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The Wi-Fi only works when it's raining

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Re: The Wi-Fi only works when it's raining

#201

Post by tgsovlerkhgsel »

user_7832 wrote: post #5

TIL about Fresnel Zones! > Interestingly, objects outside the straight line between antennas can still cause interference! For best signal quality, the Fresnel zone between the antennas should be clear of obstructions. But perfection isn't achievable in practice, so RF equipment like Wi-Fi uses techniques like error-correcting codes so that it can still work without a perfectly clear Fresnel zone. I wonder if other w…

My mind was blown when I saw the 4F experiment, where a lens transforms an image into the Fourier domain. I'm not sure if it's related to the Fresnel zone (I think it is not or only very vaguely), but it's pretty amazing:

A widened beam of collimated light (i.e. parallel beams) is sent through e.g. a slide with some image printed on it. Using a lens placed one focal length away, it is focused down to a point (one focal length from the lens). One more focal length from that point, the beam will have reached its original width again, and another lens makes it parallel again, projecting it onto a screen placed one focal length from the second lens:

     |     ()    .    ()    |
    image lens point lens  screen
This will behave exactly as expected at first glance. The image will be visible on the screen (upside down IIRC) and if you hold a piece of paper into the point, you'll just see a single bright dot. However, what's actually present (due to diffraction) is the Fourier transform of the image! If you put an iris around the point, the image on the screen becomes blurry because you just filtered the high frequencies! And what's even more impressive, if you remove the center of the point (e.g. by inserting a glass slide with a small black circle in the middle), you'll get only the high frequencies, and the image on the screen will be the edges of the original image.

Re: The Wi-Fi only works when it's raining

#202

Post by chinchilla2020 »

thekevan wrote: post #176

This is a bit ridiculous. How can you know that you have a ling of sight element to you network and not check that as the very first thing when you hear about rain effecting the wifi?

Most likely a fake story. The internet is littered with blogs that make up stories like this to get engagement. After all, this one made it to the top of hackernews.

Chances of getting proof that this happened are zero

Re: The Wi-Fi only works when it's raining

#203

Post by obi1kenobi »

Dylan16807 wrote: post #174
obi1kenobi wrote: post #131

Earlier quoted context omitted.

No antenna is directional enough to overcome n^2 scaling. Especially not the mid-tier consumer-grade hardware I would have had access to at the time. Rough rule of thumb, a consumer-grade directional antenna (at least at the time, maybe they've improved in the last 10yrs) will give your signal strength a one-digit multiplier (say ~8x), meaning ~7-10dB. But that n^2 means that improvement only takes you 2-3x farther,…

Won't 20dB at each end do the job? You can get 16-17dB antennas for $35 these days. And a wire parabolic dish promising 23dB for $200. 7-10dB matches the antennas I personally use, but those antennas are smaller than my phone. You can get a lot more out of a three foot spike or a two foot dish. (I'm assuming 2.4GHz and hoping they're not lying about the gain.)

Right. This is a multiple timelines problem.

This story happened a bit over 10 years ago at this point. At the time of the story, the Wi-Fi bridge was already almost 10 years old -- we'd had the same equipment since I first remember getting internet at home.

That means the antennas at the start of the story were mid-2000s mid-tier consumer level equipment. I don't think a 16-17dB antenna would have been $35 at the time, in either mid-2000s dollars or in today's dollars. I also grew up in a country much poorer than the US, where $35 could feed a family for a week.

At the end of the post, we upgraded to 802.11n antennas (but still tech from 10+ years ago) which solved the problem by being newer, nicer, and having beamforming + MIMO capability which let them be more tolerant of obstacles (effectively, get more dB at the same emitted power level).

Re: The Wi-Fi only works when it's raining

#205

Post by obi1kenobi »

londons_explore wrote: post #107
bongodongobob wrote: post #90

I have a hard time believing this. Wifi can go through multiple walls. And if these were directional P2P links, they can easily go through and even around trees, I've deployed them in the past and they don't need perfect line of sight. Granted the equipment could have been cheap, but this sounds questionable. He's asserting that the a few leaves at the top of a tree were blocking it when it wasn't raining? Idk.

Wifi normally uses adaptive transmit power and data rates. If the signal gets a bit weaker, your link slows down from say 300 Mbps to 260 Mbps. No biggie. But sometimes for direct links you set the modulation, power and data rate fixed. The end result is that changing channel conditions can turn the link from 'working perfectly' to 'not working at all'

(Author here) For our Wi-Fi bridge, the devices on both ends were set to max power.

I don't recall them being able to change data rates very much (if at all) because the ones at the beginning of the story were 802.11g devices, and 802.11g didn't have channel bonding capability or similar tricks up its sleeve. Newer equipment definitely has more options like this.

Re: The Wi-Fi only works when it's raining

#207

Post by obi1kenobi »

OJFord wrote: post #171
obi1kenobi wrote: post #146

Earlier quoted context omitted.

