Recently you may have been reminded how difficult it can be to get through to friends and loved ones when a major event is occurring.  Whether it be the World Cup or a terrorist attack, messages of sadness or cheer, mobile networks seem to just stop working.

Why? The short answer is, mobile networks get congested—overloaded with calls trying to get through. But the longer answer is that the mobile network is made up of different technologies, and different levels, and there are actually several different places where congestion can happen. 

The first level is the mobile switching centre that will usually cover quite a large area. By switching centre we mean a room full of racks, computers and other switching elements. The densest switch is in China, which will serve more than several million customers at a time.

Beyond the mobile switching centre are the groups referred to as radio node controllers. There are dozens to hundreds of these all controlled by one switch. They are located closer to the radios and they deal with handoffs between different radios.

Then there are the individual radios themselves. These are the antennas you see on the top of and sides of buildings. Each is a cell, and via each cell you have users who are connected to the network.

Radios have different sizes of cells—there are regular cells, smaller sub-sells and a larger overlay macro-cells. Your mobile network will try to handle your call within the small cell you're closest to. But it is a trade-off between capacity and coverage, as no network provider can put thousands of cells everywhere, and when you a ton of other people are inundating your closest cell, you may have difficulty getting your call to connect.

Often some of the calls are offloaded to a macro-cell, which helps, unless the macro cell is being bombarded as well.  When it's a planned event like the London Marathon for instance, providers may bring in additional mobile cells. They park little trucks around the edge of the course.

But the bottom line is: in different parts of the system, different things will get congested during major events. In some cases the specific cell site might be overloaded, whereas in other cases it is the connection to the radio node controllers and switching centre.

Do landlines also get congested during big events you may wonder? Historically they have, and they certainly can, but the probability is very small these days given the extensive use of mobiles instead.

If you are interested in telecoms, mobile networks or phone sytems, try checking out tutorial courses like these to learn more.