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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
or on an aeroplane, or in a hospital. Whichever, that's the shortest time from an article being posted, to a /.'ing I've seen in an age.
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Text messages are magic.
Some places are magic.
Pilots are afraid of magic.
Voicemails are magic.
Anyone that has an amateur radio license doesn't need to know this info. They already know that if voice communications fails, 99% of the time, you can send Morse code. 73's KB0GNK
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?
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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."
not RTFA yet, but id guess its because as soon as you make the connection to the tower you are connected to your service proider and they send you the data.
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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.
A phone sends a message to unregister itself right before powering of.
All right. So my phone goes out of reach if my battery falls off the phone suddenly ?
Amen, brother.
Mod parent up. In fact, crack the site, and push this one to the top.
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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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No, I fly my own airplane, usually solo. No one to hear me talk except, well, the person I'm talking to.
Hmmmm...Maybe they complained to the FCC.
MCSE? No, sir...I don't do Windows. Yes, I am an idealist. What's your point?
Have possibly managed to mention the iPhone more? Considering the market penetration, genericizing 'iPhone' to practically mean 'any old cell phone' is a tad premature...
XML is like violence. If it doesn't solve the problem, use more.
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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When a handset switches off and you are within coverage, it will signal to the cellular network that it is turning off. Similarly, if you receive a call and press the End key to reject the call, it will send a "busy" signal to the network, which can be handled differently to the usual "not available/did not answer", depending on how your network profile is set up.
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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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Your phone doesn't stay on until the battery dies, it stays on until the software determines the battery is getting too low, unregisters itself, then shuts down.
And, if the network is nearing overload, it will refuse calls but still allow textmessages. If the load gets even worse, not even textmessages will go through, possibly to maximize the chances of 911-calls actually working.
How about auto pilot drift.
An actual experienced effect some phones have caused in airplanes.
Phones that fall out of spec, or are manufactured out of spec can do this.
I know the person who evaluated confiscated phones when this occurs.
Granted, it was 10 years ago, so it might not apply anymore.
Yuor reason is really lame.
A) People can talk on the cell phone and not yell, just like they can talk to the person next to them on the plane.
B) People who talk too loud to the person next to them are asked to 'keep it down' by the attendants, this would be the same for cell users.
C) Flight attendants can have your ass arrested for not obeying.
They will change course to land sooner to ahve you removed(if you are bad enough), and the airline will sue you for the costs.
So you argument is pointless.
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Stay tuned to Slashdot for our next featured article, "The Mysterious Wheel."
When we will discover:
1. What is a wheel?
2. Why does a wheel roll?
3. What magic has created such a device?
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You mean an asshole has only one-way traffic? Because some of the Taco bell stuff i ate produced some of the largest and longest stuff i had ever seen coming out of mine.
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http://db.tidbits.com/article/9796?print_version=1
Odd that the print version on the same site works.
Actually the article explains, that their is a different transmitter for the SMS/message/misc data (not clear if it is on a separate carrier frequency, or just a lower data rate that travels better.) So the reason is this data is not sharing bandwidth with voice data. Basically if the data bandwidth gets saturated the voice path will fall apart, but if the voice path gets saturated the data part can still work.
Since cell phone voice traffic is digitized and sent as digital signal. message/sms is sent as a digital. Morse code travels well because it's a digital (granted all sent as a analog). So actually the reason voice travels poorly on amateur radio is unrelated. I assumed the same as you, that I knew, according to the article we didn't.
Holy shit! What type of balloon was your friend piloting? Was it one of those Led Zeppelins I've heard so much about?
It's been a while now. Did we bork something?
The cost of that cleanup, of course, will be borne by taxpayers, not industry.
Lol, balloon was SLOW moving - and caused many issues! Phone lockups and all sorts of network weirdness he said - calls were impossible while aloft or if they got through would misroute. This was the old analog days but the same sorts of issues might still occur with a faster moving plane in a digital world.
Now imagine a jet liner full of people *all* doing the same thing. Granted at cruise altitude it is probably not so big an issue as most won't have the range but I've been on more than one flight where a phone has rang :-O
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While the reason the GP gave isn't the only reason, your response is lame. His argument is not pointless.
A) Just because you can talk on a cell phone quietly, doesn't mean others will. Also, many mobile phones lack sidetone and because of this (and other reasons) many mobile phone users tend to speak louder than they would in person.
B) Sure an attendant can ask someone to keep it down, but do you really think they will? How about two minutes later, or when the person on the other ends says something that excites them? How annoying would it be to have to sit next to the loudmouth while they are repeatedly told to stfu?
C) Unless they make a real ass of themselves (and people like that probably don't need a cell phone to do it), do you really think the airline is going to go to all the trouble of having someone arrested or changing course over talking loudly on a cellphone?
While it would be nice to have the ability to make a call, the negatives seem to outweigh the positives, at least to me. Even with no technical barrier, I'd rather they keep cell phones turned off on planes. It's bad enough on the bus.
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."
Naaaah I don't think so. GPS does not rely on signal strength to find your location at all. In fact, it uses time difference of arrival (TDOA) information of a set of PRN sequences to trilaterate your position. And the reason cold-start takes so long is that GPS has to potentially download new alamanc and ephemeris data for every satellite, which is sent down at a whopping 50baud. And if you miss a bit, you have to wait 90 seconds, since this is how often the data is repeated. Of course, since there are usually numerous satellites in view, chances are you will be able to get complete data for 4 of them in approximately 45 seconds.
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Similes are like metaphors
Sounds like this was long enough in the past that he was using an older network, not a modern digital network. My understanding is that modern digital networks have no problems coping with phones which can view many towers at once. As a thought exercise, consider this question: does it cause chaos to use a cell phone from a high mountain peak?
I have used my cell phone in the air several times with no trouble. I even got to listen in once as my flight instructor had a nice chat with his wife while I flew the plane.
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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.
Then you are... out of reach. *rimshot*
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.
The Kruger Dunning explains most post on
I too have used my phone while in my GA aircraft. The difference is that most GA aircraft fly between 5,0000 and 15,000ft AGL (above ground level). That's only 2.5 miles up max. A commercial airliner travels around FL350-FL450 (35,000 to 45,000ft above mean sea level). You can see so many more towers 7 miles up than you can at 2.5 miles up.
Unless you are over the ocean, this point is moot. Manual cell selection is a mandatory feature in GSM/UMTS. Thus, with a limit technical limit of a bit more than 30 miles, as long as you are over a populated area you can connect to a ground based station.
why you're not allowed to use phones on airplanes
One crash in light aircraft ages ago suggested possible connection, unlikely.
My understanding is that the prohibition on cell phones is actually a FCC requirement, not a FAA requirement. It has more to do with the cell phone infrastructure than potential interference with aircraft systems.
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Yes, but he was talking about his friend the balloonist, and unless his friend's last name was Fossett he was probably not at airliner altitudes.
With the old networks, even GA planes could cause havoc. I heard one story about a guy who was approaching Chicago and called his family to tell them he would be home soon. When his bill showed up that one call had been billed to him six separate times, because he hit so many different towers that were too far apart to communicate with each other and coordinate billing. Of course airliners are far worse simply because of their speed, but at 10,000ft over flat terrain the horizon describes a circle over 250 miles in diameter, way bigger than a phone cell.
My understanding is that the older networks got really messed up by any aircraft use, and newer ones don't get messed up by any aircraft use. As evidence, I submit this article which claims:
The researchers found that on average one to four cell phone calls are typically made from every commercial flight in the northeast United States. Some of these calls are made during critical flight stages such as climb-out, or on final approach. This could cause accidents, the investigators report.
In my opinion that last sentence is crap, but there's no reason to doubt their data. If that kind of call volume is already taking place I'd think the mobile phone network would already be in shambles if it couldn't withstand phones on airliners, although it's certainly possible that it can cope with this much but not with more.
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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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he [adamengst] may not understand how they work. but for the majority here i might think the understanding is natural.
Oh yeah - natural.
I'm sure you, in your ultimate smugness, knew all this and more before reading the article (you did read it, right?). But I actually learned something from RTFA. Maybe I'm just not in the majority here...
Please don't use "umm" or "err" or "erm".
That was perfectly on topic!
How much is your data worth? Back it up now.
Cruising altitude isn't a problem at all, that's only a max of ~8 miles, I talk to a tower further away than that most of the time at home with 2 bars and an EDGE data connection. In fact we know that phones will work on a plane at full speed due to 9/11, many people called their loved ones from the plane to say their farewells.
