5GHz was introduced in 802.11a, but the radios were expensive and the band didn’t gain popularity. 802.11n was defined for both 2.4GHz and 5GHz bands, which finally launched 5GHz use. The latest 802.11ac is only defined for 5GHz but all devices still support 802.11n and most also on 2.4GHz
The 5GHz band is divided into 5MHz channels like the 2.4GHz band. 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. Most devices even cannot be tuned to the intermediate channels. The whole 5–6GHz is not available since there are some forbidden channels and some channels have special restrictions.
Originally only the four lowest channels were available in the U.S. where they are called UNII-I. Later more channels have been made available, but they have several restriction for their use in the U.S.
In Europe (or in ETSI jurisdiction) channels 36–64 are restricted for indoor use only. 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 access point use the maximum transmission power is practically irrelevant, since typical user devices have less transmit power. In WiFi the connection is always bidirectional so there is no point in receiving the access point if you cannot send a reply. 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.
On channels 100–140 the maximum transmit power is 1W (30dBm) and the channels can be used outdoors as well. 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). Weather radars use channels 120–128 and access points must yield to them. At start-up the access points will listen for radar signals for 10 minutes before transmitting. On other DFS channels 52–140 this start-up delay is one minute. If the access point detects a radar signal it will switch channel automatically. Most APs will play it safe and choose a non-DFS channel 36–48 which may result in overlaps in channel use.
The upper channels 149–165 are on every fourth odd channel. 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. 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.
Coverage and cell size
The wavelength of 5GHz is half of 2.4GHz, which implies higher attenuation. 2.4GHz will be received at 6dBm stronger or quad-fold when compared to 5GHz signal. A 5GHz access point will thus cover a smaller area in open space and won’t penetrate walls like 2.4GHz. Because of the stronger signal many devices will rather associate with the 2.4GHz AP. The simplest solution is to reduce the transmission power of the 2.4GHz AP by 6–7dBm.
The higher attenuation and poorer penetration can be turned into an advantage to reduce access point cell size. When the AP covers a smaller area there will be less users to compete for airtime, which translates to faster data transfer. You will need more APs to cover the area, but smaller cell size is the key to high performance WiFi.
802.11n introduced the concept of combining channels. Combining two 20MHz channels will yield over twice the bandwidth, since there is no need for an isolation gap between channels. In 802.11n you could combine channels on 2.4GHz as well, but there really are not enough channels available. On 5GHz combining channels is actually useful and 40MHz channels appear to be currently default on most access points.
Combined channels need to be accounted for in channel planning. If you place two adjacent access points on channels 36 and 40 and enable 40MHz channels, the APs will end up taking turns. The AP on channel 36 will use channels 36–43 and the other will use 40–47. Due to the overlap they cannot transmit or receive at the same time. You should place them on channels 36 and 44 to account for this. In the standard the 40MHz channels are numbered 38, 46, 54… to avoid overlapping, but most user interfaces seem to use 20MHz numbering.
802.11ac introduced 80MHz and 160MHz channels. They have their own channel numbers as well, because extra wide channels are so easy to set to overlap. Using such wide channels makes even the 5GHz band crowded. Another problem is wait time for the channel availability. If there are other APs in the neighborhood our AP cannot transmit before all the channels are quiet at the same time. 802.11ac provides for dynamic channel width, which turns the setting to a maximum and the AP will choose the used channel width according to the environment. The extra wide channels also require support in the user device as well to be used. 160MHz channels are a Wave 2 feature so there is not much support at this point. In reality you cannot make a channel plan with just two channels so 160MHz should be reserved for point-to-point links where they really are useful.
The maximum transmission power set by the authorities is for the whole transmission. The maximums are calculated for 20MHz channels. The maximum should be halved (-3dBm) for 40MHz, quartered (-6dBm) for 80MHz and only one eighth (-9dBm) for 160MHz. Usually this doesn’t matter for access point use, since the maximums shouldn’t be used anyways. In point-to-point links it does matter and occasionally you need to concentrate the power on fewer channels to get a stable link. A double channel with the same nominal transmit power will also consume twice the electric power, which is important factor for mobile devices. If the wide channel will respectively increase the transmission speed (i.e. shorten transmission time) then it will cancel the increase in power consumption. In practice the power consumption will increase somewhat due to retransmissions.
Compatible user devices should be steered to 5GHz since there is more capacity and less interference. The simplest way is to turn down the transmission power of the 2.4GHz to the minimum or turn it off altogether.
40MHz channels on 5GHz are well supported and increase bandwidth. You just need to plan the channel use not to cause overlaps. Even if the original channel plan is perfect, DFS may cause unexpected channel switches causing overlaps. If you have a quiet environment and your devices support them, you may use 80MHz channels. One example of consideration is the fact that the lowest 80MHz will cover all non-DFS channels. If you want to use the wider channels you have to live with DFS restrictions. Don’t use 160MHz channels except for point-to-point or other special cases.