@ymd, the problem seems to be related with reboots. If I reboot all EAPs at the same time, they won't select different channels:

 

 

 

If I reboot the EAPs one after another, they select different channels:

 

 

I guess that this is normal behavior - they just can't see each other's channel selection at scan time.

@ArthurDent113,

 

I have only EAP225-Outdoor with FW v1.7.0 at the moment, but its firmware is almost identical to EAP225 v2.7.0 and for a quick test I can set it to use indoor channels much like an EAP225 does (not recommended as permanent setting, might get you into legal troubles in the EU!).

 

If I reboot both EAPs at the same time, they sometimes choose the same channel if set to »Auto«. I think everyone can imagine that if both EAPs are doing the survey at the same time, they can end up with using the same channel which was detected as »mostly free« before the survey ends.

 

If I reboot one after another, they choose different channels, but in 20/40/80 MHz mixed mode it searches for 20 MHz wide channels.

 

Nevertheless I tried to analyse with semi profesional spektrum Analyzer and saw the signal from the AP is much broader than a typical 5G AP.

So I am assuming now that the contoller respectively the AP are using just a broader frequencyband (more than just channel 36), but claiming its just 36-

 

Yes, that's right and might be confusing.

 

If you set channel width to 80 MHz, the channel is much broader, exactly 80 MHz. The EAP uses 16 channels, so to say.

 

The spectrum of the 5 GHz band is partitioned into 5 MHz wide channels, e.g. channel 36 is 5180 MHz, channel 37 is 5185 MHz, channel 38 is 5190 MHz, channel 39 is 5195 MHz, channel 40 is 5200 MHz and so on (this are center frequencies, the spectrum used is on the left and right of this center frequencies).

 

In the EU only every fourth channel is used (e.g. 36 = 5180 MHz, 40 = 5200 MHz and so on). This guarantees non-overlapping co-existence for APs/clients with a channel width of 20 MHz (the default channel width).

 

Thus, with 20 MHz channel width an EAP set to channel 36 uses already four 5 MHz channels (from channel 34 to 38 which equals 5170–5190 Mhz). Center frequency is 5180 MHz, that's channel 36.

 

With 40 MHz channel width an EAP set to channel 36 uses 8 channels which might either below (5150–5190 MHz, that's 40-) or above (5170–5210, that's 40+) of channel 36 (5180 MHz). Let's say it selects 40+, then it uses 5170 to 5210 MHz, that's channels 34 to 42. The center frequency is 5190 MHz, which equals channel 38.

 

With 80 MHz channel width an EAP set to channel 36 uses 5170–5250 MHz, that's channels 34 to 50. Center frequency is now 5210 MHz which is channel 42.

 

But in 20/40/80 MHz mixed mode, if a client supporting only 20 MHz channel width associates with the EAP, it might negotiate to use center frequency 36. Channels 40, 44, 48 are still free and non-overlapping for other clients using 20 MHz channel width. Channel 46 is still free and non-overlapping for clients using 40 MHz channel width, while the 40 MHz channel 38 is partially overlapping with the 20 MHz channel 36 in use by the other client.

 

Let's see what this means for automatic channel selection:

 

If the EAP auto-selects channels 44 and 48 on reboot as shown here, it chooses two most unused 20 MHz channels:

 

 

 

If you now have set channel width to 20/40/80 MHz mixed mode, both EAPs needs to acquire channel 42 since the spectrum for the 80 MHz channel width does not fit elsewhere around 44 and 48. Result is two exactly overlapping WiFi cells using the same spectrum for clients using the full 80 MHz channel width.

 

But not so for clients using 40 MHz channel width: They can use channels 38 and 42 non-overlapping at the same time.

 

And clients with 20 MHz channel width can use channels 36, 40, 44 and 48 non-overlapping.

 

A good WiFi analyzer shows you the real center frequency acquired by the two EAPs (channel 42) in 80 MHz mode, no matter whether they are set to channel 36, 40, 44 or 48.

 

Conclusion:

If you want non-overlapping channels between different EAPs in the traditional style auto-selected, use 20 MHz channel width and reboot the EAPs one after each other with short delays in between.

 

But with MU-MIMO and mixed 20/40/80 MHz mode the overall throughput is way better (albeit much more complex to achieve) to reliably serve a large number of clients or to give a single client the maximum possible WiFi bandwidth.

 

ArthurDent113 wrote

Many thanks!! for your answer, it took me quite a while do understand but I think I got it'

Kind of off topic but maybe you can help me with that as well: does the "untrusted rogue AP" List somehow affect the autochannel selection? As this list is not updated after a reboot in auto mode (what basically i was expecting), this is kind of confusing for me. Shouldn't the untrusted rogue AP List be considered for the autochannel selection?

Thanks

 

You're welcome. Yes, WiFi nowadays has become a very complex technique, so you are not alone when trying to understand the nitty gritty details. Took me quite a while, too.

 

AFAIK, the »untrusted rogue AP« list is updated only when you initiate a search for them. On reboot, the channel selection still searches the frequency band for nearby APs (and ignores APs with very weak signals which will not interfere), but it does not save the results in the »Insight« database table. In contrast the search also includes APs with a weak signal.

 

To be honest, the concept of »untrusted rogue APs« vs. a simple WLAN survey is still a mystery to me regarding the terminology, since you can declare those APs as »trusted«. Unfortunately, the User's Guide does not describe what the latter function is for. Maybe, it's as simple as to just filter out known APs from the list, which is so far my best guess.