[PATCH v5 13/17] mesh: do not allow pri/sec channel switch

Jouni Malinen j at w1.fi
Mon Jun 11 11:51:52 PDT 2018

On Mon, Jun 11, 2018 at 11:23:32AM -0700, Ben Greear wrote:
> I (and some customers) needed similar behaviour for STA + AP mode in the past, the reason is that
> if you know for certain (due to user configuration typically) that a station
> has to communicate with an AP on a certain bandwidth, and you want to run an
> AP on the same radio, then you cannot have the AP interface using some different center
> frequency.  Think police cruisers that need to provide hot-spot sometimes and also
> use an STA vdev to connect to head-quarter's AP when in the parking-lot.

This is not about center frequency, but about matching primary
channels. Switching pri/sec channels does not really change the
frequency range. I'd assume it would be possible for the AP interface to
go through channel switch when the STA interface needs to move (or
connect initially), but sure, it may be simpler to hardcode channels and
pri/sec assignment. That last part, though, is likely to be
non-compliant with the standard requirements for co-existence..

> With MESH, if you are trying to join some existing mesh on a certain
> frequency, it cannot physically work if somehow the hostapd/supplicant decides to use
> a different frequency, right?

Sure, but this is similarly sounding like trying to hide the real issue
and go for something hardcoded rather than dynamically addressing needs
when the peers changes. When starting up a mesh BSS in an environment
where there is already an operating mesh BSS, matching pri/sec channels
with that existing network is obviously the correct thing to do. If
there is no mesh BSS in radio range, I find it much more difficult to
understand why there would be an exception for co-existence
requirements. The real issue comes when two originally disjoint segments
of a mesh BSS move to be within radio range of each other. That said,
pri/sec channel selection is not the only or even worse issue here since
the devices may be using completely different frequency ranges.

Jouni Malinen                                            PGP id EFC895FA

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