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How Mobile Phones Work Behind the Scenes

timothy posted more than 5 years ago | from the they-all-suck-to-different-degrees dept.

Cellphones 220

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."

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220 comments

One thing didn't get explained at this moment... (-1, Offtopic)

sam0737 (648914) | more than 5 years ago | (#25288855)

why it get slashdotted.

Oh I know now, it's because it get slashdotted...

Maybe he's in a gas station (1)

Finallyjoined!!! (1158431) | more than 5 years ago | (#25288943)

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.

Closed Specturm is Out of Gas. (1)

twitter (104583) | more than 5 years ago | (#25289183)

If we had Open Spectrum [greaterdemocracy.org] we would not have to worry about Slashdotting or sold to the highest bidder cell phone service.

Re:Maybe he's in a gas station (1)

fprintf (82740) | more than 5 years ago | (#25289951)

Just how long is an 'age'?

Is that like Libraries of Congress in measuring words or Elephants in measuring weight?

Re:Maybe he's in a gas station (0)

Anonymous Coward | more than 5 years ago | (#25290199)

Although totally offtopic, I saw a server advertising produly claming "The performance of a pile of laptops X meters tall"

Re:One thing didn't get explained at this moment.. (1, Redundant)

Kjuib (584451) | more than 5 years ago | (#25289027)

The hosting server is a mobile phone.

Re:One thing didn't get explained at this moment.. (1, Redundant)

liquidpele (663430) | more than 5 years ago | (#25289193)

It's sad when the very first post is complaining the site is down...

Already slashdotted! (4, Funny)

Gizzmonic (412910) | more than 5 years ago | (#25288875)

Should I try it from my mobile phone?

Re:Already slashdotted! (4, Funny)

old7 (564621) | more than 5 years ago | (#25288909)

Their web server must be a cell phone.

served by a Mac (the Emperor is ill) (3, Interesting)

operator_error (1363139) | more than 5 years ago | (#25289515)

http://www.tidbits.com/about/in-use.html [tidbits.com] 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.

Re:Already slashdotted! (0)

Anonymous Coward | more than 5 years ago | (#25290161)

Perhaps the author compiled Apache for his new Android phone. Should have used thttpd .. tsk tsk

Re:Already slashdotted! (4, Insightful)

electrictroy (912290) | more than 5 years ago | (#25288997)

>>>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).

Re:Already slashdotted! (4, Informative)

nwf (25607) | more than 5 years ago | (#25289109)

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.

Re:Already slashdotted! (4, Interesting)

Volante3192 (953645) | more than 5 years ago | (#25289359)

And yet still cost more than an actual call...

Re:Already slashdotted! (2, Informative)

Billhead (842510) | more than 5 years ago | (#25289825)

Not at all.
Normally, you pay for a voice plan, and if you go over you get charged a ridiculous amount per minute.
There is typically either no text messages included in that plan, or something like 200.
I use Sprint, and here is their prices:
Unlimited everything (the only way to get unlimited voice) - $100 per month.
Adding unlimited text messages to a normal plan - $20 per month.
If you don't have an unlimited voice plan, you get charged around $.40-$.45 per minute over, twice as much as the $.20 for a text message.

Re:Already slashdotted! (5, Informative)

mc900ftjesus (671151) | more than 5 years ago | (#25289969)

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.

Re:Already slashdotted! (3, Funny)

Piranhaa (672441) | more than 5 years ago | (#25290229)

You forgot the best part about texting. Assuming it's not at night, you can do it more inconspicuously while driving!

Re:Already slashdotted! (0)

Anonymous Coward | more than 5 years ago | (#25290401)

According to the article, which only specifies this for GSM (and therefore not the non-GSM US providers like Verizon), the reason text messages can get through is because they use the signaling bands instead of the data bands. My phone for sure does not buffer the text message, as I've had sending of them fail when there was no reception at all (inside buildings with thick walls, especially).

website for nerds, not norms (-1, Flamebait)

killercentipedes (782350) | more than 5 years ago | (#25288877)

he [adamengst] may not understand how they work. but for the majority here i might think the understanding is natural.

