Download True-False Questions

Chapter 2
The Modern Signal Carriers: Electricity, Light, Media, and Impairments
1
Chapter 2
The Modern Signal Carriers: Electricity, Light, Media, and
Impairments
True-False Questions
1. Electricity consists of a flow of electrons called current, whose magnitude is measured in
ampere and strength in volts.
Answer: True
Difficulty: Easy
Page Reference: 26
2. Signals progress along a medium by a process called propagation.
Answer: True
Difficulty: Easy
Page Reference: 26
3. The ampere, a unit of measure of the magnitude of electrical current, is named after Andre
Marie Ampere, a French mathematician, chemist, and physicist.
Answer: True
Difficulty: Medium
Page Reference: 28
4. The volt, a unit of measure of electrical potential or pressure, is named after the Italian
physicist Alessandro Giuseppe Volta (1794-1847), who did much pioneering work in
electricity.
Answer: False
Difficulty: Easy
Page Reference: 28
5. The ohm, a measure of resistance to current flow, is named after Georg Simon Ohm, a
German mathematician who investigated electricity and magnetism.
Answer: True
Difficulty: Easy
Page Reference: 28
6. The type of electricity most relevant to telecommunications is alternating current (AC) – the
kind that utility companies provide.
Answer: True
Difficulty: Easy
Page Reference: 28
Difficulty: Medium
Page Reference: 30
7. Sine waves are not periodic.
Answer: False
8. The distance a wave travels in one cycle is its wavelength.
Answer: True
Difficulty: Medium
Page Reference: 31
Chapter 2
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THE Modern Signal carriers: Electricity, Light, Media, and Impairments
9. Impulse noise, also called spikes, is unpredictable, usually of very short duration, and
composed of large, sudden power surges.
Answer: False
Difficulty: Medium
Page Reference: 32
10. Because delay differences are magnified by distance, delay distortion is another limiting
factor of network cable length.
Answer: True
Difficulty: Medium
Page Reference: 32
11. Because thermal noise can be eliminated, it is not a major factor in electrical signal
transmission, and does not limit the distance that a signal can travel before it attenuates too
much to be distinguished from the noise.
Answer: False
Difficulty: Medium
Page Reference: 32
12. Delay distortion stems from the way wires affect signal velocity.
Answer: True
Difficulty: Easy
Page Reference: 32
13. Thermal noise cannot be eliminated by shielding the cable.
Answer: True
Difficulty: Medium
Page Reference: 32
14. Currently, the most commonly used guided electrical medium in network communications
systems is twisted pair.
Answer: True
Difficulty: Medium
Page Reference: 33
15. The lower the cat number, the more capable the wire is of carrying higher-speed data signals
reliably and the more it costs.
Answer: False
Difficulty: Medium
Page Reference: 35
Chapter 2
The Modern Signal Carriers: Electricity, Light, Media, and Impairments
3
Multiple Choice Questions
16. What material can either resist or accommodate electrical flow?
a.
b.
c.
d.
conductor
insulator
semiconductor
cable
Answer: c
Difficulty: Hard
Page Reference:
26
17. What is used to measure the distance that a wave travels in one cycle?
a. cycle
b. period
c. frequency
d. wavelength
Answer: d
Difficulty: Medium
Page Reference:
26
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32
18. Which form of signal impairment is also called a spike?
a.
b.
c.
d.
thermal noise
impulse noise
delay distortion
crosstalk
Answer: b
Difficulty: Medium
19. Which form of signal impairment stems from the way waves affect signal velocity?
a.
b.
c.
d.
thermal noise
impulse noise
delay distortion
intermodulation distortion
Answer: c
Difficulty: Medium
Page Reference:
32
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33
20. The purpose of twists in UTP is
a.
b.
c.
d.
to reduce crosstalk.
to allow more pairs to be bundled in one cable.
to keep the wire pairs separated from each other.
to strengthen the cable.
Answer: a
Difficulty: Easy
Chapter 2
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THE Modern Signal carriers: Electricity, Light, Media, and Impairments
21. Fiber optic cables
a.
b.
c.
d.
have less capacity for carrying data than copper cables.
attenuate light signals.
are not extremely flexible.
are easier to install than copper cables.
