With the advent of channel bonding in the 802.11n standard, a mechanism had to be developed that would allow a station operating in a 20 MHz channel to co-exist with a station operating in a 40 MHz channel. The station could be a legacy pre-802.11n device or an 802.11n device that was operating in 20 MHz channel mode.

To address this issue, the 802.11 standard’s body defined mechanisms for 20/40 MHz co-existence. For an Access Point, before starting/advertising a Basic Service Set (BSS), it has to follow the below procedures

• It should find a primary and secondary channel that are not overlapping with other BSS
• If condition 1 is not met, the primary and secondary channels that should be set should be same as the primary and secondary channels of all existing 20/40 MHz BSSs
• If condition 1 is not met, the primary channel should be same as the operating channel of all 20 MHz
• The AP will perform a passive scan/active scan before starting a network to adhere to the above If the conditions are met, it will create a 40 MHz network

20/40 BSS Coexistence Element and 40 MHz Intolerance Bit