Range and Signal Strength

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• A device supporting the Wi-Fi standard should have an indoor range of at least 30 m (100 feet)
  • Outdoor range can be double or triple indoor range
• Each station uses Dynamic Rate Switching/Selection (DRS) mechanism
  • to determine an appropriate data rate based on the signal quality
  • If the signal is strong
    • the station will select the highest available data rate
      • determined by support for a given iteration of the 802.11 standard
    • if the signal is weak
      • the station will reduce the data rate
• Radio signals pass through solid objects
  • but can be weakened or blocked by thick walls of dense concrete or metal construction
• devices that can cause interference:
  • Other radio-based devices
  • devices such as fluorescent lighting, microwave ovens, cordless phones, power motors and heavy machinery
• Bluetooth uses the 2.4 GHz frequency range but a different modulation technique
  • interference is possible but not expected
• signal strength is represented as the ratio of a measurement to 1 milliwatt (mw)
  • 1 mW = 0 dBm
  • dB and dBm units can be combined to analyze losses and gains in signal strength along a communications path
  • e.g., transmit a radio signal at 1 mW and use an antenna to boost the signal
    • effective power is:
  • negative value for dBm represents a fraction of a milliwatt
    • e.g.,
      • -30 dBm = 0.001 mW
      • -60 dBm = 0.000001 mW
  • Wi-Fi outputs small amount of power

RSSI

The Received Signal Strength Indicator (RSSI) is the strength of the signal from the transmitter as measured at the client end.

• When measuring RSSI
  • dBm will be a negative value
  • values closer to zero represent better performance
    • -30 dBm is considered to be a perfect signal
    • -65 dBm represents a good signal
    • anything < -80 dBm is likely to suffer packet loss or be dropped
• must exceed the minimum receiver sensitivity

Info

• Depending on the vendor, RSSI might be measured:
  • directly in dBm
  • might be an index value related to a scale of dBm measurements
• RSSI indices can be measured as 0–60, 0–127, or as 0–255
• On a client, this index is displayed as a number of bars of signal strength on the adapter icon

SNR

Signal-to-noise ratio (SNR) is the comparative strength of the data signal to the background noise.

• Noise is measured in dBm
  • but values closer to zero are worse
    • represent higher noise levels
  • e.g., signal is ‑65 dBm and noise is ‑90 dBm
    • SNR is the difference between the two values, expressed in dB
      • 25 dB