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Free Mobile Phones - How it works

How the Cellular System works:

(Continued)

Today's cellular system divides a city into small cells.
This enables extensive frequency reuse across a city, so
that millions of people can use cell phones simultaneously.
Each cell comprises of a base station with a tower and a
building to house the radio equipment. Each cell uses one-
seventh of the available channels so it has a unique set of
frequencies and there are no collisions.

A cell-phone carrier typically has 832 radio frequencies to
use in a city. Each cell phone uses two frequencies per
call -- a duplex channel -- so there are 395 voice channels
per carrier. (The other 42 frequencies are used for control
channels) Therefore, each cell has about 56 voice channels
available. Simply put, there are about 56 people talking
from their cell phones in a cell in an analog system. In a
digital system there are more number of channels. A digital
system can carry three times as many calls as an analog
system, so each cell has about 168 channels.

The cellular approach requires a large number of base
stations in a city of any size. A city can have hundreds of
towers. Each carrier in each city runs a central office
called the Mobile Telephone Switching Office (MTSO). The
office handles the phone connections to the normal land-
based phone system, and controls the entire base stations
in that region.

How Do Calls get transmitted:

When you switch on the phone, it listens for an SID (System
Identification Code - a unique 5-digit number that is
assigned to each carrier by the respective authority) on
the control channel. The control channel is a special
frequency where the phone and base station talk to one
another about the details of call set-up and channel
changing. If the phone fails to recognize a control
channel, it knows it is out of range and displays a "no
service" message.

When it receives the SID, the phone compares it with the
SID programmed in your phone. If the SIDs match, the phone
knows that the cell it is communicating is part of its home
system. Along with the SID, the phone transmits a
registration request, and the MTSO keeps track of your
phone's location in a database so that the MTSO knows which
cell you are in when it wants to ring your phone. The MTSO
gets the call, and it tries to find you. It looks in its
database to see which cell you are in. The MTSO picks a
frequency pair that your phone will use in that cell to
take the call. The MTSO communicates with your phone over
the control channel to tell it which frequencies to use,
and once your phone and the tower switch are on those
frequencies, the call is connected and Lo! You are talking
to your friend!

As you move away from your cell, your cell's base station
notes that the strength of your signal is diminishing.
Meanwhile, the base station you are moving toward (which is
listening and measuring signal strength on all frequencies,
not just its own one-seventh) detects the strength of your
phone signal increasing. The two base stations coordinate
with each other through the MTSO, and at some point, your
phone gets a signal on a control channel asking it to
change frequencies. Thus the signal is passed from cell to
cell as you travel.

Roaming

If the SID on the control channel does not match the SID
programmed into your phone, then the phone knows it is
roaming. The MTSO of the cell that you are roaming in
contacts the MTSO of your home system, which checks its
database, to confirm whether the SID of the phone you are
using is valid. Your home system verifies your phone to the
local MTSO, which tracks your phone as you move through its
cells.

In all these processes, your voice travels in the form of
radio waves and this fact has led to controversies in using
the cell phones. Well, Are they dangerous? Here are a few
assumptions that are believed to ensue from the use of cell
phones:

Radio waves emitted from mobiles heat-up body
tissue.

Magnetic fields created by mobile phones influence
the behavior of body cells.

Long mobile phone calls cause fatigue, headaches,
and loss of concentration.

Mobile phone users are susceptible to cancer in
areas of the brain adjacent to their ears.

Radio waves from mobile phones alter the gene expression.
All the above assumptions are baseless until proved. While
scientists are still slogging on this line of infinite
research, the never-ending craze for cell phones has
reached an insurmountable peak. Since these elegant cell
phones have amplified the expediency of today's
communication, the allegations on the health front are
overlooked.

The cell phone industry is the fastest growing in today's
world. And this handheld small gadget has not only
revolutionized the modern communication but the novelty of
this small wonder has also captivated the world with its
ever-fascinating charm.

For more on up to date mobile phones go to
Mobile Phone Upgrade

This article courtesy of the MOBILE PHONE Newsletter:

http://www.mobilephonesite.co.uk