Chapter 2 Radio Frequency Fundamentals. Exam Essentials Understand wavelength, frequency, amplitude,...

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

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RF Characteristics• Wavelength

• Frequency

• Amplitude

• Phase

Pg 32

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

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.

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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.

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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

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

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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

Phase

Pg 100

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

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

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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

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

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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

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

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