15 ms·

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

299 points by arm 4y ago

BitPirate 4y ago
Quite a few devices implement Zero-wait DFS. https://docs.engenius.ai/cloud-white-papers/zero-wait-dfs#what-is-zero-wait-dfs https://docs.engenius.ai/cloud-white-papers/zero-wait-dfs#wh...
odysseus 4y ago
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
- walrus01 4y ago
  [flagged]
  - bozhark 4y ago
    E-LON is an acronym
- astrange 4y ago
  Most acronyms are more meaningful as acronyms than expanded. Like this one for instance - have you actually learned anything now that you know it’s “dynamic frequency selection”? That’s not a good description of “an extra channel that sometimes isn’t available because radar uses it”.
  - ahmedalsudani 4y ago
    It's way better when you learn the words. Now I understand it's about frequencies and channels. DFS to a software crowd will always expand to depth-first search. I was trying to figure out if that's what was being discussed here (with plenty of skepticism, admittedly) until reading GP.
    - astrange 4y ago
      It helps in this article title some, but I usually see it written as “DFS channel”. In which case you already know it’s a kind of channel, so maybe it’s best to think of it as “sometimes-available channel”.
    - jimnotgym 4y ago
      > It's way better when you learn the words. Not always. I don't think learning what REST stands for helped anyone understand web service APIs
      - ahmedalsudani 4y ago
        Yeah I'm specifically responding to the question above. Some things like REST/HTML/XML are familiar enough that the initialism is as good as a dictionary word. SOAP, on the other hand, I forget what that was. Hopefully it's not important.
        ziml77 4y ago
        It’s not about being familiar. It’s that “Representational State Transfer” is as meaningless as REST to someone who doesn’t know what REST is. (And honestly, even to those who do know what it means)
  - tleb_ 4y ago
    Another description: "dynamically picking channels to avoid radar activity". Seems more fitting with this one.
  - bagels 4y ago
    It conveys a lot more information than DFS.
  - marcosdumay 4y ago
    Most acronyms are more meaningful when you don't have dozens of them conflicting on the same 3 letter string on the same area of knowledge.
- BramEsposito 4y ago
  It’s right there in the Background section: > This mechanism is known as Dynamic Frequency Selection (DFS) and is designed to mitigate interference to 5GHz radar by WLANs.
- bee_rider 4y ago
  Collision with “depth first search.” Which almost works — I could at least be convinced that there was some problem traversing these networks.
xmddmx 4y ago
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.
- digitallyfree 4y ago
  Ah, I love 165. I've never seen it utilized even in downtown areas and I generally recommend it if you are in a congested area and are fine with the performance on 20MHz.
- spmurrayzzz 4y ago
  This is the most practical advice I tend to give folks, even if its a bit niche. Not everyone has a spectrum analyzer of course to verify this, but if you have a reasonable assurance that the channel is not being used and your devices are AC/AX that can make use of at least 2+ spatial streams, that more than makes up for the truncated channel width in most cases.
Jenda_ 4y ago
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_(802.11a/h/j/n/ac/ax) 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 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".
tapanjk 4y ago
> 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!
- smcleod 4y ago
  Oh my gosh - that's why! I always assumed that the 5Ghz radios were slower to boot their FW or something.
- deleted 4y ago
  [deleted]
can16358p 4y ago
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?
- radicaldreamer 4y ago
  Doppler weather radar shows interference from 5 Ghz networks (https://en.wikipedia.org/wiki/Dynamic_frequency_selection https://en.wikipedia.org/wiki/Dynamic_frequency_selection), but is not used in chunks of the US (don’t even exist on the west coast), but is critical in places with lots of rain storms (south, northeast, midwest).
  - tetrep 4y ago
    What's your source for areas of the U.S. without doppler? It looks like NEXRAD dopplers are "everywhere" in the U.S. https://en.m.wikipedia.org/wiki/NEXRAD https://en.m.wikipedia.org/wiki/NEXRAD
    - tjohns 4y ago
      It's not for NEXRAD. The frequencies in question are used for Terminal Doppler Weather Radar (TDWR), which is a special radar used for detecting windshear and higher resolution precipitation data at select airports. See: https://en.wikipedia.org/wiki/Terminal_Doppler_Weather_Radar https://en.wikipedia.org/wiki/Terminal_Doppler_Weather_Radar
- retrac 4y ago
  My Bluetooth headphones stop working around the train station at rush hour. While the coding used is spread spectrum and many devices can use the same frequencies, each transmitter lowers the overall effective signal-to-noise ratio for all other users in the area. At the train station with thousands of radios transmitting on the same frequencies within a stone's throw, there is so much noise my headphones can't hear my smartphone despite being one metre away from it. Same idea with radar aimed at an urban area. Thousands of WiFi devices in a high rise will add up. Tens of watts, hundreds of watts of noise. A large and dense downtown core will be transmitting kilowatts of radio noise from millions of devices. More noise means less SNR. And less SNR in radar means a blurry/noisy radar view, and lower effective range.
