The Unlicensed National Information Infrastructure (U-NII) radio band is part of the radio frequency spectrum used by IEEE-802.11a devices and by many wireless ISPs. It operates over four ranges:
- U-NII Low (U-NII-1): 5.15-5.25 GHz. Originally limited to indoor use only. Regulations required use of an integrated antenna, with power limited to 50 mW. Rules changed in 2014 to permit outdoor operation, maximum fixed power 1 watt, maximum fixed EIRP 4 watts (+36 dBm) point-to-multipoint, 200 watts (+53 dBm) point-to-point.  However, strict out-of-band emission rules limit practical point-to-point power to lower levels.
- U-NII Mid (U-NII-2): 5.25-5.35 GHz. Both outdoor and indoor use, subject to Dynamic Frequency Selection (DFS, or radar avoidance). Regulations allow for a user-installable antenna. Power limited to 250 mW
- U-NII Worldwide (U-NII-2e): 5.47-5.725 GHz. Both outdoor and indoor use, subject to Dynamic Frequency Selection (DFS, or radar avoidance). Power limited to 250 mW. This spectrum was added by the FCC in 2003 to "align the frequency bands used by U-NII devices in the United States with bands in other parts of the world". The FCC currently has an interim limitation on operations on channels which overlap the 5600 - 5650 MHz band.
- U-NII Upper (U-NII-3): 5.725 to 5.825 GHz. Sometimes referred to as U-NII / ISM due to overlap with the ISM band. Regulations allow for a user-installable antenna. Power limited to 1W
Wireless ISPs generally use 5.725-5.825 GHz.
U-NII is an FCC regulatory domain for 5- GHz wireless devices. U-NII power limits are defined by the United States CFR Title 47 (Telecommunication), Part 15 - Radio Frequency Devices, Subpart E - Unlicensed National Information Infrastructure Devices, Paragraph 15.407 - General technical requirements. Regulatory use in individual countries may differ.
The European HiperLAN standard operates in same frequency band as the U-NII.
Except where noted, all information taken from Annex J of IEEE 802.11-2007 modified by amendments k, y and n.
Countries apply their own regulations to both the allowable channels, allowed users and maximum power levels within these frequency ranges. Consult your local authorities as these regulations may be out of date as they are subject to change at any time.
In 2007 the FCC (United States) began requiring that devices in operating in channels 52, 56, 60 and 64 must have dynamic frequency selection (DFS) capabilities. This is to avoid communicating in the same frequency range as some RADAR. Edit 2014: FCC has issued a new rules change for all devices due to interference with government weather radar systems. Fines issued and equipment seized for non-compliance.
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- "15.07.2005, Heise: 5 GHz WLAN to be available all over Europe" www.heise.de
- "Cisco: Glossary" www.cisco.com
- "Dynamic Frequency Selection for 5 GHz WLAN in the US and Canada" www.cisco.com
- FCC 15.407 as of 2014 - hallikainen.com
- "15E, Dynamic Frequency Selection, DFS, DFS Approval" fcc.gov
- FCC 15.407 as of August 8, 2008 – hallikainen.com
- "802.11-2007 Japan MIC Released the new 5 GHz band (W56)" (PDF). Bureau Veritas — ADT. Retrieved 2008-02-23.
- Israel: צו הטלגרף האלחוטי (אי תחולת הפקודה) (מס' 2), התשס"ו – 2005 (PDF) (in Hebrew).
- Korea Frequency Distribution Table 2008.12.31 (in Korean)
- "Publication Number: 443999 Rule Parts: 15E". FCC. October 5, 2009.
Devices must be professionally installed when operating in the 5470 – 5725 MHz band
- "Elimination of interference to Terminal Doppler Weather Radar" (PDF). FCC. July 27, 2010.