Wireless LAN (WLAN) channels are frequently accessed using IEEE 802.11 protocols. The 802.11 standard provides several radio frequency bands for use in Wi-Fi communications, each divided into a multitude of channels numbered at 5 MHz spacing (except in the 45/60 GHz band, where they are 0.54/1.08/2.16 GHz apart) between the centre frequency of the channel. The standards allow for channels to be bonded together into wider channels for faster throughput.
802.11ah operates in sub-gigahertz unlicensed bands. Each world region supports different sub-bands, and the channels number depends on the starting frequency on the sub-band it belongs to. Therefore there is no global channels numbering plan, and the channels numbers are incompatible between world regions (and even between sub-bands of a same world region).
The following sub-bands are defined in the 802.11ah specifications:
14 channels are designated in the 2.4 GHz range, spaced 5 MHz apart from each other except for a 12 MHz space before channel 14.[2] The abbreviation F0 designates each channel's fundamental frequency.
Interference happens when two networks try to operate in the same band, or when their bands overlap. The two modulation methods used have different characteristics of band usage and therefore occupy different widths:
While overlapping frequencies can be configured at a location and will usually work, it can cause interference resulting in slowdowns, sometimes severe, particularly in heavy use. Certain subsets of frequencies can be used simultaneously at any one location without interference (see diagrams for typical allocations). The consideration of spacing stems from both the basic bandwidth occupation (described above), which depends on the protocol, and from attenuation of interfering signals over distance. In the worst case, using every fourth or fifth channel by leaving three or four channels clear between used channels causes minimal interference, and narrower spacing still can be used at further distances.[18][19] The "interference" is usually not actual bit-errors, but the wireless transmitters making space for each other. Interference resulting in bit-error is rare.[19] The requirement of the standard is for a transmitter to yield when it decodes another at a level of 3 dB above the noise floor,[20] or when the non-decoded noise level is higher than a threshold Pth which, for Wi-Fi 5 and earlier, is between -76 and -80 dBm.[19]
As shown in the diagram, bonding two 20 MHz channels to form a 40 MHz channel is permitted in the 2.4 GHz bands. These are generally referred to by the centres of the primary 20 MHz channel and the adjacent secondary 20 MHz channel (e.g. 1+5, 9+13, 13–9, 5–1). The primary 20 MHz channel is used for signalling and backwards compatibility, the secondary is only used when sending data at full speed.
Except where noted, all information taken from Annex J of IEEE 802.11y-2008
This range is documented as only being allowed as a licensed band in the United States. However, not in the original specification, under newer frequency allocations from the FCC, it falls under the 3.55–3.7 GHz Citizens Broadband Radio Service band. This allows for unlicensed use, under Tier 3 GAA rules, provided that the user doesn't cause harmful interference to Incumbent Access users or Priority Access Licensees and accepts all interference from these users,[21] and also follows of all the technical requirements in CFR 47 Part 96 Subpart E.
A 40 MHz band is available from 3655 to 3695 MHz. It may be divided into eight 5 MHz channels, four 10 MHz channels, or two 20 MHz channels.
The division into 5 MHz channels consumes all eight possible channel numbers, and so (unlike other bands) it is not possible to infer the width of a channel from its number. Instead each wider channel shares its channel number with the 5 MHz channel just above its mid frequency:
and so on.
In Japan since 2002, 80 MHz of spectrum from 4910 to 4990 MHz has been available for both indoor and outdoor use, once registered.
Until 2017, an additional 60 MHz of spectrum from 5030 to 5090 MHz was available for registered use, however it has since been re-purposed and can no longer be used.[22]
50 MHz of spectrum from 4940 to 4990 MHz (WLAN channels 20–26) are in use by public safety entities in the United States. Within this spectrum there are two non-overlapping channels allocated, each 20 MHz wide. The most commonly used channels are 22 and 26.