mmwelt 4y ago
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-using-hidden-ssids https://badfi.com/blog/2016/2/15/yet-another-reason-avoid-us...
- japanman425 4y ago
  You didn’t read the article did you. It makes no difference.
- lxgr 4y ago
  Hidden SSIDs use beacon frames too, as far as I remember. They just don‘t broadcast their SSID in it, but clients can see them and then interrogate them ("are you part of SSID xyz?").
- jeroenhd 4y ago
  The correction at the bottom of the article you linked indicates it's not that much of as problem to use hidden SSIDs onn DFS. The 100ms beacon interval latency mentioned isn't going to be much of a problem with how slow and unresponsive the WiFi stack on most devices is anyway.
dataflow 4y ago
How does a Wi-Fi chip know what country it's operating in to know what the relevant DFS channels are?
- e2le 4y ago
  It'll have a region set by the manufacturer in it's firmware. IEEE 802.11d-2001 also added country information to AP beacons and probes. Nearby access points will advertise a country code to each other. Hostapd and/or the Wi-Fi device firmware can take advantage of that. [1]: https://en.wikipedia.org/wiki/IEEE_802.11d-2001 https://en.wikipedia.org/wiki/IEEE_802.11d-2001 [2]: https://w1.fi/cgit/hostap/plain/hostapd/hostapd.conf https://w1.fi/cgit/hostap/plain/hostapd/hostapd.conf Fun fact: Access points can also advertise a timestamp and timezone. [3]: https://openwrt.org/docs/guide-user/network/wifi/basic#bss_transition_management_frames_options_80211v https://openwrt.org/docs/guide-user/network/wifi/basic#bss_t...
- Vecr 4y ago
  It's either set by the user, or it's already set on the device based on the market the product is selling into. Phones have GPS though, so I'm not sure how that factors into it.
  - sgtnoodle 4y ago
    It's usually programmed into an EEPROM inside the device. Nowadays for compliance reasons, the radio firmware may enforce it rather than the software driver.
- mjg59 4y ago
  APs are normally programmed based on the market they're sold into. Client devices receive information about which regulatory domain they're in from the AP they're connecting to.
  - meltedcapacitor 4y ago
    Can't imagine mom and pop shop on aliexpress is programming APs individually based on the recipient address.
Gigachad 4y ago
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.
- sgtnoodle 4y ago
  Many APs have buggy radio firmware that won't ever be fixed. The manufacturers hastily implemented DFS to pass FCC certification, which was a lot easier to do than implement it to work robustly. Due to detection false positives, the radios will only use any given DFS channel for a few minutes at a time. If you have the AP set to "auto", then it probably just needlessly hops around channels every few minutes; you get a brief latency spike and it's mildly annoying. If you have the AP set to a specific channel, then your wifi frequently cuts out and you go insane. It's interesting. If you're a typical consumer and use default settings, you end up with a mostly working setup that's glitchy but not quite broken enough for anyone to notice because Netflix still works. If you're a tech enthusiast and tweak seemingly harmless settings that should unambiguously improve things, you end up with a completely broken network.
  - spmurrayzzz 4y ago
    I've worked with vendors for years to debug/fix that class of bugs you reference. Cherry picking one example very recently, we discovered a certain router model shipping in calibration mode such that only 1-2 STAs could transmit at any given time. Essentially rendering an AC router into the performance class of a mixed-mode a/b/g router from 2004. Working with the upstream vendors to fix this took several weeks. Sometimes though its not even phy or mac layer bugs we find, its more banal nonsense like a poorly configured firewall that eats up all the NPU clock cycles when it sees a 30 pps rate of unsolicited UDP ingress. Or my personal favorite was the erase block management on flash that would remount the data partition as read-only whenever a single ECC cycle would fail on a given sector and permabrick the unit until reboot.
    - sgtnoodle 4y ago
      Any recommendations for modern access point hardware that is well made?
noipv4 4y ago
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!!
- bozhark 4y ago
  This just explained the massive headache I’ve had with my home network. Thanks!
- spaniard89277 4y ago
  In my home network here in Spain I just turned off all my ISP router wireless and got two 2.4 APs with OpenWRT by ethernet in different parts of the flat. There's a lot of noise but honestly I'm fine with ~30mbps, and I don't get all the weird stuff that comes with 5ghz. Everything that really needs speed is connected by ethernet anyway.
  - newZWhoDis 4y ago
    Meanwhile I run 5ghz only, completely killing the 2.4ghz band because it’s so noisy.
    - martinald 4y ago
      Agreed, even in a fairly low density street 2.4GHz is totally useless as it is so noisy, especially outdoors. 5GHz actually has better range due to the noise. Hopefully Wifi6E equipment will come down further in price.
- avidiax 4y ago
  You should see if the SRD (25mW) channels are allowed in Switzerland. They are not in BAKOM's WLAN factsheet, but I think they are supposed to gradually align with ETSI/ECC, so I expect they will eventually become available. They are low power, but unused, so they perform well.
