In normal telephone exchange systems, a pair of wires extends each telephone instrument to the telephone exchange.
Normally, copper conductors are used to give loop to indicate the exchange that the
telephone handset had been lifted for making a call or for receiving a call. In advanced countries, Optical Fibre is also used for extending the loop.
When you make a call to your friend who is connected to another exchange, the digits are sent to the exchange to which you are connected, in the form of pulses (break and make of loop) or frequencies.
The originating exchange analyses these digits and establishes a path to the terminating exchange where your friend is connected. Over this path, signals (some of the digits dialed by you and other technical information) are sent in the form of bits or frequencies.
The terminating exchange analyses and marks your friend’s line and if it is free, a ring is sent to him and ring back tone is sent to you. When the call is answered, metering takes place.
In the cell phone system, there is no permanent wire connection between the instrument and the mobile telephone exchange. The connection between the instrument and mobile exchange is established via an intermediate station called Base Station (BS) where a transmitting tower is erected. The connectivity between the mobile exchange and the BS may be wire and between BS and Cell Phone is a Radio Channel.
The connection between a mobile exchange and another mobile exchange or public telephone exchange is by wire.
Cell phones are technically called Mobile Station (MS) and its telephone exchange is called Mobile Switching Centre (MSC). The radio channel for both-way voice and control channel for controlling are established between BS and MS whenever required only.
A town or an area is divided into smaller areas called Cells. At the centre of the cell, there exists the Base Station (BS). When the MS moves around inside the cell, the signals including voice will be strong. When you use a cell phone to talk to your friend with conventional phone,you will be connected to the BS over a both-way Radio channel. From BS to MSC over a copper cable or Optic Fibre cable, from MSC to another Exchange and further to your friend over a copper cable.
As you are moving and when you enter into another cell, old BS reports to MSC that you have left the cell and your signals are weak. Immediately, the MSC orders all other Base stations to search for a new comer (as far as other cells are concerned, you are a new comer).
The BS, which is very near to you, reports to the MSC that it has found a new comer with strong signals. The MSC orders the old BS to hand off (you) to new BS. Handing off is done in less than 400 milliseconds. But you do not feel any disturbance in your conversation. You may cross many cells during your long conversation without knowing that so many hands off had taken place. Each cell phone is associated with a unique identity stored in Subscriber Identity Module (SIM). This identity is sent to MSC through Base Station when `Send’ button is pressed.
The MSC analyses this and validates as to whether cell phone is authorised to make a call. If yes, the call will be put through. When there is an incoming call to the MS, first it is received by the MSC. MSC analyses the received information that contains the MS number also. This number is paged in all cells. If the MS is kept powered on, a page response is received on the control channel from the MS. A voice channel is allotted by MSC. BS and MS are ordered to tune to that channel. Once tuned through connection information is returned to the caller who gets ring-back tone. Ringing is initiated in the MS. When the MS answers, conversation can start and metering starts in MSC.