The wavelength of 5GHz is half of 2.4GHz, which implies higher attenuation. For access point use the 14dBm would suffice and there are no DFS or other restrictions, but the sparse client support needs to tested before deployment. In Europe they can be used according to Short Range Device (SRD) specification for transmissions up to 25mW (14dBm), but most devices don’t support these channels. The upper channels 149–165 are on every fourth odd channel. Most APs will play it safe and choose a non-DFS channel 36–48 which may result in overlaps in channel use. If the access point detects a radar signal it will switch channel automatically. On other DFS channels 52–140 this start-up delay is one minute. At start-up the access points will listen for radar signals for 10 minutes before transmitting. Weather radars use channels 120–128 and access points must yield to them. In access point use 30dBm is irrelevant, but for point-to-point connections this enables long distance links (from 10km to 50km or even more). On channels 100–140 the maximum transmit power is 1W (30dBm) and the channels can be used outdoors as well. Common default for access points is maximum power, which means 2.4GHz signal will be received 3dBm stronger, which in turn will make most devices choose 2.4GHz signal instead of the 5GHz. In WiFi the connection is always bidirectional so there is no point in receiving the access point if you cannot send a reply. In access point use the maximum transmission power is practically irrelevant, since typical user devices have less transmit power. The maximum transmission power is 200mW (23dBm), which is greater than the 100mW (20dBm) allowed for 2.4GHz, but still doesn’t quite compensate for the 6dB attenuation due to higher frequency. In Europe (or in ETSI jurisdiction) channels 36–64 are restricted for indoor use only. Later more channels have been made available, but they have several restriction for their use in the U.S. Originally only the four lowest channels were available in the U.S. The whole 5–6GHz is not available since there are some forbidden channels and some channels have special restrictions. Most devices even cannot be tuned to the intermediate channels. Fortunately only every fourth channel (36, 40, 44…) is used which provides for de facto 20MHz channel width without the overlap problems of 2.4GHz. The 5GHz band is divided into 5MHz channels like the 2.4GHz band. The latest 802.11ac is only defined for 5GHz but all devices still support 802.11n and most also on 2.4GHz Channels 802.11n was defined for both 2.4GHz and 5GHz bands, which finally launched 5GHz use. 5GHz was introduced in 802.11a, but the radios were expensive and the band didn’t gain popularity.
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