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NETW-125/PW0-105 > Chapter 2 - Radio Frequency Fundamentals > Flashcards

Chapter 2 - Radio Frequency Fundamentals Flashcards

1
Q

The IEEE defines the 802.11 communications at the _____ layer and the _____ sublayer of the Data-Link layer.

A

Physical, MAC

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2
Q

Wireless communications travel across what is known as _____ _____. _____ _____ does not contain the signal, which is free to radiate into the atmosphere in all directions (unless restricted or redirected by some outside influence).

A

unbounded medium

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3
Q

The _____ _____, which is usually simply referred to as spectrum, is the range of all possible electromagnetic radiation. This radiation exists as self-propagating electromagnetic waves that can move through matter or space.

A

electromagnetic (EM) spectrum

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4
Q

_____ _____ are electromagnetic waves occurring on the radio frequency portion of the electromagnetic spectrum.

A

Radio waves

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5
Q

An RF signal starts out as an electrical _____ _____ signal that is originally generated by a transmitter. This AC signal is sent through a copper conductor (typically a coaxial cable) and radiated out of an antenna element in the form of an electromagnetic wave. This electromagnetic wave is the wireless signal.

A

alternating current (AC)

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6
Q

Changes of electron flow in an antenna, otherwise known as _____, produce changes in the electromagnetic fields around the antenna.

A

current

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7
Q

An alternating current is an electrical current with a magnitude and direction that varies cyclically, as opposed to direct current, the direction of which stays in a constant form. The shape and form of the AC signal—defined as the _____—is what is known as a sine wave.

A

waveform

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8
Q

The fluctuation of voltage in an AC current is known as cycling, or _____.

A

oscillation

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9
Q

Electromagnetic signals can travel through mediums of different materials or travel in a perfect vacuum. When an RF signal travels through a vacuum, it moves at the speed of _____, which is 299,792,458 meters per second, or 186,000 miles per second.

A

light

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10
Q

RF electromagnetic signals travel using a variety or combination of movement behaviors. These movement behaviors are referred to as _____.

A

propagation behaviors

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11
Q

A _____ is the distance between the two successive crests (peaks) or two successive troughs (valleys) of a wave pattern. In simpler words, a _____ is the distance that a single cycle of an RF signal actually travels.

A

wavelength

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12
Q

It is very important to understand that there is an _____ relationship between wavelength and frequency. The three components of this _____ relationship are frequency (f, measured in hertz, or Hz), wavelength (l, measured in meters, or m), and the speed of light (c, which is a constant value of 300,000,000 m/sec). The following reference formulas illustrate the relationship: l = c/f and f = c/l.

A

inverse

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13
Q

It is often thought that a higher frequency electromagnetic signal with a smaller wavelength will _____ faster than a lower frequency signal with a larger wavelength. In reality, the frequency and wavelength properties of an RF signal do not cause _____. Distance is the main cause of _____.

A

attenuate, attenuation, attenuation

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14
Q

All antennas have an effective area for receiving power known as the _____. The amount of RF energy that can be captured by the _____ of an antenna is smaller with higher frequency antennas. Although wavelength and frequency do not cause attenuation, the perception is that higher frequency signals with smaller wavelengths attenuate faster than signals with a larger wavelength.

A

aperture

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15
Q

Theoretically, in a vacuum, electromagnetic signals will travel _____.

A

forever

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16
Q

An electromagnetic signal with a _____ wavelength will maintain an amplitude level above the sensitivity of a receiver radio over greater distances.

A

larger

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17
Q

Higher frequency signals will attenuate _____ than lower frequency signals as they pass through various physical mediums such as brick walls.

A

faster

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18
Q

The length of a 2.45 GHz wave is about _____ inches, or 12 centimeters. The length of a 5.775 GHz wave is a distance of only about _____ inches, or 5 centimeters

A

4.8, 2

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19
Q

_____ is the number of times a specified event occurs within a specified time interval.

A

Frequency

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20
Q

A standard measurement of frequency is _____, which was named after the German physicist Heinrich Rudolf _____. An event that occurs once in 1 second is equal to 1 _____.

