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

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

wifinigel.blogspot.com

299 pointsarm182 comments

> 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!

Oh my gosh - that's why! I always assumed that the 5Ghz radios were slower to boot their FW or something.

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.

In the software world we call the overall phenomenon in the article a leaky abstraction.

We all pretend that WiFi is this magic thing that allows our devices to transmit data, but in fact there are all these little details of reality "leaking through" that abstraction.

For example, it seems that instead of power cycling everything at once, WiFi should be turned on first to ensure that there is sufficient time for 5 GHz to be available. I sometime reboot my TV, Router, WiFi, and set-top-box simultaneously by simply turning the power board they're all connected to off and on. From now on I'll do them separately and in sequence...

I started doing this about a year ago. Not because it takes longer to boot up, but because I was trying to see which single device was most often the solution.

In my setup there is a fiber modem (outside the house), a Wi-Fi router, and then all the connected devices.

Oddly, the one that I need to restart the most, seems to be my Mac. Realizing, of course, the problem could still be on the router side, but turning wifi on and off on the laptop usually solves my problem.

I rebooted one device at a time for about a year and made notes about each failure. I didn’t see the pattern until I had done this for a long time.

I am not impressed with Macbooks networking, or specifically the wireless on the Intel Macs. I've gone through three and ended up with such severe wifi issues, that I inevitably ended up switching to a wired dongle for anything serious/in need of stability.

Never really had a more fickle laptop as far as wireless goes. Only a VAIO VPCZ1 after ten years of abuse as a gaming machine/daily driver started to get close. Again though, that's after 10 years. A Macbook will manifest that behavior within the first year for me.

The frustration has gotten me to the point I'm reading up on USB/PCI-E/datasheets implementation/protocol details so I can actually try figuring out WTF could be the cause.

Restarting everything at once can break all kinds of subsystems. Even if the radio interference wasn't an issue here, you would still get high odds of them not connecting cleanly to the network because of something else.

Restarting everything should be a highly recommended test step before going to production, no matter what the system. Catches so many problems based on the assumptions that one may not fully realize. And some day it will have to be done anyway (unless the system is extremely redundant in all aspects), so best to test it sooner than later.

I’m not really sure it’s a “leaky abstraction”. Technical detail or something, I fail to see how it’s an abstraction to have this constraint in the WiFi standard.

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

No need to hack anything. There are countries where DFS does not exist. Move there, install OpenWrt, set the country code appropriately in the GUI for both radios, enjoy both the fast WiFi and the good weather.

And regarding the reasons for DFS - it is not due to the radars posing interference for access points, it is the other way round. They are supposed to be able to pick up reflections from clouds and airplanes that literally carry less than a millionth of the power that a typical AP would radiate.

Here in the Czech Republic you used to be able to see clear "wedges" on meteoradar images due to these types of interference, but it seems to be mostly solved now.

FCC fines for violating radio spectrum rules are huge, and they are authorized to confiscate equipment as well. I wouldn't recommend it.

Here is an example: https://www.fcc.gov/document/fcc-fines-hinson-dangerous-misu...

probably not the best example. the guy impersonated a first responder resulting in no one responding to a fire alarm ; https://www.arrl.org/news/fcc-fines-north-carolina-man-for-u.... https://docs.fcc.gov/public/attachments/FCC-19-82A1_Rcd.pdf

There’s also this guy: https://www.fcc.gov/document/fcc-proposes-fine-interrupting-...

If I didn't see the post I would never think it's illegal. Also the FCC does not exist outside the US so is this relevant elsewhere?

Your country very likely has a government entity that regulates radio spectrum use and has means of enforcing it.

Messing with Doppler weather radar is a usually a big thing that impacts various important services. Even in relatively backwater countries you will get someone knocking at your door if you do this.

That makes sense, but I thought I'm messing with my router to have faster WiFi and not with some weather radar that I don't even know exists

The DFS checks exist to prevent your radio from interfering with the weather radar. You’re only allowed to own a radio that could cause such interference because it has those checks.

Here’s an example of what it can look like on the weather radar side: https://www.reddit.com/r/techsupportgore/comments/lnxdji/whe...

Is this from a consumer WiFi or from some bigger antenna?

This is why spectrum in many countries requires users and/or devices to be licensed.

https://en.wikipedia.org/wiki/Ignorantia_juris_non_excusat

> the FCC does not exist outside the US so is this relevant elsewhere?

Your own country likely has an analogue. Get thee to Google. Ofcom is the UK equivalent, for instance.

