anthony morris
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Wireless Networking

The wireless spectrum

  • Most wireless devices implement one of two technologies:
    • FHSS (frequency hopping spread spectrum)
      • Frequency hopping
        • Short bursts of data are transmitted on particular frequency within the band
        • Next burst goes to next frequency in a sequence
      • Cheaper to implement than DSSS
      • More effective than DSSS in crowded/indoor environments
    • DSSS (direct sequence spread spectrum)
      • Data streams divided and encoded into small chunks (chips)
      • Chips are spread over all available frequencies within one of three channels all at the same time
  • Wireless standards in various bands
    • Wi-Fi --> DSSS
    • Bluetooth --> FHSS
    • ZigBee --> DSSS
    • ANT+ uses fixed frequency (neither DSSS or FHSS)

Antennas

  • Used to provide a path for signals to travel
    • Air doesn't provide a path like cables would
  • Signals originate from electrical current traveling along a conductor
    • Travels from transmitter to antenna
    • Antenna emits signal as a series of electromagnetic waves
    • Signal moves through air to destination
    • At destination, another antenna accepts the signal and a receiver converts it to current
  • Two antennas must be tuned to the same frequency to communicate
  • Each type of wireless service requires an antenna specifically designed for that service

Radiation Pattern

  • Relative strength over 3D area of all electromagnetic energy that antenna sends/receives
  • Unidirectional
    • Directional
    • Issues wireless signals along single direction
  • Omnidirectional
    • Sends/receives wireless signals with equal strength/clarity in all directions

Signal Propagation

  • Way in which wave travels from one point to another
  • LOS (line of site)
    • Ideal signal path
    • Signal travels in straight line directly from transmitter to receiver
    • Maximizes distance for amount of energy used
    • Results in clearest possible signal
  • When obstacles are in the way, signal may:
    • Pass through obstacle
    • Be absorbed
    • Fading
      • Energy fades as it runs into obstacles
      • Excessive fading can cause dropped connections or slow transmission
    • Attenuation
      • Signal weakens (moving away from transmission antenna)
      • How to correct
        • Increase power of transmission
        • Repeat signal from closer broadcast point (wireless range extender)
    • Interference
      • Wireless signals are vulnerable to noise
        • They have no shielding
      • SNR --> signal to noise ratio
    • Refraction
      • Alteration of direction, speed, and wavelength
      • Happens when wave is traveling into/through different mediums
    • Reflection
    • Scattering
      • Diffusion in multiple directions
    • Diffraction
      • Split into secondary waves

Topologies

  • Different than wired networks because they are not bound by cabling paths
  • Types
    • Ad hoc
      • Small number of nodes closely positioned
      • No intervening connectivity device
      • E.g. multiple computers directly communicating
    • Infrastructure
      • Uses intervening connectivity device
        • E.g. WAP or AP
    • Mesh
      • Several APs work as peer devices on same network
      • More fault-tolerant

Troubleshooting

  • Tools
    • Spectrum analyzer
      • Assess the quality of wireless signals
    • Wireless analyzer (Wi-Fi analyzer)
      • Evaluate Wi-Fi network availability
      • Optimize Wi-Fi signal settings
      • Help identify Wi-Fi security threats

Networking Computer Science