Answer: c
Difficulty: Medium
Page Reference:
39
22. What material in a fiber optic cable covers the cladding and absorbs light rays that escape the
core and are moving out through the cladding?
a.
b.
c.
d.
core
cladding
coating
sheath
Answer: c
Difficulty: Medium
Page Reference:
39
23. What material in a fiber optic cable surrounds the core and is designed to reflect the light and
keep it with in the core?
a.
b.
c.
d.
cladding
coating
jacket
sheath
Answer: a
Difficulty: Easy
Page Reference:
39
24. Historically speaking, what was the first type of fiber optic cable to be produced?
a.
b.
c.
d.
multimode
single mode
step index
graded index
Answer: a
Difficulty: Medium
Page Reference:
40
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37
25. In what EMR frequency band do cell phones transmit?
a.
b.
c.
d.
radio
microwave
infrared
visible
Answer: b
Difficulty: Medium
Chapter 2
The Modern Signal Carriers: Electricity, Light, Media, and Impairments
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26. Which EMR frequency band is utilized by most television remotes?
a. radio
b. microwave
c. visible
d. infrared
Answer: d
Difficulty:
Easy
Page Reference:
37
27. When an electromagnetic signal hits the edge of an object that is large compared to the
signal’s wavelength, the signal propagates in many directions, with the edge as the apparent
source. This is called
a. distortion.
b. noise.
c. diffraction.
d. attenuation.
Answer: c
Difficulty:
Medium
Page Reference:
37
28. About 2,000 years ago, the characteristics of light were defined by the way it behaved under
a variety of conditions. This theory is referred to as
a. wave optics.
b. geometric optics.
c. quantum optics.
d. thermal noise.
Answer: b
Difficulty:
Medium
Page Reference:
38
29. The signal impairment that results from various impurities that find their way into the fiber
during its manufacture is called
a. scattering.
b. bends.
c. coupling.
d. absorption.
Answer: d
Difficulty:
Medium
Page Reference:
7
30. The signal impairment which is caused by small contaminant and density differences in the
core is referred to as
a. bends.
b. coupling.
c. scattering.
d. absorption.
Answer: c
Difficulty:
Medium
Page Reference:
7
Chapter 2
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THE Modern Signal carriers: Electricity, Light, Media, and Impairments
Fill In the Blanks
31. Electricity consists of a flow of electrons called a current.
Difficulty: Easy
Page Reference:
26
Page Reference:
26
32. Signals move along a medium by a process called propagation.
Difficulty: Medium
33. Signals traveling through air or space are not confined; air and space are called unbounded
or unguided media.
Difficulty: Medium
Page Reference:
26
34. The process of electron flow is called conduction; opposition to flow is called resistance.
Difficulty: Medium
Page Reference:
26
35. Materials such as copper and aluminum that easily accommodate electrical flow are called
conductors.
Difficulty: Easy
Page Reference:
26
36. Materials such as rubber, plastic, and air that strongly resist electrical flow are called
insulators.
Difficulty: Easy
Page Reference:
26
37. Semi-conductors, the basis of computer chips, widely used in communications systems,
usually act as insulators, but we can make them act as conductors.
Difficulty: Hard
38.
26
One volt is the electrical pressure required to move one amp of current through one ohm of
resistance.
Difficulty: Medium
39.
Page Reference:
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27
Resistance is directly proportionate to wire length and indirectly proportionate to wire
thickness, purity, and consistency.
Difficulty: Medium
Page Reference:
28
40. The type of electricity most relevant to telecommunications is alternating current (AC) – the
kind that utility companies provide.
Difficulty: Medium
Page Reference:
28
Chapter 2
The Modern Signal Carriers: Electricity, Light, Media, and Impairments
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41. The type of electricity that comes from batteries is called direct current (DC).
Difficulty: Medium
Page Reference:
28
42. For simplicity, direction is referred to as positive and negative strength. One complete
journey from zero through positive and negative strength and back to zero is called a cycle
and traces out a sine wave pattern.
Difficulty: Hard
Page Reference:
28
43. The time it takes for a sine wave to trace one complete pattern is called its period (or cycle).
Difficulty: Easy
Page Reference:
30
Page Reference:
31
44. Noise is unwelcome energy appearing in our transmission media.
Difficulty: Easy
45. Distortion is unwanted changes in signal shapes, due to interaction between the signal and
the media.
Difficulty: Easy
Page Reference:
31
46. Electromagnetic interference (EMI) is unwanted energy induced in our line by radiation
from any external source of electromagnetic energy.
