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Wi-Fi congestion, especially in the 2.4GHz range, is a serious problem in some areas. It is widespread enough that there are many guides to choosing a less congested channel. E.g. https://www.howtogeek.com/197268/how-to-find-the-best-wi-fi-channel-for-your-router-on-any-operating-system/

Given that most routers default to automatically choosing their channel and the hardware seems capable of detecting conflicting networks, why don't they do a better job of channel selection?

Spiff
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insysion
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    Solution. Use 5GHz not 2.4. – Tetsujin Apr 05 '18 at 18:26
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    @Tetsujin That's a bit blunt, not really a solution. – Ultrasonic54321 Apr 05 '18 at 18:27
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    The problem is the environment can keep changing and it depends on the access point how often it checks the best channel. If it's just at boot and the device isn't restarted often, then it could be months between setting the best channel. So it depends on the device. – HelpingHand Apr 05 '18 at 18:32
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    @Ultrasonic54321 - it pretty much solves everything. If you live in a high-density urban environment, just check how many access points are on 2.4 & how many on 5. Here it's about a 50:1 ratio. I'm the only person in 'view' on 5, the other 50 I can see from here never changed it up from 2.4. – Tetsujin Apr 05 '18 at 18:45
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    Everyone has a different situation. Your remarks are not wrong, but they aren't right either. – Ultrasonic54321 Apr 05 '18 at 18:53
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    @Ultrasonic54321, I think **Tetsujin**s comment has merit, since the issue is not channel selection by APs but rather the overpopulation of the 2.4 GHz band, which is not the case with the 5 GHz band. – Robert Riedl Apr 06 '18 at 06:32
  • @RobertRiedl Even the most overpopulated areas have a least congested band, according to OP, the AP picks a congested space, which it shouldn't. – Ultrasonic54321 Apr 06 '18 at 07:01
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    @Ultrasonic54321 as other answers below pointed out, channel selection is not a top priority for most APs. But even if it were, you'd have to find a middle ground or otherwise your WiFi would constanly restart, as all the 10s of networks around you switch channels in an endless effort to find the least congested one, since there are so few available ! – Robert Riedl Apr 06 '18 at 07:06
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    @RobertRiedl You do have a point. – Ultrasonic54321 Apr 06 '18 at 07:09
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    @Tetsujin I wish I could, but 5ghz penetration is pretty naff, – Journeyman Geek Apr 06 '18 at 11:40
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    @Tetsujin Congratulations on posting the single most stereotypical SE post I've ever seen. Never before has someone managed to cram so much unhelpful smug into the smallest possible comment. – Monica Apologists Get Out Apr 06 '18 at 12:57
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    @Adonalsium Is that a Churchill reference?! Jokes aside, "How do I do X?" - "Do Y! Don't even try to do X!" - "But I can't do Y, because Z" - "Then remove Z" - "But I have to invest a lot of money and time!" is a very common pattern on all S.E.. Heck, it's even one of the most common complaints over on /r/ProgrammerHumor – Manchineel Apr 06 '18 at 14:10
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    To defend my quick quip still further - though frankly it wasn't meant to be anything more than "think outside the box" - In my hugely overcrowded locale, I swapped 3 x 2.4 GHz on 3 floors for 1 x 5GHz on the middle floor & have much better speed & coverage than I had before. YMMV. "Why" questions rarely gain such traction, I'm surprised this one managed to. – Tetsujin Apr 06 '18 at 15:57
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    Tetsujin is correct. The root issue in most neighborhoods isn't that APs aren't good at picking out open space, it is that there is NO open space where most of the APs reside. The correct answer is, then, to challenge the assumption and get to the root, which is precisely what Tetsujin did. What has not yet been mentioned is that a (thankfully shrinking) number of consumer devices don't yet support the 5Ghz channel (that's why my APs are on the crowded 2.4). – music2myear Apr 06 '18 at 17:47
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    @music2myear - agreed. When I swapped one of the old 2.4 APs for a 5, I made sure it could do both. Initially I ran 5 for everything that could handle it & kept 2.4 for the few devices that couldn't. As they gradually were obsoleted, I eventually could switch off the 2.4 once & for all & actually spare the neighbours' airwaves a bit by removing the 2 older APs entirely. We now live in a completely interference-free zone, whilst everybody else is still fighting over the 2.4 bandwidth. – Tetsujin Apr 07 '18 at 09:29
  • @Tetsujin It depends on the size of your home, and the quality of construction and materials. The 5Ghz band does not cross well walls. See the image in my answer. – Rui F Ribeiro Apr 08 '18 at 23:01
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    "Wear red-tinted glasses" is not a solution to "Why is the sky blue?" – leewz Apr 10 '18 at 04:51
  • @leewz but it *is* an answer to "how do I shield my eyes from the suffusion of blue?" – Will Crawford Apr 10 '18 at 09:05
  • @WillCrawford I see no such question being asked here. It'd be better to argue that a question of "why" doesn't belong in Super User SE, which is in the spirit of "how". – leewz Apr 11 '18 at 02:17
  • And I see noone asking why the sky is blue... but this is an XY-problem-style question, and "switch to red glasses" might be a good answer to "blue light is keeping me awake .... Why is the sky blue?" – Will Crawford Apr 11 '18 at 05:04
  • Or, "why is the spoon in my cup so bad at avoiding my eye when I drink tea?" is perhaps well answered "try taking the spoon out!" – Will Crawford Apr 11 '18 at 05:05

