Building Wireless Community Networks, Second Edition

Power Limits

Ideally, a well-engineered path has just the amount of power required to get from point A to point B with decent reliability. Good engineering limits the signal to the area being served, which results in reduced interference and a more efficient use of the spectrum. Using too much power will cover more area than is needed and may interfere with other users of the band.

Because 802.11 is designed for short-range use in offices and homes, it is limited to very low power.

FCC 15.247 and 80211.b

Section 15.247 of the FCC regulations covers the operation of 802.11b devices.

Point-to-multipoint communication

You are allowed up to 30dBm or 1 watt of Transmitter Power Output (TPO) with a 6dBi antenna, or 36dBm or 4 watts Effective Isotropic Radiated Power (EIRP). The TPO must be reduced 1dB for every dB of antenna gain over 6dBi.

Point-to-point communication

The FCC encourages directional antennas to minimize interference with other users. The FCC is more lenient with point-to-point links: the TPO must be reduced by one-third of a dB, instead of the full dB for point-to-multipoint communication.

More specifically, for every 3dB of antenna gain over a 6dBi antenna, you must reduce the TPO 1dB below one watt. For example, a 24dBi antenna is 18dB over a 6dBi antenna. You would have to lower a one-watt (30dBm) transmitter 6dB (one-third of 18) to 24dBm or one-quarter watt.

FCC 15.407 and 802.11a

Section 15.407 of the FCC regulations covers the operation of 802.11a devices.

Point-to-multipoint ...