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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)
That was fast...
Is he hosting his website on a cellphone?
-516
Looks like the author should have written an article about how the internet works behind the scenes. If so the webmaster might have been able to keep the site from being /.'d
Here is a mirror http://mirrors.mednor.net/slashdot/10072008/TidBITS_Networking%20_Peering_Inside_a_Mobile_Phone_Network.htm
meh
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.
If I had an Ass, I'd call it Fanny Bottom, then I could slap my Ass; Fanny Bottom, on the Arse.
Text messages are magic.
Some places are magic.
Pilots are afraid of magic.
Voicemails are magic.
why text messages work when voice calls are dropped
Retries.
why your battery lasts longer in some places than in others
Higher signal strength.
why you're not allowed to use phones on airplanes
One crash in light aircraft ages ago suggested possible connection, unlikely.
why you can be notified of a voicemail message when your phone never rang
Blah, this is a site for nerds, not "omg wow that's so cool" teenage girls (or Apple fans who think every tiny incremental step is a breakthrough).
for the majority here i might think the understanding is natural.
Really? You can tell me off the top of your head exactly why you can get a voicemail without it ringing? I honestly don't think that's common knowledge, even for nerds.
The hosting server is a mobile phone.
- Your stupidity got you into this mess, why can't it get you out? -Will Rogers
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.
Comment removed based on user account deletion
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.
IranAir Flight 655 never forget!
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 ?
New tag system apparently. Mouse over the various tags and you'll see there are three tag types--Top, System, and Type. I'm presuming that since there are more stories submitted than anything, the Story tag will be on almost every entry.
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.
When Fascism comes to America, it will call itself Anti-Fascism, and tell you to give up your guns.
http://www.tidbits.com/about/in-use.html Emperor The machine emperor.tidbits.com, also known as www.tidbits.com and just tidbits.com, is our main server. It does basically everything for us now.
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 Internet services is Web Crossing, from the company of the same name. Web Crossing can do just about anything, since it backs up its built-in Web, FTP, email, and NNTP service with plug-ins that add mailing lists (also accessible via the Web and NNTP), RSS support, weblogs, wikis, and much more. A lot of this is possible becuase at its heart, Web Crossing includes a high-performance object-oriented database and not one but two programming languages for creating dynamic sites. Web Crossing is the software that Apple uses to host their discussions.
What if you power off when you are out of reach?
You got the touch!
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.
The same reason your asshole bleeds when you stick a large object in it.
damn you slashdot. look what you're doing to these poor web pages. they may never be able to recover from this kind of shock. and i really wanted to know how cell phones work! (although i probably wouldn't have if i hadn't read it on here first)
Rumble in the Jungle
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.
Backup not found: (A)bort (R)etry (P)anic
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.
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?
"There ought to be limits to freedom." -George W. Bush
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.
"Doing what i can, with what i have." ~ Burt Gummer
The network would send a request to your phone when a call is placed to it which under normal circumstances would send a response back to the network. If your battery fell off then obviously your phone doesn't respond and the call is considered "not available".
There is no communication from your phone to the network, whereas for hitting "end" or ignoring the call information is passed back and forth.
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.
"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.
Mod Parent -1, TMI
Similes are like metaphors
This is only a problem in GSM networks. CDMA networks regular have users connected to multiple towers and it actually improves the signal quality.
GSM should have been dumped in favor of CDMA 10 years ago!
as they work in front of the scenes I should imagine.
Once I was a four stone apology. Now I am two separate gorillas.
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
The article is an incredible waste of time. If you really want (fairly) accurate basic information on how a cellular network operates go to Wikipedia or head over to www.howardforums.com . The fine people there are incredibly educated and can give you a basic understanding of what hibby jibby stuff goes on.
How do articles keep getting slashdotted when no one ever reads them? (On that note, here is a shortened version of the article.)
The path to enlightenment is truly through homemade drugs!
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.
TidBITS appears to be overloaded with traffic since you put this on /. I had no problems earlier today.
I got the error message now:
Network Timeout
The server at db.tidbits.com is taking too long to respond.
The requested site did not respond to a connection request and the browser has stopped waiting for a reply.
* Could the server be experiencing high demand or a temporary outage? Try again later.
* Are you unable to browse other sites? Check the computer's network connection.
* Is your computer or network protected by a firewall or proxy? Incorrect settings can interfere with Web browsing.
* Still having trouble? Consult your network administrator or Internet provider for assistance.
The cell phone network is like a series of tubes...
If the plane's navigational systems are in any way, shape, or form 'vulnerable' to the radio frequency emissions of a cell phone (or any other consumer device) then we need to immediately ground all commercial air travel.
If it really was an issue, the airlines would either have planes falling out of the sky, or they would already have retro-fitted the passenger compartments with faraday cages.
