CWNA · Revision Notes

CWNA Review and Chapter Notes 1 – 4

Revision Revision

During 9 months off travelling I had plenty of long bus journeys to catch up on lots of leisurely reading but I couldn’t keep away the whole time and decided to read the wireless bible (CWNA) again. I was quite surprised at how much I had forgotten within 2 years but realised part of it was due to, at the time, struggling to take in the context of some of the topics and how it applied to my work. Having utilised wireless products for 2 more years I found it very interesting going back through the book and seeing how I could now apply a lot of the knowledge to my everyday work.

After receiving a not-so-terrible-but-could-be-better 25/36 score on the pre-book quiz I decided to write out some per-chapter notes on the topics that I think are most relevant and important to understand for everyday wireless work. For me, knowing the concepts of the CWNA syllabus is highly important as it has immeasurably improved my day-to-day understanding of how everything wireless works. This knowledge can all be applied to any device that you are working with regardless of vendor which makes it extremely valuable.

Basically if you haven’t already got a copy of the CWNA text book and you want to work with wireless then do it right now.

Chapter Notes

The following chapter notes are brief and on topics I feel are important. I’ll try to keep them making sense but if you don’t understand what I’m on about then don’t be afraid to have a google :).

Chapter 1 – Overview of Wireless Standards, Organizations and Fundamentals.

There are no notes for this overview chapter

Chapter 2 – Radio Frequency Behaviour

Effects on Radio Waves

  • Absorption
    • Most common occurrence where an object absorbs the signal e.g a dry wall
  • Reflecton
    • Sky Wave (<1GHz) and Microwave (1-300GHz) Reflection
    • Where radio waves reflect off objects e.g. a mirror
  • Scattering
    • Rayleigh effect
    • Can include multiple reflections
  • Refraction
    • Bending of a signal as it passes through object
    • K-factor in bridge links indicate the amount of bend towards (<1) or away (>1) from the Earth
  • Diffraction
    • Similar to refraction but it bends around an object not as it passes through one
  • Loss/Attenuation
    • Measured in dB.
    • 3dB loss = less half the amplitude
    • 10dB loss = less ten times the amplitude
  • Free Space Path Loss
    • Natural occurrence as radio waves widen over distance.
    • Also referred to as beam divergence
    • It is logarithmic not linear
  • Multipath
    • When the above occur!
    • When radio waves are affected, they will arrive at different times
    • This time difference is called delay spread:
      • Upfade – Signals arrive in correct phase and increase the amplitude
      • Downfade – Signals arrive out of phase and decrease the amplitude
      • Nulling – Signals arrive 180° out of phase and null (cancel) each other
      • Data Corruption – Intersymbol Interference (ISI) causes corruption
  • Gain / Amplification
    • An increase in signal strength through passive or active means

Chapter 3 – RF Mathematics

Components

  • Transmitter
    • Generates the modulated signal for transmissions via the antenna
  • Antenna
    • Collects the AC signal and directs it in particular ways in order to amplify the signal passively
  • Receiver
    • Takes the signal received by the antenna and translates it to binary
  • Intentional Radiator (IR)
    • Measured in milliwatts (mW)
    • The sum of all parts except the antenna
  • Equivalent Isotropically Radiated Power (EIRP)
    • Highest RF strength transmitted from an antenna

Decibel Measurements

  • Relative measurements
    • dB – Decibels
    • dBi – Isotropic power gain from an antenna
    • dBd – Increase in power compared to a standard dipole antenna (2.14 dB)
  • Absolute measurements
    • dBm – dB relative to 1mW

Fade Margin

  • Fade Margin
    • Allowable ‘buffer’ of signal loss that can occur on a link before the receiver cannot process it anymore
    • Guidelines:
      • 3 miles – 10dBm minimum
      • 5 miles – 15dBm minimum
      • >5 miles – 25 dBm minimum

Chapter 4 – RF Signal and Antenna Concepts

Elevation Charts

  • Elevation charts are logarithmic meaning even a 3dB change dramatically affects coverage
  • 10dB = A change in distance of approximately 70% according to the 6dBi rule (6dB = 50% distance change)
  • Each concentric circle reprsents a 10dB change
    • Note: Although the book said this I did some research and it seems that the values on the circles changes per vendor or what day of the week the chart is published so be aware it could be anything but is likely to be either 3, 5 or 10 dB.

This image shows a patch antenna azimuth (horizontal/looking top down) pattern with each circle representing a 5dB change in signal strength

Patch antenna azimuth elevation chart

Antenna Patterns during power changes

  • The image below shows a dipole antenna pattern when the the power is increased (signal is slimmer and goes further) or decreased (signal is fatter and does not reach as far).
  • This is for the vertical pattern only as horizontally the pattern will always be 360°.
Dipole antenna pattern
Dipole antenna pattern

Antenna Characteristics

  • Antennas are most effective when the length of the physical element is an even fraction or multiple of the wavelength (λ).
  • Note: This isn’t a strictly important to know from my point of view but it is a pretty cool fact to err.. dazzle your co-workers with.
  • Grid antennae are spaced with wires specific to the λ of the relevant frequency it is being used for
  • One vendor, Xirrus, offers arrays of high-gain sector antennas that provide 360° coverage over fairly large areas
  • Beamforming
    • Static Beamforming – Like Xirrus with antennas statically positioned
    • Dynamic Beamforming – Increased Power in direction of client
    • Transmit Beamforming (TxBF) – Multiple phase-shifted signals sent simultaneously
      • Implicit TxBF – Uses channel sounding
      • Explicit TxBF – Does not use channel sounding
      • Both require client feedback on phase shift and therefore support for various protocols
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