Notes On Chapter Four -- Transmission Media

• 4.1 Introduction
  
  
• 4.2 Copper Wires
  
  
  • Interference:
    
    
    • current moving through a wire induces an electro-magnetic field.
      
      
    • an electro magnetic field induces flow of current in another wire.
      
      
  • Twists and Shields help solve the problem.
    
    
• 4.3 Glass Fibers
  
  
  • Pass around the sample
    
    
  • advantages are: high capacity, speed, no interference, lack of attenuation, no need for circuit.
    
    
  • disadvantages are: expensive, hard to find damage, hard to repair
    
    
  • 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
  
  
  • advantage: no wires, and long link
    
    
  • disadvantage: expensive, signal delay
    
    
  • multiple transponders operating on different frequencies recieve and re-transmit 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 36,000 kilometers up (== 20,000 miles == 2.5 earth radii), and they have to be in the plane of the equator. There is room only for 45 to 90 of them (need to avoid interference). 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 the creation of a barrier around the earth.
    
    
• Illustation of EM Spectrum
  
  
• 4.9 Microwave
  
  
  • advantages: wireless, inexpensive, good capacity and bandwidth compared to radio.
    
    
  • 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, good capacity and bandwidth compared to radio and microwave.
    
    
  • disadvantages: line of sight, low power, even fog can be a problem.
    
    
• 4.12 Summary