6 Answers6

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The failure of Wi-Fi APs to pick 2.4GHz channels well comes down to a small handful of issues:

  • Most only pick a channel at boot time, but a channel that was good when the AP was last rebooted may have become a poor choice days, weeks, or months later.
  • Most do not want to delay booting by spending long enough to truly evaluate every channel, so they use poor heuristics like "just pick the channel where we see the fewest APs", which doesn't necessarily correlate to which channel will provide the best throughput and reliability. Even worse, these oversimplified heuristics can cause problems like choosing a channel that partially overlaps with channels other APs are on, which will cause APs to interfere with each other without being able to cooperate with each other like they would if they were on the exact same channel.
  • Most don't even have the spectrum analyzer hardware necessary to truly evaluate the RF interference on each channel; they have Wi-Fi radios and focus on interference from other Wi-Fi devices, and are fairly ignorant of interference caused by non-Wi-Fi devices such as Bluetooth, microwave ovens, cordless phones, wireless subwoofers, baby monitors, wireless cameras, and more.
  • Creating an AP that has the hardware and the algorithms to choose channels well not just at boot, but to keep re-evaluating the channel choices later, and change channels when there would be benefit to do so, is both expensive and fraught with potential interop problems. Not all clients are great at honoring channel switch announcements from the AP, so an AP that changes channels on the fly risks having clients fall off the network every time it does so.
Spiff
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    And firmware developers are too chicken to lock the choices to 1/6/11. – chrylis -cautiouslyoptimistic- Apr 05 '18 at 23:25
  • Re: awful devices not following the 1/6/11 rule, is there perhaps a way to generate interference that prevents them from selecting any other channel? – R.. GitHub STOP HELPING ICE Apr 06 '18 at 00:39
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    Add in that the channel congestion where the router is situated may differ from the channel congestion where your end device is located. – Gary Apr 06 '18 at 02:27
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    @detly 1/6/11 are the only channels you should be using because they are the channels that don't overlap at all. If, for example you choose 3 because there is a lot of people on 1 and 6, you are now congesting everything between 1 and 6. I think. – Mattwmaster58 Apr 06 '18 at 03:22
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    @R.. Not legally in the US. The FCC prohibits transmitting interference intentionally. The penalties are pretty severe. – Monica Apologists Get Out Apr 06 '18 at 12:59
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    @Adonalsium Ah yes, that sticker I see on literally every device with some sort of transmitter or receiver. "This device complies with Part 15 of the FCC Rules. Operation is subject to the following two conditions: (1) this device may not cause harmful interference, and (2) this device must accept any interference received, including interference that may cause undesired operation." – Captain Man Apr 06 '18 at 13:09
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    @CaptainMan Any time the batteries died in my Gameboy while in the car I'd end up reading that label over and over. – Monica Apologists Get Out Apr 06 '18 at 13:14
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    @chrylis: In Germany a few more channels are allowed and then you have devices set for the international market and for the German market. In Germany doing 1/5/9/13 would be doable with the more modern WLAN standards that have more narrow channels. So this is hard to do right in the firmware if people use hardware from other countries. – Martin Ueding Apr 06 '18 at 13:55
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    @MartinUeding 1/5/9/13 is much, much worse than 1/6/11. 2.4GHz channels must be 5 apart to avoid interference. – chrylis -cautiouslyoptimistic- Apr 06 '18 at 15:29
  • @Adonalsium: I'm aware of FCC regulations. Arguably though configuring a router for any channel other than 1, 6, or 11 is "transmitting interference intentionally". If you called it "guiding routers to interference-free channels" rather than "generating interference", applied for a patent for it, and got VC sponsors behind it, it would probably be perfectly "legal". ;-) – R.. GitHub STOP HELPING ICE Apr 06 '18 at 16:19
  • This is the best explanation and should definitely be marked as answer. – nurchi Apr 06 '18 at 17:24
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    @chrylis: I was on the go and could not look at my home AP configuration screen. What I probably meant is that most devices use 1/6/11 although in Germany one could also do 1/7/13. People have their devices configured according to both scheme, effectively ruining the whole thing. See [this screenshot](https://imgur.com/a/r2NWK) where one can see 6 and 7 as well as 11 and 13 being chosen often. — On a closer look it seems that I am the only one on 13, so I guess I should change this to 11. – Martin Ueding Apr 07 '18 at 16:30
  • @chrylis he said "more modern WLAN standards that have more narrow channels" ... there seem to be two specs with differing channel bandwidth indeed... they probably do not mix too well though. – rackandboneman Apr 08 '18 at 02:59
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    You need to shorten days to 'the next time you have a power cut'. Think about what happens when 80 AP's all boot up after a power cut, all see that channel 3 is free completely and all then decide to use it, until the next power cut ;) – djsmiley2kStaysInside Apr 08 '18 at 10:57
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    @Adonalsium I have a DS charger in my car. Now, how do I teach my kids to read? – Dmitry Grigoryev Apr 09 '18 at 13:46
  • @R.. you're being pendantic. By that definition, transmitting on 1/6/11 is also intentionally transmitting interference for anyone else using one of those channels. Clearly, this is not the intent of the rule. – iheanyi Apr 09 '18 at 16:41
  • @iheanyi: I was half joking, but intent and practical effect are definitely part of how the rule is applied, at least in the opposite direction. For example transmitting well-formed/valid data whose contents happen to cause stations to deauth is deemed interference, and it seems plausible that malformed transmission whose intent and effect is to improve all parties' access could be deemed non-interfering. – R.. GitHub STOP HELPING ICE Apr 09 '18 at 16:46
  • It might even be possible without any "actual" interference, e.g. just running dummy beacon-only access points on all the bad channels with max legal power and very good antennas, so they don't interfere any more than an access point necessarily does, but cause other access points to select one of the good channels. – R.. GitHub STOP HELPING ICE Apr 09 '18 at 16:48
52