ThomasBb 4y ago
https://www.theregister.com/2014/05/20/getting_wifi_glitches_out_of_weather_radar/ https://www.theregister.com/2014/05/20/getting_wifi_glitches... Has an example of what interference may look like on the radar side…
the_mitsuhiko 4y ago
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.
qwertywert_ 4y ago
6GHz is coming thankfully.. wifi channel planning especially across regions is rough.
- silisili 4y ago
  6ghz has pretty poor range. I had to pull my routers off that backhaul.
  - linsomniac 4y ago
    Poor range means less interference from neighbors, which I've found is a good thing. I'm pretty sure I have a neighbor with a leaky microwave or cordless phone that wipes out 2.4 intermittently. Remember: interference isn't all WiFi.
    - silisili 4y ago
      It does, and that's generally a good thing. But I have 2 routers in a mesh, about 50 or so feet away, that work serviceably in my house on 5g, and poor connection on 6g. I think in the future people are going to need a lot more nodes. Like, twice as many. But hey, fast speeds and less interference.
      - linsomniac 4y ago
        I've been at the "have one AP in every air space where we make significant use of WiFi" for ~5 years. My 1700sqft home has 4 Google mesh nodes, and I've moved towards hard wiring them. Has really improved the wifi quality.
      - bozhark 4y ago
        So, note to new IoT device makers… give it an AP
        octotoad 4y ago
        Ugh. No, thanks.
walrus01 4y ago
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.
- suction 4y ago
  [dead]
- jimnotgym 4y ago
  Couldn't an aeroplanes onboard radar be the culprit?
  - wkat4242 4y ago
    That's possible yes around channel 96. Weather radars are pretty specific in frequency because they rely on water resonance.
  - Gordonjcp 4y ago
    I had to troubleshoot a microwave link about 15 years ago, where it would just stop passing traffic every so often. There wasn't any monitoring on it but if there had been they'd have spotted that it was running into DFS channel exhaustion. Of course, by the time I'd got to site the fault had cleared itself, but a bit of poking around and sitting on site all day eventually revealed the culprit. Not a bad day too, nice and sunny, drinking a coffee from the burger van, watching the boats racing up and down the loch (one end of the link was to the "timing tower", a cabin with all the timekeeping equipment at the far end and built a couple of hundred metres off shore), watching the planes stacking at the Lanark holding-point for Glasgow airport, wow look, there's three of them just circling now, isn't that cool? Anyway no signs of bother from the netwo<PLUNK> Scrolling up the logs, an error message about "cannot allocate DFS channel", and lo and behold all the channels are flagged as "RADAR interference". I dialled the DFS hold time down to five minutes from an hour, and the problem went away. The spot wasn't usually that busy with planes and they typically weren't stacked for more than about ten minutes ever.
giuliomagnifico 4y ago
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-spread-spectrum https://wyldnetworks.com/blog/hedy-lamarr-frequency-hopping-...
- sph 4y ago
  The more interesting fact is not that she was a woman, but she was an accomplished Hollywood actress with no formal training that went on to invent the channel-hopping technology used in Wifi, Bluetooth and military radar installations, among other inventions.
  - wheybags 4y ago
    And she's also a headcrab. A true polymath.
    - nier 4y ago
      Which dictionary does one need to consult for a positive definition of headcrab?
      - debugnik 4y ago
        Lamarr is the name of a crab-like alien pet in the game Half Life 2, its species known as headcrab.
- lxgr 4y ago
  Modern 802.11 versions don‘t really use spread-spectrum anymore (neither FHSS, which is used by some versions of Bluetooth, nor DSSS, which was used in the original 802.11 and up to 802.11b), but rather OFDM on fixed carrier frequencies. Switching channels due to DFS is not "frequency hopping" in that sense – that would be many orders of magnitude faster.
jacquesm 4y ago
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.
- nullify88 4y ago
  If you need a tool to diagnose situations like this, the Metageek Chanalyzer or perhaps a SDR with similar software can find such interferences.
  - slowmotiony 4y ago
    No way to do it from a regular laptop I suppose?
    - nullify88 4y ago
      Actually A while ago, it was discovered Atheros ath9k and ath10k based wifi cards have a spectrum scan feature. There was some work in reverse engineering the data format. I'm not sure if theres been similar developments for recent devices. https://www.bastibl.net/ath9k-spectrum-scanning/ https://www.bastibl.net/ath9k-spectrum-scanning/
    - wkat4242 4y ago
      You'll need a wi-spy dongle, SDR or something similar. A standard wifi card won't identify any interference it doesn't recognise as a wifi signal.
      - nier 4y ago
        SDR stands for software-defined radio. Goes best with a software-defined antenna. Source: https://en.wikipedia.org/wiki/Software-defined_radio https://en.wikipedia.org/wiki/Software-defined_radio
McNutty 4y ago
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.
- suction 4y ago
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- walrus01 4y ago
