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Transcript of Chapter 2 Radio Frequency Fundamentals. Exam Essentials Understand wavelength, frequency, amplitude,...
![Page 1: Chapter 2 Radio Frequency Fundamentals. Exam Essentials Understand wavelength, frequency, amplitude, and phase. –Know the definition of each RF characteristic.](https://reader034.fdocuments.in/reader034/viewer/2022042510/56649eef5503460f94bfeeaa/html5/thumbnails/1.jpg)
Chapter 2• Radio Frequency Fundamentals
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Exam Essentials• Understand wavelength, frequency, amplitude, and
phase. – Know the definition of each RF characteristic and how it can
affect wireless LAN design.• Remember all the RF propagation behaviors.
– Be able to explain the differences between each RF behavior (such as refection, diffraction, scattering, and so on) and the various mediums that are associated with each behavior.
• Understand what causes attenuation. – Loss can occur either on the wire or in the air. Absorption, free
space path loss, and multipath downfade are all causes of attenuation.
• Define free space path loss. – Despite the lack of any obstructions, electromagnetic waves
attenuate in a logarithmic manner as they travel away from the transmitter.
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Exam Essentials• Explain the difference between active and passive gain.
– RF amplifiers are active devices, whereas antennas are passive devices.
• Explain the difference between transmit and received amplitude. – Transmit amplitude is typically defined as the amount of initial amplitude
that leaves the radio transmitter. When a radio receives an RF signal, the received signal strength is most often referred to as received amplitude.
• Remember the four possible results of multipath and their relationship to phase. – Multipath may cause downfade, upfade, nulling, and data corruption.
• Know the results of intersymbol interference and delay spread. – The time differential between a primary signal and reflected signals may
cause corrupted bits and affect throughput and latency due to layer 2 retransmissions.
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Understanding Wireless• Need to understand how wireless works at
the physical layer of the OSI model
• RF Signals move through the air in an unpredictable manner– Unbounded Medium
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What is Radio Frequency (RF)• Part of the Electromagnetic Spectrum
• Starts as Alternating Current (AC) generated from a transmitter– Radiated out of an antenna element– Changes in current produce changes in
Electromagnetic FieldPg 31
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Alternating Current• Current where the magnitude and direction varies in a
cycle over time• Produces a sine wave
– Fluctuation is the oscillation
• Movement of the wave through air is the propagation behaviors– Absorption– Reflection– Scattering– Refraction– Diffraction– Amplification– attenuation
Pg 32
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RF Characteristics• Wavelength
• Frequency
• Amplitude
• Phase
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Wavelength• Distance between the peaks of the
waveform
• Distance traveled in a single cycle
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Wavelength• Represented by Greek lambda λ
• Inverse relationship between wavelength and frequency
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Wavelength• The higher the frequency, the shorter the
wavelength.
• The longer the wavelength, the shorter the frequency.
Pg 33
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Wavelength• As RF travels though space, signal
attenuate, or lose signal strength
• Shorter Wavelengths will attenuate FASTER
• Signals keep traveling, but may be below the sensitivity threshold of receiver.
Pg 34
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Comparing 5 Ghz and 2.4 Ghz• Higher Frequencies (shorter wavelength)
attenuate faster– Attenuation through the air is Free Space
Path Loss
• Higher Frequencies also don’t penetrate objects as well.
Pg 34
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Frequency• Measurement of how many times
something happens in a second– 1 hertz (Hz) = 1 cycle per second– 1 kilohertz (KHz) = 1,000 cycles per second– 1 megahertz (MHz) = 1,000,000 (million)
cycles per second– 1 gigahertz (GHz) = 1,000,000,000 (billion)
cycles per second
Pg 36
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Amplitude• Measure of signal
strength or power– λ is for wavelength– y is for amplitude
• Loss of amplitude is attenuation or loss
• Transmit Amplitude– Initial amplitude at
transmitter
• Received Amplitude– Received signal strength
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Amplitude• Different RF technologies use different
transmit amplitudes– AM Radio may use 50,000 Watts
• 802.11 Access Poitns from 1mW an 100 mW
Pg 38
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Phase• Difference in degrees separating two
overlapping sine waves– Out of phase
• Measured from 0-360– 0-in phase– 90-quarter out of phase– 180-cancels out original– Etc.
Pg 100
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Phase
Pg 100
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Phase• Differences in phase are important to
understanding multipath– Can cause interference in 802.11 signals
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RF Behaviors• Wave Propagation• How waves move through the air and obstacles
– Absorption– Reflection– Scattering– Refraction– Diffraction– Loss-Attenuation
• FSPL• Multipath
– Amplification
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Absorption• If a signal does not bounce off, move
around, or pass through an object, then 100 % absorption has occurred.– Significant cause of Loss
• Most materials absorb some level of RF signal
• Brick, Concrete, Water all absorb a lot– Even things with lots of water in them
• Drywall absorbs less
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Reflection• If a wave hits a smooth object larger than
itself it may bounce off• Sky wave reflection
– Lower Frequencies bouncing off charged particles in the ionosphere
• Microwave reflection– Higher frequencies (1 Ghz to 300 Ghz) that
bounce off smaller objects like a metal door– Important for WLAN
• Buildings, roads, water, earth’s surfacePg 41
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Reflection• Can be a problem in WLAN as reflected
signals will arrive out of phase with original signal– Multipath
Pg 41
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Scattering• Multiple reflections
– If the wavelength is longer than the medium that the wave is passing through
– Two types• If the particles are smaller than the
wavelength, minor scattering of the signal
• If the signal encounters an uneven surface and is reflected in multiple directions
– Fences, trees, etcPg 43
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Refraction• Signals can also be BENT by refraction
– Bending of a signal as it passes through a medium with a different density
– Causes the direction of the wave to change.– Issue for long distance bridging
Pg 44
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Refraction
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Diffraction• Bending of a signal AROUND an object
– Also spreads the signal– Usually caused by a partial blockage– If you are behind the blockage, you are in the
RF shadow!
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Loss (Attenuation)• Decrease of amplitude or signal strength
– On a wire due to impedance– Over the air-Free Space Path Loss
• Loss and gain are measured in Deibels
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Multipath• When multiple signals arrive at receiver
due to different obstructions/effects on RF signal
• Difference in arrival is delay spread.
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Multipath
Pg 51