ACS seems not to select the best channel in 2.4GHz.
eduardgv at gmail.com
Thu Feb 5 13:03:16 EST 2015
>> I think it's not necessary to add adjacent value. Because radio survey data
>> of each channel would detect the adjacent interference actually 20MHz.
>> In fact, the test result shows that ch.4 detect the interference of ch.1,
>> similarly ch.8 detect the interference of ch.11.
>> I attached a patch file for hostapd-v2.3 .
> I'm not sure skipping the adjacent channels completely on 2.4 GHz would
> be ideal solution here. It is clear that the channels at each end of the
> band will require something more accurate. It may also be reasonable to
> at least drop the importance of the impact from adjacent channels.
Agreed. Our experience shows that the use of partially overlapping
channels (i.e., not only 1, 6 and 11) is beneficial in dense
environments , but adjacent channel interference must be taken
> consider the following patch as a way of addressing this. If you define
> ACS_ADJ_WEIGHT 0, ACS_NEXT_ADJ_WEIGHT 0, and ACS_24GHZ_PREFER_1_6_11 1,
> you'd get the behavior you should saw with the patch you sent. If
> someone has good suggestions on what the best numbers to use here as the
> default would be, I'm certainly open to changing these. As noted in the
> commit message, I did not really do any real research on picking the
After some experiments and some math, we concluded that the
adjacent channel (i.e. my center frequency +- 5MHz) is barely filtered
(~0.5dB), so I'd suggest
#define ACS_ADJ_WEIGHT 0.85
For the case of 2-channel separation (i.e. my center frequency +
10MHz) , the attenuation was ~2.5dB, so I'd suggest
#define ACS_NEXT_ADJ_WEIGHT 0.55
In fact, I'd also suggest that the "next to the next" adjacent channel
(i.e. my center frequency +- 15MHz) should also be taken into account
with a weight of 0.22 since transmissions three channels away do have
an impact on my current channel.
> it is clear that this summing
> of three to five values together and then comparing the sum rather than
> average of some kind cannot be correct.
>Band-edge channels are
> handled in a way that takes average over the channels that were actually
> considered instead of assuming zero interference from neighboring bands.
On the one hand, I agree that summing of interference_factors doesn't
make much sense (you are summing channel utilization ratios, which
could produce values > 100% of channel utilization?!?). On the other
hand, band-edge channels are actually less interfered since they have
less adjacent channels and, hence, it makes sense they are preferred
(i.e., averaging those interference_factors is not my favorite option
What I'd like to try (at some point, when I have time and resources)
is a different formula for the interference_factor. Something like the
linear sum of the average signal level measured in all channels
(applying attenuation factors) and weighted by their utilization.
> This change results in significantly less preference for the band-edge
> channels. That is arguable good thing to do here, but it would also be
> good to avoid picking some arbitrary 2.4 GHz channel and rather give
> significant extra preference for the commonly used channels 1, 6, 11.
> This is now done by picking those channels over the other ones unless
> another channel has significantly better interference factor (0.8 or
> smaller times the one on 1, 6, 11).
Why not 1, 5, 9, 13 (where available)?
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