A

hertz (Hz),

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21
Q

A radio wave’s signal strength or power is referred to as it’s _____. _____ can be defined as the maximum displacement of a continuous wave. With RF signals, the amplitude corresponds to the electrical field of the wave. When you look at an RF signal using an oscilloscope, the _____ is represented by the positive crests and negative troughs of the sine wave

A

Amplitude

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22
Q

A variety of factors can cause an RF signal to lose amplitude, otherwise known as _____.

A

attenuation

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23
Q

_____ amplitude is typically defined as the amount of initial amplitude that leaves the radio transmitter. For example, if you configure an access point to transmit at 50 milliwatts (mW), that is the _____ amplitude.

A

Transmit

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24
Q

Cables and connectors will _____ the transmit amplitude while an antenna will _____ the transmit amplitude.

A

attenuate, amplify

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25
Q

When a radio receives an RF signal, the received signal strength is most often referred to as _____ amplitude. RF signal strength measurements taken during a site survey is an example of _____ amplitude.

A

received

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26
Q

AM radio stations may transmit narrow band signals with as much power as 50,000 watts. The radio cards in most indoor 802.11 access points have a transmit power range between _____ mW and _____ mW.

A

1, 100

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27
Q

Wi-Fi radio cards can receive signals with amplitudes as low as _____ of a milliwatt.

A

billionths

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28
Q

_____ is not a property of just one RF signal but instead involves the relationship between two or more signals that share the same frequency. The _____ involves the relationship between the position of the amplitude crests and troughs of two waveforms.

A

Phase

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29
Q

Phase can be measured in distance, time, or degrees. If the peaks of two signals with the same frequency are in exact alignment at the same time, they are said to be _____ _____. Conversely, if the peaks of two signals with the same frequency are not in exact alignment at the same time, they are said to be _____ _____ _____.

A

in phase, out of phase

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30
Q

Signals that have 0 (zero) degree phase separation actually combine their amplitude, which results in a received signal of much greater signal strength, potentially as much as _____ the amplitude.

A

twice

31
Q

If two RF signals are _____ degrees out of phase (the peak of one signal is in exact alignment with the trough of the second signal), they cancel each other out.

A

180

32
Q

The way in which the RF waves move— known as _____ _____—can vary drastically depending on the materials in the signal’s path; for example, drywall will have a much different effect on an RF signal than metal or concrete.

A

wave propagation

33
Q

The manner in which a wireless signal moves is often referred to as _____ _____.

A

propagation behavior

34
Q

The most common RF behavior is _____. If a signal does not bounce off an object, move around an object, or pass through an object, then 100 percent _____ has occurred. Most materials will _____ some amount of an RF signal to varying degrees.

A

absorption, absorption, absorb

35
Q

The _____ of an RF signal is directly affected by how much RF energy is absorbed.

A

amplitude

36
Q

When a wave hits a smooth object that is larger than the wave itself, depending on the media, the wave may bounce in another direction. This behavior is categorized as _____.

A

reflection

37
Q

There are two major types of reflections: _____ _____ reflection and ______ reflection.

A

sky wave, microwave

38
Q

_____ _____ reflection can occur in frequencies below 1 GHz, where the signal has a very large wavelength. The signal bounces off the surface of the charged particles of the ionosphere in the earth’s atmosphere. This is why you can be in Charlotte, North Carolina, and listen to radio station WLS-AM from Chicago on a clear night.

A

Sky wave

39
Q

_____ signals, however, exist between 1 GHz and 300 GHz. Because they are higher frequency signals, they have much smaller wavelengths, thus the term _____.