Your link is crap as well, the info is speculation and FUD nearly 10 years old. A simple scan of the Weak-apedia yields these items of interest:
* Boeing performed extensive tests as reported in AeroMagazine's Interference from Electronic Devices in response to reports by flight crews of anomalies that they believed to be caused by electronic devices. The flight crews had apparently confirmed the effect by switching the "suspect" device on and off and watching the effects. Despite this, and despite the fact that Boeing in many cases was able to purchase the actual offending device from the passenger and use it in extensive testing, Boeing was never able to reproduce any of the anomalies. The report concludes:
and
As of mid April 2007 Qantas have teamed up with Panasonic Avionics Corporation and AeroMobile to commence a three month trial that will "enable customers to send and receive e-mails, access the Internet and send and receive text messages from their own mobile phone"
On 18 October 2007 Ofcom published proposals for the technical and authorisation approach that would be adopted to allow this for European GSM users on the 1800Mz band on UK registered aircraft.
On 26 March 2008 Ofcom approved the use of mobile phone-supporting picocells aboard aircraft in the United Kingdom. Airline companies will have to equip the aircraft with picocells and apply for licences.
Which includes quite a bit from non-USA sources.
Don't believe all the FUD. Despite your claim that it 'can and does happen' you fail to provide a link to a citation, and all the evidence I have found contradicts yours. Not a single report EVER of a cell phone causing any dangerous interference, or leading to a crash, etc.
I saw the error and still click submit.
Maybe that is why the plane crashed~
The Kruger Dunning explains most post on
....TidBits doing an article on how web servers work and what happens when their URL appears on Slashdot?
Have gnu, will travel.
...to pay your $699 licensing fee you cock smoking twitter!
I heard this second-hand, so take with a grain of salt.
When a CDMA phone is idle, and the network supports it, the phone enters "slotted mode". Slotted mode is where the phone sleeps for a period of time (potentially quite long time -- several seconds), then wakes up to determine if anyone is calling it, then goes back to sleep until the next slot. Obviously, this feature is a key to very long battery life.
Apparently a certain CDMA carrier with quite sparse network capacity in the rural areas, switches off slotted mode on long weekends. They found out that when everyone goes out of town, their network can't handle it. So they force all the cell phones to drain their batteries by switching off slotted mode. They found their customers are very upset when calls do not go through, but not upset if they have a dead battery.
Sneaky if you ask me.
Anybody want a peanut?
Why do I need to read an article, I know how cell phones work. Dial twice, get through say 'hi' about four times, figure out whose turn it is to talk through all the break ups then once a month grease up my sphincter for a 150$ ass raping.
Peering Inside a Mobile Phone Network
by Rich Mogull
Have you ever wondered why your mobile phone can alert you to new voicemail without having ever rung? Or why a text message can get through when a call can't? Maybe you've traveled across continents and been amazed at how calls still manage to follow you? Or perhaps you've noticed that sometimes your battery only seems to last a fraction of its normal life? And why can the iPhone 3G figure out your location in 3 seconds when it takes takes your car GPS 3 minutes?
Although we normally take the ubiquitous mobile phone for granted; assuming it should work anywhere at any time, there's quite a bit of complex technology involved in sending a call to a device in your pocket. While we've all screamed in frustration over dropped calls and other annoyances, the truth is these are impressive devices, packed with amazing technology, that still hold a few tricks up their sleeves. And after you learn a little more about the inside of the system, maybe, just maybe, you'll be a little less irritated the next time you battle to make a simple call.
How Your Calls Follow You -- One of the most fascinating aspects of mobile phones is how calls manage to find us in the first place. If you think about it, you are basically wandering the planet with a tiny radio in your pocket, but by calling a single number anyone can track you down in seconds. Although there are a few different types of mobile phone networks, they all follow the same basic, yet elegant, architecture. For this article I'll be using terms for the GSM (Global System for Mobile communications) network; the one used by AT&T and other international iPhone providers. I've also simplified things a bit, and Wikipedia is a great source if you'd like to dig in deeper.
It all starts with the phone in your pocket. Every phone in the world has a unique identifier called an IMSI - your International Mobile Subscriber Identity. In most phones, this is encoded on a small smart card (yes, the same technology used by some banks and ID cards) called a SIM - Subscriber Identity Module. When you turn your phone on it tries to find the nearest base station, which is a collection of switching equipment tied to that (likely ugly) cellular antenna on the side of the highway. Your phone connects to the nearest base station, based on signal strength, and that's where the interesting stuff starts to happen.
The IMSI truly is a unique number tied to you and your mobile phone provider, and is the key to the entire system. The base station is a relatively dumb system that just passes on your information to the main brains of the system - the Mobile Switching Center (MSC). The MSC can be located pretty much anywhere, which is why in the very early days of cell phones 911 calls might have been routed to a confused emergency dispatcher in a different city or state (don't worry, that's all fixed now). While each system is a little different, a large cell phone provider will generally have a bunch of MSCs to support different phone numbers for different local areas.
At its simplest, the MSC is just two big databases and a connection to the regular phone system. One database, called your Home Location Registry (HLR) is the master database for your account, with your IMSI, phone number, and current location. The second database is called the Visitor Location Registry (VLR) and it keeps track of people that have wandered into that area (a VLR serves only a single base station). Here's how it works - your phone registers your unique IMSI with the nearest base station, and that base station tells its VLR that you are connected. The VLR then contacts your HLR and, using your IMSI, registers your location.
When someone calls you, the call is routed from the regular phone system through your MSC all the way out to the highway you're driving on, since the system always knows where you are. If you happen to be on a GSM system like AT&T (and unlike Verizon), your call can even follow you to any other GSM system
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
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
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.