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Chapter 2• Radio Frequency Fundamentals
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.
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.
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
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
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
RF Characteristics• Wavelength
• Frequency
• Amplitude
• Phase
Pg 32
Wavelength• Distance between the peaks of the
waveform
• Distance traveled in a single cycle
Pg 32
Wavelength• Represented by Greek lambda λ
• Inverse relationship between wavelength and frequency
Pg 33
Wavelength• The higher the frequency, the shorter the
wavelength.
• The longer the wavelength, the shorter the frequency.
Pg 33
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
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
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
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
Pg 37
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
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
Phase
Pg 100
Phase• Differences in phase are important to
understanding multipath– Can cause interference in 802.11 signals
Pg 38
RF Behaviors• Wave Propagation• How waves move through the air and obstacles
– Absorption– Reflection– Scattering– Refraction– Diffraction– Loss-Attenuation
• FSPL• Multipath
– Amplification
Pg 39
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
Pg 40
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
Reflection• Can be a problem in WLAN as reflected
signals will arrive out of phase with original signal– Multipath
Pg 41
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
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
Refraction
Pg 44
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!
Pg 44
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
Pg 46
Multipath• When multiple signals arrive at receiver
due to different obstructions/effects on RF signal
• Difference in arrival is delay spread.
Pg 49
Multipath
Pg 51