would be fewer hassles when it came time to figure out
such things as line speeds, line capacities, variations in
line distortion, and so on. However, there area number
of types, ranging in cost and capabilities. In the
following paragraphs, we examine the advantages and
disadvantages of twisted-wire pairs, baseband and
broadband coaxial cabling, and fiber optic cabling.
Twisted-wire Pairs
Twisted-wire pairs, also known as twisted-pair wire
or cable, is by far the least expensive transmission
media. It consists of two insulated wires twisted around
each other so that each wire faces the same amount of
interference (noise) from the environment (see fig. 1-9).
Unfortunately, this noise becomes part of the signal
being transmitted. Twisting the wires together reduces
but does not eliminate the noise.
Twisted-pair wire comes in a wide range of gauges
and pairs. Wire has an American Wire Gauge (AWG)
number based on its diameter. For network purposes,
22- and 24-gauge wires are the two most common types
of twisted-pair media. Some local-area networks use
the same inexpensive, unshielded twisted-pair cables
telephone companies use. Others require a higher data
grade quality. It’s not uncommon to have several
hundred pairs (and, in some cases, thousands) of wires
placed in a single cable. Normally, each twisted-wire
pair in a cable can accommodate a single phone call
between two people or between hardware devices.
The advantages of using telephone wires are their
relative low cost and their availability. Their
disadvantages include susceptibility to signal distortion
errors and the relatively low transmission rates they
provide over long distances. Twisted wire can handle a
data flow of up to approximately one megabit per
second (Mbps) over several hundred feet. For a small
local-area network with a limited number of users,
twisted-pair is an ideal choice because it is both
inexpensive and easy to install. A phenomenon called
Figure 1-9.—Twisted-wire pairs (2 wire pairs shown).
crosstalk exists in twisted-wire pairs whenever
transmission occurs at a high rate of speed. Crosstalk is
taking place whenever you can hear someone else’s
conversation in the background; say Mr. Frost telling
Mrs. Christmas what a great recipe he has for southern
fried chicken, or Mrs. Brush telling Mr. Smith what a
large fish she caught in the Gulf of Mexico, while you’re
trying to carry on a conversation with your party. With
voice communications this really isn’t a problem;
however, crosstalk can inhibit the high-speed
transmission required for data communications.
Twisted-wire pairs used in data communications
are either private or public lines. Private lines are those
provided by the user. Public lines are those provided by
a common carrier such as American Telephone and
Telegraph (AT&T). Generally, public lines are used
whenever distances are great or the terrain or other
environmental factors prohibit the use of private lines.
Public lines may be either switched lines or leased lines.
Switched lines are used whenever the amount of
data to be transmitted is short in duration or when many
locations must be contacted for relatively short periods
of time. There is a drawback. The telephone company
cannot guarantee you exactly which path or switching
equipment such a connection will use. Therefore, the
speed and quality of the switched connection are
questionable.
Leased lines come into play when the connection
time between locations A and B is long enough to cover
the cost of leasing, or if higher speeds than those
available with switched lines must be attained. Leased
lines can also be conditioned by the telephone company
to lower the error rate and increase transmission speeds.
Conditioned leased lines typically operate at speeds of
up to 64,000 bits per second (bps). Very-high-speed
connections are also available from the common carrier.
These are designated T1, T2, T3, and T4, and offer
transmission rates of 1.5, 6.3, 46, and 281 million bits
per second (Mbps), respectively.
Coaxial Cables
Coaxial (or coax) cable, the medium used by most
cable television companies, was developed primarily
because of the crosstalk in twisted-wire pairs when
transmission occurs at a high rate of speed. While coax
is more expensive than twisted-pair, it can transmit data
significantly faster, over much longer distances, and
with less electrical interference.
Coaxial cable is made up of one or two central data
transmission wires composed of copper surrounded by
1-16
