot 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 is 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 extension 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.