Since the antenna adds effective power, the radios themselves have a maximum power well under 1 watt. (With most WiFi chips, the driver software can control the power level).
WiFi manufacturers rate the power of their radios relative to a 1 milliwatt radio. So, for example, a 100 milliwatt radio is 100 times as powerful as the reference radio.
Instead of a simple ratio, we can convert that to decibels relative to a 1 milliwatt radio (dBm):
dBm = 10 * log (power in milliwatts)
dBm = 10 * log (100)
dBm = 10 * 2 = 20
A simple spreadsheet calculator can do this conversion.
Manufacturers rate the sensitivity of their radios in dBm, and we can use that measure in calculating power budgets for radio links.
WiFi radios automatically select a modulation scheme (and corresponding link speed) that can be sustained by the signal strength. For example, the D-Link DI-624 can adjust link speed in 11 steps ranging from 1 to 54 Mbps, as shown below:
| Speed (Mbps) | Modulation scheme | Power dBm | 802.11 b or g |
|---|---|---|---|
| 54 | OFDM | -68 | g |
| 48 | OFDM | -68 | g |
| 36 | OFDM | -75 | g |
| 24 | OFDM | -79 | g |
| 18 | OFDM | -82 | g |
| 12 | OFDM | -84 | g |
| 11 | CCK | -82 | b |
| 9 | OFDM | -87 | g |
| 6 | OFDM | -88 | g |
| 5.5 | CCK | -85 | b |
| 2 | QPSK | -86 | b |
| 1 | BPSK | -89 | b |