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:p
Sorry, i had to
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Huh...I know some airline pilots, jets like the CRJs, they make calls all the time from the cockpit when over areas with alot of towers. One guy I knew made international calls constantly because it would make the connection but not connect the billing to his call, he has done this for 7 years now.
Maybe there us is why the plane crashed~
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I saw the error and still click submit.
Maybe that is why the plane crashed~
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....TidBits doing an article on how web servers work and what happens when their URL appears on Slashdot?
Have gnu, will travel.
They will change course and land if you disobey the attendant enough. Yes, it needs to be a big deal, most of the time they will ahve the police remove them at their next stop whether or not that was the passengers final destination.
The bus doesn't have someone that can get the people in a lot of trouble.
The attendant can confiscate the device. If you dis obey that, you can bet your ass you will change to the closest safe landing area and land.
If that was the reason, why do the have phones on the plane?
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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.
Anybody want a peanut?
Instead we hear both sides of those loud-talking assholes talking on the flight. Only now you hear both sides of it...
Not my problem. Why is it the airline's problem?
30,000 feet gives you a lot of horizon and unobstructed path.
I have a friend who launches amateur weather balloons that get up to over 100,000 feet. We track them by a GPS and a 300mW ham radio data transmitter. At 300mW and 100,000 feet, we're regularly lighting up every repeater within 400 to 500 miles. A cell phone can run at 1 or 2 watts, depending on the band.
Obviously, it does work, to a fashion (9/11), but the system will not handle very many of those calls at all.
All that white noise from the engines, aircon, and wind is plenty enough to isolate me from passengers further than three seats away. The only thing I can hear from the other end of the plane is the urgent cries of infants with stuffed up sinuses.
Seriously, you should have to apply for a special permit to bring an infant on an airplane. Unless it's an emergency, it's basically child abuse.
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If a GSM phone has good reception signal, and doesn't move, then it only polls the tower every hour to 90 minutes. To prove this just leave you phone next to something that's susceptible to GSM interference
Not if it was the pilot making the call.
Yes, I think people often forget that, if the phone ban was lifted, it wouldn't be one or two phone calls being made. Every single commercial airliner would have hundreds of phones on it constantly connecting and reconnecting to mobile phone base stations.
Personally, I don't understand the constant obsession with having to be connected to the outside World 24 hours a day. I have to travel by air a lot and it never ceases to amaze me how many people have to switch their phones on the second the aircraft has come to a halt on the stand. Perhaps it helps them maintain the delusion that they are not totally insignificant in the World.
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I only skimmed the article, but one thing I looked for that I'm hoping to understand someday is why a different protocol wouldn't have worked just as well and gotten rid of the problem of interference with mp3 players in your pocket and other devices (until you actually get a call or message). My idea is simple: All towers have unique id sequences that are broadcast on some type of beacon. Phones can be on some type of schedule where they wake up every few seconds to few minutes, demodulate and decode the beacon and determine if the id sequence has changed. Only if it has changed, does the phone need to contact the network to let it know it has moved to a different cell. I'm sick and tired of sitting at my desk and having my phone cause interference every 10-20 minutes. This is a waste of power and it annoys me.
Am I missing something?
Dara
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."
You mean, as opposed to the sound of big loud roaring jet engines? That would probably mask most of the sound of people talking on cellphones anyway?
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That transmission every 10-20 minutes is called a "registration" and is basically a keep-alive timer for your phone. In order to not waste resources paging a user that has left the area or has turned off their phone the system will automatically deactivate the user (and send straight to voicemail) if the phone hasn't checked in for so many registrations (usually an hour or two.) Granted it was mentioned in another post of the "registration" and explicit "de-registration" when you power off, but de-registration messages don't need an ack so they sometimes fail to go through completely.
You'll have an opposite problem if you are on the border between two systems (two cities) or two regions. Your phone will scan, pick up one network and immediately register. After a few seconds it might pick up the other network and then register there. That constant ping-ponging will drain your battery in no time. But it has to be done, because the network has to know where to send your call notification. The amount of time between when you are on the new network and have not updated your location database in the network means that you will miss calls in that short amount of time.