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Fri, 13 Feb 2026 09:46:31 +0000 Date: Fri, 13 Feb 2026 10:46:27 +0100 From: Alexander Wilhelm To: Alex Gavin Cc: Johannes Berg , linux-wireless@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: wifi: mac80211: question about 20/40 coex support Message-ID: References: Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.12 (2023-09-09) X-ClientProxiedBy: GVX0EPF00014AF5.SWEP280.PROD.OUTLOOK.COM (2603:10a6:158:401::319) To FRWP192MB2997.EURP192.PROD.OUTLOOK.COM (2603:10a6:d10:17c::10) Precedence: bulk X-Mailing-List: linux-wireless@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: FRWP192MB2997:EE_|VI2P192MB3124:EE_ X-MS-Office365-Filtering-Correlation-Id: 0cdf9a40-a2d2-4169-5174-08de6ae4c4ad X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014; 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X-MS-Exchange-CrossPremises-SCL: 1 X-MS-Exchange-CrossPremises-Processed-By-Journaling: Journal Agent X-OrganizationHeadersPreserved: VI2P192MB3124.EURP192.PROD.OUTLOOK.COM X-Proofpoint-GUID: eSpo2tw2xlZsbZByTom2TSPsPveCrbEU X-Proofpoint-ORIG-GUID: eSpo2tw2xlZsbZByTom2TSPsPveCrbEU X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMjEzMDA3NSBTYWx0ZWRfX8roO4c1lYs6U RH6hIjkOCvz+yEZ/n3SjHQ8ECu3O+JkTkv1+IFiTyA308oRS82oxntK+ehpBEO1vxeHw6NoUmFk Zi0TKOPJ9SJrX2FYXNIY8kRMSweATFUs/KC3xg0+1BmqQSTw8o6Lt6PWpyqXOaLnhcv+jlrExoK s98pIRr3L5XHYgTgOy3YxpFYp2bqBKFgzHVxUIN9r769EuApni0m4yukLF/2Hmppe0lhNDkoiol ETz9v9gY16HqPK+lvPIUKkwuhm0gdNvDmDfY9KwoivqVG/IGU5XmfxBD7CqGIcaNowo9gQcewv0 EQq7pODMbGK8LP+BkYpD7Tn8i1EbWhZkzWHwa688QWp7m9GUMampRthauKbdRajgesLZZn1C4pt vegAx+YJCBo9UG8ODRMOtfUGqAp6V5UaSihsbhgNTymjlamhTHMV5U05NPl0IpUxp5vcl8hAaBZ /4xNxdBd+j8osvfPpLg== X-Authority-Analysis: v=2.4 cv=J+inLQnS c=1 sm=1 tr=0 ts=698ef2fa cx=c_pps a=rLet0NO1Vy5TTuje2zMcmA==:117 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=HzLeVaNsDn8A:10 a=8gLI3H-aZtYA:10 a=VkNPw1HP01LnGYTKEx00:22 a=Mpw57Om8IfrbqaoTuvik:22 a=GgsMoib0sEa3-_RKJdDe:22 a=IJ9QT1cfHbmz9b9oiCkA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 On Fri, Feb 13, 2026 at 12:14:16AM -0800, Alex Gavin wrote: > On 2/12/26 23:05, Alexander Wilhelm wrote: > > > > 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 > > > > From my understanding, this is implemented in userspace (i.e. hostapd), at least in upstream code. I've alredy seen the code in `hostapd` that implements a fallback to 20 MHz. But that implementation is based only on scan results, not on 20/40 coexistence feautre. Here the snippet from `ieee80211n_check_scan` function: [...] if (iface->current_mode->mode == HOSTAPD_MODE_IEEE80211A) oper40 = ieee80211n_check_40mhz_5g(iface, scan_res); else oper40 = ieee80211n_check_40mhz_2g4(iface, scan_res); wpa_scan_results_free(scan_res); iface->secondary_ch = iface->conf->secondary_channel; if (!oper40) { wpa_printf(MSG_INFO, "20/40 MHz operation not permitted on " "channel pri=%d sec=%d based on overlapping BSSes", iface->conf->channel, iface->conf->channel + iface->conf->secondary_channel * 4); iface->conf->secondary_channel = 0; if (iface->drv_flags & WPA_DRIVER_FLAGS_HT_2040_COEX) { /* * TODO: Could consider scheduling another scan to check * if channel width can be changed if no coex reports * are received from associating stations. */ } } [...] > In hostapd, the '20/40 BSS Coexistence Management Support' bit you reference is set in the > 'hostapd_ext_capab_byte()' function in 'src/ap/ieee802_11_shared.c' when 'obss_interval' is set in > the AP config file. > > For more information on the 'obss_interval' config item see the following from the hostapd config > reference here [1]: > > # If set non-zero, require stations to perform scans of overlapping > # channels to test for stations which would be affected by 40 MHz traffic. > # This parameter sets the interval in seconds between these scans. Setting this > # to non-zero allows 2.4 GHz band AP to move dynamically to a 40 MHz channel if > # no co-existence issues with neighboring devices are found. > #obss_interval=0 Great, I found the the configuration in my `hostapd` version and the code that implements the handling of this kind of management frames. I'll give the configuration option a try. > The following sections from the 802.11 standard may be helpful to understand this topic in more > depth, although I'm sure there's information online that may be easier to digest: > > - 9.4.2.58 20/40 BSS Coexistence element > - 9.4.2.54.2 HT Capability Information field > - 11.15.12 Switching between 40 MHz and 20 MHz > > From 11.15.12: > > TE-B: On any of the channels of the channel set defined in Clause 18, reception of a 20/40 BSS > Coexistence Management, Beacon, Probe Request, or Probe Response frame that contains a value of 1 in > a Forty MHz Intolerant field and that has the Address 1 field equal to the receiving STA’s address > or to a group address, with no further addressing qualifications. Thank you for the support, Alex. The information helps me a lot. Best regards Alexander Wilhelm