Cells, identifiers and how the network can locate you

My first post tried to be an approach to the cell idea, so in this second part I am going to explain what is, in fact, a cell, how it works and how the service provider can determine in which cell you are at every moment.

Despite there are a number of differences depending on the technology (2G, 3G, 4G), the mobile network model always follow a hierarchical scheme. It is usual to see the network as two separated segments:

• The 'core' segment, the wired part of the network, responsible for communications routing (through packet or circuit switching), mobility management and billing. It is usually composed of a central element (the switching center) and a number of databases for storing information about users. 
• The 'radio access' segment, which is the true 'wireless' part, responsible for the establishment of the radio link between network and users. The basic element of the radio access network is the base station, which is the set of devices (transmitters, receivers, modulators, antennas...) that provide service to a single cell, and several base stations are managed by higher level elements. These are the base station controllers, whose main task is to distribute network resources among its dependent base stations.

A very simplified model of the mobile network architecture.

The total service area of the network is partitioned into smaller geographical regions, to help tracking users location. In GSM/GPRS/UMTS technologies, these regions are known as 'Location Areas', in the case of circuit switching, and 'Routing Areas', for packet switching, whereas LTE introduces the concept of 'Tracking Area'. This logical division is pretty useful in order to optimize signalling, paging procedures, etc.

Each cell, as well as the location area it belongs to, are unambiguously identified by a pair of numbers: the cell ID (CID) and the Location Area code (LAC). These two identifiers allow the service provider to know exactly in which cell the user is. Even though this location method is far from being as accurate as GPS, it is very resource-saving for the network.

Apparently, cells, location areas and all the related stuff are transparent to the user. Is there any way we can access this concealed information? Can we get to know the cell we are connected to? Well, we can write a really simple Android code to show a lot of network parameters (such as CID, LAC, mobile network code, mobile country code, and so on...), it isn't difficult at all, but for the moment, to keep it simple, here is a quick way to reveal this information.

If you own an Android device, try to type *#*#4636#*#* in the dial screen. This code will open the Engineering test menu, where you can check, among other things, network related information.

Phone info screen

In the screenshot above we can see the phone's IMEI, our service provider and, of course, the stuff we are looking for: the network details. We can find the LAC and the CID of the tower we are connected to, the signal strength our antenna is receiving (measured both in dBm and asu), and a list of neighboring cells in ID@strength format. iOS presents a very similar feature, just by typing *3001#12345#*, but the results it retrieves are slightly more extensive, including bandwidth usage and other interesting data.

Easy, right? In the next post I will show you how to implement a simple application to get a more detailed insight of the network parameters.