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Chapter 2

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.

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Chapter 2

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  1. Chapter 2 • Radio Frequency Fundamentals

  2. 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.

  3. 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.

  4. 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

  5. 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 Field Pg 31

  6. 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

  7. RF Characteristics • Wavelength • Frequency • Amplitude • Phase Pg 32

  8. Wavelength • Distance between the peaks of the waveform • Distance traveled in a single cycle Pg 32

  9. Wavelength • Represented by Greek lambda λ • Inverse relationship between wavelength and frequency Pg 33

  10. Wavelength • The higher the frequency, the shorter the wavelength. • The longer the wavelength, the shorter the frequency. Pg 33

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. Phase Pg 100

  18. Phase • Differences in phase are important to understanding multipath • Can cause interference in 802.11 signals Pg 38

  19. RF Behaviors • Wave Propagation • How waves move through the air and obstacles • Absorption • Reflection • Scattering • Refraction • Diffraction • Loss-Attenuation • FSPL • Multipath • Amplification Pg 39

  20. 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

  21. 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 surface Pg 41

  22. Reflection • Can be a problem in WLAN as reflected signals will arrive out of phase with original signal • Multipath Pg 41

  23. 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, etc Pg 43

  24. 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

  25. Refraction Pg 44

  26. 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

  27. 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

  28. Multipath • When multiple signals arrive at receiver due to different obstructions/effects on RF signal • Difference in arrival is delay spread. Pg 49

  29. Multipath • Downfade • This is 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 downfade. • Upfade • This is 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 upfade. Please understand, however, that the fnal received signal can never be stronger than the original transmitted signal because of free space path loss. • Nulling • This is 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 nulling. Nulling is the complete cancellation of the RF signal. • Data corruption • 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. 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 intersymbol interference (ISI). Pg 49

  30. Multipath Pg 51

  31. Amplification Pg 39

  32. 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. Pg 54

  33. 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. Pg 54

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