  In my experience as an ISP 95% of small businesses have no money or inclination to hire someone such as yourself to architect their LAN, sadly, resulting in worse results for them. They just buy whatever has more spikey antennas at best buy and hope it works.
- wheybags 4y ago
  As a total amateur when it comes to electronics, let alone radio, what do you think would be the consequences if one were to hack their openwrt router to ignore dfs and just use the reserved channels without checking for radar? Not on a huge corporate network, just something like a single suburban home. Would the interruptions from interference of radar be less intrusive than the drops from changing channel? And is that sort of violation the kind of thing your local regulator would be scanning for and send someone out to shut you down? Obviously it would be illegal, and I promise I'm not gonna do it whatever your answer is :p
  - quetzthecoatl 4y ago
    even if you set a dfs channel manually (editing hostapd config or cli) if you are using qualcomm/atheros or broadcomm chipsets, they will automatically move you out once it detects radar. At least with QCA/Atheros chipsets they used to have both compiletime and runtime flags to enable/disable most features (including dfs). Also, FCC fines are huge when it comes to dfs violation - as dfs spectrum is for emergency communication (military, natural disasters etc). There are radar simulators with which you can test this.
glogla 4y ago
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.
theshrike79 4y ago
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.
- knorker 4y ago
  I think Unifi resets the detector daily with a cronjob on the AP.
robocat 4y ago
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.
- dragonwriter 4y ago
  > Is it legal to spoof radar, clear the DFS channels of other users, and then use those channels for your own devices? Not in the US. While incidental interference with unlicensed use of spectrum is legal in the US, intentionally blocking, jamming, or interfering with authorized (including unlicensed) radio communication is prohibited. https://www.fcc.gov/general/jammer-enforcement https://www.fcc.gov/general/jammer-enforcement
- phoronixrly 4y ago
  Interfering with other users of ISM bands is illegal at least in my country.
- hansel_der 4y ago
  how would you limit the interference to others?
  - robocat 4y ago
    Regulations really depend on jurisdiction. For the case of WiFi (unlicensed users) in the US you see “(1) This device may not cause harmful interference, and (2) this device must accept any interference received, including interference that may cause undesired operation.". So what is harmful interference? Poorly defined, but from what I can tell, it means “Unlicensed users may not cause harmful interference to primary or secondary users”. So, interfering with other unlicensed users seems okay, so long as you never interfere with licensed users e.g. weather radar. However, there are other laws related to jamming (wilful or malicious interference) that seem likely to cover the situation. https://www.fcc.gov/general/jammer-enforcement https://www.fcc.gov/general/jammer-enforcement Perhaps you get away if caught if you could show you didn’t have criminal intent? An especially important example might be if you caused someone’s VoIP phone to not work, since that is related to “safety”. Because you only need a short signal, it would be very hard to detect, unlike noisy full-time jammers. One place to check on the legalities would be conference centres that have WiFi systems that interfere with guest hotspots - I seem to remember that Cisco||Meraki had that functionality? Presumably the manufacturers would have thoroughly vetted the legalities - especially since selling jamming devices has heavy penalties.
blackhaz 4y ago
Interesting. Does anybody know how to list available channels in FreeBSD?
- hansel_der 4y ago
  if your wifi-ap runs freebsd you probably have more serious problems
- erdewit 4y ago
  For Linux the list is here: https://git.kernel.org/pub/scm/linux/kernel/git/sforshee/wireless-regdb.git/tree/db.txt https://git.kernel.org/pub/scm/linux/kernel/git/sforshee/wir... It also shows the maximum power (in mW or dB) for the channels, which is also key to get a good reception. The dB notation is for example (20), which corresponds to 100 mW. Every 3 dB is a factor 2 in power, so (23) is 200 mW.
Deathmax 4y ago
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-on-the-5ghz-band https://draytek.co.uk/information/blog/ofcom-relax-the-rules...
- glogla 4y ago
  I use those because they are the only ones that work around here. They have lower broadcast energy and thus lower performance but better than 2.4 or getting disconnected every ten minutes.
meltedcapacitor 4y ago
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...
IYasha 4y ago
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!
wkat4242 4y ago
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
- ugjka 4y ago
  I disabled DFS CAC completely because I live in kind of rural area and Wifi congestion is not a thing here
  - Dylan16807 4y ago
    You could just set it to use channel 42 if nobody is competing with you.
deleted 4y ago
[deleted]
uxx 4y ago
sounds like a vulnerability is included
Aerroon 4y ago
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.
yupis 4y ago
Finally I found the answer to my dropped wifi connection
giomasce 4y ago
Is there a way to know how often DFS are sent in your zone?