Difficulty: Easy
Page Reference:
32
47. Crosstalk is the result of energy induced in one wire by signals radiating from one another.
Difficulty: Medium
Page Reference:
32
48. Because delay differences are magnified by distance, delay distortion is another limiting
factor of network cable length.
Difficulty: Hard
Page Reference:
32
49. A ground wire, or simply a ground, is a return path for the electricity, so that a circuit is
created.
Difficulty: Hard
Page Reference:
33
50. Generally speaking, a backbone is a high-capacity common link to which networks and
communications devices are attached.
Difficulty: Hard
Page Reference:
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Chapter 2
THE Modern Signal carriers: Electricity, Light, Media, and Impairments
8
Essay Questions
51. What are some of the properties of electricity and electrical media?
Electricity consists of a flow of electrons called a current, whose magnitude is measured in
amperes and strength (pressure) in volts. The process of electron flow is called conduction;
opposition to flow is called resistance. Materials such as copper and aluminum that easily
accommodate electrical flow are called conductors. Materials such as rubber, plastic, and air
that strongly resist electrical flow are called insulators.
Difficulty: Hard
Page Reference:
26
52. Describe three signal impairments in electrical transmission.
Impairments are caused by noise and distortion; they change and deform the signals carrying
our information. If impairments are severe enough, the receiver will not be able to interpret
our signals correctly.
1. Noise is unwelcome energy appearing in our transmission media. Picture yourself in
a room full of loudly talking people. If you want your friend standing next to you to
understand what you are saying, you need to speak louder than the level of the
“noise” coming from all the other conversations in the room. Similarly, if we expect
the receiver to properly interpret our signals, the received signal energy must exceed
the energy of the noise in the line.
2. Distortion is unwanted changes in signal shapes due to interactions between the
signals and the media. If you stand in front of a funhouse mirror, your image will be
distorted because of the way the mirror is bent. Media can distort our signals.
3. Attenuation is a form of distortion in which signal energy is lost as it travels –
signed shapes get smaller and smaller, flatter and flatter. Attenuation is primarily due
to the resistance of the medium to electrical flow. Overly attenuated signals will not
be recognized by the receiver.
Difficulty: Hard
Page Reference:
31
Chapter 2
The Modern Signal Carriers: Electricity, Light, Media, and Impairments
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53. Describe different types of antenna and how they are regulated.
All unguided media use antennas for transmission and receipt of signals. The word antenna
conjures up images of thin metal wands extending from automobile fenders, tall towers with
rectangular panels for cell phones, satellite TV dishes, and the like. Indeed, all of these are
antennas, but as we have seen, so is anything that conducts electricity and therefore can be
the transmitter or recipient of induced radiation. In fact, even your body can act as an
antenna. You can experience this phenomenon by touching a radio antenna connection or a
TV rabbit-ear antenna -- weak reception may improve.
Antennas come in a wide variety of shapes and sizes, designed for specific applications based
on the portion of the electromagnetic radiation (EMR) spectrum that is used. The EMR
spectrum, much of which is regulated by the Federal Communications Commission (FCC),
has been divided into bands described by EMR frequency ranges or their associated
wavelengths. Broadly speaking, there are three EMR groupings relevant to communications:
radio waves, microwaves, and infrared light. In this grouping, radio waves have the lowest
frequencies and longer wavelengths, and infrared light has the highest frequencies and
shorter wavelengths.
Difficulty: Hard
Page Reference:
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54. What is the basic nature of light?
Over the centuries, people have struggled to understand what light is. So elusive this has
been that a great many theories have been proposed, only to be discarded. About 2,000 years
ago, the characteristics of light were defined by the way it behaved under a variety of
conditions (geometric optics). The premise was that light consists of rays that move outward
from their source in straight lines as long as they are traveling in a consistent medium (air,
water, space, or glass, for example). When a ray of light hits another medium, as when
sunlight strikes the surface of a lake, it may be reflected or refracted (bent).
This premise was good enough for some time, but it concentrated on descriptions of behavior
rather than explanations. We now accept two complimentary explanations. One considers
light to be waves of energy (wave optics), which explains interference and diffraction. The
other depicts light as particles (quantum optics), which explains how we can produce light
by imposing electrical energy on some materials and how we can produce electricity by
shining light onto others. We may consolidate these theories by thinking of light as a particle
whose motions are wavelike. When we wish to describe a particular phenomenon, we use
that aspect of light that most easily lets us analyze the situation.
