from Hacker News

The 5GHz “Problem” for Wi-Fi Networks: DFS (2018)

by arm on 12/26/22, 5:36 AM with 182 comments

by tapanjk on 12/26/22, 6:40 AM
> When powering up an AP that uses a DFS channel, you will see that the 2.4GHz radio becomes available as soon as the AP has completed its boot sequence, but the 5Ghz radio may not available for another minute. This is due to the AP performing its channel availability check, if the AP is trying to use a DFS-impacted 5GHz channel.
This always baffled me. Mystery solved!
by McNutty on 12/26/22, 9:05 AM
As a Wi-Fi networking engineer who often scratches his head at a lot of the software posts on HN, it is super fun to be on the other side and see the types of comments being thrown around by you lot.
Also FYI you can rest assured that if you've engaged a semicompetent Wi-Fi professional to design the coverage in your office/campus/warehouse/etc then you don't need to worry about "the DFS problem" as it is well known and will have been accounted for in the design.
by noipv4 on 12/26/22, 7:33 AM
Using 5GHz is a real pain here in Switzerland. Only 4 non DFS channels are available 36, 40, 44 and 48 (output power is also highly restricted in these 4 channels). Rest 14 channels are DFS. Channel planning is a nightmare; never know when a AP on DFS channel will hop back to a non DFS channel and start sharing bandwidth there, and will also lower its output power. Moreover adding to the confusion is that all Outdoor APs are restricted to using DFS channels only!!
by jacquesm on 12/26/22, 9:04 AM
I had a pretty nasty wifi problem here recently and it took me forever to track it down to a forgotten Zigbee dongle that was broadcasting at its maximum power to find all those non-existent devices. It made several WiFi channels completely unusable. Pull the dongle, problem gone. Just in case it helps someone else.
by odysseus on 12/26/22, 7:57 AM
Maddening when you have to scroll through several paragraphs and repeated mentions of DFS just to find out what DFS stands for:
Dynamic Frequency Selection
To quote Elon: Acronyms Seriously Suck
by ThomasBb on 12/26/22, 7:33 AM
https://www.theregister.com/2014/05/20/getting_wifi_glitches... Has an example of what interference may look like on the radar side…
by Deathmax on 12/26/22, 9:55 AM
In the UK Ofcom recently (2020) relaxed the rules on Band C channels (channel 140+) to allow for indoor non-licensed use. These have been a godsend in a crowded environment, as the lower non-DFS channels are very crowded, while the DFS channels are, well, DFS.
https://draytek.co.uk/information/blog/ofcom-relax-the-rules...
by xmddmx on 12/26/22, 4:03 PM
In the USA, there is one non-DFS 5GHz channel which is somewhat unique: Channel 165. It's limited to 20MHz bandwidth, and if you google it you will see warnings against using it.
On the other hand, since nobody uses it, it's generally totally clear, and if your AP and client devices support it (and you can live with the lower bandwidth) it can be a nice "secret" channel to use in a noisy 5GHz environment.
by glogla on 12/26/22, 9:20 AM
Yup. It is really bad.
Where I live, the channels 36-48 - which is really just one channel once you go 80 Mhz - is chock-full of networks and completely unusable. The DFS channels are also complete unusable, because they jump around like crazy due to some noise source (I checked, and there's no radars around). Trying to use them means you see disconnects ten+ times a day.
My router can use channels 149+, which are not DFS and most consumer routers don't use them so that part of the spectrum is clear, but those channels have smaller allowed broadcast power so I get slower speeds. Or I could use 2.4 which works reliably, but is even slower than the low power 5 Ghz channels.
Interesting consequence is that while my ISP started offering gigabit connection, I see no reason to upgrade, since my Wi-Fi is now the limiting aspect, unless I start using wires or replace my devices with ones that can do Wi-Fi 6e or something. Even the 300 Mbps I pay for I only get in the living room.
by mmwelt on 12/26/22, 6:52 AM
I've found the main problem is when on a DFS channel and doing something real-time -- a Zoom/Skype call, for instance. Very noticeable then.
Also, don't use hidden SSIDs on a DFS channel: https://badfi.com/blog/2016/2/15/yet-another-reason-avoid-us...
by theshrike79 on 12/26/22, 9:24 AM
I used to live near a secondary arrival path to the nearby airport.
Every time the main route was unavailable, due to wind or something I would lose my Wifi because of DFS. That was really fun to debug until I finally figured it out.
Unifi especially may take hours to get the 5GHz network back for some reason.
by BitPirate on 12/26/22, 9:45 AM
Quite a few devices implement Zero-wait DFS.
https://docs.engenius.ai/cloud-white-papers/zero-wait-dfs#wh...
by giuliomagnifico on 12/26/22, 8:22 AM
The best fact about this, is that it is a technology that was "thinked" in 1940s by a woman: Hedy Lamarr > https://wyldnetworks.com/blog/hedy-lamarr-frequency-hopping-...
by can16358p on 12/26/22, 6:49 AM
