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MERLIN LEGEND Communications System Release 5.0
System Manager’s Guide
555-650-118
Issue 1
June 1997
About Telecommunications
Page B-3Transmission Facilities
B
Although there are still some rotary-dial telephones in use today, most modern
telephones have touch-tone dialing that involves the creation of unique tones
produced by pressing buttons on the dialpad. Touch-tone dialing is faster and,
with the advent of services available from touch-tone phones, more versatile.
Today’s telephones still consist of the components described above. But, with
continued innovations, these basic elements have been enhanced to include
many other features and components, for example, built-in speakerphones,
programmable features and buttons, and even the capability to transmit and
receive digital signals.
Transmission Facilities 2
The telephone network can transmit various types of information which originates
in either of two forms:
analog
(continuously variable physical signals, for example,
speech or video signals) or
digital
(representation of signals in discrete elements
such as zero and one, for example, signals from computers). This information is
conveyed from one place to another in the network over communications paths
provided by transmission facilities. These facilities involve different types of media
as well as electronic equipment.
There are various types of media, including:
■ Open Wire. Strung on poles, uninsulated copper wire was used in the early
days of telecommunications until physical congestion became a problem. It
is still found, though rarely, in rural areas.
■ Paired Wire. Commonly called
twisted pair
. Consists of two copper wires,
individually insulated with wood pulp or plastic, twisted together.
■ Paired Cable. Combines many twisted pairs (from 6 to 3600) into a single
cable, originally sheathed in lead but now insulated with plastic. Cable can
be strung on poles, buried underground, or installed in a conduit of either
long blocks of concrete or plastic pipe. The first transoceanic underseas
cable was laid by AT&T in 1958.
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