> https://en.wikipedia.org/wiki/Ignorantia_juris_non_excusat

What do I care about some law student latin?

I know the analog of the FCC in my country. I have not ever heard about them knocking on anyone's door, ever so that's why I'm asking questions?

Ignorance of the law does not excuse one from breaking. There is no freebie.

The world is a bitch in that it is your oyster, right up until you start impacting something everyone else is counting on. In that case, it is not infrequent for one to be made an example of.

That law student Latin there is a reminder that in spite of you "not knowing" something, the world still turns, you are expected to do some level of research, and there are people whose job is to just search for violations, and dispense a Notice of Apparent Liability to the perpetrator.

Hell, Amateur radio operators run what they call foxhunts when they notice something odd going on, because spectrum misuse impacts everyone.

Thank you for explaining something that's not very complicated and was not relevant to my question I guess? Just because we have analogous agencies doesn't mean they are a carbon copy of each other and the radio spectrum stuff is not exacty mainstream knowledge.

Given that 5.8GHz WiFi and 5.8GHz weather radar are both licence-exempt services within an ISM band, you literally cannot pay Ofcom to take an interest.

Given the level of corruption within the current UK government, I would not necessarily agree with your use of the word literally here. But at least in law and in principle yes.

Pick a channel outside the DFS range. Not every 5GHz channel is affected, only a subset of channels is.

If you do cause interference with weather radar, there's actually a chance that the authorities take action. WiFi interference isn't hard to scan for at all, even or you disable broadcasting your SSID on the channels you're interfering on.

The interruption when the FCC knocks on your door would be pretty damn intrusive.

Radars are pretty good at locating things. So, if that rogue wifi device interferes much enough to be noticed by the radar operator on some nearby airfield, the reaction from radio authorities will likely be swift.

This is not how it works. The AP is not actually responding to or reflecting the radar ping so the range detection won't work. If it even sees it it'll show up as a line from the center of the radar screen in the direction of the AP. It won't pinpoint it. But most likely it'll just add to the noise floor.

It'll still be possible to locate it with that info but it's more work unless you manage to interfere with two radars and they can triangulate. That's pretty unlikely for an indoor access point with only a few milliwatts though.

I saw meteorologists publicly shame rogue APs by name. This implies they do have means (directional antenna) to probe the source of interference and get information such as AP name, MAC address and distance.

https://www.shmu.sk/sk/?page=2037

They wouldn't get this from the actual radar though. It must have been the result of further investigation.

I was mainly referring to the 'radars are good at pinpointing things' comment from the OP :) I wanted to clarify that an AP on this frequency won't show up as a dot on a radar screen. For the distance to be captured it must have an actual reflection.

You can even see some of the lines I mentioned in that pic!

Radar receivers have a secondary output (or at least a waveguide coupler) where you can connect arbitrary equipment. So you connect standard wifi card to it, enable monitor mode and listen to all the MAC addresses and SSIDs (if they are not hidden) - directly though the giant radar antenna. (I really need to try it with our radar, I currently only detect packets based on their wide spectrum, but I have never decoded them)

There is even a software-based wifi demodulator (https://www.nuand.com/bladerf-wiphy/) so you can try to decode it from the IQ data, but they usually don't have enough bandwidth.

The way this works is someone notices a disruption and notifies the relevant authorities, which have detector vans to track down the emission sources. And they're quite good at that, too.

Indeed, I just wanted to clarify that the radar itself won't identify you as a 2D dot just like that. It needs further investigation.

The comment mentions "something like a single suburban home". I suspect a line drawn on the map in the direction of the signal may give a good enough idea which few suburban homes may be the culprits, simplifying the task of locating it from a van.

I may overestimate the angular precision of that though.

The beamwidth of a standard weather radar is 1 degree. You can get to maybe 0.3° accuracy if you are pinpointing the maximum carefully. So this is 400m at 70km distance - which is not that bad. You then need to evaluate which parts of this sector are actually visible from your radar site (using a digital elevation model) and then search in them (I have never done this). Depending on the location, the entire 100km line may be visible...

But you need to specially do this scan and carefully evaluate the maximum (tricky if it's transmitting only once in a while). When you are scanning normally, you are pointing a bit above the ground (you are scanning the sky, of course) and the beam may have several degrees at the ground level.

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.

> Would the interruptions from interference of radar be less intrusive than the drops from changing channel?

depends, hence the checking.

