Tuesday, November 20, 2012

Notes - Guided Transmission Media

The following are notes from Computer Networks written by Tanenbaum 5th edition.

Magnetic Media

  • write to magnetic tape or removable media
    • VHS, DVDs
    • transmission involves physical delivery to destination
  • Due to reduction in cost of data storage, transmission rate, is actually very powerful, a lot of bandwidth is possible
    • ultrium tape storage, goes out to 70 Gbps
    • however, delivery time uncertain/requires manpower to sort
Twisted Pairs
  • twisted pair transmission lines
    • consists of two insulated copper wires about 1 mm thick
    • helical form, helps limit crosstalk
    • can be used to transmit analog or digital information
    • Category 5 twisted pair
      • two insulated wires twisted together
    • different LAN standards use the twisted pairs differently
  • Full-duplex
    • links that can be used two directions at the same time
  • Half-duplex
    • links that can only be used one direction at a time but can go both forward and back
  • Simplex
    • links that can only every be used in one direction
  • Category 3 cables
    • more twists per meter than cat 5
  • Category 6/7
    • handle signals with greater bandwidths
  • UTP(Unshielded Twisted Pair)
    • Only have wires and insulators, no extra shielding
Coaxial Cable
  • better shielding and bandwidth than unshielded twisted pairs
    • spans longer distances
  • 50 ohm cable used for digital transmission
  • 75 ohm used for analog transmission and cable tv

Power Lines
  • electrical power lines most common wiring
  • use for communication X10 standard
  • issues
    • extremely convenience superimpose low frequency data signal into wiring
    • difficult to transmit, not original purpose of grid
    • wiring changes from house to house
    • signal reflections as appliances turned on/off
  •  still possible to send 100Mbps over typical household wiring
Fiber Optics
  • Moore's Law increase in computer technology
  • original IBM PC ran at 4.77 MHz, 28 years later 3 HHz, about factor of 16 per decade
  • Fiber technology bandwidth can achieve up to 50,000 Gbps, currently at 100 Gbps
  • FttH(Fiber to the Home)
    • light pulse is a 1 nothing is a 0
    • transmission takes advantage of internal reflection, light ray incidence at or above critical value
  • Multimode fiber
    • each ray has a different mode, light ray allowed to bounce at different angles
  • Single-mode fiber
    • light can only propagate in a straight line no bounce
    • more expensive, can transmit longer distance
  • Transmission of light through Fiber
    • made of glass
    • no more than 1mm thick
    • light attenuation depends on physical property of glass and the wavelength of light
    • wavelength bands most commonly used are
      • 0.85, 1.30, 1.55 microns
      • relatively flat attenuation
    • Chromatic Dispersion
      • light spreading through propagation
      • making pulses in a special shape, called solitons can cancel this effect
  • Fiber cables
    • construction shown in the following diagram
  • fiber connections
    • connectors
      • when fiber is connected, loses about 10-20% of light
    • spliced mechanically
      • lay two end to end, and clamp
      • about 10% lgith loss
    • fused into one
      • small attenuation
  • light sources
    • LED(light emitting diodes) semiconductor lasers
    • these light beams can be tuned using Fabry-Perot or Mach-Zhender interferometers
    • receiving end is a photodiode
  • Comparison of Fiber Optics and Copper wire
    • Fiber very advantageous
      • more bandwidth
      • low attentuation
        • distance to amplifier/repeater 50km for fiber
        • 5 km for copper
      • thin and lightweight
    • disadvantages
      • less familiar
      • unidirectional, can't have two way communication without a separate wire

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