Computer Networking Lecture Notes

21 February 2013 • Wireless Transmission Media

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Outline

• Wireless media.
• The electromagnetic spectrum.
• Wireless frequencies.
  • Radio, microwave, and infrared.

Wireless Transmission

• Wireless transmission propagates signals without constraint.
• Wireless transmission media have no interfaces (or trivial interfaces).
  • Air, water, earth, fire, …
• Wireless media are a noisy, complex environments.
  • But wireless deployment is easy and cheap.
  • And wireless supports mobility.

Difficulties

• Copious interference sources.
  • Physical: weather, birds, clouds, …
  • Electrical: lightening, anything electrical, other wireless systems, …
• It’s hard not to eavesdrop on wireless transmissions.
• Multipath distortion delivers redundant signals.
• A dynamic environment, changing rapidly from many factors.

In the Beginning…

James Clerk		Heinrich Rudolph
Maxwell		Hertz
1831–1879		1857–1894

Electromagnetic Radiation

• Moving electrons produce electromagnetic radiation.
  • Predicted by Maxwell (1865), observed by Hertz (1887).
• Antennas broadcast and receive low-frequency radiation (radio).
  • Broadcast signals are orders of magnitude more powerful than received signals.
• Radiation characteristics vary with frequency.

Frequency and Wavelength

• Frequency (f): cycles per time (Hz).
• Wavelength (λ): length per cycle
• Electromagnetic waves travel at c, the speed of light.
  • In a vacuum, c ≈ 186,000 miles/sec, 300,000 km/sec, 1 ft/nsec.
  • In matter, it’s ≈ 2/3c.
• λf = c
  • For fun, λm/c · f Mc/s = 300 m/s.

Electromagnetic Spectrum

Transmission Spectrum

Radio Frequencies

• Frequencies from 10⁴ to 10⁸ Hz or so (λ = 3 to 3·10⁵ m).

  

Radio Transmission

• Low-frequency radio radiates omnidirectionally.
• Wave-front path loss is proportional to r², r = distance to the antenna.
  • Wireless signals attenuate more slowly than do wired signals.
  • Wireless signals travel further than wired signals.
• Low-frequency waves penetrate deeply everywhere.

Line-of-Sight

• Higher frequency radio (HF and VHF) radiates in straight lines (line of sight).
  • And are absorbed by big things, such as the earth.
• Sufficiently high-frequency radio may bounce off the ionosphere and return to earth.
  • What hams call DXing.
  • Also known as sky waves.

Microwave Frequencies

• Frequencies from 10⁹ to 10¹⁰ Hz or so (λ = 0.3 m to 3 cm).

  

Microwave Transmission

Like light, microwaves move in a straight line. And can be focused. And do diverge. Repeater towers “bend” microwaves to follow the earth.

Historic New Jersey.

Historic New Jersey..

Microwave Characteristics

• Microwaves are bullied by most everything.
  • Can’t penetrate buildings, absorbed by water (rain).
• Microwaves bounce off things, causing multipath distortion (fading).

Infrared Frequencies

• Frequencies from 10¹¹ to 10¹⁴ Hz or so (λ = 3 mm to 3 μm).

  

• A proto-Bluetooth, slowly disappearing.

Infrared Transmission

• Infrared wavelengths are essentially light.
  • Infrared works well with silicon-based fiber.
• Like light, infrared is easily blocked (you) or absorbed (water).

Summary

• Wireless media has fast, cheap deployment and supports mobility.
  • But is a complex, finicky, expensive environment.
• The three major wireless bands: radio, microwave, and infrared.
  • All on the low side of the visible spectrum.
• Future networks may be fiber backbones with wireless last miles.

References

• Three Degrees Above Zero by Jeremy Bernstein in The New Yorker, 20 August 1984.
• Project Echo: A Horn-Reflector Antenna for Space Communication by Arthur Crawford, D. Hogg and Loyd Hunt in The Bell System Technical Journal, July 1961.

  The July 1961 issue of The Bell System Technical Journal is devoted to the Bell system’s efforts in Project Echo, an experiment in microwave communication using passive satellites.

Credits

• James Clerk Maxwell from Wikimedia Commons, in the public domain.
• Heinrich Hertz from Wikimedia Commons, in the public domain.
• dsc_0068 by Curtis Anderson under a Creative Commons 2.0 license.
• Homdel Horn Antenna by Erik Dunham under a Creative Commons BY SA license.
• Homdel Horn Antenna by Erik Dunham under a Creative Commons BY SA license.

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This page last modified on 2013 February 22.