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How Mobile Phones Work Behind the Scenes
adamengst writes "We seldom think about how our mobile phones actually work, but in this TidBITS article, Rich Mogull pulls back the covers and peels away the jargon to explain why text messages work when voice calls are dropped, why your battery lasts longer in some places than in others, why you're not allowed to use phones on airplanes, why you can be notified of a voicemail message when your phone never rang, and more."
Should I try it from my mobile phone?
(-1, Raw and Uncut is the only way to read)
Here is a mirror http://mirrors.mednor.net/slashdot/10072008/TidBITS_Networking%20_Peering_Inside_a_Mobile_Phone_Network.htm
Text messages are magic.
Some places are magic.
Pilots are afraid of magic.
Voicemails are magic.
And here we see illustrated why a reading the article isn't always a good thing. This summary is obviously designed to drive people to the site hosting this article (and lots of ads I'm sure), but by forcing people to read the article you've taken down your site and most of us will now leave this page. Nice.
On a side note, what we do have in the way of a summary suggests that there's very little for us to learn here.
1. Text messages work when voice calls are dropped for the same reason Morse can get through when SSB voice can't.
2. Your battery lasts longer in some places than in others because the phone automatically adjusts its transmit strength based on the distance from the tower.
3. You're not allowed to use phones on airplanes because of paranoid ignoramuses and the insightful people who realize how bad it could get when people in a flying bomb know what's going on (and how annoying cell phones are).
4. You can be notified of a voicemail message when your phone never rang because the network was too busy to initiate the connection, your phone was on vibrate or it didn't have a connection at the moment.
There. Now you can get on with your day.
The government can't save you.
why you're not allowed to use phones on airplanes
One crash in light aircraft ages ago suggested possible connection, unlikely.
How about "You're a loud-talking asshole and you're enclosed in a tight, cylindrical object for several hours with a couple of hundred other people who don't want to hear about your stupid business plan."
Does it make you happy you're so strange?
Whilst that may be a perfectly valid reason to you, the real reason is that the airlines just haven't figured out how to charge for it yet.
They will soon:
http://www.guardian.co.uk/news/blog/2008/sep/25/ryanair.mobilephones
which includes a classic quote from Ryanair boss Michael O'Leary:
"If you want a quiet flight, use another airline."
How do mobile-phone servers distinguish between a switched off mobile phone and a one that is 'out of reach' of the mobile towers ? I never understood how I get those two different messages. What mechanism is used to differentiate between a switched off phone and a one that is out of reach ?
3. You're not allowed to use phones on airplanes because of paranoid ignoramuses and the insightful people who realize how bad it could get when people in a flying bomb know what's going on (and how annoying cell phones are).
This only half the story. There are a couple technical limitations also.
1. Airplanes are metal tubes. Ever try to make a call in an elevator? A singlewide trailer? It's difficult or impossible.
2. Even if you could get a signal in a plane, you're several tens of thousand feet up. You can see dozens of cell towers but go into and out of their range very quickly at 600mph. Cell tower networks aren't designed for this.
Even those who arrange and design shrubberies are under considerable economic stress at this period in history.
1. Dial number, tower recieves signal and discards number.
2. Dial again, tower connects and routes call around the world before connecting to the called number.
3. Tower waits for conversation to begin and injects random noise, removes every third word, and then disconnects.
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Disclaimer: this is for GSM -- other network types may be similar, though.
When a handset is turned on, it sends an IMSI* Attach message to the cellular network. When you turn it off, instead of immediately powering down it sends an IMSI Detach message to let the network know that it is no longer available.
If you lose signal, or just take the battery out, the network doesn't know that the handset is unavailable. It sends out a paging message to the last cell it was known to be in, and eventually to the whole network before giving up and returning an 'unavailable' message.
* Or TMSI if it has already been assigned a temporary ID to use instead of its IMSI.
Do not think that is true either. However a friend of mine who is a balloonist years ago told me what happened when he used a cell phone in flight - chaos! It would try and talk to many many towers at once and it was a mess. This article supports that theory and I think they have the reason right - multiple cell towers cannot easily handle being contacted by a single phone moving 500miles an hour. Now multiply that by the numbers of people that fly every day and you can see why the cell companies sure as heck don't want this occurring! I've still done it though :-) They explain how in-plane cell calls would work too if you read the article. http://mirrors.mednor.net/slashdot/10072008/TidBITS_Networking%20_Peering_Inside_a_Mobile_Phone_Network.htm
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why you're not allowed to use phones on airplanes
"The Economist" explained a while back that mobile phones interfere with ground networks.
