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. A.5.1 FCC 15.247 and 80211.bSection 15.247 of the FCC regulations covers the operation of 802.11b devices. A.5.1.1 Point-to-multipoint communicationYou 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. A.5.1.2 Point-to-point communicationThe 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. A.5.2 FCC 15.407 and 802.11aSection 15.407 of the FCC regulations covers the operation of 802.11a devices. A.5.2.1 Point-to-multipoint communicationAs described previously, the U-NII band is chopped into three sections. The "low" band runs from 5.15GHz to 5.25GHz and has a maximum power of 50mW (TPO). This band is meant for in-building use only, as defined by sections 15.407 (d) and (e):
The "middle" band runs from 5.25GHz to 5.35GHz, with a maximum power limit of 250mW. Finally, the "high" band runs from 5.725GHz to 5.825GHz, and has a maximum transmitter power of 1 watt and an antenna gain of 6dBi/36dBm/4 watts EIRP. A.5.2.2 Point-to-point communicationAs with 802.11b, the FCC does give some latitude to point-to-point links in 15.407(a)(3). For the 5.725GHz to 5.825GHz band, the FCC allows a TPO of 1 watt and up to a 23dBi gain antenna without requiring reduction of the TPO 1dB for every 1dB of gain over 23dBi. 15.247(b)(3)(ii) does allow the use of any gain antenna for point-to-point operations without reduction of the TPO for the 5.725GHz to 5.825GHz band. Look at the Part your equipment is certified under to see which EIRP restrictions apply. |