> 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?

highly doubtfull

Well, you are both showing that you know it is illegal, yet asking "what would be the consequences ?". Of course if you get caught you would both have your hardware confiscated and experience a very severe fine (and possibly worse than just a fine if there were consequences in terms of interferences).

People who operate radars (in particular radars for weather forecasts) are now really upset of the recurring interferences that come from wifi devices with (illegally) disabled DFS. Keep in mind that weather forecasts is not only about consumer people watching TV to know whether tomorrow will be sunny or rainy, but is also safety-related (e.g. for flights and navigation. You don't want your flight to be caught in a storm !).

Those issues (significant interferences that are now almost impossible to solve) are periodically raising debates within RF administrations about whether to require locked hardware (locked bootloader and signed firmwares) in order to ensure that DFS cannot be disabled. That would be a pity for any embedded linux geek (openwrt is great both with regards to freedom, privacy, long-term support with security updates and therefore avoiding obsolescence, etc.), but a lot of people now think that it is the only way...

I understand the shortcomings of DFS, yet you should really understand the 5 GHz wifi band is a band dedicated to radars as primary users where wifi has been tolerated under the condition of DFS, so wifi users should really not complain too much about the consequences and limitations of DFS (if there was no DFS, there would be no 5 GHz wifi band at all ! ;)

No, there are plenty of modern weather radars now that have nothing to do with 5 GHz. This is a good example of how the FCC managing spectrum is the furthest thing possible from a good allocation; trillions of economic activity are driven by use of unlicensed bands, yet because there is nobody in particular ready to put up billions for a slice of spectrum, we end up with the FCC "design by committee" process for releasing them. That means whoever happened to be there is considered some sort of "lifetime incumbent" that must be worked around at all costs, even if their use of prime spectrum is absurdly wasteful. Challenged with the choice between "incur $ costs on few" or "incur $$$ costs on many", the FCC will consistently pick "incur $$$$ on all".

This comment above is a good example of someone having stereotypes about RF administrations without actually knowing their field of expertise... It's not the matter of "unlicensed application don't pay license fees so administrations don't want them" (even though whenever you want to move an application out of a band, someone has to pay for the new equipment. But that's another topic and various countries have various process for that), but a matter of the laws of physics : various frequency bands have various physical properties and this applies not only to radio propagation but also (maybe more importantly) to measurements related to the molecules you want to observe. There are reasons why some bands and not others are dedicated to radars (and even more : some bands are fully passive because they are required for calibration and scientific measurements). So yes there are weather radars in other bands than 5 Ghz... so what ? Different radars measure different things and they are complementary (just like having different low/high bands for mobile operators are complementary for them). Before saying that the use of spectrum is "absurdly wasteful", you should show your thorough knowledge of the topic...

Keep in mind that whenever a new application appears, detailed technical analysis and simulations have to be done with regards to the other applications in co-channel and adjacent channel/bands, both with regards to direct interference, aggregated interference taking into account the projected density of the new spectrum use, clutter/terrain, propagation models, geometries of antennas, filter characteristics, etc. And even then, administrations do their best to find the right balance and enable as many applications as possible (including unlicensed and SRD), but those are tradeoffs and sometimes administration take risks to foster innovation (and sometimes it proves it was too permissive for the new user, such as what is happening now with regards to wifi vs weather radars).

(N.B. I am not in the US, but I guess FCC works similarly to EU administrations - which I know very well).

Ensuring pilots and air traffic control can monitor for microbursts is very important. https://www.kahuna-fi.com/post/dfs-tpc-tdwr

Following the rules (DFS) is fair ask to be able to access these additional 5 GHz channels.

I certainly get why the FCC would tread lightly around "lifetime incumbents". In a civilized country people would look at the overall issues, recognize it as a collective problem, and support using shared money to reorganize things to society's long-term benefit. But in the US the incumbents would squawk about government overreach and creeping socialism and how President John Wayne gave their grandpappy that spectrum and it was their god-given right to use it without communist agitators trying to take it away, using taxpayer money to boot.

> 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.

What does this mean? The DFS check doesn’t exist? Or they just reboot nearby devices several minutes apart to allow enough time for DFS to complete?

Some chipsets can have the main cores operating in non DFS channels while one of the cores performs the CAC and only move into DFS ISM after (they implement what's called zero-wait DFS)

I was wondering the same. What does a 'competent professional' do differently here?

Not all 5Ghz channels are affected by the DFS requirement. The DFS channels are more country specific than other WiFi band frequency restrictions. Usually the best course of action (or at least the one my coworkers and myself take) is to simply deny the band plan the use of those channels.

