| A Mobile Telephone (also known as a Cellular | | | | tens of metres using the principle of triangulation. A |
| Telephone) is defined as a 'portable electronic device | | | | control panel signal can be broadcast to your phone |
| for the purpose of telecommunications over long | | | | from the three cells nearest to you. The time it takes |
| distances'. Which boils down to 'a telephone you can | | | | for the signals to reach your phone is measured and |
| roam freely with'. Most current mobile phones actually | | | | this can be used to pinpoint the location of your phone. |
| connect to a cellular network of base stations (the cell | | | | Once the call has been connected then he call still has |
| sites themselves) which overlap to yield coverage and | | | | to be maintained. The data are transmitted to and |
| which also link to the standard landline public switched | | | | from your phone via traffic channels which are the |
| telephone network. | | | | data transmission and reception channels of the mobile |
| Most radio transmitters work by transmitting and | | | | network. To maximize the number of phones that can |
| receiving on the same radio channel so that you | | | | be used on a network these traffic channels are |
| cannot talk and listen at the same time. However, | | | | different between adjacent cells. Therefore if you |
| mobile phones are what's called 'full duplex' radio | | | | move from one cell to the next and your phone wasn't |
| devices. The radio channels for transmitting and | | | | told you had moved then because of the different |
| receiving data are separate. As a result you can both | | | | traffic channels used in each cell your phone actually |
| talk and listen to a conversation at the same time. | | | | wouldn't work! |
| In fact, mobile phones use three channels for | | | | To overcome this problem a process called |
| communication. The first of these, the control channel | | | | 'hand-over' is employed. Whilst your mobile phone is in |
| is completely dedicated to the network. It is this | | | | use it actually constantly monitors the control channels |
| channel that the network uses to communicate with | | | | of up to sixteen of the neighbouring cells closest to it. |
| your phone. It is this that informs the network where | | | | The phone then works out which of these cells |
| your phone is so that you can connect to the | | | | produce the strongest signals and broadcasts a list of |
| appropriate cell. It's the network's the control channel | | | | the top six cells by signal strength back to the base |
| that tells the system which cell you are in, so that this | | | | station to which it is currently connected. If a phone |
| information can be stored in the system's database. It | | | | moves away from the base station then the signal the |
| is entirely because of this that an incoming call can be | | | | phone produces is boosted to generate a clearer |
| routed to your mobile phone. When an incoming call is | | | | signal. However, if this boosting of signal strength no |
| received by the network the network then sends a | | | | longer works then the network consults its database |
| message to your phone saying that there's an | | | | and instructs your mobile phone to switch to another |
| incoming call. This message is routed to the cell that | | | | cell. This triggers a handover where the phone adjusts |
| represents the last location that the system knows | | | | its reception and transmission frequencies so that it |
| your phone to have been in. Just to be sure the same | | | | can now work with the closest cell that will give the |
| message is also transmitted to that cell's immediate | | | | best signal strength. |
| neighbours, just in case you have moved since the last | | | | As you can see, mobile communication occurs by |
| time your location was recorded. Even if you have | | | | radio communication and many complex processes |
| wandered out of this cluster of cells the network | | | | occur to give you a seamless calling experience, even |
| should have recorded this and updated its database. In | | | | when you are on the move. |
| fact your mobile phone can be located to within a few | | | | |