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Comment by sigwinch28 | Hacker News Reader

> Note that the reason for airport mode is a courtesy to cell-phone carriers.

This doesn’t track. What about trains?

Cell phone cells are (ideally) shaped to account for the expected pattern of how handover will occur. Along roads and train lines, the cells are (at least in GSM) supposed to be tailored to allow for easy routing and handover as the devices travels in the direction of the way.

While I have never read anything concretely analyzing the handover pattern of devices on airplanes, I would expect that since a very large number of cells are almost equally visible/equal signal strength, the network would have to frequently handover the device from one cell to another.

The handover process is, for voice traffic, very resource intense. (in GSM) it involves duplication of traffic to the neighboring cell and a lot of coordination.

I think that could be the reason for why mobile operators find airplane-borne devices annoying.

The cell tower network is designed to cope with devices located on the surface of the planet, i.e. where the geometry between the handsets and the tower is roughly two-dimensional.

I am also unconvinced that handsets on aeroplanes are really a problem for the network, but trains aren't the counterexample you're making them out to be.

Flight patterns are also pretty consistent. Tower planners have probably given some thought to it.

That doesn't address the 2 vs 3D problem the person you’re replying to outlined.

Cell towers are designed for terrestrial users because their radios aren’t pointed at the sky.

Gogo provides inflight Wi-Fi in the US through its ground based towers and all the antennas are pointed at the sky :)

Flights (at least in the US) fly roughly a direct route, but ATC spaces out planes and small weather diversions are common. So it’s more like a band of coverage needed between cities. Also, the combinatorics of considering every US city pair that currently or has previously had commercial service between gets long. Then you would have to consider every route that an airline in the future may offer, even just seasonally. Then if you calculated the optimal route between each pair and figured out the width of the cellular coverage at 30,000 ft from a single cell tower below, I don’t think it would be possible to offer service to every domestic flight in the US without achieving complete coverage of the US.

China on the other hand has a limited number of very narrow flight corridors that all commercial aviation is restricted to because the majority of the airspace is government controlled. That’s the only nation I can think of that would have enough air traffic to justify terrestrial tower installation but whose domestic flights are confined to a such a small part of the airspace that it would be possible to provide service to all domestic flights without achieving coverage of all Chinese airspace.

Line of sight and curvature of the earth.

At 20 miles out, a device on the ground starts losing line of sight, at 20 miles out, a device on an airplane starts receiving horizontal line of sight.

This. Aloft, your phone sees many, many towers.

You're supposed to put your phone on airplane mode in a train or subway too, but most people don't:

https://pocketnow.com/airplane-mode/

The problem is significantly lessened for trains vs. planes though because you're traveling an order of magnitude slower and hence you switch cells an order of magnitude less frequently. Also many commuter rails run alongside major highways or boulevards, where the cell systems are built assuming lots of cell handoffs at roughly highway speeds. If enough people used high-speed rail to overwhelm the cell system we'd probably see announcements to put phones in airplane mode there too, but very few folks will travel long distances at high speeds on a train.

That article sounds like a bunch of hogwash to me. The CTA in Chicago specifically has cell phone infrastructure in the subway lines, and that predates that article.

https://www.transitchicago.com/cta-to-bring-4g-wireless-serv...

> You're supposed to put your phone on airplane mode in a train or subway too,

That's a pretty insane claim in most of the world. Lots of people expect and enjoy full coverage on train or subway rides. I work at a carrier and the signal strength and cell capacity on trains are part of the top metrics we use to benchmark the quality of our network and the customer experience.

> You're supposed to put your phone on airplane mode in a train or subway too,

Why would you be expected to do this when we don't require drivers to put their phones in airplane mode? I get that US trains are slow compared to other parts of the world, but does the Autobahn require airplane mode? Doubtful since my German car happily connects to my phone in multiple ways.

>> You're supposed to put your phone on airplane mode in a train or subway too,

> That's a pretty insane claim in most of the world.

So is this:

> but very few folks will travel long distances at high speeds on a train.

> If enough people used high-speed rail to overwhelm the cell system we'd probably see announcements to put phones in airplane mode there too, but very few folks will travel long distances at high speeds on a train.

Millions of people travel long distances on trains: 353 million/year on the Shinkansen network, 1.5+ billion/year on China high-speed rail, 110 million on France's TGV. These passengers use their phone without overloading the cell networks.

The operator can just pay for the high speed handover feature license ;)

It doesn't track in any way. In the airport there's small cells or femtocells by the operators. In the air, the only cells you can reach are the one pointing upwards.

Also, they don't really ask for airplane mode anymore, at least not when I'm flying.