A lot of consumer routers I've seen don't even expose the DFS channels as an option to avoid this problem (looking at you Motorola mesh products). It's not a problem just kind of funny to see.

Put simply there will be two aspects taken into account in the 5GHz Wi-Fi design.

Firstly, a coverage aspect of the design which will ensure that any given spot has a primary and secondary (and sometimes tertiary) AP radio providing appropriate RSSI.

Secondly, a channel plan that ensures that the DFS channels are not allocated to the primary and secondary radios. So if a client station's primary AP get taken offline for a minute by DFS then that station can immediately roam to the stable channel and maintain a decent RSSI.

Additionally, there may be some environments where the DFS channels are not enabled on the equipment at all. For example, many cheaper devices (think low end android phones) don't support DFS channels at all, so if there are many such devices in the area (e.g. College campus populated by poor students) then every radio running a DFS channel is effectively a coverage hole and therefore better to not use those channels at all.

obtaining the govt certifications for using DFS channels does add to the cost of the chipset so cheapest chipset simply don't do it.

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.

As someone who just "plugs and hopes" what steps should one take in building a small network - from running basic scans to see what else is in the area, strengths, and simply going "screw it lay down some cable"

What's the best practise in other words

The trick is to choose a list of potential noise-free channels in the 5 GHz band, instead of one channel. Ideally channels that require DFS in at least one country, because a lot of cheap customer-grade network equipment (other than wireless cards) has them hard-disabled. E.g. TP-Link Archer C7 cannot be set to channel 56 using the stock firmware, even though the hardware itself is perfectly capable of transmitting on this channel and of radar detection. In short, DFS channels are good because you know your neighbors will not be using them.

Then install OpenWrt (even on that TP-Link Archer C7 if you want), set "channels" to the list of channels that you identified (unfortunately this property is available only from the command line), and "channel" to auto. Then OpenWrt will select one of these channels after a one-minute delay, and jump away (but only to another known-good channel) if there is a radar.

Do you have a reference or setup guide that walks you through best practices on how to most efficiently setup a home WiFi network?

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!!

This just explained the massive headache I’ve had with my home network.

Thanks!

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.

Meanwhile I run 5ghz only, completely killing the 2.4ghz band because it’s so noisy.

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.

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.

Seems SRD channels are allowed in Switzerland:

https://www.ofcomnet.ch/api/rir/1008/12

https://kernel.googlesource.com/pub/scm/linux/kernel/git/sfo...

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.

If you need a tool to diagnose situations like this, the Metageek Chanalyzer or perhaps a SDR with similar software can find such interferences.

No way to do it from a regular laptop I suppose?

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/

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.

SDR stands for software-defined radio. Goes best with a software-defined antenna.

Source: https://en.wikipedia.org/wiki/Software-defined_radio

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

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”.

It conveys a lot more information than DFS.

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.

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”.

> 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

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.

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)

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.

Another description: "dynamically picking channels to avoid radar activity". Seems more fitting with this one.

Collision with “depth first search.” Which almost works — I could at least be convinced that there was some problem traversing these networks.

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.

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

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...

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.

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.

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.

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.

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.

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...

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.

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?").

You didn’t read the article did you. It makes no difference.

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.

I think Unifi resets the detector daily with a cronjob on the AP.

Quite a few devices implement Zero-wait DFS.

https://docs.engenius.ai/cloud-white-papers/zero-wait-dfs#wh...

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-...

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.

And she's also a headcrab. A true polymath.

Which dictionary does one need to consult for a positive definition of headcrab?

Lamarr is the name of a crab-like alien pet in the game Half Life 2, its species known as headcrab.

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.

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?

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.

Is this why there’s such shitty service when the local college is having a home game?

That's a slightly different thing with mobile cells with CDMA style techniques reducing in range when they're busy. Called cell breathing.

https://en.wikipedia.org/wiki/Cell_breathing_(telephony)

This is because CDMA is basically a cacaphony of stations screaming for attention and as a cell gets busier the furthest stations (phones) won't make it through the noise.

GSM was a lot better at this with its fixed timeslots, however as a result it had much more flexible capacity limits. From 3G onwards all standards are based on CDMA (not just for technical reasons, also because of Qualcomm's lobby who are the ones that own many CDMA patents).

The radar is looking at reflected analog signal bouncing off rain drops. I'm not too surprised an active transmitter blasting digital data can cause interference. Raw 802.11 packets can go several miles and be successfully picked up without corruption. The useful range is simply limited by the bit error rate exceeding what the forward error correction is capable of correcting. Also, each 802.11 frame preamble is transmitted at a relatively low symbol rate, maybe like 6Mbps for 5Ghz channels. Even if the data itself is at a higher rate that's spread over more spectrum, the preamble is going to maximally stick out; that's the whole point of it, to be maximally received by all radios.

Yes, because the radar is looking for a return signal from a cloud, which can be pretty faint. A powered transmitter can be more powerful than a radar return.

Doppler weather radar shows interference from 5 Ghz networks (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).

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

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

The radar will likely use Gold codes or another sequence with strong autocorrelation properties to reject any signals other than its own reflected beam. It generally won't be a problem.

This is incorrect. Wifi systems that don't comply with DFS show up brightly on the radar display. Here's an example from a real-world system of what a misbehaving 5 GHz AP looks like:

https://journals.ametsoc.org/view/journals/bams/97/7/full-ba...

The FCC has a list of WISP companies they've gone after for misconfiguring their wireless equipment:

https://www.fcc.gov/general/u-nii-and-tdwr-interference-enfo...

Interesting, I'm genuinely surprised that's necessary. WiFi looks like white noise, and should be more or less trivial for a radar to reject.

If removing “noise” was trivial, we would all have perfect radios.

What makes you think that would be trivial?

Years of engineering experience in RF and DSP applications.

A radar that can be jammed by a WiFi access point amounts to an HR failure at the company that made it.

It's not jammed, it's dealing with a high noise floor and a weak signal.

You can only do so much to identify your own signal when it's smeared over a trillion water droplets.

I think you’ve misunderstood what we are talking about.

This isn’t jamming.

If it was trivial to identify signals from high noise situations all our radios would be perfect.

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

I disabled DFS CAC completely because I live in kind of rural area and Wifi congestion is not a thing here

You could just set it to use channel 42 if nobody is competing with you.

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.

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.

> 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.

But Skype/Teams/Zoom gets visibly glitchy, which may be annoying during remote schooling or work. In the past two years, a sizeable fraction of "typical consumers" started to use real-time applications a lot.

Not to mention, while Netflix "still works", TikTok may not. Or YouTube, if you happen to switch between videos at the moment. Or Facebook or Twitter or anything that relies on the "infinite scroll" dark pattern.

Point being, regular, non-tech users definitely notice. We may think that they don't, but that's because they don't know how to frame what's going on, and have been conditioned to accept that digital technologies are just shitty and glitchy. They think it's their fault, or the problem with "their Internet", or that their computer "has viruses" - and won't tell you until you're close enough with them they feel they can vent to you, or hope you can fix it for them.

Oh for sure, it's one of the main reasons why people believe wifi sucks.

One of my favorite visualizations is Steam Remote Play's debug graph that shows the latency stack up. Playing a game over wifi on a smart phone, it can make subtle stutters significantly easier to track down. In one case, I was able to attribute a periodic latency spike to the Bluetooth radio.

I've been trying to go pretty deep understanding 802.11 the last couple years, between trying to make my home wifi setup work flawlessly, and using it extensively at work for real-time communication between embedded systems.

At home, I have converged on WRT1x00ac devices (with useless DFS). Three as APs, one as the main router, and a couple more to play around with. They can be had for $30 on ebay, have dual core ARM CPUs, and run OpenWRT well. The only reason they work well for me, though, is because I am an embedded engineer and stubborn. Recently I had to desolder failed TVS diodes from a couple of them...

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.

Any recommendations for modern access point hardware that is well made?

Hardware isn't often the issue, its usually firmware/driver related when the problems occur in my experience. So no matter what hardware you have, running something like OpenWRT et al. can be a massive quality-of-life improvement.

With that said, I've had the best luck with Broadcom chipsets over the years. I'm currently using a Netgear R8000P (which is the BCM4906 chipset), but not on stock firmware.

EDIT: should add an addendum to my initial comment that hardware and software is obviously connected because the radio vendors are writing the driver software in most cases, but my Broadcom recommendation still stands even with that throat clearing.

Cool, I just ordered one to play with. $40 on ebay. The prospect of effective beam forming is appealing. Do you run OpenWRT?

I do. yes. But the latest stock firmware on that router is surprisingly stable and performant. Not a bad idea to do some burn-in testing with that to see if it meets your needs. If you need to tweak phy layer knobs like tx power etc, you'd have to switch but otherwise there's probably not a need for most folks.

Also, open driver support for broadcom chipsets is very lean, so getting openwrt to work on the wireless side is a bit of an effort. Doable, but definitely not simple out of the box.

