Updated May 29,
here to go to our page on biological hazards of radiation
New for June 2009! On
this page we will (hopefully) start explaining how cell phones work.
If anyone has any suggestions please send
them our way!
We are not experts on cell phones,
but that never stopped us from blabbing on and on about a microwave
Most cell phones use GaAs HBT
power amplifiers and pHEMT transmit/receive switches. Silicon germanium
may replace GaAs HBT due to its lower cost some day.
Most cell phones operate around
Because cell phones are made
with lead-free processes due to ROHAS, don't expect them to last
more than a few years.
This info came from Rick... thanks!
CB radios and walkie-talkies
are half-duplex devices. One frequency is used for talking and listening.
This means that only one person can talk at a time. Cell phones
are full-duplex devices which means that one frequency is used for
talking and a separate frequency for listening.
The word "cell" in
cell phones comes from the fact that an area is divided into cells
each with a tower in the middle. Each cell is typically 10 sq. mi.
A cell-phone carrier gets about 832 frequencies to use in a city:
790 for voice and 42 for data. A channel is a pair of frequencies.
So there are 395 voice channels and 21 data channels. Each cell
uses a seventh of these channels (56 people can talk on their cell
phones at once in a cell).
Each cell phone has special codes
that identify the service provider (SID--system identification code
is a 5-digit number given to each carrier by the FCC), the phone
(ESN--an electronic serial number is a 32-bit number programmed
into each phone by the manufacturer), and the owner (MIN--mobile
identification number is a 10-digit number based on the phone's
When you turn on your phone,
it listens for its SID on the control channel (a channel used for
organizing channel changes during a call and for other data transfers)
and transmits a registration request. Your service provider's Mobile
Telephone Switching Office (MTSO) keeps track of your phone's location
in its database so that it will know which cell to direct an incoming
If your phone number is dialed,
the MTSO gets the call and looks in its database to see which cell
the phone is in. Then the MTSO communicates with your phone via
the control channel to coordinate the frequencies to use. Once those
frequencies are used, the call to your phone is placed. Your base
station (cell phone tower) will notice the strength of your phone's
signal. If you move to the edge of your cell, your base station
will coordinate (through the MTSO)with the base station you are
moving towards, and at some point, your phone will receive a signal
on a control channel telling it to change frequencies. At this point,
you have switched cells.
If you move into a cell which
is out-of-range of your service provider's towers, your call is
handed to a third-party service provider. If the SID on the control
channel does not match the SID programmed to your phone, you are
roaming. The MTSO of the cell you are roaming contacts your home
MTSO to verify that your SID is valid. If confirmed, the roaming
MTSO tracks your phone as you move through cells.