Re: [PATCH v2 2/2] mac80211: passively scan DFS channels if requested

[Simon Wunderlich <sw@simonwunderlich.de>]

[-- Attachment #1: Type: text/plain, Size: 1987 bytes --]

On Wednesday, October 26, 2016 2:58:56 PM CEST Johannes Berg wrote:
> > (NOTE: going off-channel while operating is a different topic).
> 
> Why do you think it's different?
> 
> 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 operating 
on that channel, then the channel becomes an Available Channel." (4.7.1.4 
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 the 
channel operating, but try to do CAC-checking on a different channel by 
shortly switching to that new channel from time to time. The new channel must 
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 channel" 
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, and 
personally I haven't seen it in the wild yet.

From 4.7.2.3.1:

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

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

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

Cheers,
     Simon

[-- Attachment #2: This is a digitally signed message part. --]
[-- Type: application/pgp-signature, Size: 801 bytes --]