Trains move significantly slower than planes. They're also on the ground, mostly below the towers, while planes are in the air, moving very fast, with multiple towers in line of sight.

Doubly doesn't track as airplane mode is often required during takeoff and landing, which barring controlled or uncontrolled flight into terrain usually happens at the airport, where you and the so many folks taking the same airplane as well as several other airplanes are not required to take their cell phone off as a courtesy to operators.

I can get the EM interference angle just fine, which stays (or stayed until recently) in place as a vestigial CYA rule of a time when cell phones were crude enough to be able to produce a spark which is what makes gas stations display a "no cellphone" sign to this day because back then it could ignite gasoline vapour.

Line-of-sight. If you're 4-5 miles up with no blockers except the aircraft skin, you get to confuse a lot of towers at once.

Must not be very much of an issue since there’s basically zero messaging around what airplane mode is for.

Not once has any of my carriers even said “please turn on airplane mode to help the network”. Instead it’s an in-passing recommendation made by air stewardesses so boilerplate it feels like superstition.

I wonder what the actual impact is and how that relates to the low effort messaging around airplane mode.

Most trains only go a tenth the speed of planes. And frequently much slower than that if they're making a lot of stops.

And movie theaters, malls, stadiums. Maybe I’m missing something?

If you're in a stadium travelling at 600mph, you have a heck of a problem!

I think the challenge isn't so much "lots of people in one place" as "lots of people travelling extremely fast with line-of-sight to hundreds of cellular towers at the same time".

Most, if not all, of the movie theaters I've been in have been fairly stationary.

Stadiums are actually challenging problems for mobile network builders. That's tens of thousands of people. 300 people in a passing airplane are not a problem. Computers are fast. 3MPH, or 300MPH no longer matters to mobile networks until you actually overwhelm the number of calls/connections you can pack into one place's infrastructure.

I used to live a few blocks from the AA Arena in Miami. I could always tell when a Heat game had just finished because my cell reception would drop to 1 bar or lose connection altogether. Then a few seconds later you'd hear the crowd of people reaching the parking lot right by my building.

Ideally mobile devices in theaters, malls, and stadiums aren't traveling at hundreds of miles an hour with a vantage such that they're in range of multiple towers simultaneously. It used to be (and maybe still is with 5G) that adjacent cells used different channel pairs. That way a handset at the edge of cell A could talk to both cell A and B to negotiate a handoff. Handoffs can happen pretty quickly.

If cells A and F are on the same channel pair and a handset can see both towers the tower the handset is not talking to just has a higher noise floor. It's like in an apartment if the next unit over is on the same WiFi channel.

You'll note that in older generation cell systems theaters, malls, and stadiums were terrible places for cell signals. Now many have micro/nano cells that cover much smaller areas to deal with congestion. Handsets reduce their output power when the received signal strength is better. So a handset will readily talk to a microcell at a stadium rather than try to talk to a tower further away (with a worse signal).

But for planes, hundreds of passengers hitting dozens of towers simultaneously on the same channel pairs would be rough on switching. Carriers need to know where in the network to route messages to handsets. If a handset is handing off to multiple towers quickly or worse hopping seemingly randomly to towers as a jet flies over it'll be difficult to keep it connected. Multiple that by thousands of people in the air at any given time. Areas near busy airports would have absurdly unreliable connections for people in the air and on the ground.

Trains aren’t traveling at 400-700 mph ground speed. At best they might be doing 225mph. The rate you pass by towers is thus lower.

Trains also have trees, hills, buildings, earth curvature, etc obstructing how many towers your phone can see. Planes generally have a clear LOS to the tower over a significantly larger area because of the altitude.

If the radiation pattern of the tower wasn’t the shape that it is, the problem would be even worse in fact.

Compare the speeds of both modes of transport

US resident here. What is a train? Those things that move cows, oil, and cars across those rusty bits of metal? I don't think those are suitable for human transportation.

Can we avoid this here and at least have one website where we focus on the technicals and problem being discussed vs. the Americans’ interjecting their current complaints?

Trains are what keep groceries on your shelves. They almost had general strike last week because the drivers only have 5 days a year not working. No weekends, no sick days, etc.

>> I don't think those are suitable for human transportation.

> Trains are what keep groceries on your shelves. They almost had general strike last week because the drivers only have 5 days a year not working. No weekends, no sick days, etc.

Are you trying to disagree? That sounds extremely human-hostile.

If this were a thread on ever declining US education outcomes, this comment would be relevant.

Please, just don’t.

Dude, faux ignorance is rude and anti-conducive to useful conversation; please don't. (Unless, of course, you're sincerely unaware of trains being a widely used mode of transportation even in the States, in which case you're not rude but you should go ask Google to inform yourself.)

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