Mobile Phones – How they Work

A Mobile Telephone (also known as a Cellular Telephone) is defined as a ‘portable electronic device for the purpose of telecommunications over long distances’. Which boils down to ‘a telephone you can roam freely with’. Most current mobile phones actually connect to a cellular network of base stations (the cell sites themselves) which overlap to yield coverage and which also link to the standard landline public switched telephone network.

Most radio transmitters work by transmitting and receiving on the same radio channel so that you cannot talk and listen at the same time. However, mobile phones are what’s called ‘full duplex’ radio devices. The radio channels for transmitting and receiving data are separate. As a result you can both talk and listen to a conversation at the same time.

In fact, mobile phones use three channels for communication. The first of these, the control channel is completely dedicated to the network. It is this channel that the network uses to communicate with your phone. It is this that informs the network where your phone is so that you can connect to the appropriate cell. It’s the network’s the control channel that tells the system which cell you are in, so that this information can be stored in the system’s database. It is entirely because of this that an incoming call can be routed to your mobile phone. When an incoming call is received by the network the network then sends a message to your phone saying that there’s an incoming call. This message is routed to the cell that represents the last location that the system knows your phone to have been in. Just to be sure the same message is also transmitted to that cell’s immediate neighbours, just in case you have moved since the last time your location was recorded. Even if you have wandered out of this cluster of cells the network should have recorded this and updated its database. In fact your mobile phone can be located to within a few tens of metres using the principle of triangulation. A control panel signal can be broadcast to your phone from the three cells nearest to you. The time it takes for the signals to reach your phone is measured and this can be used to pinpoint the location of your phone.

Once the call has been connected then he call still has to be maintained. The data are transmitted to and from your phone via traffic channels which are the data transmission and reception channels of the mobile network. To maximize the number of phones that can be used on a network these traffic channels are different between adjacent cells. Therefore if you move from one cell to the next and your phone wasn’t told you had moved then because of the different traffic channels used in each cell your phone actually wouldn’t work!

To overcome this problem a process called ‘hand-over’ is employed. Whilst your mobile phone is in use it actually constantly monitors the control channels of up to sixteen of the neighbouring cells closest to it. The phone then works out which of these cells produce the strongest signals and broadcasts a list of the top six cells by signal strength back to the base station to which it is currently connected. If a phone moves away from the base station then the signal the phone produces is boosted to generate a clearer signal. However, if this boosting of signal strength no longer works then the network consults its database and instructs your mobile phone to switch to another cell. This triggers a handover where the phone adjusts its reception and transmission frequencies so that it can now work with the closest cell that will give the best signal strength.

As you can see, mobile communication occurs by radio communication and many complex processes occur to give you a seamless calling experience, even when you are on the move.



Source by Dyfed Lloyd Evans

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