Different radio bands require different regulatory power limits. Each device you can buy, must not exceed any power limits given by the regulatory domain you want to deploy it in. The values given here are taken from the ETSI standards and apply to european countries. Some of the terms related to regulatory demands or transmission power used in this post are explained in this post.
There exists two EIRP power limits for the 2.4 GHz band, one for 802.11b rates with CCK modulation (1, 2, 5.5 and 11 Mbps) and one for 802.11g/n rates with OFDM modulation. The limit is set to 20 dBm (100 mW) for OFDM and 18 dBm (63 mW) for CCK.
The spectral power limitation of 10 dBm/MHz (10 mW/MHz) causes the lower power limit for 802.11b. As the spectral mask of the CCK modulation looks more like a sombrero, we see a high spectral power per MHz at the center and a lower one at the edges. So if you don’t lower the Tx power generally to 18 dBm, you exceed the spectral power limitation at the center of a 802.11b 20 MHz channel. For OFDM, the spectral mask looks more like a rectangular, so the power is nearly distributed equally, with an idealistic 7 dBm/MHz (5 mW/MHz) over a 20 MHz channel for example, and the maximum power limit of 20 dBm can be used.
Since the 5 GHz band is divided into two different ETSI Radio Local Area Network (RLAN) bands of 5150 to 5350 MHz and 5470 to 5725 MHz, which can be compared to the FCC Unlicensed National Information Infrastructure (U-NII) bands in the US, each band can have different power limits. As 802.11 only uses OFDM modulation in this radio band, there are no modulation specific regulations, only frequency specific.
RLAN band 1 (5150 to 5350 MHz)
Indoor only sub-band I (5150 – 5250 MHz)
The first RLAN sub-band includes the channels 36 to 48 and has an EIRP power limit to 23 dBm (200 mW). These channels are considered for indoor only usage and do not require any Dynamic Frquency Selection (DFS) or Transmit Power Control (TPC) features. It is comparable to FCC U-NII-1.
Indoor only sub-band II (5250 – 5350 MHz)
In the second sub-band of the RLAN band 1 with channels 52 to 64, the ETSI has set the EIRP power limit to 23 dBm (200 mW) for devices with TPC and 20 dBm (100 mW) for devices without TPC. For a device with TPC, the mean EIRP at the lowest power level of the TPC range must not exceed 17 dBm (50 mW). This band requires DFS support and is comparable to FCC U-NII-2.
RLAN band 2 (5470 to 5725 MHz)
Channels from 100 to 140 are part of the second RLAN band and have an EIRP power limit of 30 dBm (1000 mW) for TPC and 27 dBm (500 mW) for non-TPC devices or 20 dBm (100 mW) for devices without any TPC or DFS support. The mean EIRP power level for a slave device with TPC must not exceed 24 dBm at the the lowest TPC power level if the device is also capable of radar detection or 17 dBm otherwise. This band can be used for in- and outdoor deployments as well and is comparable to FCC U-NII-2e.
RLAN band 3 / Broadband Radio Access Networks (BRAN) (5725 – 5875 MHz)
Comparable to the FCC U-NII-3 (5725 – 5825 MHz) band with a higher upper frequency range, the ETSI has defined the channels 155 to 171 (155, 159, 163, 167, 172) for Broadband Wireless Access (BWA) use. The idea is to give internet access to locations without any wired access network available. The maximum EIRP output power has been set to 36 dBm (4000 mW) with the limitation of RF power into antenna of 30 dBm (1000 mW).
Furthermore, with the ERC Recommendation 70-03, it is allowed to deploy WLAN in this band with a maximum EIRP output power of 14 dB (25 mW) in this frequency band.
Clarification: The RF output power is defined as the mean equivalent isotropic radiated power (e.i.r.p.) of the equipment during a transmission burst. In general, the limits are valid for the device with antenna gain and cable loss and not only the output power of WLAN module.
ETSI documents used for this post
EN 300 328 v1.8.1 for 2.4 GHz
Document 32005D0513 Article 4 on Indoor only usage of RLAN band 1.
ERC Recommendation 70-03 for short range devices in U-NII-3
Advice: The author hopes that the values given here are correct. If you can prove otherwise, feel free to comment or contact me directly.
*Update (2014-11-27): Use the term EIRP power instead of Tx power, I also added a clarification.*
*Update (2014-12-02): Substituted U-NII band definitions with ETSI RLAN bands.*
*Update (2014-12-09): Added link to post with term definitions, better definition of “lowest” power level with TPC*
*Update (2016-01-07): Correct indoor only use for frequency range 5250 – 5350 MHz, thanks to comment by Stefan Schneider.
*Update (2018-09-21): Added paragraph regarding usage of the U-NII-3 band with 25 mW EIRP.