1. The Wonders of
Wireless
Professor Philip Levis
Stanford eDay 2012
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. 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. 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.
6. Describing a Wave
Amplitude
Phase
Wavelength
Frequency: how many waves per second (e.g., 1,000Hz)
Wavelength: speed of wave / wave frequency
Phase: timing of waves within a wavelength
7. 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
8. 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