802.11n: A Survival Guide by Matthew Gast

Author:Matthew Gast [Matthew Gast]
Language: eng
Format: epub, pdf
Tags: COMPUTERS / Networking / Network Protocols
ISBN: 9781449312039
Publisher: O'Reilly Media
Published: 2012-04-01T16:00:00+00:00

40 MHz Intolerance for Channel Width Interoperability

Because 40 MHz transmissions may cause significant impairment to overlapping 20 MHz networks, 802.11n includes a mechanism for a 20 MHz network to request of overlapping networks, “Please don’t use 40 MHz transmissions near me.” This mechanism, Forty MHz Intolerance, shares its name with the bit in the HT Capabilities IE used for signaling (see Figure 5-2), and applies only to networks operating in the 2.4 GHz spectrum. Within the 5 GHz band, there are a sufficient number of non-overlapping channels that 40 MHz intolerance is not required.

Any device can use the 40 MHz Intolerant bit to disable 40 MHz channels in its immediate vicinity. When a client device sets the 40 MHz Intolerant bit, it instructs the AP to which it is associated that the AP should use only 20 MHz transmissions provided the AP is operating a 20/40 MHz BSS as described in the previous section. Access points that have been restricted to 20 MHz operation will then transmit a 20/40 Coexistence Management frame to indicate to neighboring APs that they should also switch to 20 MHz operation, if possible. Through the use of the 40 MHz intolerance bit, it is possible for an 802.11 device to shut down 40 MHz operation in its immediate vicinity, even on networks that merely overlap.

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