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Mon, 16 Feb 2026 09:50:24 +0000 Date: Mon, 16 Feb 2026 10:50:21 +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: <61276f01-5fdb-48d9-8c5a-cc3a765bb642@icloud.com> <261dcee1-e5b9-4b9c-9052-f726fe5beb69@icloud.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <261dcee1-e5b9-4b9c-9052-f726fe5beb69@icloud.com> User-Agent: Mutt/2.2.12 (2023-09-09) X-ClientProxiedBy: GVZP280CA0028.SWEP280.PROD.OUTLOOK.COM (2603:10a6:150:276::9) 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_|AS2P192MB2117:EE_ X-MS-Office365-Filtering-Correlation-Id: fe790581-693a-4e92-aa44-08de6d40cea3 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|366016; 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X-MS-Exchange-CrossPremises-SCL: 1 X-MS-Exchange-CrossPremises-Processed-By-Journaling: Journal Agent X-OrganizationHeadersPreserved: AS2P192MB2117.EURP192.PROD.OUTLOOK.COM X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMjE2MDA4MiBTYWx0ZWRfXxkeGgyXLXzHP j0TNUyZkUB2YsVwH2So/GG+Sh9NPKf6wgeF5tVlA2Jr0UIF+AnRd7IC40W7U17zT3w7zPY0n/eQ 315FdXdbBP/sZKuJ4PggjZmCrJslZJEovF+QnlYb/xgV47MFZXsvikFlsRb/Ivwmfo5sK20wSkP XE9s2pzC1plaMZcZ/5Fp8yV0WUzJdfMwKIpwAE2WbtT4ZXuD/rjoibfeVKY/bnPcoVZnDEFDnfe r+SwKQmwEsnw01xE+jQsuZ0NIeoruWCZKhxLHgE9YvYc6J6kym2iX/ZXanf58EHt2LkP4y+w4ib SoTy3ZfAg06z4HD3gFhWUFsi8jUjvPUEb9Ehi81J/Q6k2a+a4w4j7ApvgApiqYswF1JW7MFPwYK 811srM7q6BVuKTmfbZC006zaTdykqKQunTLXpZdlLL3wsFiLX2UaKFSI5Oo2YpFJbROuzFohHQz +Q2puKYd5slPTChIIOA== X-Authority-Analysis: v=2.4 cv=OpdCCi/t c=1 sm=1 tr=0 ts=6992e863 cx=c_pps a=pcOzTpqVKfWy+bLO9R5M0A==: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=yaAG3qJ-AAAA:8 a=SXzkmgPmAAAA:8 a=i9Wy5rNffVyLRxwbNK8A:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 a=oLVlbjkABFOu4cUI0CGI:22 a=EWLf6cg6Bh5aS0AxDgDu:22 X-Proofpoint-GUID: 5-c8u7Cc97uSiN6w_t1Lgpju1O8a1NZw X-Proofpoint-ORIG-GUID: 5-c8u7Cc97uSiN6w_t1Lgpju1O8a1NZw On Fri, Feb 13, 2026 at 02:01:37PM -0800, Alex Gavin wrote: > On 2/13/26 13:44, Alex Gavin wrote: > > On 2/13/26 01:46, Alexander Wilhelm wrote: > >> 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. > >> */ > >> } > >> } > >> [...] > > > > There are ways the BSS can switch from 40 MHz to 20 MHz operation beyond just scan results. > > > > For example, a newly-associated, 40 MHz intolerant station could cause hostapd to reconfigure to 20 > > MHz operation, assuming the WPA_DRIVER_FLAGS_HT_2040_COEX is set by radio driver capabilities. > > > > Directly handling association: > > > > handle_assoc() > > -> update_ht_state() > > -> update_sta_ht() > > -> ht40_intolerant_add() > > -> ieee802_11_set_beacons() > > -> ieee802_11_set_beacon() > > -> __ieee802_11_set_beacon() > > -> ieee802_11_build_ap_params() > > -> hostapd_build_ap_extra_ies() > > -> hostapd_eid_ext_capab() > > -> hostapd_ext_capab_byte() > > > > > > Or from driver callback: > > > > hostapd_notif_assoc() > > -> ht40_intolerant_add() > > -> ieee802_11_set_beacons() > > ... > > -> hostapd_ext_capab_byte() > > > > > > Additionally, receipt of a 20/40 MHz Coex Management Action frame in 'hostapd_2040_coex_action()' > > may trigger the AP to reconfigure to 20 MHz using a similar code path. > > > > For testing, wpa_supplicant supports configuring 40 MHz intolerant stations. See below from its > > config file reference [1]: > > > > # ht40_intolerant: Whether 40 MHz intolerant should be indicated. > > # 0 = 40 MHz tolerant (default) > > # 1 = 40 MHz intolerant > > > > I've uploaded some example hostapd and wpa_supplicant configs here [2], if you'd like something to > > reference or play around with. Associating the intolerant station config should trigger hostapd to > > switch from 40 MHz to 20 MHz with this configuration, assuming radio driver support as mentioned above. > > > > [1] https://urldefense.com/v3/__https://git.w1.fi/cgit/hostap/plain/wpa_supplicant/wpa_supplicant.conf__;!!I9LPvj3b!AsA2QqDGWXpvNJ_Z3N2Trl0qQHOI1jDydzfs8Xjv-GME96gVt-Q4hgdZTkUlQISMAQmn7HqwqqSmayxFDxGc-woJ$ > > [2] https://urldefense.com/v3/__https://codeberg.org/a-gavin/hostap-confs/src/branch/main/open/20-40MHz-bss-coexistence__;!!I9LPvj3b!AsA2QqDGWXpvNJ_Z3N2Trl0qQHOI1jDydzfs8Xjv-GME96gVt-Q4hgdZTkUlQISMAQmn7HqwqqSmayxFD70l2TKr$ > > Since you mentioned packet capture analysis, to observe a 40 MHz BSS switch to 20 MHz operation, pay > close attention to the HT Operation IE in the Beacon frames. > > Assuming you're using Wireshark, you should see the 'HT Operation Information Subset (1 of 3)' > section switch from showing 'Supported channel width: Channel of any width supported' to '20 MHz > channel width only'. I'm sure other programs are similar. > > Using the 40 MHz intolerant association example, you should see this switch immediately after > station association. > > > 40 MHz operation: > > Tag: HT Operation > Tag Number: HT Operation (61) > Tag length: 22 > Primary Channel: 1 > HT Operation Information Subset (1 of 3): 0x05 > .... ..01 = Secondary channel offset: Secondary channel is above the primary channel (0x1) > .... .1.. = Supported channel width: Channel of any width supported > .... 0... = Reduced Interframe Spacing (RIFS): Prohibited > 0000 .... = Reserved: 0x0 > HT Operation Information Subset (2 of 3): 0x0000 > HT Operation Information Subset (3 of 3): 0x0000 > Rx Supported Modulation and Coding Scheme Set: Basic MCS Set > > > 20 MHz operation: > > Tag: HT Operation > Tag Number: HT Operation (61) > Tag length: 22 > Primary Channel: 1 > HT Operation Information Subset (1 of 3): 0x00 > .... ..00 = Secondary channel offset: No secondary channel (0x0) > .... .0.. = Supported channel width: 20 MHz channel width only > .... 0... = Reduced Interframe Spacing (RIFS): Prohibited > 0000 .... = Reserved: 0x0 > HT Operation Information Subset (2 of 3): 0x0000 > HT Operation Information Subset (3 of 3): 0x0000 > Basic HT-MCS Set: Reserved: 00000000000000000000000000000000 > > > Hope this helps! Hi Alex, thank you again for a great support. And yes, I'm using Wireshark or tshask to analyze the packets. Unfortunatelly I'm currently struggling with my noisy environment. I cannot realy start an 40 MHz AP without force it with `noscan` option. But then the Coex mechanism does not work as well. But I think I'll get it working with some workarounds. Best regards Alexander Wilhelm > > >>> 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 > > > > >