Are you sure you don't have an R8000 rather than an R8000P? As far as I can tell, the only alternative firmware for the R8000P is OpenWRT, and the documentation claims only 802.11b/g rates are supported. DD-WRT and Tomato, which would have better driver support, only target the R8000.

At least I had fun connecting to the console UART in order to flash the latest firmware on this R8000P. The one I bought was stuck on an older version with a bug that prevented it from accepting firmware ever again.

I got the hardware in hand. Did you need to connect a UART in order to flash OpenWRT? Any tips or links you can recommend would be appreciated.

The virtues of knowing enough to be dangerous but not enough to be safe.

Though one will never learn without at least some modicum of living dangerously.

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

to use full power on 6GHz, the router needs to upload its GPS location to a central server to get the available frequencies. Worse than 5GHz in my opinion

I assume that can be also done offline, right? Forcing an intranet device to connect to the outside just to get a list of frequencies would be less than optimal.

No, it can't. Which is precisely the problem.

There is an exemption for indoor low-power APs, but the manufacturer has to prove that the housing will not survive outdoor weather (usually rain). They have to deliberately make it not-rainproof.

Just lie

I’m not: it’s called AFC. https://www.extremenetworks.com/extreme-networks-blog/wi-fi-... https://documentation.meraki.com/MR/Other_Topics/Wi-Fi_6E_Fr... (Point 4)

It has been approved in November: https://www.rcrwireless.com/20221103/network-infrastructure/...

No, lie to the router about what frequencies and power is okay.

You don't have the crypto keys to do that.

AP firmware is 100% closed-source, vendor-signed blobs these days.

I don't know the situation in the US and I don't think EU administrations require firmwares to be locked at the moment, yet I can tell that requesting locked hardware/firmware is a recurring debate within RF administrations since there were too many abuses (cf. this thread "just tell the hardware to lie" or "just use openwrt to disable DFS" above...) and too many interferences to other services (such as wifi-DFS-disabled interfering with weather-forecast radars). The 6 GHz wifi 6E has to coexist with fixed links, and administrations obviously don't want to repeat the mistakes and issues that are happening today at 5 GHz with radars...

6ghz has pretty poor range. I had to pull my routers off that backhaul.

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.

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.

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.

So, note to new IoT device makers… give it an AP

Ugh. No, thanks.

Yeah. Imagine trying to hunt down misbehaving devices when every single one is an ap

A 6 GHz radio is pretty much equivalent to a 5-point-something GHz one.

As a counterexample, 2.4 GHz is completely unusable in our penthouse office. I'd rather deal with limited cell size than continuous interference issues.

Yes, in an apartment, more available frequencies and less ability to penetrate walls is a good thing. Otherwise there’s immense amounts of interference. And it ends up being made worse when people try to improve their own experience by turning their transmit power all the way up. It necessitates that everyone turn theirs up, which then just makes the interference return and spread further.

There's a reason wired backhaul is considered best practice.

Were you using directional antennas (or at least parabolic reflectors around them) for your backhaul?

No. I just bought an off the shelf 6e kit.

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.

Interfering with other users of ISM bands is illegal at least in my country.

> 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

how would you limit the interference to others?

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 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.

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.

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".

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.

I had to revert firmware on some of my ubnt equipment for a bit because the false positives were killing me. I live near a hospital, so maybe it was caused by the helicopters flying over? I believe that there was also a bug where once it detected "radar" it wouldn't actually recover until a nightly cron job ran on the AP, so it was very noticeable once it had happened.

I've moved to current firmware as a test and it is no longer a problem for me.

Couldn't an aeroplanes onboard radar be the culprit?

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.

That's possible yes around channel 96. Weather radars are pretty specific in frequency because they rely on water resonance.

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!

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...

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

Finally I found the answer to my dropped wifi connection

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

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.

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.

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

[2]: 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_t...

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.

Can't imagine mom and pop shop on aliexpress is programming APs individually based on the recipient address.

There's no technical mechanism to prevent someone operating an AP inappropriately. That's more an opportunity to discover how effective your local enforcement agency is.

The brand I use has a drop down menu to select from.

Also, regarding WiFi equipment. Finding out the quality of consumer WiFi gear is near impossible before buying, because of so many other factors affecting wifi performance (especially that I use mostly 2.4ghz). If anyone has a foolproof method for this please let me know.

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

For Linux the list is here: 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.

if your wifi-ap runs freebsd you probably have more serious problems

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.

sounds like a vulnerability is included

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