From: Simon Wunderlich <sw@simonwunderlich.de>
To: Johannes Berg <johannes@sipsolutions.net>
Cc: Antonio Quartulli <a@unstable.cc>, linux-wireless@vger.kernel.org
Subject: Re: [PATCH v2 2/2] mac80211: passively scan DFS channels if requested
Date: Wed, 26 Oct 2016 15:30:44 +0200
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On Wednesday, October 26, 2016 2:58:56 PM CEST Johannes Berg wrote:
> > (NOTE: going off-channel while operating is a different topic).
>=20
> Why do you think it's different?
>=20
> Seems exactly the same to me, since you come back to the channel and
> start sending without any new checking?

There are two ways to leave a channel:

1.) Leave the operating channel "permanently". Then the ETSI 301 893 says:
"If no radar was detected on an Operating Channel but the RLAN stops operat=
ing=20
on that channel, then the channel becomes an Available Channel." (4.7.1.4=20
Master Devices (c))

Then we can select a new operating channel and re-start the process.

2.) Off-Channel CAC: This is an optional feature. Simply spoken, we keep th=
e=20
channel operating, but try to do CAC-checking on a different channel by=20
shortly switching to that new channel from time to time. The new channel mu=
st=20
be checked for a longer time than the standard CAC time (6 minutes for non-
weather radar channels in ETSI).

The main difference is that we keep the current channel as "operating chann=
el"=20
and are also required to be able to detect radars with the same probability.

Also note that this feature is not implemented in Linux as far as I know, a=
nd=20
personally I haven't seen it in the wild yet.

=46rom 4.7.2.3.1:

"Off-Channel CAC is defined as an optional mechanism by which an RLAN devic=
e=20
monitors channel(s), different from the Operating Channel(s), for the prese=
nce=20
of radar signals. The Off-Channel CAC may be used in addition to the Channe=
l=20
Availability Check defined in clause 4.7.2.2, for identifying Available=20
Channels.

Off-Channel CAC is performed by a number of non-continuous checks spread ov=
er=20
a period in time. This period, which is required to determine the presence =
of=20
radar signals, is defined as the Off-Channel CAC Time.

If no radars have been detected in a channel, then that channel becomes an=
=20
Available Channel."

Cheers,
     Simon
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