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Why is almost everything negative in Wireless?

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If you have used wireless for a while or have just ventured into the wireless environment, one thing that you will notice is that your numbers that are associated with the signal strength (RSSI) and noise are represented with negative numbers.

My first question when I saw this was, “Why negative numbers?”

The reason you see negative values is that they are representing small but positive numbers, on a logarithmic scale. In logarithms, the value indicated represents an exponent... for example, under a log 10 scale, a value of -2 represents 10 to the -2 power, which equals 0.01.  Likewise, a negative Decibel-milliwatt (dBm) means that you're applying a negative exponent in your power calculations; 0 dBm equals 1 milliwatt (mW) of power, so -10 dBm equates to 0.1 mW, -20 dBm equates to 0.01 mW, and so forth.  It's a lot easier, and more useful in some calculations, to describe a weak signal as -100 dBm as opposed to 0.0000000001 mW.

Signal Strength

The signal strength is the wireless signal power level received by the wireless client.

  • The stronger the signal strength the more reliable the connections and higher speeds are possible.
  • Signal strength is represented in -dBm format (0 to -100). This is the power ratio in decibels (dB) of the measured power referenced to one milliwatt.
  • That means the closer the value is to 0, the stronger the signal. For example, -41dBm is better signal strength than -61dBm.

Signal Strength

 

Required for

-60 dBm

Minimum signal strength for applications that require very reliable, timely packet delivery.

VoIP/Video

-65 dBm

Minimum signal strength for reliable packet delivery.

Email, Basic Surfing

-70 dBm

Minimum signal strength for basic connectivity. Packet delivery may be unreliable.

N/A

-80 dBm

Mostly background noise. Any functionality is very unlikely.

N/A

Note: The numbers in this chart are suggestions only.

The desired signal strengths will vary, based on the requirements for the network.

Noise Level

The noise level indicates the amount of background noise in your environment.

  • The higher the noise level, the more likely hood of degraded strength and performance for your wireless signal strength.
  • The closer the value to 0, the greater the noise level.
  • Negative values indicate less background noise. For example, -96dBm is a lower noise level than -20dBm.

So how are those numbers used? I am glad you asked. They are used to determine the Signal to Noise Ratio.

Signal to Noise Ratio

The signal-to-noise ratio (SNR) is the power ratio between the signal strength and the noise level. Here is where things switch.

  • This value is represented as a +dBm value.
  • In general, you should have a minimum of +25dBm signal-to-noise ratio. Lower values than +25dBm result in poor performance and speeds.

For example:

If you have a -41dBm signal strength, and a -50dBm noise level, this results in a poor signal-to-noise ratio of +9dBm.

If you have a -41dBm signal strength, and a -96dBm noise level, this results in an excellent signal-to-noise ratio of +55dBm.

 

In my next document I will talk about how we can gather these numbers and what we can do with them.

Comments
Beginner

Nice post, and yes this is 3 years later I'm seeing it.  In it though there is a reference to a subsequent article, but the Cisco site doesn't seem to hvae a way to search your 1100+ posts for me.  Manual URL editing puts July/Auguest 2015 at around page 25-30 or so of your 78 pages of posts but anwyay, would be great to have a clean way to locate these things :)