Wi-Fi

From HFUnderground

Revision as of 13:50, 14 May 2021

Wireless LAN / Wi-Fi WiFi frequency bands UHF and higher.

900 MHz (802.11ah)

802.11ah operates in sub-gigahertz unlicensed bands. Each world region supports different sub-bands, and the channels number depends on the starting frequency of the sub-band it belongs to. Thus, 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:

Template:Anchor 2.4 GHz (802.11b/g/n/ax)

Template:Seealso Fourteen 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.<ref>Template:Cite book</ref>

Nations apply their own RF emission regulations to the allowable channels, allowed users and maximum power levels within these frequency ranges. Network operators should consult their local authorities as these regulations may be out of date as they are subject to change at any time. Most of the world will allow the first thirteen channels in the spectrum.

To guarantee no interference in any circumstances the Wi‑Fi protocol requires 16.25 to 22 MHz of channel separation (as shown below). The remaining 2 MHz gap is used as a guard band to allow sufficient attenuation along the edge of the band. This guardband is mainly used to accommodate older routers with modem chipsets prone to full channel occupancy, as most modern Wi‑Fi modems are not prone to excessive channel occupancy. In 22 MHz channel width situations, there is 3 MHz free/unused spectrum between NON-overlapping channels, this is not called guard band but channel spacing.

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):

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However, the exact spacing required when the transmitters are not colocated depends on the protocol, the data rate selected, the distances and the electromagnetic environment where the equipment is used.<ref name="adjacentchannels"/>

The overall effect is that if there is considerable overlap between adjacent channels transmitters they will often interfere with each other. However, using every fourth or fifth channel by leaving three or four channels clear between used channels can cause less interference than sharing channels, and narrower spacing still can be used at further distances.<ref name="compnetworking.about.com">Template:Cite web</ref><ref name="adjacentchannels">Template:Cite conference</ref>

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3.65 GHz (802.11y)

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.

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, as follows: