hostapd: dfs: Limitation of VHT80 channels

Janusz Dziedzic janusz.dziedzic at
Mon Apr 9 02:29:16 PDT 2018

2018-04-09 9:03 GMT+02:00 Sven Eckelmann <sven.eckelmann at>:
> Hi,
> we were asking ourselves why you've introduced in 58b73e3dd96c ("hostapd: DFS
> with 40/80 MHz channel width support"), 1dc17db34ac4 ("DFS: Fix available
> channels list for VHT80") and  4f1e01b8e3ff ("DFS: Add VHT160 available
> channels") special limitations for CSA target channels.
> The example from Daniel Bailey was a US VHT80 configuration (ath10k) which
> received a relevant radar signal pattern and now has to switch the channel.
> The channel list contained a the non-DFS channel 44 and he expected that the
> device switches to this channel. But instead, the channel was only marked as
> unusable and hostapd did nothing (aka: was still sending at the now "unusable"
> channel range). Not the best idea when you want to get a DFS certified AP.
Should switch to 36 (or 149 in US).
So, you would expect we will switch to VHT80 (36-48) and use 44 as a
control one?

That was simple implementation with limitation:
- using same BW - CSA did't have BW we can specify (that was 5 years ago).
- always using first channel as a control one

Now for BW change, probably we can use extended channel switch.

Know this limitation AP should start always on first control channel.
In other such scenario will fail:
- start DFS on 56 (VHT80)
- connect HT40 station - will use HT40-
- trigger radar
- we will switch to 36 (VHT80)
- HT40 sta will not work while need to change channel 56 -> 36 and
also HT40-  to HT40+

We plan to improve this but get different tasks :)

So, you can create patches that will fix/improve:
 - control channel choose (secondary_channel)
 - BW change (but here probably some cards eg. Intel don't work
correctly when change channel and bw).

While that was long time ago, I am not sure regarding OBSS scan - if
we care before CSA ...

Best check the code, I didn't monitor dfs/csa changes someone
add/could add there, during this 5 years.


> This could be worked around by replacing channel 44 with channel 36. But it
> would be good to understand why VHT80 channels were limited to 36, 52, 100,
> 116, 132, 149
> Kind regards,
>         Sven
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> Hostap at

Janusz Dziedzic

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