A long time ago, before cell phones, there were
mobile radio telephones that were used in cars if you absolutely needed some
type of mobile communication. Two problems arise with radio telephones; there is
only one really large central antennae in the city, with only 25 channels. This
translates to only 25 people communicating at once on a radio telephone system.
To broadcast 40-50 miles, a very large transmitting antennae must be installed on
the user's car which poses the second problem.
A good way to show how superior cell phones are to radio telephones, walkie-talkies,
and CB radios, is comparing their duplexes. Cell phones use "full duplex" meaning
that the device uses one frequency to talk and one frequency to listen. CB radios
and walkie-talkies use "half duplex." This uses one duplex for both listening and
talking. Because there is only one frequency, only one person can talk at one time,
while the other person must listen.
Cell phones are also superior to CB radios and walkie-talkies in range and
number of channels. Motorola Talkabout T6320 and Audiovox GMRCPS are two
modernized walkie-talkies that have a range of about 2 miles, usually 14
channels, and numerous modern-day features: digital compass, altimeter, thermometer,
NOAA weather broadcasts, and a GPS. CB radios have 40 channels and since they are
much more powerful than walkie-talkies, can generally transmit up to 5 miles. Cell
phones typically have 1664 channels to communicate on and a range of hundreds of miles
due to the use of cell towers.
Cell towers usually transmit within a 10 mile radius and are generally thought of
as hexagons in a huge
hexagonal grid. Each cell phone carrier receives up to 800 frequencies in
any given city. The reason that a car can travel hundreds of miles and "remain connected"
is because the transition between "cells" is seamless as the signal carries over.
There are
three principal
generations of cell phones. 1G is the oldest, and the least efficient. On
a 1G or normal analog network, there are only 56 voice channels per cell. With the
2G network there are triple the voice channels because of digital transmission.
It's very important to know that both the cell phone and the base stations at the
cell towers transmit at low wattage. The importance of this is that the signal
transmitted does not make it very far out of the cell, allowing frequencies to be
reused from cell to cell. A 3G network has a wider service area as well as a wider range of services, including voice data, video data, and broadband wireless data.
A cell tower network requires numerous base stations per city. There can be
hundreds of towers in large cities. Because there are so many base stations
and towers, there is a central office called Mobile Telephone Switching Office (MTSO)
for each provider or carrier.
Every cell phone contains special codes that are used to identify that particular
cell phone. There are 3 types of cell phone codes: Electronic Serial Number (ESN),
Mobile Identification Number (MIN), and System Identification Code (SID). The SID is a 5 digit code unique to your network provider.
Shockwave animation and caption courtesy of
How Stuff Works
When you initially turn on your cell phone, the cell that you are in looks for your
SID. The sole purpose of the control center is to help the base station and cell
phone communicate back and forth. If the cell phone cannot find a control channel
to listen to, than it knows it's out of range. If it connects to a control channel
and the SID is confirmed, then the phone knows it is in one of it's "home systems."
Your phone then sends a registration request to the MTSO. When your phone registers,
MTSO logs which cell your phone was in and puts it into the database, so that it
knows what cell to ring you in. The MTSO also keeps track of the last cell you were
in so it knows where to look first. When you call someone, the MTSO picks the two
frequencies that your phone will be working with. After all this process, you're
connected with your friend on what is basically two-way radio. The reason you're
able to go from cell to cell is because of the technology in the base station of
the towers. These base stations can tell that your signal is diminishing and so
it "hands off" your signal to another tower. Say you go into another city, or into
another branch of your city that uses a different MTSO, the MTSO you're currently
in recognizes that you're not in the right place and then goes back to your home
MTSO to make sure your SID is valid.