The overarching problem here is that the 2.4GHz band is completely saturated in any moderately populated area. In addition, there are only 14 channels, depending on country, available to use. Out of those 14 only 3 channels don’t overlap and interfere with each other. And that is only true if the device uses only 20MHz of bandwidth and not the 40MHz bandwidth available on some access points.

All properly configured Wi-Fi routers should only use channel 1, 6, or 11 at 20MHz bandwidth. An access point stomps on the signals of any nearby access points for at least 2 channels higher and 2 channels lower from itself. Worse if it’s on 40MHz bandwidth.

When access points can see each other, on the same channel, they will cooperate and share the air space. If two access points are using nearby, but different channels, then they stomp on each other and each collision results in lost data.

Unfortunately, most modern Wi-Fi routers, for simplicity, default to auto-channel selection. However, they do not adhere to the 1, 6, or 11 rule. Instead they use a proprietary algorithm that is probably based on the usage of each channel. This causes severe and unavoidable interference of nearby networks, practically rendering the 2.4GHz band useless in some areas. In addition, the auto-channel selections usually only happen during a reboot or rarely at all. So the channel selection can quickly become stale as nearby access points also jump channels and compete to find the “cleanest” channel. To make things worse, the channel selection is based on what the AP hears, and not what the client hears, which may be closer to a different set of APs.

So, the problem is not the selection mechanism, but the fact that the 2.4GHz band is just completely saturated. Not only by Wi-Fi access points, but by cordless phones, microwaves, Bluetooth, baby-monitors, wireless cameras, and any number of other technologies.

The answer is to use the 5GHz band. There are dozens of 5GHz channels available. None of which overlap with others if the standard 20MHz bandwidth setting is used. This means that all devices using the 5GHz band can cooperate with each other without interfering. Unfortunately, Wireless-N and especially Wireless-AC allow for wider channels which overlap in an attempt to provide greater throughput. So, even in the 5GHz band, you should be conscious of co-channel interference and choose your settings wisely, rather than utilizing auto-channel selection.