(Author here) I didn't forget, it just didn't seem like the most likely problem. Like I said in a reply to a sibling to your comment, the gear was ~10yrs old at the time and had been working fine until then. It was perched in a very inconvenient spot because it had to "look around the corner" of the building, so checking the line of sight wasn't just a case of looking out the window. I went in order of "most likely t…

But rain is more obviously related ^ to a point to point link than aging hardware or some kind of bad update? (^though of course the improvement is surprising! I assumed there was antennae involved, whether point to point or LTE or whatever, just from the title. The story to me was from the outset why's it better not worse in rain.)

Related, sure!

But it could be rain helping close a circuit on a rusty antenna connector port. Or rain improving the grounding of some neighboring circuit that otherwise drains through the metal scaffolding the antenna is attached to. Or rain attenuating a neighbor's own Wi-Fi that otherwise might have been aggressively transmitting on the same channel as our units.

The rain and the Wi-Fi devices were clearly related. How they were related, was not clear. Aging hardware rusts, breaks, gets yanked around or unseated or pulled out of the ground, or has water enter in places where it shouldn't be.

I was already running diagnostics on everything to figure out which devices might be faulty (local AP, local bridge unit, local antenna, remote antenna, remote bridge unit, remote switch, remote modem/router, upstream connection to ISP) so checking for "update gone wrong" was a 3 second job: I was already in the admin UI, so check logs, nope, no recent updates, done. I'd rather spend 3 seconds checking something that probably isn't the problem but I can know for sure in 3 seconds than risk climbing precariously up a scaffold 30ft in the air only to realize it was just something I could have solved at a keyboard instead.

Risk/reward. Low risk, low reward is okay too if it's super fast and already on the way.

Re: The Wi-Fi only works when it's raining

#208

Post by obi1kenobi »

tgsovlerkhgsel wrote: post #201
user_7832 wrote: post #5

TIL about Fresnel Zones! > Interestingly, objects outside the straight line between antennas can still cause interference! For best signal quality, the Fresnel zone between the antennas should be clear of obstructions. But perfection isn't achievable in practice, so RF equipment like Wi-Fi uses techniques like error-correcting codes so that it can still work without a perfectly clear Fresnel zone. I wonder if other w…

My mind was blown when I saw the 4F experiment, where a lens transforms an image into the Fourier domain. I'm not sure if it's related to the Fresnel zone (I think it is not or only very vaguely), but it's pretty amazing: A widened beam of collimated light (i.e. parallel beams) is sent through e.g. a slide with some image printed on it. Using a lens placed one focal length away, it is focused down to a point (one foc…

Wow! Are you maybe able to find a demo video of this? I'd love to see it!

(I'm the author of the post btw)

Re: The Wi-Fi only works when it's raining

#209

Post by dhosek »

obi1kenobi wrote: post #143
dhosek wrote: post #130

Earlier quoted context omitted.

Yes, it’s not a competition, but if you have a line of sight network connection and the network only works when it’s raining, the obvious thing to check is that line of sight.

(Author here) I didn't think to check the line of sight because I was primed by the fact the bridge had been running fine for 10 years. With networking gear that old, it seemed more likely that a device/cable/power brick had just gone bad with age. Also, the antenna is on some metal scaffolding propped out 6ft past the edge of our balcony, because it needs to "look around the corner" of the building. It's 30ft in the…

Fair enough. My current router is less than a year old (my ex-wife got the house and the networking equipment in it).

Re: The Wi-Fi only works when it's raining

#210

Post by obi1kenobi »

femto wrote: post #190
obi1kenobi wrote: post #138

Earlier quoted context omitted.

(Author here.) Yeah, I get where you're coming from. Ultimately, this was an editing decision first and foremost. Beamforming is cool and magical, and MIMO even more so. The post wasn't intended as a primer on wireless technology, just as a fun read for folks to enjoy. I tried to sprinkle in some nerd-snipe-quality technical detail and offer links for folks who might want to dig in, and MIMO is explicitly discussed i…

I get where you're coming from too. I felt the need to mention the difference as so many people equate beamforming with MIMO and claim that there is no difference, when in fact that difference, which Foschini discovered in 1996, is responsible for spatial multiplexing and the high WiFi data rates we enjoy. I figured the typical HN reader might be more technical and interested in the difference, or maybe not.

Haha sadly from reading many of the rest of the comments, the typical HN reader seems unconvinced that trees or rain could impede Wi-Fi signals, because their Wi-Fi at home goes through walls just fine. A few even suggested I shouldn't have bothered with Wi-Fi and instead just laid cable across a few city blocks instead :)

But I do appreciate precise language and the desire to help people learn new things, so thanks for helping make the distinction between MIMO and beamforming clear! I hope at least a few more people know about it now thanks to your comments.

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