The 20/40 BSS Coexistence element is used by STAs to exchange information that affects 20/40 BSS coexistence. The 20/40 co-existence element is shown below.
Fig Courtesy: 802.11 Standard
As per the standard, the fields are defined below
Information request field
The Information Request field is used to indicate that a transmitting STA is requesting the recipient to transmit a 20/40 BSS Coexistence Management frame with the transmitting STA as the recipient
Forty MHz Intolerant
The Forty MHz Intolerant field is set to 1 to prohibit an AP that receives this information or reports of this information from operating a 20/40 MHz BSS. When equal to 0, it does not prohibit a receiving AP from operating a 20/40 MHz BSS. This field is used for inter-BSS communication. The definition of this field is the same as the definition of the Forty MHz Intolerant field in the HT Capabilities element
20 MHz BSS width request
The 20 MHz BSS Width Request field is set to 1 to prohibit a receiving AP from operating its BSS as a 20/40 MHz BSS. Otherwise, it is set to 0. This field is used for intra-BSS communication.
OBSS scanning exemption request
The OBSS Scanning Exemption Request field is set to 1 to indicate that the transmitting non-AP STA is requesting the BSS to allow the STA to be exempt from OBSS scanning. Otherwise, it is set to 0.
OBSS scanning exemption grant
The OBSS Scanning Exemption Grant field is set to 1 by an AP to indicate that the receiving STA is exempted from performing OBSS Scanning. Otherwise, it is set to 0.
In addition to the Co-existence Element, the 40 MHz intolerance bit is also part of the HT-Capabilities sent out by a WLAN station. The 40 MHz intolerance bit is a notification that should be adhered and it brings the network down to a lower channel bandwidth configuration.
Forty MHz intolerant Bit in HT – capability
The bit and its description is provided below
