dynamic frequency selection

{{See also|List of WLAN channels}}

When starting operation, an access point automatically selects channels with low interference levels in a phase known as Channel Availability Check (CAC). During this phase, the access point is in a passive state scanning for radar signals. This commonly takes one to two minutes, but could take up to ten minutes. Thereafter, the access point performs In-Service Monitoring (ISM) to detect active radar signals; if radar is detected, and the access point is configured to automatically select a channel, it broadcasts a switch-channel event to its clients and follows by switching the channel.

The actual mechanism, durations, radar pulse pattern, power levels, and frequency bands on which DFS is enforced vary by jurisdiction. DFS is mandated for the 5470–5725 MHz U-NII band in United States by the FCC.{{cite web |url=http://www.ntia.doc.gov/ntiahome/press/2003/5ghzagreement.htm |title=5GHz Agreement |website=Ntia.doc.gov |date=2003-01-31 |access-date=2012-08-29 |archive-date=2011-07-19 |archive-url=https://web.archive.org/web/20110719174039/http://www.ntia.doc.gov/ntiahome/press/2003/5ghzagreement.htm |url-status=dead }} DFS is mandatory for the 5250–5350 and 5470–5725 MHz bands in India.{{cite web |url=https://dot.gov.in/sites/default/files/License%20Exemption%20in%205%20GHz%20G_S_R_1048%28E%29%20dated%2022nd%20October%2C%202018_0.pdf |title=G.S.R. 1048(E) dated 18th October 2018 |website=dot.gov.in |date=2018-10-18 |access-date=2022-01-05 }}

Prior to the introduction of Wi-Fi, one of the biggest applications of the 5 GHz band was Terminal Doppler Weather Radar.{{Cite web| title = Winning Back the Weather Radio Channels Adds Capacity to 5GHz Wi-Fi Spectrum| first = Chris| last = Spain| publisher = Cisco| date = July 10, 2014| access-date = 4 December 2019| url = https://blogs.cisco.com/wireless/winning-back-the-weather-radio-channels-adds-capacity-to-5ghz-wi-fi-spectrum| quote = The FCC ruling is re-opening the Terminal Doppler Weather Radar (TDWR) band (channels 120, 124, 128) with new test requirements for DFS protection.}}{{cite journal| last = Saltikoff| first = Elena| title = The Threat to Weather Radars by Wireless Technology| journal = Bulletin of the American Meteorological Society| volume = 97| issue = 7| year = 2016| pages = 1159–1167| issn = 0003-0007| doi = 10.1175/BAMS-D-15-00048.1| bibcode = 2016BAMS...97.1159S| quote = Since 2006, interference to C-band radars from RLAN is increasingly experienced by most OPERA members. ... The South African weather services initially tried to implement specific software filtering to improve the situation but then decided in 2011 to move its meteorological radar network to S band.| doi-access = free}} The decision to use 5 GHz spectrum for Wi-Fi was finalized in the World Radiocommunication Conference in 2003; however, the meteorological community was not involved in the process.{{Cite web| title = C-band meteorological radars - Threats related to RLAN 5 GHz| first = Philippe| last = Tristant| publisher = EUMETNET| via = itu.int| date = 23–24 October 2017 |access-date = 5 December 2019 |url = https://www.itu.int/en/ITU-R/study-groups/workshops/RSG7-ITU-WMO-RSM-17/Documents/C-band%20meteorological%20radars%20Threats%20related%20to%20RLAN%205%20GHz.pdf}} Implementation and configuration problems caused significant disruption in weather radar operations in countries around the world. In Hungary, the weather radar system was declared non-operational for more than a month. Due to the severity of interference, South African weather services ended up abandoning C band operation, switching their radar network to S band.{{Cite web| title = RLAN 5 GHz interference to weather radars in Europe| first = Philippe| author = Tristant| publisher = International Telecommunication Union| date = 16–18 September 2009| access-date = 4 December 2019| url = https://www.itu.int/dms_pub/itu-r/md/09/sem.wmo/c/R09-SEM.WMO-C-0019!!PDF-E.pdf| quote = More than 12 European countries experienced such interference cases (other cases have now been reported in number of countries in the world). Definitively harmful interference (in Hungary, the radar was declared as non-operational for more than 1 month)}}

Uniform spreading is a mechanism used to provide an aggregated uniform load across all allocated channels within the declared spectrum mask. There are several ways in which this can be achieved, either on-device or in a WLAN controller.

{{Reflist}}

• [https://www.itu.int/rec/R-REC-M.1652 Recommendation M.1652 : Dynamic frequency selection] – ITU
• [https://www.wi-fi.org/knowledge-center/faq/what-is-dynamic-frequency-selection-dfs What is dynamic frequency selection (DFS)?] – Wi-Fi Alliance

Category:IEEE 802.11

Category:Radio resource management