Chapter 2 Radio Frequency Fundamentals. Exam Essentials Understand wavelength, frequency, amplitude, and phase. –Know the definition of each RF characteristic.

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

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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.

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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.

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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

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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

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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.

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Phase

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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

Reflection• Can be a problem in WLAN as reflected

signals will arrive out of phase with original signal– Multipath

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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

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

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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

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