Difference between revisions of "WiFi notes"

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Contrast this to [[#Signal Strength|Signal Strength]].
 
Contrast this to [[#Signal Strength|Signal Strength]].
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==  SNR ==
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The '''Signal to Noise Ratio''' (SNR) in WiFi  can be characterized by the following values:
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;>40 dB: excellent signal; almost always full speed.
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;25 - 40 dB: good signal; usually full speed, but may sometimes drop to lower speed.
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;15 - 25 dB: moderate signal; fast, but not always full speed.
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;10 - 15 dB: lowest useful signal; slow data speeds; may sometime loose association.
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;<10 dB: AP may be detectable, but rarely useful signal; rarely maintains association.
  
 
== Signal Strength ==
 
== Signal Strength ==
  
Signal Strength is usually expressed in '''dBm'''. This is an absolute value (not dimensionless) for measuring milliwatts. The reference power is 0 dBm = 1 mW. The range of received signal power for WiFi stations is typically in the range or -80 to -60 dBm. Contrast this to [[#Received Signal Strength Indicator|Received Signal Strength Indicator]].
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Signal Strength is usually expressed in '''dBm'''. This is an absolute value (not dimensionless) for measuring milliwatts. That is, '''dBm is another way to measure milliwatts'''. The reference power is 0 dBm = 1 mW. The range of received signal power for WiFi stations is typically in the range or -80 to -60 dBm. Contrast this to [[#Received Signal Strength Indicator|Received Signal Strength Indicator]].
  
 
*A reading of -80 dBm equals 10 pW of received radio power.
 
*A reading of -80 dBm equals 10 pW of received radio power.

Revision as of 16:50, 5 October 2011


Received Signal Strength Indicator

Received Signal Strength Indicator (RSSI) is dimensionless. It is a measure of relative signal strength; thus, it is only useful in comparing measurements made with the same hardware and drivers. Measurements are probably not comparable even between two different receivers made with identical equipment. The values returned be different hardware and drivers can have vastly different ranges and interpretations of minimum and maximum.

Contrast this to Signal Strength.

SNR

The Signal to Noise Ratio (SNR) in WiFi can be characterized by the following values:

>40 dB
excellent signal; almost always full speed.
25 - 40 dB
good signal; usually full speed, but may sometimes drop to lower speed.
15 - 25 dB
moderate signal; fast, but not always full speed.
10 - 15 dB
lowest useful signal; slow data speeds; may sometime loose association.
<10 dB
AP may be detectable, but rarely useful signal; rarely maintains association.

Signal Strength

Signal Strength is usually expressed in dBm. This is an absolute value (not dimensionless) for measuring milliwatts. That is, dBm is another way to measure milliwatts. The reference power is 0 dBm = 1 mW. The range of received signal power for WiFi stations is typically in the range or -80 to -60 dBm. Contrast this to Received Signal Strength Indicator.

  • A reading of -80 dBm equals 10 pW of received radio power.
  • A reading of -70 dBm equals 100 pW. This is 10 times stronger than -80 dBm.
  • A reading of -60 dBm equals 1000 pW, or 1 nW. This is 10 times stronger than -70 dBm.

The inside of a microwave oven can have a signal strength of 60 dBm, which is 1 kilowatt. There is a 120 dB difference between -60 dBm and 60 dBm. That's a factor of 10^12, so there is one trillion times more radio energy inside a microwave oven than what is received by a WiFi station. But, you have to be careful when trying to impress people with big numbers. The range of human hearing is also 120 dB. That ranges from the most quiet sound the ear can hear to the loudest sound the ear can hear before it is damaged.

1 milliwatt = 1.0 × 10-6 kilowatts

Location by RSSI

Trilateration

Bilinear Interpolation

Shapely