Across the world, mobile phones are revolutionalising the way people communicate. They are being used for all manner of things, particularly in conservation circles (t4cd is testament to that). What on the surface may appear to be complex piece of equipment, however, hides a device which is not totally dissimilar to the humble radio. Indeed, in the days before mobiles phones as we now know them, one of the few ways to communicate on-the-move was via ‘radio telephone’. Physically wired into a car or other vehicle, the user could communicate via one of only 25 channels via fixed transmitters. Not only were these handsets far too heavy to carry around, use was limited by range and, with only a limited number of channels, the system would often be too busy.
The introduction of the ‘cell’ system changed all that. By dividing an area up into smaller areas, or cells, range could be increased, handsets would need less power to communicate with multiple and widely-spread base-stations, and far more channels could be made available to users. Now, rather than having a single tower, stations were installed in a fixed pattern, like a honeycomb, providing blanket coverage. Although the number of base stations required in an area depends on factors such as user density and terrain – inner city or mountainous areas typically require more – each cell can typically cover 10 square miles, or 26 square kilometres.
As a mobile phone user moves from one cell to another, the call is passed onto the new cell automatically using a special signal sent to the handset. If the user isn’t making a call, then details of the particular cell they are moving into is still passed on – this way the network knows exactly where the phone is should someone wish to call. (This is also how location-based services work – “where is the nearest train station?”, for example – and why mobile phones are now beginning to be used instead of GPS systems to determine location). Some handset manufacturers, such as Nokia, often allow the user to ‘Display Cell Info’ on their mobile screen – this gives the cell name or number that the user is currently in.
This ‘cell’ approach requires a large number of base stations in a city of any size, with a typical large city having hundreds of towers. Each carrier (network operator) in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all of their mobile connections to the normal land-based phone system, and controls all of the base stations in the region.
So, what happens when a user receives a call on a mobile phone? Well, all cell phones have special codes associated with them. These codes are used to identify the phone, the phone's owner and the service provider, and are passed around the network seamlessly, without the user knowing or having to do anything. The simple act of switching on a mobile actually calls into play a sequence of events, and these are outlined below.
When a user first powers up their phone, it listens for an SID (System Identification Code – a unique number linked to their network operator) on a particular frequency (know as the “control channel”). The control channel is a special frequency that the phone and base station use to talk to one another about things like call set-up and channel changing. If the phone cannot find any control channels to listen to, it knows it is out of range and displays a "No service" message. If the handset does receive an SID, it compares it to the SID programmed into SIM card in the users phone. If the SID matches, the phone knows that the cell it is communicating with part of its own network. If not it will search for others, until it finds one. If no valid SID is found, a “No service” message will be displayed.
Along with the SID, the phone also transmits a registration request, informing the MTSO of your location – via the base station you are communicating through – so that it can then keep track of your phone's location. This location is kept in a database and is constantly updated as you move around the network. Without this the MTSO would not know where your phone was located, and would be unable to route any calls through. When someone does call, the MTSO first picks up the call and then tries to find you. It looks in its database to see which cell you are in, and then picks a frequency pair (one for incoming voice, and one for outgoing) that your phone will use in that cell to take the call.
The MTSO then communicates with your phone over the control channel to tell it which two frequencies to use, and once your phone and the tower switch to those frequencies, the call is connected and the handset rings. The user then decides whether or not to take the call. While the call is in progress, the handset constantly monitors signal strength and location, and uses this information to keep the two users connected.
As a user moves towards the edge of their active cell, the base station notes that signal strength is diminishing. Meanwhile, the base station in the cell the user is moving towards (which is also ‘listening’ and measuring the signal strength) sees the phone approaching. The two base stations co-ordinate with each other through the MTSO and, at some point, the handset gets a signal on the control channel telling it to change frequencies. This hand off automatically switches the phone to the new cell, and so the process repeats
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When a user travels abroad and uses a ‘non-home’ network, the registration process is slightly different. If the SID on the control channel does not match the SID programmed into the SIM card then the phone knows it is roaming. The MTSO of the cell that you are roaming in contacts the MTSO of your home system, which then checks its database to confirm that the SID of the phone you are using is valid. Your home system verifies your phone to the local MTSO, which then tracks your phone as you move through its cells.
Mobile images courtesy of the Mobile Gallery at kiwanja.net