299 points by arm 4y ago

Quite a few devices implement Zero-wait DFS. https://docs.engenius.ai/cloud-white-papers/zero-wait-dfs#what-is-zero-wait-dfs https://docs.engenius.ai/cloud-white-papers/zero-wait-dfs#wh...

odysseus 4y ago

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

walrus01 4y ago
[flagged]
- bozhark 4y ago
  E-LON is an acronym
astrange 4y ago
Most acronyms are more meaningful as acronyms than expanded. Like this one for instance - have you actually learned anything now that you know it’s “dynamic frequency selection”? That’s not a good description of “an extra channel that sometimes isn’t available because radar uses it”.
- ahmedalsudani 4y ago
  It's way better when you learn the words. Now I understand it's about frequencies and channels. DFS to a software crowd will always expand to depth-first search. I was trying to figure out if that's what was being discussed here (with plenty of skepticism, admittedly) until reading GP.
  - astrange 4y ago
    It helps in this article title some, but I usually see it written as “DFS channel”. In which case you already know it’s a kind of channel, so maybe it’s best to think of it as “sometimes-available channel”.
  - jimnotgym 4y ago
    > It's way better when you learn the words. Not always. I don't think learning what REST stands for helped anyone understand web service APIs
    - ahmedalsudani 4y ago
      Yeah I'm specifically responding to the question above. Some things like REST/HTML/XML are familiar enough that the initialism is as good as a dictionary word. SOAP, on the other hand, I forget what that was. Hopefully it's not important.
      - ziml77 4y ago
        It’s not about being familiar. It’s that “Representational State Transfer” is as meaningless as REST to someone who doesn’t know what REST is. (And honestly, even to those who do know what it means)
- tleb_ 4y ago
  Another description: "dynamically picking channels to avoid radar activity". Seems more fitting with this one.
- bagels 4y ago
  It conveys a lot more information than DFS.
- marcosdumay 4y ago
  Most acronyms are more meaningful when you don't have dozens of them conflicting on the same 3 letter string on the same area of knowledge.
BramEsposito 4y ago
It’s right there in the Background section: > This mechanism is known as Dynamic Frequency Selection (DFS) and is designed to mitigate interference to 5GHz radar by WLANs.
bee_rider 4y ago
Collision with “depth first search.” Which almost works — I could at least be convinced that there was some problem traversing these networks.

xmddmx 4y ago

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.

digitallyfree 4y ago
Ah, I love 165. I've never seen it utilized even in downtown areas and I generally recommend it if you are in a congested area and are fine with the performance on 20MHz.
spmurrayzzz 4y ago
This is the most practical advice I tend to give folks, even if its a bit niche. Not everyone has a spectrum analyzer of course to verify this, but if you have a reasonable assurance that the channel is not being used and your devices are AC/AX that can make use of at least 2+ spatial streams, that more than makes up for the truncated channel width in most cases.

Jenda_ 4y ago

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_(802.11a/h/j/n/ac/ax) 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 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".

tapanjk 4y ago

> 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!

smcleod 4y ago
Oh my gosh - that's why! I always assumed that the 5Ghz radios were slower to boot their FW or something.
deleted 4y ago
[deleted]

can16358p 4y ago

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?

radicaldreamer 4y ago
Doppler weather radar shows interference from 5 Ghz networks (https://en.wikipedia.org/wiki/Dynamic_frequency_selection https://en.wikipedia.org/wiki/Dynamic_frequency_selection), but is not used in chunks of the US (don’t even exist on the west coast), but is critical in places with lots of rain storms (south, northeast, midwest).
- tetrep 4y ago
  What's your source for areas of the U.S. without doppler? It looks like NEXRAD dopplers are "everywhere" in the U.S. https://en.m.wikipedia.org/wiki/NEXRAD https://en.m.wikipedia.org/wiki/NEXRAD
  - tjohns 4y ago
    It's not for NEXRAD. The frequencies in question are used for Terminal Doppler Weather Radar (TDWR), which is a special radar used for detecting windshear and higher resolution precipitation data at select airports. See: https://en.wikipedia.org/wiki/Terminal_Doppler_Weather_Radar https://en.wikipedia.org/wiki/Terminal_Doppler_Weather_Radar
retrac 4y ago
My Bluetooth headphones stop working around the train station at rush hour. While the coding used is spread spectrum and many devices can use the same frequencies, each transmitter lowers the overall effective signal-to-noise ratio for all other users in the area. At the train station with thousands of radios transmitting on the same frequencies within a stone's throw, there is so much noise my headphones can't hear my smartphone despite being one metre away from it. Same idea with radar aimed at an urban area. Thousands of WiFi devices in a high rise will add up. Tens of watts, hundreds of watts of noise. A large and dense downtown core will be transmitting kilowatts of radio noise from millions of devices. More noise means less SNR. And less SNR in radar means a blurry/noisy radar view, and lower effective range.

