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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

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2. Signals progress along a medium by a process called propagation. Answer: True

Difficulty: Easy

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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

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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

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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

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6. The type of electricity most relevant to telecommunications is alternating current (AC) – the kind that utility companies provide. Answer:True

Difficulty: Easy

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Difficulty: Medium

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7. Sine waves are not periodic. Answer:False

8. The distance a wave travels in one cycle is its wavelength. Answer:True

Difficulty: Medium

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9. Impulse noise, also called spikes, is unpredictable, usually of very short duration, and composed of large, sudden power surges. Answer:False

Difficulty: Medium

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10. Because delay differences are magnified by distance, delay distortion is another limiting factor of network cable length. Answer: True

Difficulty: Medium

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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

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12. Delay distortion stems from the way wires affect signal velocity. Answer: True

Difficulty: Easy

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13. Thermal noise cannot be eliminated by shielding the cable. Answer: True

Difficulty: Medium

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14. Currently, the most commonly used guided electrical medium in network communications systems is twisted pair. Answer: True

Difficulty: Medium

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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

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Full file at http://testbank360.eu/test-bank-principles-of-computer-networks-and-communications-1stedition-dumas

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

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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

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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

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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


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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

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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

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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

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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

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25. In what EMR frequency band do cell phones transmit? a. b. c. d.

radio microwave infrared visible

Answer:b

Difficulty: Medium


Full file at http://testbank360.eu/test-bank-principles-of-computer-networks-and-communications-1stedition-dumas

26. Which EMR frequency band is utilized by most television remotes? a. radio b. microwave c. visible d. infrared Answer: d

Difficulty:

Easy

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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

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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

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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

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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

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Fill In the Blanks 31. Electricity consists of a flow of electrons called a current. Difficulty: Easy

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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

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34. The process of electron flow is called conduction; opposition to flow is called resistance. Difficulty: Medium

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35. Materials such as copper and aluminum that easily accommodate electrical flow are called conductors. Difficulty: Easy

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36. Materials such as rubber, plastic, and air that strongly resist electrical flow are called insulators. Difficulty: Easy

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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

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Resistance is directly proportionate to wire length and indirectly proportionate to wire thickness, purity, and consistency. Difficulty: Medium

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40. The type of electricity most relevant to telecommunications is alternating current (AC) – the kind that utility companies provide. Difficulty: Medium

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Full file at http://testbank360.eu/test-bank-principles-of-computer-networks-and-communications-1stedition-dumas

41. The type of electricity that comes from batteries is called direct current (DC). Difficulty: Medium

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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

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43. The time it takes for a sine wave to trace one complete pattern is called its period (or cycle). Difficulty: Easy

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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

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46. Electromagnetic interference (EMI) is unwanted energy induced in our line by radiation from any external source of electromagnetic energy. Difficulty: Easy

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47. Crosstalk is the result of energy induced in one wire by signals radiating from one another. Difficulty: Medium

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48. Because delay differences are magnified by distance, delay distortion is another limiting factor of network cable length. Difficulty: Hard

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49. A ground wire, or simply a ground, is a return path for the electricity, so that a circuit is created. Difficulty: Hard

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50. Generally speaking, a backbone is a high-capacity common link to which networks and communications devices are attached. Difficulty: Hard

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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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Test bank principles of computer networks and communications 1st edition dumas  

test bank principles of computer networks and communications 1st edition dumas. Full file at http://testbank360.eu/test-bank-principles...

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