Transmission

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Chapter 7
Transmission Media
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OBJECTIVES
After reading this chapter, the reader should be
able to
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7.1
GUIDED MEDIA
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Figure 7-1
Categories of guided media
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Figure 7-2
Twisted-pair cable
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Note
A twisted pair consists of two conductors each
surrounded by an insulating material.
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Technical Focus Why Twisted-Pair Cable Reduces
Noise

Interference from devices such as a motor can
create unequal noise over two parallel lines.
The line that is closer to the device receives
more interference than the one that is farther.
If however, the wires are twisted around each
other at regular intervals, each wire is closer
to the noise source for half of the time and
farther for the other half. Both receive the same
amount of noise. The noise from two wires cancel
each other out at the receiver.
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Technical Focus Categories of Twisted-Pair
Cables

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Business Focus Use of UTP in Telephone Network
In the early days of the telephone network, the
local loop, the two wires that connect a
residence to the telephone office, was made of
two parallel wires. The telephone companies later
changed the local loop to a pair of twisted-pair
wires, which is less prone to noise than the
previous parallel wires.

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Figure 7-3
Shielded twisted-pair cable
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Technical Focus Shielding and Lightning

We can be immune from lightning in a desert if we
are sur-rounded by a metallic cover (inside a
car, for example). The reason is that
electromagnetic energy (lightning) can penetrate
insulators, but not metals. Lightning on a
metallic shell creates a current on the outside
surface, which can be dangerous the inside is
protected. Note that this phenomenon is opposite
from what happens with electricity. The
electrical current passes through a metal, but
it is stopped by an insulator. Electromagnetic
energy passes through an insulator, but
is stopped by a metal. The same idea is used in
shielded twisted-pair cable. The shield is a
metal that protects the cable from outside
noise, electromagnetic interference.
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Figure 7-4
Coaxial cable
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Figure 7-5
Refraction and reflection
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Figure 7-6
Propagation modes
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Technical Focus Advantages of Fiber Optic

Noise resistance Because fiber optic
transmission uses light rather than electricity,
noise is not a factor. Less signal attenuation
fiber optic transmission distance is
significantly greater than that of other guided
media. A signal can run without requiring
regeneration. Higher Bandwidth bandwidths are
very high in fiber optics.
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Technical Focus Disadvantages of Fiber Optic

Cost manufacturing is painstakingly precise,
hence costly. Installation and maintenance
roughness and cracking in the Core of the optic
cable diffuses light and alters the signal. All
splices must be polished and precisely
fused. Fragility glass fiber can break easily
making it useless.
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7.2
UNGUIDED MEDIA
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Technical Focus Unguided Media
Also called wireless communication. It transports
electromagnetic Waves without using a physical
conductor. Signals are broadcast through the air
( in some cases water). There are available to
anyone who can receive them. Radio Frequency
Allocation the selection of the
electromagnetic spectrum defined as radio
communication is divided into 8 ranges, called
bands, each regulated by government. These bands
are rated from very low frequency (VLF) to very
high frequency (EHF) VLF and HF this propagate
very close to the surface. MF and HF upper
layers and then reflect back to the earth VHF and
UHF transmitted using line of sight EHF and
SHF space propagation.

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Figure 7-7
Terrestrial microwave
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Figure 7-8
Satellite communication
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Figure 7-9
Satellites in geosynchronous orbit