mmwelt 4y ago

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-using-hidden-ssids https://badfi.com/blog/2016/2/15/yet-another-reason-avoid-us...

japanman425 4y ago
You didn’t read the article did you. It makes no difference.
lxgr 4y ago
Hidden SSIDs use beacon frames too, as far as I remember. They just don‘t broadcast their SSID in it, but clients can see them and then interrogate them ("are you part of SSID xyz?").
jeroenhd 4y ago
The correction at the bottom of the article you linked indicates it's not that much of as problem to use hidden SSIDs onn DFS. The 100ms beacon interval latency mentioned isn't going to be much of a problem with how slow and unresponsive the WiFi stack on most devices is anyway.

dataflow 4y ago

How does a Wi-Fi chip know what country it's operating in to know what the relevant DFS channels are?

e2le 4y ago
It'll have a region set by the manufacturer in it's firmware. IEEE 802.11d-2001 also added country information to AP beacons and probes. Nearby access points will advertise a country code to each other. Hostapd and/or the Wi-Fi device firmware can take advantage of that. [1]: https://en.wikipedia.org/wiki/IEEE_802.11d-2001 https://en.wikipedia.org/wiki/IEEE_802.11d-2001 [2]: https://w1.fi/cgit/hostap/plain/hostapd/hostapd.conf https://w1.fi/cgit/hostap/plain/hostapd/hostapd.conf Fun fact: Access points can also advertise a timestamp and timezone. [3]: https://openwrt.org/docs/guide-user/network/wifi/basic#bss_transition_management_frames_options_80211v https://openwrt.org/docs/guide-user/network/wifi/basic#bss_t...
Vecr 4y ago
It's either set by the user, or it's already set on the device based on the market the product is selling into. Phones have GPS though, so I'm not sure how that factors into it.
- sgtnoodle 4y ago
  It's usually programmed into an EEPROM inside the device. Nowadays for compliance reasons, the radio firmware may enforce it rather than the software driver.
mjg59 4y ago
APs are normally programmed based on the market they're sold into. Client devices receive information about which regulatory domain they're in from the AP they're connecting to.
- meltedcapacitor 4y ago
  Can't imagine mom and pop shop on aliexpress is programming APs individually based on the recipient address.

Gigachad 4y ago

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.

sgtnoodle 4y ago
Many APs have buggy radio firmware that won't ever be fixed. The manufacturers hastily implemented DFS to pass FCC certification, which was a lot easier to do than implement it to work robustly. Due to detection false positives, the radios will only use any given DFS channel for a few minutes at a time. If you have the AP set to "auto", then it probably just needlessly hops around channels every few minutes; you get a brief latency spike and it's mildly annoying. If you have the AP set to a specific channel, then your wifi frequently cuts out and you go insane. It's interesting. If you're a typical consumer and use default settings, you end up with a mostly working setup that's glitchy but not quite broken enough for anyone to notice because Netflix still works. If you're a tech enthusiast and tweak seemingly harmless settings that should unambiguously improve things, you end up with a completely broken network.
- spmurrayzzz 4y ago
  I've worked with vendors for years to debug/fix that class of bugs you reference. Cherry picking one example very recently, we discovered a certain router model shipping in calibration mode such that only 1-2 STAs could transmit at any given time. Essentially rendering an AC router into the performance class of a mixed-mode a/b/g router from 2004. Working with the upstream vendors to fix this took several weeks. Sometimes though its not even phy or mac layer bugs we find, its more banal nonsense like a poorly configured firewall that eats up all the NPU clock cycles when it sees a 30 pps rate of unsolicited UDP ingress. Or my personal favorite was the erase block management on flash that would remount the data partition as read-only whenever a single ECC cycle would fail on a given sector and permabrick the unit until reboot.
  - sgtnoodle 4y ago
    Any recommendations for modern access point hardware that is well made?

noipv4 4y ago

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!!

bozhark 4y ago
This just explained the massive headache I’ve had with my home network. Thanks!
spaniard89277 4y ago
In my home network here in Spain I just turned off all my ISP router wireless and got two 2.4 APs with OpenWRT by ethernet in different parts of the flat. There's a lot of noise but honestly I'm fine with ~30mbps, and I don't get all the weird stuff that comes with 5ghz. Everything that really needs speed is connected by ethernet anyway.
- newZWhoDis 4y ago
  Meanwhile I run 5ghz only, completely killing the 2.4ghz band because it’s so noisy.
  - martinald 4y ago
    Agreed, even in a fairly low density street 2.4GHz is totally useless as it is so noisy, especially outdoors. 5GHz actually has better range due to the noise. Hopefully Wifi6E equipment will come down further in price.
avidiax 4y ago
You should see if the SRD (25mW) channels are allowed in Switzerland. They are not in BAKOM's WLAN factsheet, but I think they are supposed to gradually align with ETSI/ECC, so I expect they will eventually become available. They are low power, but unused, so they perform well.