Re:website for nerds, not norms (0)

Anonymous Coward | more than 5 years ago | (#25289015)

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.

Re:website for nerds, not norms (1)

RiotingPacifist (1228016) | more than 5 years ago | (#25289287)

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.

/.ed (0)

Anonymous Coward | more than 5 years ago | (#25288883)

That was fast...

How the internet works behind the scenes. (0)

Anonymous Coward | more than 5 years ago | (#25288907)

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

Re:Mirror (2, Funny)

commodoresloat (172735) | more than 5 years ago | (#25288975)

Here's a much better mirror [artandanti...porium.com].

Re:Mirror (1)

iamdrscience (541136) | more than 5 years ago | (#25289043)

Mod parent down. (Score: -1, Ugly Mirror).

Re:Mirror (0)

Anonymous Coward | more than 5 years ago | (#25289875)

Mod parent down. (Score: -1, Ugly Mirror).

That's not the mirror...

Re:Mirror (0)

Anonymous Coward | more than 5 years ago | (#25289931)

Actually, I would mod it (Score: -1, Made Me Expect Lame Animated GIF Gag That Never Happened). It's disappointing when you stare at a picture waiting for a giraffe [nedmartin.org] or something but never get to see it.

Re:Mirror (1, Funny)

eln (21727) | more than 5 years ago | (#25289741)

Just what exactly are you trying to pull? That mirror is defective! I can't even see my reflection in it!

/.'d already (0, Redundant)

Anonymous Coward | more than 5 years ago | (#25288929)

meh

Server error! (-1, Offtopic)

Anonymous Coward | more than 5 years ago | (#25288959)

The server encountered an internal error and was unable to complete your request.

Error message:
couldn't create child process: 35: index.cgi

If you think this is a server error, please contact the webmaster.
Error 500

Shortlist of answers: (5, Funny)

iamdrscience (541136) | more than 5 years ago | (#25288963)

The real answers:

why text messages work when voice calls are dropped

Text messages are magic.

why your battery lasts longer in some places than in others

Some places are magic.

why you're not allowed to use phones on airplanes

Pilots are afraid of magic.

why you can be notified of a voicemail message when your phone never rang

Voicemails are magic.

Re:Shortlist of answers: (0, Offtopic)

.sig (180877) | more than 5 years ago | (#25289067)

It's the magic smoke inside the cell phones that do it, it's ionized such that it can it maintains polarity with the magic cell towers. That's why if you break one, letting the magic smoke out, it won't work anymore.

Re:Shortlist of answers: (0)

Anonymous Coward | more than 5 years ago | (#25289623)

Why battery lasts or doesn't last, ?
  A cellphones transmitter is not fixed output power ,
  When close to a cell tower or where the received signal is strongest, the phones p RF output power is reduced .
As the cell tower signal gets weaker or remains weak, the phones RF power is increased in increments and sometimes to full power
So if your nearby to the cell tower(s) more often , the battery will last much longer because less energy is used from the battery per unit time .

wrong audience, buddy (0)

Anonymous Coward | more than 5 years ago | (#25288995)

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).

Re:wrong audience, buddy (5, Insightful)

Kenshin (43036) | more than 5 years ago | (#25289165)

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."

Re:wrong audience, buddy (4, Interesting)

Curmudgeonlyoldbloke (850482) | more than 5 years ago | (#25289273)

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."

Re:wrong audience, buddy (5, Informative)

BLKMGK (34057) | more than 5 years ago | (#25289599)

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]

Re:wrong audience, buddy (4, Funny)

shadow349 (1034412) | more than 5 years ago | (#25290111)

However a friend of mine who is a balloonist years ago told me what happened when he used a cell phone in flight - chaos! ... multiple cell towers cannot easily handle being contacted by a single phone moving 500miles an hour.

Holy shit! What type of balloon was your friend piloting? Was it one of those Led Zeppelins I've heard so much about?

Re:wrong audience, buddy (1)

BLKMGK (34057) | more than 5 years ago | (#25290273)

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

Re:wrong audience, buddy (0, Troll)

BobMcD (601576) | more than 5 years ago | (#25290195)

But they worked SO WELL on the September 11th flights...

