Editing Wi-Fi (section)

=== Interference ===
{{Further|Electromagnetic interference at 2.4 GHz#Wi-Fi}}
[[File:Wi-Fi Allocations 4.gif|thumb|upright=0.9|Network planning frequency allocations for North America and Europe. Using these types of frequency allocations can help minimize co-channel and adjacent-channel interference.]]

[[File:Co-Channel Wi-Fi Interference 01.png|thumb|In the 2.4&nbsp;GHz wavebands as well as others, transmitters straddle multiple channels. Overlapping channels can suffer from interference unless this is a small portion of the total received power.]]

Wi-Fi connections can be blocked or the Internet speed lowered by having other devices in the same area. Wi-Fi protocols are designed to share the wavebands reasonably fairly, and this often works with little to no disruption. To minimize collisions with Wi-Fi and non-Wi-Fi devices, Wi-Fi employs [[Carrier-sense multiple access with collision avoidance]] (CSMA/CA), where transmitters listen before transmitting and delay transmission of packets if they detect that other devices are active on the channel, or if noise is detected from adjacent channels or non-Wi-Fi sources. Nevertheless, Wi-Fi networks are still susceptible to the [[hidden node]] and [[exposed node problem]].<ref>{{Cite journal|last1=Chakraborty|first1=Sandip|last2=Nandi|first2=Sukumar|last3=Chattopadhyay|first3=Subhrendu|date=22 September 2015|title=Alleviating Hidden and Exposed Nodes in High-Throughput Wireless Mesh Networks|journal=[[IEEE Transactions on Wireless Communications]]|volume=15|issue=2|pages=928–937|doi=10.1109/TWC.2015.2480398|s2cid=2498458 }}</ref>

A standard speed Wi-Fi signal occupies five channels in the 2.4&nbsp;GHz band. Interference can be caused by overlapping channels. Any two channel numbers that differ by five or more, such as 2 and 7, do not overlap (no [[adjacent-channel interference]]). The oft-repeated adage that channels 1, 6, and 11 are the ''only'' non-overlapping channels is, therefore, not accurate. Channels 1, 6, and 11 are the only ''group of three'' non-overlapping channels in North America. However, whether the overlap is significant depends on physical spacing. Channels that are four apart interfere a negligible amount{{snd}}much less than reusing channels (which causes [[co-channel interference]]){{snd}}if transmitters are at least a few metres apart.<ref name="villegas">{{cite book |chapter=Effect of Adjacent-Channel Interference in IEEE 802.11 WLANs |first1=Eduard Garcia |last1=Villegas |first2=Elena |last2=Lopez-Aguilera |first3=Rafael |last3=Vidal |first4=Josep |last4=Paradells |title=2007 2nd International Conference on Cognitive Radio Oriented Wireless Networks and Communications |date=2007 |pages=118–125 |doi=10.1109/CROWNCOM.2007.4549783 |hdl=2117/1234 |isbn=978-1-4244-0814-6 |s2cid=1750404 }}</ref> In Europe and Japan where channel 13 is available, using Channels 1, 5, 9, and 13 for [[IEEE 802.11#802.11g|802.11g]] and [[IEEE 802.11#802.11n|802.11n]] is viable and [[IEEE 802.11#Channels and frequencies|recommended]].

However, multiple 2.4&nbsp;GHz 802.11b and 802.11g access-points default to the same channel on initial startup, contributing to congestion on certain channels. Wi-Fi pollution, or an excessive number of access points in the area, can prevent access and interfere with other devices' use of other access points as well as with decreased [[signal-to-noise ratio]] (SNR) between access points. These issues can become a problem in high-density areas, such as large apartment complexes or office buildings with multiple Wi-Fi access points.<ref>den Hartog, F., Raschella, A., Bouhafs, F., Kempker, P., Boltjes, B., & Seyedebrahimi, M. (November 2017). [http://unsworks.unsw.edu.au/fapi/datastream/unsworks:50254/bin458a10d9-f568-479c-a9b5-5c185ef64e78?view=true A Pathway to solving the Wi-Fi Tragedy of the Commons in apartment blocks] {{Webarchive|url=https://web.archive.org/web/20200713111300/http://unsworks.unsw.edu.au/fapi/datastream/unsworks:50254/bin458a10d9-f568-479c-a9b5-5c185ef64e78?view=true |date=13 July 2020 }}. In 2017 27th International Telecommunication Networks and Applications Conference (ITNAC) (pp. 1–6). IEEE.</ref>

Other devices use the 2.4&nbsp;GHz band:<ref name="wired" /> microwave ovens, ISM band devices, [[security camera]]s, Zigbee devices, Bluetooth devices, [[video sender]]s, cordless phones, [[baby monitor]]s,<ref>{{cite web|url=https://www.monitorshq.com/6-easy-steps-to-protect-your-baby-monitor-from-hackers/|title=6 Easy Steps To Protect Your Baby Monitor From Hackers|last=Caravan|first=Delia|date=12 September 2014|website=Baby Monitor Reviews HQ|archive-url=https://web.archive.org/web/20141018042051/https://www.monitorshq.com/6-easy-steps-to-protect-your-baby-monitor-from-hackers/|archive-date=18 October 2014|url-status=dead|access-date=12 September 2014}}</ref> and, in some countries, [[amateur radio]], all of which can cause significant additional interference. It is also an issue when municipalities<ref>{{cite web|url=https://computer.howstuffworks.com/municipal-wifi.htm|title=How Municipal WiFi Works|last=Wilson|first=Tracy V.|date=17 April 2006|website=[[HowStuffWorks]]|archive-url=https://web.archive.org/web/20080223120947/http://computer.howstuffworks.com/municipal-wifi.htm|archive-date=23 February 2008|url-status=live|access-date=12 March 2008}}</ref> or other large entities (such as universities) seek to provide large area coverage. On some 5&nbsp;GHz bands interference from radar systems can occur in some places. For base stations that support those bands they employ Dynamic Frequency Selection which listens for radar, and if it is found, it will not permit a network on that band.

These bands can be used by low power transmitters without a licence, and with few restrictions. However, while unintended interference is common, users that have been found to cause deliberate interference (particularly for attempting to locally monopolize these bands for commercial purposes) have been issued large fines.<ref>{{Cite web|url=https://www.networkworld.com/article/949170/wi-fi-hotspot-blocking-persists-despite-fcc-crackdown.html|title=Wi-Fi hotspot blocking persists despite FCC crackdown|last=Brown|first=Bob|date=10 March 2016|website=[[Network World]]|archive-url=https://web.archive.org/web/20190227033935/https://www.networkworld.com/article/3042454/wi-fi-hotspot-blocking-persists-despite-fcc-crackdown.html|archive-date=27 February 2019|url-status=live}}</ref>