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Wireless Transmission Media

Also called Unguided Media

Sub-topic Outline:

• The Electromagnetic Spectrum
• Radio Transmissions
• Microwave Transmissions
• Other options: Infrared, Lightwave

Electromagnetic Waves

Properties:

• Created from movement of electrons.
• Includes visible light.
• Like all waves, they have amplitude, frequency and phase.
• Wavelength - distance between peaks.
• Travel through free space
  • low frequency - travels through obstacles, direction bends with medium.
  • high frequency - bounces off solid obstacles, straight line.

For electronic communication,

• Attach antenna to electrical circuit - bradcast and receive electromagnetic waves.

The Electromagnetic Spectrum

Fig 2-11 Tanenbaum textbook p95

• Frequencies used for wireless communications are in the range 30KHz to 300 GHz.
• Above 300 GHz (infrared light) can be used, but ot normal.

Regulation on the Use of the Electromagnetic Spectrum

• ITU-R allocates different parts of the spectrum for different uses (AM/FM radio, TV, cellular phone, etc).
• National bodies not obligated to follow ITU-R recommendations - but most do.

Radio Transmission

Some good properties of radio waves

• Easy to generate
• Travels long distances
• Moves through solid obstacles
• omni-directional

Some bad properties of radio waves

• Subject to interference from motors and electrical devices
• At low frequencies,
  • power falls off rapidly with distance.
• At high frequencies,
  • moves in straight line.
  • bounces off solid obstacles.
  • absorbed by rain.

Radio Transmission at Different Frequencies

Fig 2-12 Tanenbaum textbook p98

Microwave Transmission

• Approx 100 MHz to 100 GHz
• Very high frequencies - straight line, no obstacles.
  • Use repeaters if necessary
• Possible problems:
  • Multipath fading - component waves refracted by atmospheric layers and goes out of synch.
  • Above 8 GHz, absorbed by water (ie. rain)
• Advantages of Microwave over Fiber Optics:
  • No need to dedicate complete physical path on land.
  • Putting up simpel "towers" cheaper than laying cables.
  • Some frequency bands do not need licensing to use.

Using Higher Frequencies than Microwaves

• Infrared (or milimeter) waves:
  • Used by remote controls for TV, VCRs, etc.
  • Cheap and easy to build.
  • Straight line, no obstacles - even more so than microwaves.
  • Used for wireless LANs within a room.
• Lightwave:
  • Connect up buildings
  • High frequency lasers and photodetectors on rooftops
  • Very high accuracy in aligning the transmitter and detector required
  • Direction of beams affected by fog, rain, and even temperature changes.
• Neither used very widely.

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