Re: Channel interference scan and manual selection

```There is no "1, 6, 11" anymore.  You're not listening.

Back in the day, the adjacent  (1 on 6, 6 on 1, 11 on 6, 6 on 11)
channel rejection spec for 802.11b was 35dBm at 11Mbps modulation
There never was an alternate (1 on 11, 11 on 1) channel rejection spec
in the days of 802.11b.```
```
Most decent super-het designs back then used a SAW filter in the
middle of the downconversion chain, and got around 41dBm of adjacent
channel rejection.

But we all moved to direct-conversion receivers ... because: cost.  We
all loves us some cheap 802.11 gear.

The adjacent channel rejection spec for 802.11g (and 802.11a) is -1dBm
@ 54Mbps modulation.
The alternate channel rejection spec for 802.11g/a is 15dBm @ 54Mbps modulation.
Just FYI, minimum ACR for 802.11g/a at 6Mbps is 16dB (alternate is
16dB more, for 32dB)

If you're underwhelmed by the difference between 41dBm and -1dBm, then

Free Space Path Loss = 20 log(4*p*r/λ) dB, where
r = distance between transmitter and receiver
λ = wavelength

Path loss in the first meter @ 2.4GHz is 41dB. At 10m it's 60dB.

Lets say you've got a garden-variety radio that puts up 32mW (15dBm)
of tx power, and ignore antenna gain for now (so 0 dBi antennas on

15dBm - 60dB - 41dB = -86dBm This is the in-channel 'noise power' of
Notice that it is at least 15dB above the thermal noise floor.

New 802.11g/a (direct conversion receivers) world:
15dBm - 60dB - -1 dBm = -44dBm.   This is about 20dB higher than what
is necessary to recover a 54Mbps receiver, but remember, it's the
802.11 OFDM receivers are EVM-limited at 48Mbps and higher, but I
digress.)

Note as well that we're 57dB above the thermal noise floor, at 10m (33')

Even if you back off to alternate channels (1 and 11), you're still at -60dBm.

> On 3 December 2013 19:32, Tek Wiz <tx2...@gmail.com> wrote:
>> Yes, the interference can be calculated by averaging the beacon RSSI (dB) of
>> the overlapping channels, e.g. for channel 6, the beacon RSSI of channels 4,
>> 5, 6, 7, 8 are averaged. Normally channels 1, 6, and 11 are chosen, although
>> other channels may have less interference, in order to be a 'good neighbor'.
>
> Sure. But we can also look at what kinds of frames we actually receive
> during a sample window.
>
> What I'd also like to do is finally add receive power histogram
> support. Ie, look at the RSSI of all the frames you receive, stick
> them in buckets, respond to the power histogram action request frames,
> use them for this.. that doesn't require any driver support. Just
> net80211 work in the RX path.
>
>
>