ThomasBb 4y ago

https://www.theregister.com/2014/05/20/getting_wifi_glitches_out_of_weather_radar/ https://www.theregister.com/2014/05/20/getting_wifi_glitches... Has an example of what interference may look like on the radar side…

the_mitsuhiko 4y ago

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.

qwertywert_ 4y ago

6GHz is coming thankfully.. wifi channel planning especially across regions is rough.

silisili 4y ago
6ghz has pretty poor range. I had to pull my routers off that backhaul.
- linsomniac 4y ago
  Poor range means less interference from neighbors, which I've found is a good thing. I'm pretty sure I have a neighbor with a leaky microwave or cordless phone that wipes out 2.4 intermittently. Remember: interference isn't all WiFi.
  - silisili 4y ago
    It does, and that's generally a good thing. But I have 2 routers in a mesh, about 50 or so feet away, that work serviceably in my house on 5g, and poor connection on 6g. I think in the future people are going to need a lot more nodes. Like, twice as many. But hey, fast speeds and less interference.
    - linsomniac 4y ago
      I've been at the "have one AP in every air space where we make significant use of WiFi" for ~5 years. My 1700sqft home has 4 Google mesh nodes, and I've moved towards hard wiring them. Has really improved the wifi quality.
    - bozhark 4y ago
      So, note to new IoT device makers… give it an AP
      - octotoad 4y ago
        Ugh. No, thanks.

walrus01 4y ago

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.

suction 4y ago
[dead]
jimnotgym 4y ago
Couldn't an aeroplanes onboard radar be the culprit?
- wkat4242 4y ago
  That's possible yes around channel 96. Weather radars are pretty specific in frequency because they rely on water resonance.
- Gordonjcp 4y ago
  I had to troubleshoot a microwave link about 15 years ago, where it would just stop passing traffic every so often. There wasn't any monitoring on it but if there had been they'd have spotted that it was running into DFS channel exhaustion. Of course, by the time I'd got to site the fault had cleared itself, but a bit of poking around and sitting on site all day eventually revealed the culprit. Not a bad day too, nice and sunny, drinking a coffee from the burger van, watching the boats racing up and down the loch (one end of the link was to the "timing tower", a cabin with all the timekeeping equipment at the far end and built a couple of hundred metres off shore), watching the planes stacking at the Lanark holding-point for Glasgow airport, wow look, there's three of them just circling now, isn't that cool? Anyway no signs of bother from the netwo<PLUNK> Scrolling up the logs, an error message about "cannot allocate DFS channel", and lo and behold all the channels are flagged as "RADAR interference". I dialled the DFS hold time down to five minutes from an hour, and the problem went away. The spot wasn't usually that busy with planes and they typically weren't stacked for more than about ten minutes ever.

giuliomagnifico 4y ago

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-spread-spectrum https://wyldnetworks.com/blog/hedy-lamarr-frequency-hopping-...

sph 4y ago
The more interesting fact is not that she was a woman, but she was an accomplished Hollywood actress with no formal training that went on to invent the channel-hopping technology used in Wifi, Bluetooth and military radar installations, among other inventions.
- wheybags 4y ago
  And she's also a headcrab. A true polymath.
  - nier 4y ago
    Which dictionary does one need to consult for a positive definition of headcrab?
    - debugnik 4y ago
      Lamarr is the name of a crab-like alien pet in the game Half Life 2, its species known as headcrab.
lxgr 4y ago
Modern 802.11 versions don‘t really use spread-spectrum anymore (neither FHSS, which is used by some versions of Bluetooth, nor DSSS, which was used in the original 802.11 and up to 802.11b), but rather OFDM on fixed carrier frequencies. Switching channels due to DFS is not "frequency hopping" in that sense – that would be many orders of magnitude faster.

jacquesm 4y ago

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.

nullify88 4y ago
If you need a tool to diagnose situations like this, the Metageek Chanalyzer or perhaps a SDR with similar software can find such interferences.
- slowmotiony 4y ago
  No way to do it from a regular laptop I suppose?
  - nullify88 4y ago
    Actually A while ago, it was discovered Atheros ath9k and ath10k based wifi cards have a spectrum scan feature. There was some work in reverse engineering the data format. I'm not sure if theres been similar developments for recent devices. https://www.bastibl.net/ath9k-spectrum-scanning/ https://www.bastibl.net/ath9k-spectrum-scanning/
  - wkat4242 4y ago
    You'll need a wi-spy dongle, SDR or something similar. A standard wifi card won't identify any interference it doesn't recognise as a wifi signal.
    - nier 4y ago
      SDR stands for software-defined radio. Goes best with a software-defined antenna. Source: https://en.wikipedia.org/wiki/Software-defined_radio https://en.wikipedia.org/wiki/Software-defined_radio