What would be a realistic effect of Wi-Fi still being on 5GHz when a radar scans the area?
Any actual radar would probably be far away from my home 5GHz Wi-Fi anyway, which has relatively low tx power. Is there a realistic scenario where a 5GHz-scanning radar sends a pulse from far away, and my home Wi-Fi's signal gets picked up by it?
by wkat4242 on 12/26/22, 11:05 AM
Some channels even have a 10 minute wait here in Europe. Really annoying but the flip side is they're really clean because nobody uses them :P
by Gigachad on 12/26/22, 7:22 AM
Fascinating article. Doesn’t seem terribly concerning though. I’ve personally never noticed wifi dropping out like this. Although if I did I’d likely attribute it to some other unexplainable event.
by qwertywert_ on 12/26/22, 7:44 AM
6GHz is coming thankfully.. wifi channel planning especially across regions is rough.
by robocat on 12/26/22, 9:31 AM
Is it legal to spoof radar, clear the DFS channels of other users, and then use those channels for your own devices? I could see that being useful in dense 5GHz environments, so long as you don’t mind being a selfish bastard/bitch.
by the_mitsuhiko on 12/26/22, 7:37 AM
Some Wifi routers only allow you to pick non DFS channels. An example of this is the Orbi range from Netgear. I assume that supporting DFS channels either results in suboptimal behavior or it’s too much work for them.
by Jenda_ on 12/27/22, 12:49 AM
The overlapping of WiFi and radars in Europe and America is a HUGE headache. In comparison, for example in Australia, they have channels 116-128 forbidden (see the table at https://en.wikipedia.org/wiki/List_of_WLAN_channels#5_GHz_(8... ) and it is so much easier to operate there (yeah, I get it, with lower available bandwidth to 5GHz wifi).
As you can see in the article, the radar pulse sequence to trigger DFS requires pretty short pulse widths, but you want to use pulse compression (with longer pulses) in modern radars, so you won't trigger DFS. Additionally, you may scan over particular location only once per 5 minutes, so again you have lower chance to trigger it. And another challenge are ever-changing atmospheric conditions - maybe sometimes you won't see the wifi device and so you won't trigger the DFS (https://en.wikipedia.org/wiki/Anomalous_propagation) and then the conditions change and you suddenly start getting interference until the DFS on the remote end re-evaluates.
We have a radar on the Czech-German border and it's interesting that we have way worse interference coming from the Czech side. But this may also be because of some local conditions.
In the comments, it is suggested to transmit a pseudorandom code and then correlate it with the received signal to filter out uncorrelated interference. I'm of course doing this, but it helps for point targets (towers, airplanes) - not so much for distributed targets (clouds), especially when they are not stable in time (the droplets all vibrating in turbulences) - so the replicas you receive are distorted by various means, instead of a single reflection you get from e.g. an airplane.
What helps A LOT is a wifi packet detector, which completely blanks the data when the remote station is transmitting, so our radar is basically operating in the gaps between the packets. For some products (such as reflectivity), we have enough oversampling so we can interpolate the gaps; for other products this degrades the data. But at least you get gaps, not giant "lightsabers".
by walrus01 on 12/26/22, 7:59 AM
It's even more annoying if you're trying to use cheap consumer grade 5 GHz point to point outdoor radio equipment (like a pair of $160 Ubiquiti 802.11ac based radios) to span a few km of distance, and choose a certain DFS part of the 5 GHz band because it's cleanest, but the link keeps dropping due to false "DFS hits" when there is actually no DFS weather radar nearby.
by IYasha on 12/30/22, 7:25 PM
A bit off-topic, but: while watching some very suspicious advertisement on this blog page I've noticed it's a HTTP connection. Please don't post HTTP links!
by meltedcapacitor on 12/26/22, 10:10 AM
Does it help to put the access point in the basement of a small building so that the AP and radars (or other sources of false positives) don't "see" each other, while still having a short line to client devices?
My intuition so far was always to put the AP as high as practical...
by giomasce on 12/26/22, 1:40 PM
Is there a way to know how often DFS are sent in your zone?
by yupis on 12/26/22, 1:29 PM
Finally I found the answer to my dropped wifi connection
by dataflow on 12/26/22, 7:05 AM
How does a Wi-Fi chip know what country it's operating in to know what the relevant DFS channels are?
by blackhaz on 12/26/22, 9:43 AM
Interesting. Does anybody know how to list available channels in FreeBSD?
by Aerroon on 12/26/22, 12:49 PM
Reading this thread gives me the impression that WiFi is doomed. It doesn't seem like it's going to be able to keep up with 5G or beyond, let alone wired connections.
by uxx on 12/26/22, 11:27 AM
sounds like a vulnerability is included