Best channel choice for multi-AP wifi

Hi all,
I flashed two Tp-Link Archer C7 v4 with the latest available snapshot OpenWRT firmware, then I setup same SSID for both AP and for radios (5 and 2.4 GHz), same subnet for LAN address, DHCP disabled (provided by external firewall) and both authenticate on a radius server.

Everything was working fine, then I remembered to change channel on the radio, so I choose channel 11 and 13 for 2.4 GHz, and ch. 36 and 64 for 5GHz. But after applying changes, radio0 (5GHz) on AP number two reverted back automatically to channel 36. So now both are working in the same channel. Is this a bad thing ? Actually I see no issues at all.
Can I leave it in this config or is better to force a change ? And in case, which channels are the best choice ?

Thanks and bye,
Dk

Your 5GHz radio moved to channel 35 probably because it detected a radar on channel 64, or it is not available in your country; have a look at https://en.wikipedia.org/wiki/List_of_WLAN_channels#5_GHz_(802.11a/h/j/n/ac/ax) for more info.

If you use the same channel on two APs, then the total bandwidth available is divided between them, but devices will jump from one AP to the other (if you move around the place) easily.

also note (from top of the wikipage) that 11 and 13 overlap, which maybe even worse than using the same channel outright.
safe to use 1,6,11

roaming between ap's on different channels also works works fine (and you will have double the bandwidth and no interference).

Yes, it works with different channels; but some devices tend to jump faster if you use the same channel. It's a compromise between speed and convenience.

Don't see any difference in roam speed if i use different channels for ap's with the same ssid.

Setup 802.11r or enable wds for your two ap's to get a better roaming between different ap's.
Preferably use 802.11r.

Thanks for the replies, I'll try a channel without DFS/TPC limitations (for example 40, 44, 48 for Europe) in the 5GHz band and 1,6,11 for the 2.4GHz.

Now last question: let's say for a moment that bandwidth is not an issue, so using the same channel for roaming APs is safe and can even give a small benefit (faster jump). But if the same channel is shared across access points of completely different networks, can this create problems, also from a security point of view ?
This is to understand whether it is better to fix channels but leaving the possibility that some neighbour uses the same, or let the AP decide which to choose on a "least populated" basis ?

Thanks,
Dk.

AFAIK, OpenWrt does not really support auto-configuring AP channels, and the "auto" value is interpreted as "first channel in that band"; so, you are going to have to choose a channel yourself. Get one of those "wifi analyzer" app for your phone, see which channels are used by your neighbors around you, and choose one channel.

I am not aware of any security implications on choosing channels.

If you APs allow channel 13 then according to https://en.wikipedia.org/wiki/List_of_WLAN_channels 1, 5, 9, 13 allow 4 (almost) non-interfering channels. In reality use a wifi scanner like kismet to figure out what the APs in your surround are using.
Sidenote it seems better to have two APs on the same channel as compared to two directly neighboring channels as in the first case they can arbitrate their tx-slots while in the second case they simply add noise to each other. In both cases bandwidth is going to be reduced if the neighbor is active...

The security thing is not affected by that in any way.

I believe it would be best if people would manually select the channels, or better if there was a way of restricting the auto method to restrict itself to a sane subset of channels (1, 5, 9 in the US, 1, 5, 9, 13 in europe). The big problem simply is that the nominal 12-14 channel center frequencies one can select do not map into the same number of non-interfereing/non-overlapping 20MHz channels. This probably is great in areas without other APs around as it will allow to select the best frequency with a nice granularity, but it certainly is suboptimal in the more typically RF-crowded areas.

Best Regards

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In my experience, one is either in an area where there is RF congestion, or one that is quiet. If in a congested area, it seems to change by the minute. A channel scan might suggest that one channel is "clearer" than others (as many APs seems to default to channel 1), but getting two, "non-overlapping" channels free is a rarity. I'm not aware of any successful, non-impactful implementations of dynamic channel selection in consumer-grade APs.

If you're in a quiet area, there might be a channel to avoid (due to that one neighbor), but again, it changes by the day.

I go with the US non-overlapping channels of 1, 6, and 11 with a preference to avoid channel 1 as it seems to be the "auto" selection of other APs (OpenWRT or not). I used to try to use a channel scanner when I was in the city, but found it a futile effort, even before the explosion of wireless APs.

On 5 GHz, things will be very different due to radar/DFS. I just avoid the impacted channels as I'm close enough to airports and all that I assume that they're all filled one way or another. As I use wireless backhaul between APs, I can't have channel switching.

And I wish everybody would go with the 1, 5, 9, (13) instead, as that scheme will work virtually everywhere. If there are no other APs/Stations in the vicinity it does not matter one whit and unless everybody else is using 1, 6, 11 the probability is high that those users on channels 2,3,4,5, and 7,8,9,10 will cause more interference than the slightly larger sideband overlap in the 1,5,9 scheme as compared to 1,6,11. But I realize that is wishful thinking...
Avoiding 1, seems like a good idea; my experience with a channel scanner is more positive than yours, but the main thing I use it for is to detect whether there are other APs around or not, not so much for exactly selecting a channel as most other APs are pretty variable...

Ah, that is sad, I was, naively under the impression that this is what openwrt's "auto" setting would do. Thanks for the grounding :wink:

This is what the wiki says: "Specifies the wireless channel to use. "auto" defaults to the minimum channel available" :frowning: .

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While you are right in principle, it's a losing game. Way too many APs use 1, 6, 11 - and way too many wlan cards sold in non-FCC regions don't allow channel 12, 13. Unless you are far away from other wireless stations, 1, 6, 11 is sadly the only option that doesn't create even more havoc.

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I see, I should have dome more research. I do wonder though which use-case this auto mode caters to, are there that many environments where channel 1 does not work at all?

But there are enough roughe APs on the other channels to make the argument change is impossible somewhat moot :wink:

Hence my recommendatuin for the US to use 1,5,9, which is still the same 3 "non-interfering" channels as with 1,6,12, but will also work without channels 12 and 13.

But I know that you are basically right, already 1, 6, 11 does not work sufficiently well...

Couldn't we just have openwrt auto pick randomly from 1,6,11 ? Here in the US it's actually more and more common to see these as the main channels, I think vendor firmwares default settings seem to hop between them on some basis.

If we can look in the region config and change to 1,5,9 for regions where that makes sense, good.

But it seems random assignment makes better sense to me

Have to agree, my neighborhood in germany currently exclusively uses 1, 6, 11. I really really wish the initial recommendation would have been the world usable 1,5,9,(13), but that train has left the station. On the upside this looks better than the broad mixture of all channels I have seen in the past, so progress!

Best Regards

Yep, definitely progress. The default algorithm for many vendor firmwares I'd say does hop, because I've seen my neighbors hopping around. What I've found is that picking a setting 1,6,11 and sticking to it is best, let the vendors hop around me.

On the other hand, if people are going to select "auto" on OpenWRT or if it's going to be the default, I think something like randomly selecting 1,6,11 at the time the wifi interface comes up would be a good idea. I have no idea how to change that in OpenWRT but if someone does, perhaps they can?