Notes On Chapter Four -- Transmission Media

• 4.1 Introduction -- to understand network software, it helps to know a little about the hardware.
  
  
• 4.2 Copper Wires
  
  
  • Cheap, easy to work with, high conductivity/low resistance
    
    
  • Interference:
    
    
    • current moving through a wire induces an electromagnetic field.
      
      
    • an electromagnetic field induces flow of current in another wire.
      
      
  • Twists and Shields help solve the problem.
    
    
• 4.3 Glass Fibers
  
  
  • Pass around sample??
    
    
  • advantages: high bandwidth and speed, no interference, low attenuation, no need for circuit, quite flexible.
    
    
  • disadvantages: expensive, difficult to install and repair, hard to locate damaged portion.
    
    
  • light source is LED or Laser
    
    
• 4.4 Radio
  
  
  • advantage: wireless! i.e. no physical link required
    
    
  • disadvantages: weak signals, interference, lack of privacy
    
    
  
• 4.5 Satellites
  
  
  • advantages: no wires & long link
    
    
  • disadvantages: expensive & signal delay
    
    
  • multiple transponders operating on different frequencies receive and retransmit in another direction.
    
    
• 4.6 Geosynchronous Satellites
  
  
  • advantage: easier to do the aiming necessary -- stationary relative to the surface of the earth.
    
    
  • disadvantage: they have to be 36,785 kilometers up (== 20,236 miles == 2.5 earth radii), and they have to be in the plane of the equator. There is room only for a limited number - to avoid interference. They have to be something like 4 to 8 degrees apart. This implies a limit on the order of 45 to 90 GEO satellites. Signal delay is high because of the great distance from the surface of the earth.
    
    
• 4.7 Low Earth Orbit Satellites
  
  
  • advantage: less delay, and there can be more of them so more bandwidth is possible in total.
    
    
  • disadvantage: aiming is harder because they move -- ground antennas have to move to "track" the satellite.
    
    
• 4.8 Low Earth Orbit Satellite Arrays
  
  
  • An array of low earth orbit satellites could maintain a constant communication flow worldwide. (66 of them). Under this regime, any two points on the earth could communicate at any time.
    
    
  • Requires complex ground-to-"bird" and bird-to-bird communications because of all the relative motion.
    
    
  • There is concern that a great deal of "space junk" could be created by chain reactions of collisions, resulting in a barrier around the earth.
    
    
  • Iridium Information from Iridium.com
    
    
  • Globalstar Wikipedia Article
    
    
  • Teledesic Capitulates
    
    
• Illustration of EM Spectrum
  
  
• 4.9 Microwave
  
  
  • advantages: wireless, inexpensive, higher bandwidth than RF, directional
    
    
  • disadvantages: line of sight, low power, even vegetation can be a problem.
    
    
• 4.10 Infrared
  
  
  • advantages: wireless, inexpensive, no antenna needed, very nice for networking all the computers within a single room.
    
    
  • disadvantages: very short range
    
    
• 4.11 Light From A Laser
  
  
  • advantages: wireless, inexpensive, higher bandwidth than radio and microwave.
    
    
  • disadvantages: line of sight, low power, even fog can be a problem.
    
    
• 4.12 Summary