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

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

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