Forum

Question: What are the power requirements for the 5Ghz UNII bands(low, middle, and high) in a point to multipoint environment. I was originally told it was as follows:
IEEE FCC
low 40 50
mid. 200 250
high 800 1000

This is also how it is answered in the CWNP practice test. In contrast, the new revision 3 study guide is telling me that all three bands follow what was listed under the IEEE column above for both the IEEE and FCC.

Did this change over time? Which is the correct answer?

Prior to the 3rd edition of the CWNA study guide, our assessment of the power output guidelines for the UNII band was the same as you list above. We stated that the FCC's requirements were 50/250/1000 mW and the IEEE's requirements were 40/200/800 mW.

While researching for the 3rd edition, we discovered section 17.3.9.1 of 802.11a, which states that, "The maximum allowable output power according to FCC regulations is shown in Table 89." Table 89 then gives the lower numbers that had previously been quoted as IEEE numbers. We were initially surprised to see the IEEE state that those lower numbers were actually based on an FCC requirement.

Attempting to resolve this inconsistency, we double-checked CFR 15.407. CFR 15.407 does give the higher numbers (50/250/1000 mW), but it actually says that power output must be the lower of either those numbers or "4 dBm + 10logB, where B is the 26-dB emission bandwidth in MHz." (for UNII-1). The formula for UNII-2 is 11 dBm + 10logB and the formula for UNII-3 is 17 dBm + 10logB.

It's possible that the IEEE's lower power requirements are the result of the second clause of the "or" statement: 4 dBm + 10logB. If, for an 802.11a transmitter, 4 dBm + 10logB was 40 mW, then the IEEE would have to use that as its maximum power output in UNII-1. We can't think of any other way to explain why 802.11a states that the 40/200/800 mW limit is "according to FCC regulations".

The bottom line is that an 802.11 engineer in the U.S. needs to follow the power output requirements given in 802.11, which are essentially guaranteed to be in compliance with FCC guidelines. Those requirements are 40 mW, 200 mW, and 800 mW in the low, middle, and high UNII bands, respectively.

Also: you make reference to a point to multipoint environment. It's important to keep in mind that UNII requirements for power output in a point-to-multipoint vs. point-to-point configuration differ depending on which band you're in, unlike the 2.4 GHz ISM band, where the requirements are the same throughout.

Here is the full text of the relevant part of 15.407, for reference, from this URL:

http://frwebgate.access.gpo.gov/cgi-bin/get-cfr.cgi?TITLE=47&PART=15&SECTION=407&YEAR=2000&TYPE=TEXT

Sec. 15.407 General technical requirements.

(a) Power limits:

(1) For the band 5.15-5.25 GHz, the peak transmit power over the frequency band of operation shall not exceed the lesser of 50 mW or 4 dBm + 10logB, where B is the 26-dB emission bandwidth in MHz. In addition, the peak power spectral density shall not exceed 4 dBm in any 1-MHz band. If transmitting antennas of directional gain greater than 6 dBi are used, both the peak transmit power and the peak power spectral density shall be reduced by the amount in dB that the directional gain
of the antenna exceeds 6 dBi.

(2) For the band 5.25-5.35 GHz, the peak transmit power over the frequency band of operation shall not exceed the lesser of 250 mW or 11 dBm + 10logB, where B is the 26-dB emission bandwidth in MHz. In addition, the peak power spectral density shall not exceed 11 dBm in any 1-MHz band. If transmitting antennas of directional gain greater than 6 dBi are used, both the peak transmit power and the peak power spectral density shall be reduced by the amount in dB that the directional gain of the antenna exceeds 6 dBi.

(3) For the band 5.725-5.825 GHz, the peak transmit power over the frequency band of operation shall not exceed the lesser of 1 W or 17 dBm + 10logB, where B is the 26-dB emission bandwidth in MHz. In addition, the peak power spectral density shall not exceed 17 dBm in any 1-MHz band. If transmitting antennas of directional gain greater than 6 dBi are used, both the peak transmit power and the peak power spectral density shall be reduced by the amount in dB that the directional gain of the antenna exceeds 6 dBi. However, fixed point-to-point U-NII devices operating in this band may employ transmitting antennas with directional gain up to 23 dBi without any corresponding reduction in the transmitter peak output power or peak power spectral density. For fixed, point-to-point U-NII transmitters that employ a directional antenna gain greater than 23 dBi, a 1 dB reduction in peak transmitter power and peak power spectral density for each 1 dB of antenna gain in excess of 23 dBi would be required. Fixed, point-to-point operations exclude the use of point-to-multipoint systems, omni directional applications, and multiple collocated transmitters transmitting the same information. The operator of the U-NII device, or if the equipment is professionally installed, the installer, is responsible for ensuring that systems employing high gain directional antennas are used exclusively for fixed, point-to-point operations.

BRAVO! I'm not sure anything can be done here except to congratulate Joshua on a killer answer! Thanks Joshua!

Devinator