My Study notes from Chapter 2 of CWNA Study Guide

RF Characteristics

• Wavelength
  • A wavelength is the distance between the two successive crescents (peaks) or two successive throughs (valleys)

• Frequency
  • Is how often an RF signal cycles in a certain time period
  • Standard measurement of frequency is hertz (Hz). An event that occurs once in a second has a frequency of 1 Hz.
    • 1 Hertz (Hz) = 1 Cycle per second
    • 1 Kilohertz (KHz) = 1000 cycles per second
    • 1 megahertz (MHz) = 1,000,000 (million) cycles per second
    • 1 gigahertz (GHz) = 1,000,000,000 (billion) cycles per second

 

• Amplitude
  • The height of the wave.
  • Can be characterised simply as the signal’s strength or power

 

• Phase
  • Is not a property of just one signal but instead involves the relationship between two or more signals that share the same frequency.

RF Behaviours

• Wave Propagation
  • The way the wave moves is known as wave propagation.
  • This can vary drastically depending on the type of material the signal is traversing.
• Absorption
  • Most common RF behaviour
  • If signal does not bounce off an object, move around an object, or pass through an object, then 100% absorption has occurred.
  • Most materials absorb some RF signal, the amount depends on the material.
• Reflection
  • Most important RF behaviour to be aware of.
  • Causes serious performance problems in legacy 802.11a/b/g WLANs
  • When wave hits smooth object that is larger than the wave itself, dependant on the material the signal may bounce in another direction.
  • Two Types:
    • Sky wave reflection: Signals below 1 GHz – signal bounces off charged partials in the ionosphere in the earth’s atmosphere.
    • Microwave: 1GHz – 300 GHz, can bounce off smaller objects like a metal door
• Scattering
  • Described as multiple reflections
  • Is reflection off an object with multiple sides (think mirror ball)
  • Two types:
    • Lower level has lessor effect on signal quality and strength
    • Second type occurs when RF signal encounters some type of uneven surface and is reflected into multiple directions (Chain link fences, wire mesh in stucco walls)
• Refraction
  • Signal is bent into behaviour known as refraction.
  • when signal passes through a medium with different density thus causing the wave to change direction.
  • Three most common causes:
    • Water Vapour
    • Changes in air temperature
    • Changes in air pressure
• Diffraction
  • Signal is bent around an object
  • Sitting directly behind the object is an area known as the RF shadow. Depending on the change in direction of the signal this area can become a dead zone of coverage.
• Loss (Attenuation)
  • Decrease in amplitude
  • Signal may lose strength on the wire or in the air
  • Signal can be absorbed into materials it passes through.
• Free Space Path Loss (FSPL)
  • Because of the laws of physics the signal will attenuate as it travels despite the lack of attenuation caused by obstructions, absorptions, reflection, diffraction and so on.
  • Loss of signal strength caused by the natural broadening of the waves.
• Multipath
  • Occurs when 2 or more signals arrive at the receiving station at the same time or within nanoseconds of each other.
  • Four results of multipath
    • Upfade
      •  Multiple RF signal paths arrive at the same time and are in phase (phase differences of 0 to 120 degrees) will cause upfade
      • Results in increased signal strength
    • Downfade
      • Multiple RF signals arrive at the same time but are out of phase (phase difference of 121 to 179 degrees)
      • Results in decrease signal strength
    • Nulling
      • This is signal cancellation
      • Multiple RF signals arrive at the same time and are 180 degrees out of phase of each other
    • Data Corruption
      • Multiple signals arriving but not at the same time the receiver might have trouble demodulating the signal.
• Gain (Amplification)
  • Increase in amplitude or signal strength
  • Two types:
    • Active – increase to signal on the transmitter or transceivers side through the use of an amplifier. IE more power is applied
    • Passive – done by focusing the antenna, the inner workings of the antenna make the signal stronger.