Editing IEEE 802.11 (section)

===Channel spacing within the 2.4&nbsp;GHz band===
In addition to specifying the channel center frequency, 802.11 also specifies (in Clause 17) a [[spectral mask]] defining the permitted power distribution across each channel. The mask requires the signal to be [[Attenuation|attenuated]] a minimum of 20&nbsp;[[decibel|dB]] from its peak amplitude at ±11&nbsp;MHz from the center frequency, the point at which a channel is effectively 22&nbsp;MHz wide. One consequence is that stations can use only every fourth or fifth channel without overlap.

Availability of channels is regulated by country, constrained in part by how each country [[Frequency allocation|allocates radio spectrum]] to various services. At one extreme, Japan permits the use of all 14 channels for 802.11b, and {{nowrap|1–13}} for 802.11g/n-2.4. Other countries such as Spain initially allowed only channels 10 and 11, and France allowed only 10, 11, 12, and 13; however, Europe now allow channels 1 through 13.<ref name="CNAF">{{cite web|title=Cuadro nacional de Atribución de Frecuencias CNAF|publisher=Secretaría de Estado de Telecomunicaciones |url=http://www.mityc.es/Telecomunicaciones/Secciones/Espectro/cnaf |access-date=2008-03-05 |archive-url=https://web.archive.org/web/20080213092618/http://www.mityc.es/Telecomunicaciones/Secciones/Espectro/cnaf <!-- Bot retrieved archive --> |archive-date = 2008-02-13}}</ref><ref name="ART">{{cite web|title=Evolution du régime d'autorisation pour les RLAN|publisher=French Telecommunications Regulation Authority (ART)|url=http://www.arcep.fr/uploads/tx_gspublication/evol-rlan-250703.pdf|access-date=2008-10-26|url-status=dead|archive-url=https://web.archive.org/web/20061209172043/http://www.arcep.fr/uploads/tx_gspublication/evol-rlan-250703.pdf|archive-date=9 December 2006}}</ref> North America and some Central and South American countries allow only {{nowrap|1 through 11.}}

[[File:2.4 GHz Wi-Fi channels (802.11g WLAN).svg|thumb|center|upright=3.25|Spectral masks for 802.11g channels {{nowrap|1–14}} in the 2.4&nbsp;GHz band]]

Since the spectral mask defines only power output restrictions up to ±11&nbsp;MHz from the center frequency to be attenuated by −50&nbsp;dBr, it is often assumed that the energy of the channel extends no further than these limits. It is more correct to say that the overlapping signal on any channel should be sufficiently attenuated to interfere with a transmitter on any other channel minimally, given the separation between channels. Due to the [[near–far problem]] a transmitter can impact (desensitize) a receiver on a "non-overlapping" channel, but only if it is close to the victim receiver (within a meter) or operating above allowed power levels. Conversely, a sufficiently distant transmitter on an overlapping channel can have little to no significant effect.

Confusion often arises over the amount of channel separation required between transmitting devices. 802.11b was based on [[direct-sequence spread spectrum]] (DSSS) modulation and utilized a channel bandwidth of 22&nbsp;MHz, resulting in ''three'' "non-overlapping" channels (1, 6, and 11). 802.11g was based on OFDM modulation and utilized a channel bandwidth of 20&nbsp;MHz. This occasionally leads to the belief that ''four'' "non-overlapping" channels (1, 5, 9, and 13) exist under 802.11g. However, this is not the case as per 17.4.6.3 Channel Numbering of operating channels of the IEEE Std 802.11 (2012), which states, "In a multiple cell network topology, overlapping and/or adjacent cells using different channels can operate simultaneously without interference if the distance between the center frequencies is at least 25&nbsp;MHz."<ref>{{cite web|title=Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications|url=http://standards.ieee.org/getieee802/download/802.11-2012.pdf|access-date=2013-12-05|url-status=dead|archive-url=https://web.archive.org/web/20130626000833/http://standards.ieee.org/getieee802/download/802.11-2012.pdf|archive-date=2013-06-26}}</ref>
and section 18.3.9.3 and Figure 18-13.

This does not mean that the technical overlap of the channels recommends the non-use of overlapping channels. The amount of inter-channel interference seen on a configuration using channels 1, 5, 9, and 13 (which is permitted in Europe, but not in North America) is barely different from a three-channel configuration, but with an entire extra channel.<ref>{{cite web|title=Choosing the clearest channels for WiFi... continued|url=https://www.wirelesswaffle.com/index.php?m=02&y=13&entry=entry130212-072406|access-date=2020-08-24}}</ref><ref name="adjacentchannels">{{Cite conference |last1=Garcia Villegas |first1=E. |title=Effect of adjacent-channel interference in IEEE 802.11 WLANs |conference=CrownCom 2007 |publisher=ICST & IEEE |year=2007 |url=https://upcommons.upc.edu/e-prints/bitstream/2117/1234/1/CrownCom07_CReady.pdf |display-authors=etal |url-status=dead |archive-url=https://web.archive.org/web/20110720102658/https://upcommons.upc.edu/e-prints/bitstream/2117/1234/1/CrownCom07_CReady.pdf |archive-date=20 July 2011 |access-date=17 September 2008 }}</ref>

[[File:NonOverlappingChannels2.4GHz802.11-en.svg|thumb|802.11 non-overlapping channels in the 2.4&nbsp;GHz ISM band]]

However, overlap between channels with more narrow spacing (e.g. 1, 4, 7, 11 in North America) may cause unacceptable degradation of signal quality and throughput, particularly when users transmit near the boundaries of AP cells.<ref name="ciscodeployissues">{{cite web |title=Channel Deployment Issues for 2.4&nbsp;GHz 802.11 WLANs |publisher=Cisco Systems, Inc |url=http://www.cisco.com/en/US/docs/wireless/technology/channel/deployment/guide/Channel.html |access-date=2007-02-07 |url-status=dead |archive-url=https://web.archive.org/web/20140209230927/http://www.cisco.com/en/US/docs/wireless/technology/channel/deployment/guide/Channel.html |archive-date=2014-02-09 }}</ref>