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."

Comments

Mirror

[Exstatica]

Here is a mirror http://mirrors.mednor.net/slashdot/10072008/TidBITS_Networking%20_Peering_Inside_a_Mobile_Phone_Network.htm

Short summary isn't always good

[bendodge]

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

[bws111]

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

[SuiteSisterMary]

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

[Free the Cowards]

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.

Re:Already slashdotted!

[nwf]

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.

Phones on airplanes

[dj245]

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:I don't understand this about mobile networks .

[Isvara]

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:wrong audience, buddy

[BLKMGK]

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

Re:Already slashdotted!

[mc900ftjesus]

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.

GPS explanation is total BS

[harrkev]

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.