CELL PHONE

   "NECESSITY IS THE MOTHER ON INVENTION"

Today, every human has an extra organ called cell phone... which is of variety of models and size...i say it as organ since it is being with us all around the clock..
 Every one is using it but very few know how it works..
as i said already i love to again knowledge... i put little effort to understand it... here is that...

CELL PHONE

This is the advanced version of radio,but sophisticated and secured one... As like as the radio each network operator gets their own unique carrier frequency with which they modulate the signals...

• A cell-phone carrier typically gets 832 radio frequencies to use in a city.
• Each cell phone uses two frequencies(one to receive and other to transmit) per call -- a duplex channel -- so there are typically 395 voice channels per carrier. (The other 42 frequencies are used for control channels -- more on this later.)      

The carrier chops up the city into cells. Each cell is typically sized at about 10 square miles (26 square kilometers). Cells are normally thought of as hexagons on a big hexagonal grid, like this:

The about 395 channels are divided into 7. each cell has about 56 channels (i.e. 56 people can be talking on their cell phone at a same time). the cell around will not have this same 56 channel.. but the cell outside the cells around the center can have the same this same 56 channel.. this is to avoid the repetition of channel
.

when the customer goes from one cell to another the signal to the current base station will diminish and the base station of the next cell will find the arrival of other channel with in its range and the two base stations coordinate with each other through the MTSO, and at some point, your phone gets a signal on a control channel telling it to change frequencies. This hand off switches your phone to the new cell.

Let's say you're on the phone and you move from one cell to another -- but the cell you move into is covered by another service provider, not yours. Instead of dropping the call, it'll actually be handed off to the other service provider.this is roaming

once the mobile is switched ON... our mobile will search for same SID(system identification number).. once the same SID is found, it will setup a connection with the base station using the control channels.. else it will say "NO SERVES" or " NO NETWORK COVERAGE"..

our mobile will communicate with base station using the control channels about call set up and channel changing.. for this communication sets up only if our phone system identification number(SID) should match with the base station SID... always there will be a connection between our mobile phone and the base station..with this the mobile telephone switching office(MTSO) will maintain a database of their customers..

once a call arrives.. the MTSO will find the location of the specific customer with the help of the data base.. and divert the call to the nearest base station... then the base station will allocates two frequency(one freq. to receive and another to send the date) which is support by the customer's phone.. then the call will be transferred..

when the customer wants to make a call the reverse operation will be done..