Re:wrong audience, buddy (2, Interesting)

BLKMGK (34057) | more than 5 years ago | (#25290311)

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:wrong audience, buddy (1)

element-o.p. (939033) | more than 5 years ago | (#25289541)

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.

Re:wrong audience, buddy (2, Interesting)

PolygamousRanchKid (1290638) | more than 5 years ago | (#25289747)

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 (1)

geekoid (135745) | more than 5 years ago | (#25289791)

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.

Re:wrong audience, buddy (1)

MetalPhalanx (1044938) | more than 5 years ago | (#25290285)

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.

Re:wrong audience, buddy (1)

repvik (96666) | more than 5 years ago | (#25289773)

why text messages work when voice calls are dropped

Retries.

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.

Re:wrong audience, buddy (0, Flamebait)

Hikaru79 (832891) | more than 5 years ago | (#25289805)

It's amazing how you can be so smug and condescending while still being wrong on every single one of your throaway answers. I'd think you were trying to make a joke if I could find any sort of humour in your post, but I can't. Try reading the article, because it seems that your knowledge on these issues is less than those teenage girls you look down on.

Why does morse code work? (1)

p51d007 (656414) | more than 5 years ago | (#25289047)

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

Re:Why does morse code work? (0)

Anonymous Coward | more than 5 years ago | (#25289217)

Good, except that 73 doesn't come with apostrophe and an "s". One of the first things my elmer taught me once I got on the air.

Re:Why does morse code work? (1)

JDHawg (800829) | more than 5 years ago | (#25289219)

This is exactly why I finally got my amateur license. Two weeks with little communication to the outside world (cell phones worked for about 15 minutes each night) following Hurricane Katrina to let family know we were OK was unacceptable. Now my father and I have a scheduled frequency and time to make contact during hurricanes. 73's W5KDH

Re:but it's all digital (well binary states) (1)

Dare nMc (468959) | more than 5 years ago | (#25290089)

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.

Short summary isn't always good (5, Informative)

bendodge (998616) | more than 5 years ago | (#25289101)

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 (3, Insightful)

mmontour (2208) | more than 5 years ago | (#25289259)

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.

Re:Short summary isn't always good (1)

AlejoHausner (1047558) | more than 5 years ago | (#25290427)

Actually, maybe airlines don't allow phones on planes because they are being considerate to other passengers who don't want to spend 6 hours sandwiched next to a blabbermouth. Then again, why would the companies who fly sardine cans with wings care about the comfort of their prisoners? Alejo

Re:Short summary isn't always good (5, Informative)

bws111 (1216812) | more than 5 years ago | (#25289289)

Well, at least your number 3 is wrong. Cell phones are not allowed on planes because a few hundred phones simultaneously hopping from tower to tower at several hundred MPH wreaks havoc on the phone system. It is an FCC rule, not an FAA rule.

Re:Short summary isn't always good (0)

Anonymous Coward | more than 5 years ago | (#25289809)

Glad someone read the article!

Re:Short summary isn't always good (1)

snowraver1 (1052510) | more than 5 years ago | (#25290201)

Yet I bet every day, 1,000's of phones are left on in the air unintentially, without consequence.

Re:Short summary isn't always good (0)

Anonymous Coward | more than 5 years ago | (#25290449)

This is not a limitation on CDMA network. In fact, you are often connected to more than one tower with CDMA and it helps improve the signal quality.

GSM really should be abandoned for CDMA, but there is so much invested in it that no one is willing to dump it and buy all new equipment.

Phones on airplanes (4, Informative)

dj245 (732906) | more than 5 years ago | (#25289343)

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.

Re:Phones on airplanes (2, Insightful)

Anonymous Coward | more than 5 years ago | (#25289495)

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:Phones on airplanes (1)

at_slashdot (674436) | more than 5 years ago | (#25289731)

So you want to say that you are not _allowed_ to use phones on the ariplanes because you _can't_ use them?

