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Comment by WalterBright | Hacker News Reader

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> more effective to just install a cell phone jammer in the schools

It would be very difficult to engineer this to not jam cell phones just beyond school property. (You’d also need federal approval. Banning is easier.)

> It would be very difficult to engineer this to not jam cell phones just beyond school property

Most schools are set back a fair ways from the street, with lawns and playgrounds. I doubt it would be that difficult. Or one could use jammers with an effective range of 30 or 40 feet and have several of them to flesh out the coverage.

> You’d also need federal approval

Yup.

Here in Germany, most stores have effective zero cell reception immediately once you enter in the store and everything is normal once you are near the exit. In stores where people can order online, they offer free wifi(sometimes). I can see a similar approach in schools.

> most stores have effective zero cell reception

Is this intentional or a consequence of closed brick construction?

I think it has to do with the building, perhaps there is a Faraday cage caused by the copper net layer in the walls. From a similar experience, the doorstep of my apartment is really close to my other 2 neighbours (ground floor), and they all have WiFi routers above the entrance, inside — it was very hard to connect and use Wifi in the house on 2.4GHz due to the bandwidth, even if I changed the channels manually, so I was forced to buy a 5GHz router to circumvent the shortage. Also, my LTE doesn't work at all at my doorstep, so it's pretty apparent.

This is a highly questionable statement without any proof.

And then teachers would not be able to call 911 if there is an emergency in class. Next suggestion, please, this one was no good.

Not that I like the idea but teachers typically also had/have phones at their desk (for the last couple decades at least) it's just easier for the teacher to have the central office deal with contacting the parents and getting the kid with a fever picked up or whatever. Similarly, it's easier for the parents to call the central number and be immediately responded to and directed accordingly than try to find the specific teacher's number and hope it's a good time for them to pick up directly. For 911 level emergencies in class the teacher would call direct as the lesson plan and class conduct is a bit less important than the kid who can't breathe.

The other problems with this solution is there is no way to just jam cell phone signals inside the school only, the signals that need to be jammed are different each place and change over time, and it's unpopular with both anyone that had a legitimate use case for cell service as well as the FCC doesn't like doing it. It's also a generally complicated solution, even if you only attempt to get "some" of it to work right.

Put a landline on the teacher's desk.

Do classes no longer have a wired phone?

i think the fcc might have something to say about that

Fix the fcc.

The FCC is already on a pretty optimal answer in this case, it's physics you'd want to fix. Completely jamming all cell signals disrupts the public and private airspace well beyond the school.

I'm not an electromagnetic radiation expert, but I know my garage door opener has a useful range of about 10 feet, and my wifi about 30 feet.

It's difficult to explain accurately without making one into an RF expert but you've essentially said the radio equivalent of "my headphones aren't really useful past 10 feet" and concluding any sound so loud as to make the headphone use unintelligible across an entire school would also cease to exist a few feet away.

If one absolutely had to get rid of usable signals originating from outside the school a much more practical way would be to add RF shielding to the exterior elements. After all, if you want to get rid of a signal then subtracting it even intuitively feels a lot more reasonable than trying to add so much other signal you don't notice the original anymore.

Another way to jam would be to spoof being a tower. The cell phones would uselessly connect to the school tower. Since phones transmit their location, the spoofer could "geofence".

Or even ask the local cell phone tower people to geofence the school.

Spoofing is, by definition, not jamming but in terms of the goal if you combined a multi carrier spoofer with a DAS you could theoretically get things down to ~a million $ per high school on average and not horrifically leak signal as if you just stood up a spoofer and centrally blasted it out. It's also still carrier and technology generation specific though. I.e. if a new carrier/MVNO appears or an existing carrier rolls out new frequency bands then you need to replace the system. If it's a new technology (e.g. 5G) then you need to replace the system. If things just change in general (new MVNO on existing bands), you at least need to maintain the system accordingly (but might not have to replace it). You'd also need to convince the majority of people this is something all public companies should be mandated to support, which is a separate issue from convincing the majority of people this general approach should be taken, or you'll never get much compliance. All of that accounted for, the solution still brushes against needing to fix physics a bit in that lowers SNR in a many times larger area than the accepted dead zone - though not as badly as a jammer system.

Geofencing with cooperation of the local carriers is technically possible (with big downsides) in that if a phone is connected to 3+ towers you can decently get the position tower side to within several hundred meters (or worse) in an urban area. That means to reliably geofence a building you're still going to screw up anyone in the surrounding city blocks trying to drive by, use their phones at home/work, and so on regularly. It also still impacts noise (therefore connection quality), though not as bad, over a much large area than is excluded. You could also theoretically mandate that all devices have device side geofencing which uses the extra data from phone side (more than just tower connection data, BSSID or other local signal data) were you convince people to mandate public companies on this one as well (and even then there will be "old" devices for many years a la retiring 3G lest you want to convince everyone to throw their current devices away on top of the extra forced hardware monitoring ability). The cost for this would largely escape the schools by becoming a cost to carriers/hardware manufacturers.

Or you could, again, just build schools with RF shielding if this is really the path you want to go down. It'll probably have a higher up front cost but it'd achieve the goal for every high speed cellular service current and future without requiring any external mandates or external interference. It's still a bit silly compared to other funding requests schools typically have but it would solve the problem if the public prioritized it.

Thanks for the info.

Many schools are set back a ways from other buildings.

I would just set up a jammer, and adjust its power so the nearest buildings could still use their phones, while the school phones would have problems connecting.

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