Most 802.11 analysis tools and vendors' client management utilities provide a representation of signal strength. Four units of measurement are used to represent RF signal strength in 802.11. These are: mW (milliwatts), dBm ("db"-milliwatts), RSSI (Received Signal Strength Indicator), and a percentage measurement. All of these measurements are related to each other, some more closely than others, and it's possible to convert from one unit to another.
The first two units to consider are the mW and the dBm (pronounced "dee-bee-em" or spoken as "dee-bee milliwatts"). Although these are not the most common units in 802.11, we discuss them first because they are the most basic. Just like a pound is a basic unit for measuring weight, a watt is a basic unit for measuring energy (and, in keeping with metric conventions, a mW is one one-thousandth of a watt). It turns out that measuring RF energy in mW units is not always convenient. This is due, in part, to the fact that signal strength does not fade in a linear manner, but inversely as the square of the distance. This means that if you are a particular distance from an access point and you measure the signal level, then you move twice as far away, the signal will have decreased by a factor of four. This relationship can be characterized as logarithmic, and one can say that "RF power drops off logarithmically."
The "dBm" is a logarithmic measurement of signal strength. Since it is logarithmic, just like the power of the RF signal, as the RF signal's strength changes (logarithmically), the dBm value changes linearly. To put it more generally, if you measure a quantity that changes logarithmically (RF power) with a linear unit (mW), the unit will change logarithmically, which is inconvenient. If you measure a quantity that changes logarithmically with a logarithmic unit (dBm), the unit will change linearly, which is more convenient.
dBm values can be exactly and directly converted to and from mW values. Just like miles and kilometers can be converted directly, so can mW and dBm. The formulas to convert are:
dBm = log(mW) * 10
mW = 10^(dBm/10)
We use dBm because it's much easier to say, and write, "-96dBm" than have to say "0.000 000 000 25 mW". That's a lot of zeroes! You should realize that convenience and ease-of-understanding are two fundamental reasons why the dBm metric is used for RF signal strength, rather than mW.
The IEEE 802.11 standard defines a mechanism by which RF energy is to be measured by the circuitry on a wireless NIC. In 802.11b, g, and a, this numeric value is an integer with an allowable range of 0-255 (a 1-byte value) called the Received Signal Strength Indicator (RSSI). Notice that nothing has been said here about measurement of RF energy in dBm or mW. RSSI is an arbitrary integer value, defined in the 802.11 standard and intended for use internally by the physical and data link layers (the hardware in the card and its drivers). For example, when an adapter wants to transmit a packet it must be able to detect whether or not the channel is clear (i.e.: nobody else is transmitting). If the RSSI is below some very low threshold then the chipset decides that the channel is clear. 802.11 does not require that a particular RSSI value correspond to any particular mW value, so each vendor makes this decision on its own. This means that you probably can't compare "signal strength" values between two vendors' chipsets, because those values are based on the RSSI, and different vendors' chipsets associate different power levels with different RSSI values.
To circumvent the complexities (and potential inaccuracies) of using RSSI as a basis for reporting dBm signal strength, it is common to see signal strength represented as a percentage. The percentage represents the RSSI for a particular packet divided by the maximum RSSI value (multiplied by 100 to derive a percentage). If all vendors used that formula for converting RSSI to signal strength percentage, then percentage for signal strength would provide a reasonable cross-vendor metric for use in network analysis and site survey work. However, if vendors do not consistently use the formula above, then we once again end up in a scenario where it's impossible to compare numbers from different vendors. For example, a vendor might hypothetically use a logarithmic function to map RSSI to signal strength, which would cause the signal strength to stay at high values longer as RSSI decreased, and then to drop off very rapidly as RSSI approached zero. Frankly, we don't know the exact details, on a model-number-by-model-number basis, of how each of the many NIC manufacturers map RSSI to signal strength percentage, so its difficult to draw concrete conclusions on this matter.
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