Re:Phones on airplanes (2, Informative)

DaveV1.0 (203135) | more than 5 years ago | (#25290017)

While you are correct about #2, #1 is demonstrably false.

As far as the GPP goes, the reason cell phones were banned on airplanes was concern about possible interference with avionics and instrumentation. The ban goes back to when cell phones were first popularized.

Re:Short summary isn't always good (1)

ethanms (319039) | more than 5 years ago | (#25289391)

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 (4, Informative)

SuiteSisterMary (123932) | more than 5 years ago | (#25289655)

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.

Re:Short summary isn't always good (1)

CyrusOmega (1261328) | more than 5 years ago | (#25289513)

Actually the site came up fine and I didn't notice any ads. Furthermore the content was non-trivially interesting

For example (2) isn't just distance but also signaling competition with other phones in the area.

The writer expands (3) to the include technical issues as well.

Re:Short summary isn't always good (1)

BLKMGK (34057) | more than 5 years ago | (#25289627)

Well you managed to get one right pulling ideas out of your ass!

Re:Short summary isn't always good (0)

Anonymous Coward | more than 5 years ago | (#25290193)

1. Text messages work when voice calls are dropped for the same reason Morse can get through when SSB voice can't.

not according to the article, according to the article it's all digital (voice is digitized), just a separate transmitter for data than voice, in a different portion of the spectrum. So SMS cannot be saturated by too many voice calls (but voice calls can't be placed if this data channel is saturated, so voice will never work when SMS doesn't.)

why the redundant "story" in all the tags?? (-1, Offtopic)

CodeMunch (95290) | more than 5 years ago | (#25289285)

tags are sucking worse than usual this week. Why is "story" in every damn ARTICLE posting? who runs this place?

Why is whatcouldpossiblygowrong laser and sharks not a permanent tag on every article?

Re:why the redundant "story" in all the tags?? (0, Offtopic)

KookyMan (850095) | more than 5 years ago | (#25289455)

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.

Next up, (-1, Redundant)

Hawthorne01 (575586) | more than 5 years ago | (#25289295)

an article on why websites crash when slashdot links to them.

Re:Next up, (0, Troll)

legoman666 (1098377) | more than 5 years ago | (#25289639)

The same reason your asshole bleeds when you stick a large object in it.

I don't understand this about mobile networks .. (3, Interesting)

instinct71 (1076915) | more than 5 years ago | (#25289321)

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 ?

Re:I don't understand this about mobile networks . (1, Informative)

Anonymous Coward | more than 5 years ago | (#25289355)

A phone sends a message to unregister itself right before powering of.

Re:I don't understand this about mobile networks . (1)

instinct71 (1076915) | more than 5 years ago | (#25289419)

All right. So my phone goes out of reach if my battery falls off the phone suddenly ?

Re:I don't understand this about mobile networks . (1)

ipb (569735) | more than 5 years ago | (#25289753)

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.

Re:I don't understand this about mobile networks . (0)

Anonymous Coward | more than 5 years ago | (#25289939)

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.

Re:I don't understand this about mobile networks . (5, Informative)

Isvara (898928) | more than 5 years ago | (#25289589)

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.

Re:I don't understand this about mobile networks . (1)

timmyf2371 (586051) | more than 5 years ago | (#25289685)

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.

How it Works for Me (5, Funny)

sycodon (149926) | more than 5 years ago | (#25289475)

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... (1)

Junta (36770) | more than 5 years ago | (#25289555)

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...

/.ed (0, Offtopic)

ray13eezy (1237106) | more than 5 years ago | (#25289661)

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)

You have 1 Missed Call (-1, Troll)

Anonymous Coward | more than 5 years ago | (#25289853)

Frist psot!!!

Next on Slashdot. (5, Funny)

SeNtM (965176) | more than 5 years ago | (#25289901)


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?

signal strength? (2, Informative)

Anonymous Coward | more than 5 years ago | (#25290359)

"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?

GPS explanation is total BS (4, Informative)

harrkev (623093) | more than 5 years ago | (#25290363)

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.

GPS uses signal strength? (1)

stevenm86 (780116) | more than 5 years ago | (#25290389)

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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