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

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

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

9. Spectrum

10. Spectrum