They went on to say, that if mobile phones where really dangerous for avionics, then we all would be anal-probed for the things before entering the plane, because some dickhead always forgets to turn it off.
How about "You're a loud-talking asshole and you're enclosed in a tight, cylindrical object for several hours with a couple of hundred other people who don't want to hear about your stupid business plan."
And "The Economist" also mentioned in another article, that the airlines were really afraid off riots on the plane caused by the asshole that you mentioned.
Do you work at "The Economist?"
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http://db.tidbits.com/article/9796?print_version=1
Odd that the print version on the same site works.
Holy shit! What type of balloon was your friend piloting? Was it one of those Led Zeppelins I've heard so much about?
Actually no they didn't. The plane that crashed after the passengers learned what was up didn't have reliable communication with the cell phones if the stories I recall are correct. They DID manage to get through but I do not believe that the calls were for long or that they weren't suffering from drops. It was enough though to tell folks what was going on though at least.
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"Your GPS looks for special signals from satellites, and then compares the strength of those signals to triangulate your position."
No, GPS doesn't use the signal strength to calculate your position, it uses the relative arrival time of time signals from the GPS signals.
So can I trust the author to get anything else right?
In TFA, the explanation of GPS is total BS. The person writing the article does not even have the faintest idea how real GPS works.
Here is the real story:
Unlike in the article, determining the GPS position does not use strength of the signal, but the timing of the signals along with a knowledge of exactly where the GPS satellites are.
There are two types of data needed by a GPS: almanac and ephemeris. Almanac just gives the satellite's orbit. This stuff does not change, unless a satellite dies or the government changes the orbits for some reason. Given a rough location and time, the GPS can use the almanac data to know which satellites it should be looking for. This is why an older GPS may ask for the time, date, and state you are in when first turning it on. The GPS can figure out this stuff by itself, but it will take a few extra minutes.
Ephemeris data, on the other hand, needs to be refreshed every hour or two, and pins the satellite's location down fine enough to be useful. This data is encoded on the GPS signal, and may take a couple of minutes to get (very slow data rate). This is why getting a lock can take some time when first turning on a GPS. If you turn off a GPS and then turn it on 30 minutes later (even if you traveled 100 miles in that time), then the GPS will get a fix in under a minute.
The reason that phones can get a GPS lock almost instantly is that they can get the ephemeris data from the cell tower. It is true the cellular network can have a pretty good idea where the phone is even without the GPS, but that extra information does not help the phone's GPS at all.
"-1 Troll" is the apparently the same as "-1 I disagree with you."
My experience from flying in private aircraft (both prop and VLJ) is that rarely do you get service above 6000' AGL. You get blips to about 10000' AGL so a text message can come in or out, but a phone call is pretty much out of the question.
A) Cell Phones are only licensed for ground mobile. Using them in the air is actually a crime.
B) They can interfere with the navigational systems.
C) It's not just cell phones.
Here is some real world reports:
http://www.airnig.co.uk/emi.htm
Studies have been conducted on confiscated equipment. While there are a lot of variables, it can and has happened and has happened in repeatable tests.
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How do articles keep getting slashdotted when no one ever reads them? (On that note, here is a shortened version of the article.)
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That doesn't quite constitute using (overall) signal strength, and neither is it the primary location method. Yes, it is possible to use the carrier phase information as well as the L2 carrier phase (and L1/L2 discrepancy) to get a more accurate fix, but this information is only used to adjust the TDOAs of the PRN signals and compensate for varying ionospheric delays. Signal strength of each satellite is much more affected by random low clouds and even the receiver's immediate environment, than by distance from the satellite. In fact, if your receiver provides an SNR readout for each satellite, you can get an idea about just how dramatically these values are affected by, say, a tree that partially obscures a portion of the sky.
I heard this second-hand, so take with a grain of salt.
When a CDMA phone is idle, and the network supports it, the phone enters "slotted mode". Slotted mode is where the phone sleeps for a period of time (potentially quite long time -- several seconds), then wakes up to determine if anyone is calling it, then goes back to sleep until the next slot. Obviously, this feature is a key to very long battery life.
Apparently a certain CDMA carrier with quite sparse network capacity in the rural areas, switches off slotted mode on long weekends. They found out that when everyone goes out of town, their network can't handle it. So they force all the cell phones to drain their batteries by switching off slotted mode. They found their customers are very upset when calls do not go through, but not upset if they have a dead battery.
Sneaky if you ask me.
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