A

Microwave

40
Q

_____ reflection is what we are concerned about in Wi-Fi environments.

A

Microwave

41
Q

As a wave radiates from an antenna, it broadens and disperses. If portions of this wave are reflected, new wave fronts will appear from the reflection points. If these multiple waves all reach the receiver, the multiple reflected signals cause an effect called _____.

A

multipath

42
Q

802.11n radios utilize _____-_____ _____-_____ antennas and advanced digital signal processing (DSP) techniques to take advantage of multipath.

A

multiple-input multiple-output (MIMO)

43
Q

_____ can most easily be described as multiple reflections. These multiple reflections occur when the electromagnetic signal’s wavelength is larger than pieces of whatever medium the signal is reflecting from or passing through.

A

Scattering

44
Q

In addition to RF signals being absorbed or bounced (via reflection or scattering), if certain conditions exist an RF signal can actually be bent in a behavior known as _____. A straightforward definition of _____ is the bending of an RF signal as it passes through a medium with a different density, thus causing the direction of the wave to change. RF _____ most commonly occurs as a result of atmospheric conditions.

A

refraction

45
Q

When you are dealing with long-distance outdoor bridge links, an instance of refractivity change that might be a concern is what is known as the _____-_____. A _____-_____ of 1 means there is no bending. A _____-_____ of less than 1, such as 2/3, represents the signal bending away from the earth. A _____-_____ of more than 1 represents bending toward the earth. Normal atmospheric conditions have a _____-_____ of 4/3, which is bending slightly toward the curvature of the earth.

A

k-factor

46
Q

Not to be confused with refraction, another RF propagation behavior exists that also bends the RF signal; it is called _____. _____ is the bending of an RF signal around an object (whereas refraction, as you recall, is the bending of a signal as it passes through a medium). _____ is the bending and the spreading of an RF signal when it encounters an obstruction. The conditions that must be met for _____ to occur depend entirely on the shape, size, and material of the obstructing object as well as the exact characteristics of the RF signal, such as polarization, phase, and amplitude.

A

diffraction

47
Q

Typically, diffraction is caused by some sort of partial _____ of the RF signal, such as a small hill or a building that sits between a transmitting radio and a receiver.

A

blockage

48
Q

In terms of diffraction caused by a blockage, sitting directly behind the obstruction is an area known as the _____ _____. Depending on the change in direction of the diffracted signals, the area of the _____ _____ can become a dead zone of coverage or still possibly receive degraded signals.

A

RF shadow

49
Q

Loss, also known as _____ , is best described as the decrease of amplitude, or signal strength. A signal may lose strength when transmitted on a wire or in the air. On the wired portion of the communications (RF cable), the AC electrical signal will lose strength because of the electrical impedance of coaxial cabling and other components such as connectors.

A

attenuation

50
Q

It is important to understand that an RF signal will also lose amplitude merely as a function of distance due to _____ _____ _____ _____.

A

free space path loss

51
Q

Free Space Path Loss is the loss of signal strength caused by the natural broadening of the waves, often referred to as _____ _____. RF signal energy spreads over larger areas as the signal travels farther away from an antenna, and as a result, the strength of the signal attenuates.

A

beam divergence

52
Q

The _____ _____ rule states that doubling the distance will result in a loss of amplitude of _____ _____.

A

6 dB

53
Q

All radio devices have what is known as a _____ _____ level. The radio receiver can properly interpret and receive a signal down to a certain fixed amplitude threshold.

A

receive sensitivity

54
Q

In addition to being able to receive and interpret a signal, the received signal must not only be strong enough to be heard, but also strong enough to be heard above any RF background noise, typically referred to as the _____ _____. The signal must be louder than any background noise.

A

noise floor

55
Q

_____ is a propagation phenomenon that results in two or more paths of a signal arriving at a receiving antenna at the same time or within nanoseconds of each other. Because of the natural broadening of the waves, the propagation behaviors of reflection, scattering, diffraction, and refraction will occur differently in dissimilar environments. A signal may reflect off an object or scatter, refract, or diffract. These propagation behaviors can all result in multiple paths of the same signal.

