The Mobile Revolution
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The Mobile Revolution
Rakesh Mohan Hallen
By March 2004 the total number of cell phone users in India exceeded 3 crores. Experts call it a mobile revolution. People from all walks of life, of all ages use them. They carry it in their hands, pockets or purses. One can find advertisements related to cell phones anywhere: roads, newspapers, and television. Theoretically one can contact a person carrying a mobile (cell phone) anywhere, anytime.
But few are aware of the technology that makes it possible. It is not very difficult to become aware of its nitty-gritties.
How it works
To use a cell phone one needs a handset. The handset of a cell phone, as you might have seen, has a small screen at one end and a panel of buttons below it. Some handsets have a small antenna that is protruding out from its, but it is not universal. Some modern handsets are foldable so that when not in use the panel of buttons is not visible, these handsets have a larger screen that can display colour pictures also.
The essential component of the technology that make cell phone possible are networks of a large number of radio transmitters/receivers. Each cell phone company has a network of this kind that is controlled in each city/state by its central office known as MTSO (Mobile Telephone Switching Office) . You know that our body is made up of many cells. These networks are also cellular akin to the body of any animal. The area covered by a cell phone company, a city or a state, for its services is divided into zones that are called its cells. Thus the network of a cellular company is made up of many such cells. Each of these cells has a moderately sized radio transmitter/receiver and a microprocessor based instrument somewhat like a computer, located at a place known as its base station. Base stations can communicate with each other. They can easily located in a town/city because of the prominent size of their antenna above some buildings.
A cell phone handset is also a low power radio transmitter/receiver. It can transmit as well as receive electromagnetic waves from its closest base station when it is powered on.
As soon as one turns on a cell phone handset it listens for SID (System Identification Code – a unique 5-digit number that is assigned to each carrier by the central authority say TRAI (Telecom Regulatory Authority of India) and is programmed into each handset when it is activated by a company.) on 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. The SID helps a cell phone company to identify cell phone handsets registered with it that are powered ON.
It also transmits a signal that includes the ESN to the base station of its parent company every few seconds. The computers attached to these transmitters/receivers maintain latest information about all the handsets distributed/registered by the parent company. This information at each base station is constantly updated. Thus base station of each cell knows the location of every powered on handset at any instance of time, through the codes SID and ESN.
When one dials a number on a handset signals are first converted into electricity and then into electromagnetic waves that are transmitted by it. The radio receiver of the cell in which handset is located at that time receives this signal. It is in then communicated to its computer to find the nature of the addressed party. If it is addressed to another cell phone serviced by the parent company, it finds out the location of the addressed handset and transmits signals to that base station, which in turn alerts the handset about the incoming call.
In case the call is addressed to a cell phone serviced by another company or to a landline telephone the cell directs the call into relevant network, which then directs the call in a similar fashion.
If a handset moves toward the edge of a cell, the cells base station senses that the strength of the signals from that handset is diminishing. Meanwhile, the base station in the cell towards which it is moving (because is also listening and measuring signal strength on all frequencies) sees that the strength of radio waves from it is increasing. The two base stations coordinate with each other through the MTSO, and at some point, the handset gets a signal on a control channel telling it to change frequencies. Thus the handset is switched to the new cell.
If the handset moves to another city/state the SID on the control channel does not match the SID programmed into the handset. If the cellphone company provides roaming facility then the phone knows it is roaming. The MTSO of the cell that you are roaming in contacts the MTSO of the 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. The amazing thing is that all of this happens within seconds!
Inside the handset
A cell phones handsets is somewhat like a palmtop computer. It is the most intricate device common people play with daily. But, if you were to take a cell phone apart, you will be surprised to discover very few parts inside it. They are:
A circuit board that has several chips (ICs) on it.
An antenna
A liquid crystal display (LCD)
A keyboard (not unlike the one you find in a TV remote control)
A microphone
A speaker
A battery
Let us explore the function of each one of them. One chip on the circuit board translates the sound that is spoken near the microphone from analog to digital and the incoming digital signal back to sounds that can be heard through its speaker. Cell