McNutty 4y ago

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.

suction 4y ago
[dead]
walrus01 4y ago
In my experience as an ISP 95% of small businesses have no money or inclination to hire someone such as yourself to architect their LAN, sadly, resulting in worse results for them. They just buy whatever has more spikey antennas at best buy and hope it works.
wheybags 4y ago
As a total amateur when it comes to electronics, let alone radio, what do you think would be the consequences if one were to hack their openwrt router to ignore dfs and just use the reserved channels without checking for radar? Not on a huge corporate network, just something like a single suburban home. Would the interruptions from interference of radar be less intrusive than the drops from changing channel? And is that sort of violation the kind of thing your local regulator would be scanning for and send someone out to shut you down? Obviously it would be illegal, and I promise I'm not gonna do it whatever your answer is :p
- quetzthecoatl 4y ago
  even if you set a dfs channel manually (editing hostapd config or cli) if you are using qualcomm/atheros or broadcomm chipsets, they will automatically move you out once it detects radar. At least with QCA/Atheros chipsets they used to have both compiletime and runtime flags to enable/disable most features (including dfs). Also, FCC fines are huge when it comes to dfs violation - as dfs spectrum is for emergency communication (military, natural disasters etc). There are radar simulators with which you can test this.

glogla 4y ago

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.

theshrike79 4y ago

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.

knorker 4y ago
I think Unifi resets the detector daily with a cronjob on the AP.

robocat 4y ago

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.

dragonwriter 4y ago
> Is it legal to spoof radar, clear the DFS channels of other users, and then use those channels for your own devices? Not in the US. While incidental interference with unlicensed use of spectrum is legal in the US, intentionally blocking, jamming, or interfering with authorized (including unlicensed) radio communication is prohibited. https://www.fcc.gov/general/jammer-enforcement https://www.fcc.gov/general/jammer-enforcement
phoronixrly 4y ago
Interfering with other users of ISM bands is illegal at least in my country.
hansel_der 4y ago
how would you limit the interference to others?
- robocat 4y ago
  Regulations really depend on jurisdiction. For the case of WiFi (unlicensed users) in the US you see “(1) This device may not cause harmful interference, and (2) this device must accept any interference received, including interference that may cause undesired operation.". So what is harmful interference? Poorly defined, but from what I can tell, it means “Unlicensed users may not cause harmful interference to primary or secondary users”. So, interfering with other unlicensed users seems okay, so long as you never interfere with licensed users e.g. weather radar. However, there are other laws related to jamming (wilful or malicious interference) that seem likely to cover the situation. https://www.fcc.gov/general/jammer-enforcement https://www.fcc.gov/general/jammer-enforcement Perhaps you get away if caught if you could show you didn’t have criminal intent? An especially important example might be if you caused someone’s VoIP phone to not work, since that is related to “safety”. Because you only need a short signal, it would be very hard to detect, unlike noisy full-time jammers. One place to check on the legalities would be conference centres that have WiFi systems that interfere with guest hotspots - I seem to remember that Cisco||Meraki had that functionality? Presumably the manufacturers would have thoroughly vetted the legalities - especially since selling jamming devices has heavy penalties.

blackhaz 4y ago

Interesting. Does anybody know how to list available channels in FreeBSD?

hansel_der 4y ago
if your wifi-ap runs freebsd you probably have more serious problems
erdewit 4y ago
For Linux the list is here: https://git.kernel.org/pub/scm/linux/kernel/git/sforshee/wireless-regdb.git/tree/db.txt https://git.kernel.org/pub/scm/linux/kernel/git/sforshee/wir... It also shows the maximum power (in mW or dB) for the channels, which is also key to get a good reception. The dB notation is for example (20), which corresponds to 100 mW. Every 3 dB is a factor 2 in power, so (23) is 200 mW.

Deathmax 4y ago

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-on-the-5ghz-band https://draytek.co.uk/information/blog/ofcom-relax-the-rules...

glogla 4y ago
I use those because they are the only ones that work around here. They have lower broadcast energy and thus lower performance but better than 2.4 or getting disconnected every ten minutes.

meltedcapacitor 4y ago

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...

IYasha 4y ago

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!

wkat4242 4y ago

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

ugjka 4y ago
I disabled DFS CAC completely because I live in kind of rural area and Wifi congestion is not a thing here
- Dylan16807 4y ago
  You could just set it to use channel 42 if nobody is competing with you.

deleted 4y ago

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uxx 4y ago

sounds like a vulnerability is included

Aerroon 4y ago

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.

yupis 4y ago

Finally I found the answer to my dropped wifi connection

giomasce 4y ago

Is there a way to know how often DFS are sent in your zone?