If someone calls me from across the US is my phone pinging the local tower every so often? And then there is a central database that says my phone is now in the area and then it rings?
Yes, while it is turned on, your cell phone continually contacts the cell towers near you, to establish its location, and determine from which tower it’s getting the strongest signal. As you move around (e.g., if you’re driving), your phone will be “handed off” from one cell tower to another, always attempting to establish its connection with the strongest signal.
This is my understanding of what kenobi_65 wrote: While I’m moving around, the phone continually contacts the cell towers, and at some point, the phone notices that “Cell tower X5G64Q used to be the one closest to me, but now I am nearest to 7K9PQ7.”
But that information is pretty useless. I think the OP was asking if there is a central database somewhere which records which tower is closest to cell phone 212-555-4829, and it updates that record from “X5G64Q” to “7K9PQ7” so that incoming phone calls can be routed to that phone.
If such a database exists, where is it and who runs it? And if not, how do incoming calls find me?
Just to clarify my previous post: When my phone gets close to tower 7K9PQ7, it doesn’t really care which tower it’s close to, only that there is a clear signal from somewhere. But perhaps the tower is now aware that 212-555-4829 is nearby, and sends that info somewhere.
I have a MSc in this and fixed a bug in Ericsson’s network equipment as a summer job in 1998, but I still found this a difficult question to answer. (Mind you I haven’t worked with the mobile infrastructure since and haven’t worked in telecom for 15 years.)
The database (in GSM and UMTS) is the Home Location register and is a part of the network infrastructure, so it’s owned by whomever runs the network.
Mind you your phone also does data, which doesn’t use all the same bits as a voice call does. I’m not sure the HLR is part of that. And when the network operators were forced to let other companies sell cell phone subscriptions without owning their own hardware that had to be accommodated somehow, and I don’t know exactly how.
It feels like a few steps are missing. If I dial a phone number, “The Phone System” has a number of things to figure out:
- Is this a mobile number or a landline?
- If mobile, which carrier? AT&T, Verizon, or…?
- Once a carrier is known, which tower is currently in contact with the phone?
It sounds like you’re describing #3, which assumes that The Phone System knows this is a mobile number and also knows which carrier owns the number.
How are steps #1 and #2 determined?
Sorry, I don’t have an answer, but I’ve often pondered this as well. However, I’d expand the question from:
to how can I be found anywhere in the world.
This really hit me a few years ago; we were travelling in a backwater village in Burma (no electricity, running water, sanitary systems etc., but great cell signal) and I got a call from my son in Canada. Of course, I knew this could happen, but it really made me wonder about the technology and the processes involved.
The thought that across the world computers are somehow constantly tracking billions of cell phones is mind boggling.
From my very very short stint as customer tech support for verizon, everything, calls text messages etc to and from a specific phone goes like this
From address(your phone #) goes to the tower which then sends it to a central processor (variously known as the switch, also called a computer) for you physical area. That switch does its thing via the internet to determine the correct routing for your traffic, text emails etc are sent via what are essentially email servers, voice calls get routed over ATT’s land lines to the recieving station. When you are recieving traffic, the central computer for the system nationwide knows what region you’re in based on cell tower reporting and sends out a query to all the towers in that region to see which has the strongest lock on you and then routes the traffic to that tower.
A lot of this is done at the tower itself and most of the rest is done over the internet.
This is my hazy memory from many years ago and is thus pretty vague and probably wrong about many of the details.
This fails when I suddenly appear out of nowhere. The best example of this is when my phone has been turned off for an airline flight - or even if the battery ran down - and I don’t reconnect to the system until I am hundreds or thousands of miles away.
At that point I imagine that my local cell tower gets my ping, and tells all its friends, “Yo! Everybody! Cell phone 212-555-4829 just popped up out of nowhere, i.e., none of you passed it off to me. So go tell the Central Database that if they get any incoming calls, I’m your guy.” And the question remains: exactly what and where is the Central Database.
I think you have the order reversed here. The phone system starts by figuring out which carrier/phone company the number belongs to and sends the request to them, and they then, if they operate both landlines and cell-phones will check which it is.
Both calling while connected to a different providers network in the US and in a different country is called roaming. How it works is … complicated: Roaming
How can they do that without a Central Database?
Where did I say there were no central databases?
