The Wonders of Wireless: Phil Levis

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Associate Professor Philip Levis discusses the history of wireless and the basics of modern phone and WiFi systems.

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The Wonders of Wireless: Phil Levis

  1. 1. The Wonders of Wireless Professor Philip Levis Stanford eDay 2012
  2. 2. Today You’ll Learn• What are wireless signals?• How does wireless work?• Why does wireless seem so capricious?• How can you improve your signal?
  3. 3. Today You’ll Learn• What are wireless signals?• How does wireless work?• Why does wireless seem so capricious?• How can you improve your signal?
  4. 4. Electromagnetic Radiation• Visible light, x-rays, gamma rays, ultraviolet light, cosmic rays, and radio waves• Intangible (almost!) waves of energy• Travel at “speed of light” ‣ 180,000 miles/second ‣ 1,000,000,000 ft/second (1 Gft/sec) ‣ 1 ft/nanosecond
  5. 5. Describing a Wave Amplitude Phase WavelengthFrequency: how many waves per second (e.g., 1,000Hz)Wavelength: speed of wave / wave frequencyPhase: timing of waves within a wavelength
  6. 6. Example: Audio • Waves through a physical medium: air • Audible: 20Hz to 20,000Hz • Speed of sound ≈ 1,100 ft/second ‣ Wavelength of piano’s lowest C (33Hz)? ‣ Wavelength of middle C (261Hz)? ‣ Wavelength of highest flute note (2100Hz)?Wavelength = speed of wave / wave frequency
  7. 7. Light is Different• Not waves through a medium: pass through vacuum like empty space ‣ Michelson-Morley and the luminiferous aether• Waves in water and air are energy in the form of motion of matter• Electromagnetic radiation is “pure” energy: it has no mass ‣ Carried by photons
  8. 8. Spectrum
  9. 9. Spectrum

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