A

Multipath

56
Q

As it relates to multipath, it usually takes a bit longer for reflected signals to arrive at the receiving antenna because they must travel a longer distance than the principal signal. The time differential between these signals can be measured in billionths of a second (nanoseconds). The time differential between these multiple paths is known as the _____ _____.

A

delay spread

57
Q

With RF signals, the effects of multipath can be either constructive or destructive. Quite often they are destructive. Because of the differences in phase of the multiple paths, the combined signal will often attenuate, amplify, or become corrupted. These effects are sometimes called _____ _____, another phenomenon named after British physicist Lord Rayleigh.

A

Rayleigh fading

58
Q

_____ refers to increased signal strength. When the multiple RF signal paths arrive at the receiver at the same time and are in phase or partially out of phase with the primary wave, the result is an increase in signal strength (amplitude). Smaller phase differences of between 0 and 120 degrees will cause _____. Please understand, however, that the final received signal can never be stronger than the original transmitted signal because of free space path loss. _____ is an example of constructive multipath.

A

Upfade

59
Q

_____ refers to decreased signal strength. When the multiple RF signal paths arrive at the receiver at the same time and are out of phase with the primary wave, the result is a decrease in signal strength (amplitude). Phase differences of between 121 and 179 degrees will cause _____. Decreased amplitude as a result of multipath would be considered destructive multipath.

A

Downfade

60
Q

_____ refers to signal cancellation. When the multiple RF signal paths arrive at the receiver at the same time and are 180 degrees out of phase with the primary wave, the result will be _____. _____ is the complete cancellation of the RF signal. A complete cancellation of the signal is obviously destructive.

A

Nulling

61
Q

Because of the difference in time between the primary signal and the reflected signals known as the delay spread, along with the fact that there may be multiple reflected signals, the receiver can have problems demodulating the RF signal’s information. _____ _____ is the most common occurrence of destructive multipath.

A

Data corruption

62
Q

The delay spread time differential can cause bits to overlap with each other, and the end result is corrupted data, as seen in Figure 2.16. This type of multipath interference is often known as _____ _____. Data corruption is the most common occurrence of destructive multipath.

A

intersymbol interference (ISI)

63
Q

Multipath has a constructive effect with 802.11n transmissions that utilize multiple-input, multiple-output (MIMO) antenna diversity and _____ _____ _____ signal processing techniques.

A

maximum ratio combining (MRC)

64
Q

_____ can best be described as the increase of amplitude, or signal strength.

A

Gain

65
Q

Gain is sometimes referred to as _____.

A

Amplification

66
Q

The two types of gain are known as _____ gain and _____ gain.

A

active, passive

67
Q

_____ gain is usually caused by the transceiver or the use of an amplifier on the wire that connects the transceiver to the antenna. Many transceivers are capable of transmitting at different power levels, with the higher power levels creating a stronger or amplified signal. An amplifier is usually bidirectional, meaning that it increases the AC voltage both inbound and outbound. _____ gain devices require the use of an external power source.

A

Active

68
Q

_____ gain is accomplished by focusing the RF signal with the use of an antenna. Antennas are passive devices that do not require an external power source. Instead, the internal workings of an antenna focus the signal more powerfully in one direction than another.

A

Passive

69
Q

A _____ _____ _____ can be used to measure amplitude in a finite frequency spectrum.

A

frequency domain tool

70
Q

The frequency domain tool used by WLAN engineers is called a _____ _____.

A

spectrum analyzer

71
Q

A _____ _____ _____ can be used to measure how a signal’s amplitude changes over time.

A

time domain tool

72
Q

The conventional name for a time domain tool is an _____.

A

oscilloscope

73
Q

_____ _____ are often used by WLAN engineers during site surveys, whereas _____ are used in laboratory environments.

A

Spectrum analyzers, oscilloscopes

NETW-125/PW0-105 (4 decks)

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