wifi: mac80211: question about 20/40 coex support

[Alexander Wilhelm]

Hello wireless devs,

I am analyzing a wireless management frame taken from the beacon of an
access point, and in this frame the `20/40 Coexistence Management Support`
bit is set to 0, indicating that the feature is not supported. See frame
below:

    Frame 2: 217 bytes on wire (1736 bits), 217 bytes captured (1736 bits)
    Radiotap Header v0, Length 26
    802.11 radio information
    IEEE 802.11 Beacon frame, Flags: ........
    IEEE 802.11 Wireless Management
        Fixed parameters (12 bytes)
        Tagged parameters (155 bytes)
            Tag: SSID parameter set: "SSID"
            Tag: Supported Rates 6(B), 9, 12(B), 18, 24(B), 36, 48, 54, [Mbit/sec]
            Tag: DS Parameter set: Current Channel: 6
            Tag: Traffic Indication Map (TIM): DTIM 0 of 2 bitmap
            Tag: Country Information: Country Code DE, Environment All
            Tag: ERP Information
            Tag: QBSS Load Element 802.11e CCA Version
            Tag: Supported Operating Classes
            Tag: HT Capabilities (802.11n D1.10)
            Tag: HT Information (802.11n D1.10)
            Tag: Extended Capabilities (10 octets)
                Tag Number: Extended Capabilities (127)
                Tag length: 10
                Extended Capabilities: 0x04 (octet 1)
                    .... ...0 = 20/40 BSS Coexistence Management Support: Not supported
                    .... ..0. = General Link (GLK): 0x0
                    .... .1.. = Extended Channel Switching: Supported
                    .... 0... = GLK-GCR: 0x0
                    ...0 .... = PSMP Capability: Not supported
                    ..0. .... = Reserved: 0x0
                    .0.. .... = S-PSMP Support: Not supported
                    0... .... = Event: Not supported
                Extended Capabilities: 0x00 (octet 2)
                Extended Capabilities: 0x00 (octet 3)
                Extended Capabilities: 0x02 (octet 4)
                Extended Capabilities: 0x00 (octet 5)
                Extended Capabilities: 0x00 (octet 6)
                Extended Capabilities: 0x01 (octet 7)
                Extended Capabilities: 0x0040 (octets 8 & 9)
                Extended Capabilities: 0x40 (octet 10)
            Tag: Vendor Specific: Microsoft Corp.: WMM/WME: Parameter Element

I reviewed several driver implementations, and none of them seem to
advertise this capability. I also could not find any definition for this
bit in `include/linux/ieee80211.h`, unlike many other capability bits that
are defined (e.g. `WLAN_EXT_CAPA1_EXT_CHANNEL_SWITCHING` or
`WLAN_EXT_CAPA3_MULTI_BSSID_SUPPORT`). From what I can see, none of the
`mac80211`‑based drivers appear to support this feature, is that correct?

Additionally, I want to confirm whether my understanding of the feature is
accurate: the 20/40 Coexistence mechanism implements the “good neighbor”
policy, meaning that an AP using a 40 MHz channel in the 2.4 GHz band must
fall back to 20 MHz when it detects an overlapping BSS (OBSS). Is that
right?

Any clarification would be greatly appreciated.


Best regards
Alexander Wilhelm