Difficulty: Hard
Page Reference:
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Chapter 2
THE Modern Signal carriers: Electricity, Light, Media, and Impairments
10
55. Discuss the construction of optical fiber cables.
In an optical fiber cable, the signal carrying fiber, called the core, runs through the cable
axially. It is surrounded by cladding designed to reflect light to keep it within the core’ a
coating layer covering the cladding absorbs light rays that escape the core and are moving
out through the cladding.
Typically, hundreds and even thousands of fibers are bundled together. Because the
extremely thin fibers are easily broken, strengthening and protecting material is added to the
cable – non light-conducting material such as Kevlar and steel wires. Each strand or the
entire cable may be jacketed as well.
Difficulty: Hard
Page Reference:
39
56. Why is fiber optic cable installation more difficult and costly?
Fiber optic cable installation requires even more care and expertise than electrical cable
installation and should be done only by fully qualified professionals. It is critical not to curve
the cable more than its specified maximum bend radius to avoid damaging the fiber, which
will substantially reduce its effectiveness. Even if the bend is not too severe, the sharper the
curve, the more likely the light beams will be refracted into the cladding, and therefore lost to
the transmission.
Careful attention must also be paid to avoid excessive stretching. Simply lowering a cable
down a shaft, if not done properly, can cause it to stretch too much just from its own weight.
Aside from breaking the fibers, even moderate stretching narrows the fiber, which increases
attenuation. Special devices are used to avoid this problem. Joining (splicing) fiber cables
and attaching them to connectors also requires special care and devices.
For the above reasons, installation of fiber-optic cable is an exacting and expensive
proposition. Making corrections or revisions to a poor installation is also a very expensive
proposition. Because labor cost is the biggest expense item, extra fiber should be installed,
since adding more at a later date will be much more costly as well as potentially disruptive to
normal business. Finally, no job should be accepted or final payment made until the
installation is fully labeled, tested, and proved to be functioning properly.
Difficulty: Hard
Page Reference:
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Chapter 2
The Modern Signal Carriers: Electricity, Light, Media, and Impairments
11
57. Discuss the value of lasers and the safety precautions that must be practiced when using
them.
Lasers of many different types have found their way into a wide variety of applications.
Lasers can cut steel, repair torn retinas, improve vision, pay our CDs and DVDs, and make
very good printers. They also produce the light beams that carry our information in optical
communications systems.
Whether laser light is in a visible wavelength range or not, even the relatively low-powered
lasers use for communications can severely damage an unprotected eye and can even cause
blindness. Therefore, never look into an optical fiber (or any laser-producing device) without
proper eye protection unless you know with absolute certainty that no laser light is being
transmitted!
Difficulty: Hard
Page Reference:
44
58. What are the signal impairments in light transmission?
One of the great advantages of optical fiber and light waves is that they are not subject to
interference or distortion from electrical or magnetic forces, or do they radiate any electrical
and magnetic disturbances. But just as with any electromagnetic radiation, as light travels it
loses power (attenuation). This is the principal source of optical transmission impairment.
The four main sources of light attenuation are absorption – results from various impurities
that find their way into the fiber during its manufacture (primarily water); scattering –
caused by small contaminants and density differences in the core; bends – caused by
excessive curve in the cable around objects or when extra cable is hung in loops; and
coupling – caused by improper splicing (joining) cables and attaching cables to connectors.
Difficulty: Hard
Page Reference:
44
59. What is the source of delay distortion which causes signal impairment during electrical
transmission?
Delay distortion stems from the way wires affect signal velocity. If we send various
frequencies down our wire, we will see that they travel at different speeds. Because signals
are composed of a range of frequencies, their frequency components arrive at the receiver at
somewhat different times than each other, even though they were transmitted at the same
time. If the delays are large enough, our signals will be distorted beyond recognition.
Difficulty: Hard
Page Reference:
32
Chapter 2
THE Modern Signal carriers: Electricity, Light, Media, and Impairments
12
60. Why is thermal noise a problem during electrical transmission?
Thermal noise, also called background noise, white noise, Gaussian noise, and hiss, is
unwanted energy in out transmission line caused by random movement of electrons of the
media (and, in fact, in all electronic devices) and cannot be eliminated. Thermal noise is
distributed uniformly over the entire electromagnetic spectrum proportional to temperature
and the bandwidth (capacity) of the line, but independent of line length and signal
frequencies.
Difficulty: Hard
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