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(Mis)Uses of Technology

by Mike Masnick


Filed Under:
data, in-flight, voip, wifi



Voice Is Data: Tech Won't Be Able To Stop VoIP In The Air

from the of-course-not dept

With increasing attempts to turn internet access on in the sky, there's been some concern about people making VoIP calls from airplanes, just as there is a concern over mobile phone use in the sky being too "annoying." Some of the companies providing internet-in-the-sky have claimed that they would block VoIP calls, but that's going to be pretty difficult. As we've pointed out in the past voice is just data and you can always find a way to disguise the data, such that it won't be blocked. And, indeed, that seems to be exactly what's happening. Andy Abramson talks about how he got around AirCell's VoIP blocking when talking to a friend who was on one of these wired airplanes. There's always going to be away around those things, so unless Congress really decides to ban all voice calls on phones, why not wait and see if people chatting really is a problem?

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  1. icon
    Derek Kerton (profile), 26 Aug 2008 @ 8:43am

    Cellular Networks Are Terrestrial

    Poster #15 is onto the real reason you can't use a cellphone in the air: cellular networks are NOT designed for this, and it screws them up.

    That's why the FCC has a ban on airplane cell use, and the FAA's ban is merely window dressing with "safety" as a justification. The people above who doubt the safety issue are right, given the evidence: how many phones do you think are forgotten and left on on any given flight? If planes are that vulnerable, I'd never get in one. What would that imply? A terrorist could crash a plane by using common cell phones, why bother looking for explosives at the security check?!! Why bother rushing the cockpit!? Just call 1-900-hijack at the coordinated time.

    The reason cellphones at 30,000 feet are bad for cellular networks has to do with the network design, specifically two things endemic to cellular phone networks: 1) frequency re-use, and 2) tower handoff.

    1) Cellular nets use frequency re-use to get more people onto their system using just the limited amount of spectrum they are leasing from the FCC. You can picture it this way: a guy in NYC uses Verizon Wireless (VZW) channel "1" to make a call. Obviously, a guy in Chicago, Denver, and San Francisco could also use VZW channel 1 to make a call at the same time without causing any interference to NYC. This is frequency re-use. Now extrapolate that to a much higher use level. Think of a guy in Manhattan using channel 1, another guy in the Bronx, and another guy in Brooklyn. Actually, think of the honeycomb-like cells in a beehive overlaid on the country, and then understand that so long as any two cells are non-contiguous, they may re-use the same channels. This concept is so important to cellular networks, that the networks are named after the concept. Any single user can drive around on the ground, moving from one cell to another, using channels, and never broadcasting so strongly that it interferes with someone else using the same channel a few miles away. Now imagine an airplane, overhead, and equidistant to 3 towers that are all re-using the same channels. If the airplane's phone signal could reach one tower, it would also reach the others, causing interference.

    Note that the towers have antennae that are aimed along the surface of the earth, not into the sky. No cellular operator wants to waste energy transmitting to the sky! This further makes it difficult for airplanes to connect.

    Commercial airplanes at cruise fly about 4-5 miles up. That's a long way. Then don't forget the tower antennas aren't aimed up there. Signal quality will be poor at best.

    2) Now, for tower hand-off. Usually, when a person is talking on the cellphone while driving or on a fast train, the network is capable of understanding where the person is, which tower they are using, and how to transfer their call to the next tower along their route. Take the speed up to 500mph, and the network was never engineered to expect that, accommodate that, or make the hand-off. Now picture a situation where the network expects that this 500mph phone is on the ground, yet can't get a good idea of where it is, since it is visible to way more towers than normal. Which tower should it hand the call to? Yikes. Call drops.

    Thus, cellular phones won't crash airplanes, but they will adversely affect the cellular network. They will at best provide spotty, poor, and short connections inside of airplanes, but usually nothing.

    Remember the 9/11 phone calls from the hijacked airliners? This was a sad demonstration of how well cellphones work in the air. The planes were at lower altitudes, but still the calls were garbled, cut off, or not connected.

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