The stronger the signal strength the more reliable the connections and higher speeds are possible.The signal strength is the wireless signal power level received by the wireless client. 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. 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. for example, under a log 10 scale, a value of -2 represents 10 to the -2 power, which equals 0.01. In logarithms, the value indicated represents an exponent.

The reason you see negative values is that they are representing small but positive numbers, on a logarithmic scale. My first question when I saw this was, “Why negative numbers?” 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.

Wifi signal strength