In a densely populated area, the use of wide channels will provide little, if any benefit and could actually make things worse.

Nayuki
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Appleoddity
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    Suggesting people use tiny old 20MHz channels in 5GHz is poor advice unless you warn them that it'll cut their 802.11ac performance to less than a quarter of what it could be. – Spiff Apr 05 '18 at 19:11
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    Also, what the AP sees/hears is different to what the AP clients here, so it is making decisions on slightly wrong information. – davidgo Apr 05 '18 at 19:26
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    @davidgo Agreed. Channel selection should be planned. Auto selection only causes issues. However, the 5Ghz band eliminates a lot of it. – Appleoddity Apr 05 '18 at 19:38
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    @Spiff I appreciate that it might reduce performance. But minimizing co-channel interference is one of the biggest goals in wifi planning. In any urban, or moderately populated area, this is going to mean using 20Mhz bandwidth channels. Attempting to use anything more will either make things worse, or not do much of anything at all. 802.11AC can use per-frame bandwidth decisions to avoid co-channel interference but is rendered useless by wireless-N devices. Ultimately, you will see little difference trying to use larger bandwidth channels in dense areas. – Appleoddity Apr 05 '18 at 19:46
  • ...And the problem - stomping on anything else extra-hard, bundling to try and dominate all channels just in case ... is gladly being sold as a solution by AP makers :) – rackandboneman Apr 06 '18 at 12:08
  • Interestingly, in my area I see several 5GHz wireless routers which have all managed to choose the same channel via whatever flawed auto-selection process they use. – Michael Hampton Apr 08 '18 at 02:10
  • There are 4 non overlapping channels. 1,5,9,13. Suggesting 1,6,11 is actually bad, as it makes 1 channel less available. – user3549596 Apr 08 '18 at 03:57
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    @user3549596 Not true in the slightest. The minimum channel bandwidth is 22Mhz and each channel, except for 14, is 5Mhz apart. Using your suggestion there would be 2Mhz of overlap on both sides of the channel. You won't find a single, accurate reference suggesting anything different, but I'm happy to read it if you can provide it. 1, 6, and 11 is well known. If you live in Japan your additional option is channel 14 which doesn't overlap channel 11 due to it's 12Mhz spacing from channel 13. – Appleoddity Apr 08 '18 at 04:13
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    22 MHz is only for 802.11b compatibility, which you don't need anymore (hopefully). So 20 MHz still stands correct. You can read a recommendation from RTR: https://www.rtr.at/de/tk/Spektrum2400MHz – user3549596 Apr 08 '18 at 04:18
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    @user3549596 Again, I can understand the confusion, including that of whoever wrote the article you linked to. But, it is simply not true. The 802.11g standard calls for 25Mhz of separation between occupied channels. FM wireless signals cause interference beyond a 20Mhz or 22Mhz bandwidth. What is important is that the interfering signal is sufficiently attenuated below the good signal. You might be able to get away with 1,5,9, and 13 - but only if the other cells are spaced sufficiently apart. https://en.wikipedia.org/wiki/IEEE_802.11#Channel_spacing_within_the_2.4_GHz_band – Appleoddity Apr 08 '18 at 04:34
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    The sources are down. It seems very conservative to use 25 MHz spacing, when the actual channel is using 16.25 with the rest being guard band already. The reality is different, where 1,5,9,13 is actually usable. The other sources in that paragraph are either old (Cisco using 22 MHz) or unavailable or don't test 1,5 (or similar) channels. – user3549596 Apr 08 '18 at 10:55
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    Read this by chance, when I suddenly remembered that my connection went from 10 to 1 Mbps the other day and that it never recovered. I attributed it to old hardware. Just now tried changing from channel 11 to 6 and 1... yep. WRT54GS still going strong. – Andreas Apr 10 '18 at 01:43
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Just adding a visual representation about the 2.4GHz congestion vs the 5GHz band to the already excellent answers.

I live in an European capital with a strong Internet and Wifi market penetration.

Furthermore, most local ISPs also add an extra roaming SSID/network by default in their router/modems/CPE and so often it is at least 1 SSIDx2 per home/neighboor. Do keep in mind, that besides the APs broadcasting signals, clients also broadcast.

So as an example, only listening with a normal notebook without any amplification in a fixed point in my bedroom, without walking around home, I can see at least 136 SSIDs (around 70-90 APs). It would not be a long stretch that led me to suspect I might have around me aprox. 200 equipments (APs+clients) broadcasting signals in the 2.4GHz band.

