Slashdot Log In
How Mobile Phones Work Behind the Scenes
Posted by
timothy
on Tue Oct 07, 2008 12:20 PM
from the they-all-suck-to-different-degrees dept.
from the they-all-suck-to-different-degrees dept.
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."
Related Stories
This discussion has been archived.
No new comments can be posted.
The Fine Print: The following comments are owned by whoever posted them. We are not responsible for them in any way.
Full
Abbreviated
Hidden
Loading... please wait.
Already slashdotted! (Score:4, Funny)
Should I try it from my mobile phone?
Re:Already slashdotted! (Score:4, Funny)
Parent
served by a Mac (the Emperor is ill) (Score:3, Interesting)
Dual 1.33 GHz Xserve G4 - [Our server, sic]Emperor runs on a normal dual 1.33 GHz Xserve G4 (2 GB of RAM). Emperor is still running Mac OS X Server 10.2.8, which came with it and handles the load just fine, so we haven't had any reason to upgrade.
Web Crossing - The server software that powers all of our Inter
Re:Already slashdotted! (Score:5, Insightful)
>>>Or why a text message can get through when a call can't?
This is no great mystery. A test message can just sit in a buffer until your phone is within broadcast distance, and then it's sent. But a call has to be done in realtime; if reception is poor the caller gets a busy signal (and then send a text instead).
Parent
Re:Already slashdotted! (Score:5, Informative)
This is no great mystery. A test message can just sit in a buffer until your phone is within broadcast distance, and then it's sent. But a call has to be done in realtime; if reception is poor the caller gets a busy signal (and then send a text instead).
And they require much less bandwidth and don't tie up a phone line out of the cell tower. Just data, which can go over a shared data line asynchronously.
Parent
Re:Already slashdotted! (Score:5, Interesting)
And yet still cost more than an actual call...
Parent
Re: (Score:3, Informative)
The ratio of signaling channels to voice channels is something around 1:21, hence the signaling channel is a scare resource compared to the voice channels (and therefor more expensive than voice calls).
Re: (Score:3, Insightful)
Except that a text messages takes FAR less than 1/21 of the data that a similarly priced voice call does.
Re:Already slashdotted! (Score:5, Informative)
Text (SMS) are sent over paging channels, not data channels. This is why they're still 160 characters. Yes, it's data but it's send in messaging protocols used for voice signaling. They can still get through if there are no voice channel available since they never need to setup a whole call.
Telecom is old, don't assume things work the way they seem to as lots of legacy protocols are still in use.
Parent
Re:Already slashdotted! (Score:4, Funny)
You forgot the best part about texting. Assuming it's not at night, you can do it more inconspicuously while driving!
Parent
Working link (Score:3, Interesting)
http://db.tidbits.com/article/9796?print_version=1 [tidbits.com]
Odd that the print version on the same site works.
Re: (Score:3, Funny)
Mirror (Score:5, Informative)
Re: (Score:2, Funny)
Here's a much better mirror [artandanti...porium.com].
Re: (Score:2)
Shortlist of answers: (Score:5, Funny)
Text messages are magic.
Some places are magic.
Pilots are afraid of magic.
Voicemails are magic.
Re:Shortlist of answers: (Score:5, Funny)
Pilots are afraid of magic.
And so they sould be, since they emit pilot killer rays [penny-arcade.com]
Parent
Re: (Score:2)
god did it
Short summary isn't always good (Score:5, Informative)
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.
Re:Short summary isn't always good (Score:4, Insightful)
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).
Or, just possibly it's because:
1. GSM phones are known to emit strong pulses of RF that interfere with nearby electronics (audio amplifiers, televisions, speakerphones, etc).
2. Airplanes contain quite a few important electronic systems for navigation, communication, flight control, etc.
3. Considering the number of passengers who are carried by airplanes each year, even something with a one-in-a-million chance of causing a problem would be a very bad thing.
Parent
Re:Short summary isn't always good (Score:4, Informative)
Considering the number of passengers who are carried by airplanes each year, even something with a one-in-a-million chance of causing a problem would be a very bad thing.
This is blatantly false. Airliners are chock full of things with much higher odds of failure than one in a million. Airliners achieve their extremely good safety record through redundancy and robustness, not through avoiding failure at all costs. Airliners have things fail all the time, it's just that the vast majority of the time the inherent redundancy and robustness of the aircraft make it such a minor event that the passengers don't even know it happened.
Parent
Re:Short summary isn't always good (Score:5, Informative)
Parent
Phones on airplanes (Score:5, Informative)
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.
Parent
Re: (Score:2, Insightful)
Then why were people able to make cell phone calls on the 9/11 planes with no problem. The couple I've heard were long and clear.
Re: (Score:2)
So you want to say that you are not _allowed_ to use phones on the ariplanes because you _can't_ use them?
Re: (Score:3, Insightful)
Sigh, mythbusting time...
1) The "metal tube" myth: Get in an elevator, and compare the performance of a 2G (GSM, CDMA) phone and a 3G (UMTS) phone -- you might be surprised. In the 2100MHz band at least, most 3G phones work just fine.
2) The "hundreds of MPH" myth: Nope. Phones are not banned on high speed trains in Europe or Asia, which also travel at hundreds of MPH. The story I heard was that it's not the speed of the handoffs that's the problem, it's the fact that a phone in an airplane at cruising a
Re: (Score:2)
1. Text messages work when voice calls are dropped for the same reason Morse can get through when SSB voice can't.
At this point it's all data... so why would TXT get thru and not voice? only explanation might be that more data doesn't get thru, or that re-tries make it happen...
but I don't buy the tone-signaling vs. voice argument... it's just bits...
Re:Short summary isn't always good (Score:5, Informative)
Lets say a cell tower has 64 voice channels available. Lets say there are sixty-four people on that cell tower holding conversations. Lets say somebody calls your cell. Ooops, no available voice channel; they get your voice mail. You get a 'new voicemail' notification through the dedicated signalling channel.
Parent
Re: (Score:2)
Well you managed to get one right pulling ideas out of your ass!
I don't understand this about mobile networks .. (Score:3, Interesting)
Re:I don't understand this about mobile networks . (Score:5, Informative)
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.
Parent
Re: (Score:2)
How it Works for Me (Score:5, Funny)
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.
Could the article... (Score:2)
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...
Next on Slashdot. (Score:5, Funny)
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?
GPS explanation is total BS (Score:5, Informative)
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.
/.'s ignorance on the plane subject is staggering (Score:3, Informative)
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 [airnig.co.uk]
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.
Providers Will Sometimes Kill Your Battery Life (Score:3, Interesting)
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.
Maybe he's in a gas station (Score:2)
Re: (Score:2, Redundant)
Re:wrong audience, buddy (Score:5, Insightful)
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."
Parent
Re:wrong audience, buddy (Score:5, Interesting)
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 [guardian.co.uk]
which includes a classic quote from Ryanair boss Michael O'Leary:
"If you want a quiet flight, use another airline."
Parent
Re:wrong audience, buddy (Score:5, Informative)
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 [mednor.net]
Parent
Re:wrong audience, buddy (Score:5, Funny)
Holy shit! What type of balloon was your friend piloting? Was it one of those Led Zeppelins I've heard so much about?
Parent
Re: (Score:3, Interesting)
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.
Re: (Score:2)
Amen, brother.
Mod parent up. In fact, crack the site, and push this one to the top.
Re: (Score:2)
Hmmmm...Maybe they complained to the FCC.
Re: (Score:2, Interesting)
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?"
Re:wrong audience, buddy (Score:4, Interesting)
Parent
Re: (Score:2)
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.