As mentioned above the central databases for knowing where a cell phone is is the HLR(s?) in each provider’s network. How it’s decided what provider has each number is a different question, and will vary from country to country because it builds on what was put into place to digitize the fixed network when that was the dominant network.
Very roughly, when a phone is first turned on it looks about for any cell it can talk to. It looks for any cell tower broadcasting away with its “hi there, would you like to talk to me” message. It will initiate a conversation with the cell towers, providing identity via its International Mobile Subscriber Identity (IMSI) stored in the SIM, from here the cell system will work out who it is, and most importantly, if they are able to bill anyone.The IMSI contains a country code, and a network code - which identifies each phone company, as well as a subscriber code (you). So, if your own phone company has a deal with another carrier in the country you have popped up in, that company’s connection processing will recognise that your phone’s SIM is one from a phone company it has a deal with, and it will negotiate back to you home phone company for the needed info. So now your home telco knows where to direct calls, and the local company knows how to bill for the calls. (This is going to be more complicated, as it needs to deal with both the carriers of the call and the actual companies that are selling the calls, and they are often not the same. But once the carriers know about one another, calls can be routed.)
If a cell is owned by a company your phone company doesn’t have a deal with, it won’t be able to set things up. Usually any cell is meant to allow emergency calls, so if needed, no matter what, a cell tower will allow an emergency connection request. But there is no way of calling the phone. (You can often see all the cells your phone can see, both useful and not, but it depends on you individual phone how easy this is.)
Once all the billing info and location info is set up the phone will “camp”. It listens, but won’t transmit again unless it sees it is moving to another cell, when handover occurs.
One very annoying issue is that if you are in another country, if a local calls your mobile phone, the call goes back to your home network, and then comes back to your phone. Everyone gets charged for an international call, even if you are in the same room.
As we move to more IP based protocols this will slowly improve (we hope.)
Ah there are the fine details I couldn’t remember, from Francis Vaughan.
The central database was a bit of a misnomer on my part. the system has nodes and stuff but it’s not all located in one place and a lot of what happens is pretty decentralized and no not really, the cell towers, at least for verizon, actually talk to each other, so as you move from cell to cell, you don’t just suddenly switch towers, verizon uses a “soft hand off” the cell tower you are leaving talks to the tower you are going to and the recieving tower picks you up before the sending tower drops you.
There are two different situations: making calls, and receiving calls. When I initiate a call, I talk to whatever cell tower is nearby, and I’m connected to the system, and for that stuff I’m not asking any questions.
The part that’s confusing to me is receiving calls. When someone is calling my call phone, how does the system know where I am. There are two possibilities: There is a central database, or there isn’t.
Possibility #1: There is a central database of all phone numbers
If there IS some sort of central database, then the caller [more specifically, the caller’s phone, or the caller’s cell tower, or the caller’s service provider, but let’s just use the term “caller” for simplicity] could ask it about my specific phone number, and it would tell the caller where my phone is located, or it could at least tell the caller who my service provider is, and then the caller would ask the same questions to my service provider, who has a their own central database, but only of their own customers. With all this information, the system knows how to route the call.
I imagine this was relatively simple when everything was a landline, and one company ran the phone system for each area. It got a little more complicated with the introduction of exchanges and area codes, but as long as it was just one company, it was manageable. When multiple phone companies were allowed, each bunch of 10,000 phone numbers in each area code / exchange were assigned to one specific company, and (if my memory serves me correctly) there definitely WAS a list whereby the system could look up any area code / exchange, and thus know who the service provider was, and the call would go through.
Possibility #2: There is no central database of all phone numbers (like some posters say)
But nowadays, even if I know your phone number, I won’t have a clue who your service provider is. You might have switched multiple times just in the last year. The phone number will give me a clue about where you live, but it is more accurately a clue about where you used to live when you got that phone number. And in any case, your number has no information about where the phone was located when you turned it on after your flight landed a few minutes ago.
So if there isn’t any central database, how does the system find a phone? The only way I can think of is by going through ALL the providers one by one, and asking each, “Is this phone number one of yours?” That will work okay most of time, because the vast majority of phones are serviced by a relatively small number of providers. [I’m being deliberately vague because (a) I don’t know what the numbers are, and (b) They don’t matter anyway,] But what if the target phone belongs to a small company, perhaps one that has been in business only a short while? There’s got to be a central database somewhere - if not of every single phone number, then at least of all the providers.