Compare the graphics in the left side, 2.4Ghz, with the right side, in the 5GHz band.

wifi

Rui F Ribeiro
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As Spiff mentioned, channel selection is usually only done during boot time, as periodic re-evaluation of utilization of alternative channels requires additional or better hardware. There is also no accepted standard on how APs should cooperate when selecting their channel. What would happen if all the APs in an area suddenly see that channel 6 is being less utilized than channels 1 and 11? Right. A few seconds later channel 6 has become unusable, and every AP is jumping back to channels 1 and 11...leaving channel 6 open as the prime target for the next AP invasion.

In the 5GHz band, Dynamic Frequency Selection (DFS) may be required for some channels (channels 52-64 and 100-140 in Germany and the USA). This is, however, not meant to improve the cooperation of APs, but to prevent APs to affect weather radars. An AP using DFS has to constantly monitor the channel for weather radar and, if it detects something which could be a weather radar, has to leave that channel immediately (typically switching to a channels from 36 to 48, as these are not used for weather radar and do not require DFS...in other words, the AP does not select the best alternative channel, but rather just a channel which is guaranteed to be safe from weather radar).

It may be possible that some manufacturers of APs have algorithms which can optimize the channel assignment when an area is covered by a number of their (and only their) APs. A "Rogue Access Point" (which is not taking part in this optimization process) can significantly disturb the network. Some companies periodically perform hunts for Rogue APs on their premises.

Klaws
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In a high-congestion area where there are dozens of APs on 2.4GHz channels 1, 6 and 11, I sometimes get a more reliable connection by forcing 802.11b (the slowest mode), especially on less-used channels like 4 and 8. The bandwidth overlap diagram from wikipedia (below) suggests tantalizing clues why this might work, since the round bandwidth profile of 802.11b (DSSS) makes it look like it would care the most about the middle of its own channel even if overlapping channels were present. Of course this approach is too zany for the router to do on its own. Your mileage may vary.

enter image description here

krubo
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  • It is a good idea to disable 802.11b ; when you have at least a device talking that protocol, all the remaining 2.4GHz devices sharing the same medium of connection are downgraded to 802.11b. – Rui F Ribeiro May 22 '20 at 08:10
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The real reason is that 2.4gHZ is a junk band and it never should have been used for anything. And the reason is it is a junk band is that it is the same frequency as the water molecule, it resonates. That is why radio astronomers use the band extensively searching for exoplanets and nebula. Corporate didn’t want the band because they knew it was useless. So thanks to the corporate toadyness of the FCC, 2.4ghZ became public domain by default. Sort of like the town dump being inagurated as a city park, but without the improvements.

The problem of the water molecule can’t be understated. Anything wet will interfere, including humans, dogs, house plants, microwave ovens, aquariums, snow and plastic water pipes. Competing transmitters cause “interference bubbles” that wander around over the course of minutes. There is no solution to this wandering, its part of the resonance of 2.4 with the water molecule. The more competing transmitters in your neighborhood the worse the wandering gets.

Sorry to say it but the only solution is really to move up the spectrum to 5.6.

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    Actually, 2.4GHz was largely chosen because it was already an ISM band and was low enough in frequency to make the RF chipsets cheap. The water thing is not actually true, water having many, many resonant modes, most of them fairly low Q. The absorption bands do not become a major issue until you hit the oxygen one at about 60GHz which can cost you 15dB per km of increased path losses. 2.4GHz was a compromise based of magnetron size, harmonic considerations and some experiments on penetration into foodstuffs back in the 1940s, an oven works perfectly well anywhere between about 1 and 20GHz. – Dan Mills Apr 06 '18 at 17:18
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    There is usefulness to having bands that get absorbed. The whole point of having low power APs in such a band is that they're not going to interfere with others too far away. Unfortunately, there's a ton of stuff in that band. – Brad Apr 07 '18 at 23:47
  • Let me quickly check the resonant frequency of uncondensed water vapor...right...: "The peak is 10GHz to 50GHz depending on temperature." I think that the resonant frequency at room temperature is around 22GHz. This resonance doesn't affect WiFi much, but...it affects 5GHz signals more than 2.4GHz signals. Even worse for 5.6GHz and higher, obviously. Note that 5GHz WiFi has (at the same output power) a smaller range than 2.4GHz WiFi. However, the limited range of 2.4 and 5GHz WiFi is beneficial...as a longer range would mean that you'd pick up **more** interference from neighbors. – Klaws Apr 11 '18 at 08:19