Computer Networking Lecture Notes

13 November 2012 • Unguided Transmission Media

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Outline

Unguided Transmission

• Unguided transmission propagates signals without constraint.
• Unguided transmission media have no interfaces (or trivial interfaces).
  • Air, water, earth, fire, …

Difficulties

• External interference: weather, birds, clouds, …
• It's hard not to eavesdrop on unguided transmissions.
• Multipath distortion delivers redundant signals.

Electromagnetic Radiation

• Moving electrons produce electromagnetic radiation (radio waves).
  • Predicted by Maxwell (1865), observed by Hertz (1887).
• Antennas broadcast and receive radio waves.

Frequency and Wavelength

• Frequency (f): cycles per second (Hz).
• Wavelength (λ): inches per cycle
• Radio waves travel at c, the speed of light.
  • In a vaccuum; in matter it's c2/3.
  • c ≈ 186,000 miles/sec, 300,000 km/sec, 1 ft/nsec.
• λf = c

Electromagnetic Spectrum

Transmission Spectrum

Summary

References

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This page last modified on 2011 October 9.