Believe it or not -- Cellular telephones ("cell phones")

have been around since the early 1980s!

They were actually introduced by AT&T! The basic idea of cell

phones is to overcome the scarcity of radio frequency channels by dividing

a service area up into relatively small "cells". These are relatively small

regions each served by a "base station", a radio installation connected

to the cellular network. Cell sites are easily seen in any metropolitan

area, especially along highways and other areas where there are many users.

Each cell site is given a subset of the channels available to the

whole system. In the US Advanced Mobile Phone System (AMPS, the technical

term for conventional analog cellular), a total of 832 channels are available.

Each channel is actually a pair of channels 45 MHz apart. The mobile phone

transmits on the reverse channel of the pair while the base station transmits

on the forward channel. The US cellular system therefore occupies a total

of 50 MHz of spectrum, from 825-850 MHz for the reverse channels and 870-895MHz 

for the forward channels.

416 channel pairs, half of the total, are assigned to each of two

service providers, "A" and "B", in each metropolitan serving area. Originally

the "B" or "wireline" carrier was to be the local telephone company, while

the "A" carrier was to be a separate company to give the local phone company

some competition. Most of the "A" carriers were bought up by McCaw (doing

business as Cellular One), which was in turn acquired by AT&T. So an

easy way to remember the distinction is that "A" generally stands for AT&T

while "B" generally stands for "Bell". Obviously there are exceptions;

for example, in the San Diego area where I live, the "A" carrier used to

be US West while the "B" carrier was PacTel. Then PacTel spun off Airtouch,

which inherited the "B" system, and Airtouch and US West merged their cellular 

operations, selling off US West's interest in the "A" system to GTE tomeet FCC 

competition rules.

In any event, each cell in a system uses only a fraction of the totalchannels.

 That's because the channels used in adjacent cells must be avoided to prevent 

interference; typically only 1/7 of the channel pairs are used in each cell. 

That means a fully equipped cell can theoretically carrya maximum of about 60 calls.

Some channels are set aside for overhead functions, specifically

paging and access. These are digital control channels (10 kb/s data rate)

used to request service and to notify users of incoming calls. The cells

transmit continuously on the paging channels; whenever your phone is on,

it is monitoring the strongest one it can find that belongs to the desired

system. (If it can't find a usable paging channel, the phone lights the

NO SERVICE indicator).

The reverse channels paired with the paging channels are used by

the phones to request service. When you dial a number and hit the SEND

button, your phone sends a short digital message over the access channel

corresponding to the paging channel it has been monitoring. If the system

receives the request, it acknowledges it on the paging channel and sends

further commands to transfer to a "traffic" channel for the actual call.</p>

<p>As an aside, the messages over the access and paging channels are

not encrypted in standard analog AMPS. This has made them extremely vulnerable

to eavesdroppers who intercept the electronic serial numbers and telephone

numbers and then program ("clone") other phones to uses these same numbers to 

obtain fraudulent service. Only now, with the introduction of digital cellular

phones, which are cryptographic authentication techniques, being introduced to derail

these types of attacks.

It is now common knowledge that analog cellular systems are extremely

vulnerable to eavesdropping, as the voice is sent with ordinary Frequency 

Modulation (FM). Cryptographic authentication does nothing to protect the privacy

of the communication itself.   Analog isn't very popular today and is generally 

only used in sparsly populated areas where the digital age hasn't fired the wires

yet so to speak.

 To learn a lot more, please go here:

 http://www.cell-phone-numbers.com/cell-phone-tracking.htm

and here also:

http://www.cell-phone-numbers.com/cell-phone-facts.htm

All the Best!  Melanie Kozik

Thanks in part to Phil Karn for his great input for this article.