Initial automatic channel selection implementation

Luis R. Rodriguez mcgrof at
Wed May 25 16:01:14 EDT 2011

On Wed, May 25, 2011 at 12:27 PM, Luis R. Rodriguez <mcgrof at> wrote:
> On Wed, May 25, 2011 at 12:24 PM, Luis R. Rodriguez <mcgrof at> wrote:
>> On Wed, May 25, 2011 at 12:20 PM, Felix Fietkau <nbd at> wrote:
>>> On 2011-05-25 5:01 PM, Luis R. Rodriguez wrote:
>>>> On Wed, May 25, 2011 at 7:45 AM, Helmut Schaa
>>>> <helmut.schaa at>  wrote:
>>>>>  On Wed, May 25, 2011 at 4:37 PM, Luis R. Rodriguez<mcgrof at>
>>>>>  wrote:
>>>>>>  On Wed, May 25, 2011 at 5:19 AM, Helmut Schaa
>>>>>>  <helmut.schaa at>  wrote:
>>>>>>>  On Wed, May 25, 2011 at 12:54 AM, Luis R. Rodriguez<mcgrof at>
>>>>>>>  wrote:
>>>>>>>>  Yes, thanks this is a lot of work already done. Now we just need a
>>>>>>>>  basic algorithm to parse this, quantify how ideal this channel is and
>>>>>>>>  then spit out a desired optimal channel.
>>>>>>>  That's what I've hacked some time ago (in form of the attached _ugly_
>>>>>>> shell
>>>>>>>  script that does auto channel selection with rt2x00, not sure if it
>>>>>>> works
>>>>>>>  correct with other drivers as the survey dump differs somehow between
>>>>>>>  rt2x00, ath5k and ath9k, and it only does channel 1-11):
>>>>>>>  - Iterate over all channels and stay on each channel for some time
>>>>>>  Nice, yeah I was thinking of using the offchannel operation if we want
>>>>>>  to spend more time there for inspection and if associated. For first
>>>>>>  iteration it should be possible to just move around. In fact for AP
>>>>>>  purposes I suppose one will want to just start AP mode ASAP and then
>>>>>>  later do offchannel operations to do the inspection on the ideal
>>>>>>  channel. Otherwise we sit there idle until we complete the Automatic
>>>>>>  Channel Selection thingy.
>>>>>  Correct, especially if we also consider 5Ghz channels. Offchannel
>>>>> operations
>>>>>  would be nice but how can we ensure AP mode while being offchannel?
>>>> Notification of absence.
>>>>>>>  - Store busy time stats for each channel
>>>>>>>  - Choose the channel with the lowest busy time (and on 2.4Ghz also
>>>>>>>   check the busy times on adjacent channels)
>>>>>>  So I was reviewing this -- if we are TX'ing or RX'ing it seems to me
>>>>>>  we should skip that time from the busy time, otherwise the "busy" time
>>>>>>  includes time we induced on TX'ing or RX'ing ourselves. So I was
>>>>>>  thinking of using the:
>>>>>>   (active time - rx time - tx time)  / busy time
>>>>>  Looks good ;) just one problem from a rt2x00 POV: We can't report rx/tx
>>>>> busy
>>>>>  time separately, we can only advice the hw to include or exclude rx/tx
>>>>> time
>>>>>  from the busy time statistics.
>>>> Doh, I see.. well in order for the above math to be useful we'd have
>>>> to be consistent across drivers. What is being done by rt2x00 right
>>>> now? If the later then the math would still work, otherwise then we'd
>>>> need to adjust.
>>> Excluding rx time isn't even a good idea, since it makes no distinction
>>> between local BSS or other activity in the area.
>> What if we do an offchannel operation?
> Once we figure out that...
> The missing piece is how to deal with noise info here. In short the
> lower noise we have the better signal we'll get. The challenge then is
> to take into consideration the noise mathematically in such a that a
> high noise value would nullify any clean idle air time ratio
> conditions from the formula postulated before. Let me review again
> with some modifications.
> Active time is the time we spend on the channel, so to get an idea of
> how "busy" that channel is we have to remove the tx and rx time from
> that channel. That gives us the time we spent idle on that channel.
> Then the busy time is a subset of the entire active time but we should
> also exclude the time we spent tx'ing and rx'ing as well. We then
> have:
>  (busy time - rx time - tx time) / (active time - rx time - tx time)
> This is a bounded ratio already, given that if we spent 0 time tx'ing
> 0 time rx'ing, but 10 ms on a channel, and all that time we had busy
> time as well we'd have a ratio of 10 / 10 = 1. In the best case we'd
> have  0 / 10 = 0.
> What I'd like to do is to affect the ratio to nullify it if the noise
> is very low on the channel. Given that noise is logarithmic we'd have
> to use a logarithmic function as well. Working on that now.

I also noticed that the survey results will stale out after a period
of time. As such the offchannel operation should ideally be followed
imediately by a survey of the channel. Our current API though allows
only for an entire survey, not a channel specific survey query. Not a
big deal but just something to keep in mind if we intend on using this
further for repetitive heuristics on a channel. We'll have to extend
the survey dump later to accept channel specific queries.


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