# How does it work?

Each mobile unit is identified by the following sets of numbers.

The first number is the Mobile Identification number (MIN). This 34 bit binary number is derived from the units telephone number, MIN1 is the last seven digits of the telephone number and MIN2 is the area code. /

For demonstrative purposes, we'll encode 617-637-8687.

Here's how to derive the MIN2 from a standard area code. In this example, 617 is the area code. All you have to do is first convert to modulo 10 using the following function. A zero digit would be considered to have a value of 10.

100(first number) + 10(second) + 1(third) - 111= x

100(6) + 10(1) + 1(7) -111= 506

(or you could just - 111 from the area code.)

Then convert it to a 10-bit binary number: 0111111010

To derive MIN1 from the phone number is equally as simple. First encode the next three digits, 637.

100(6) + 10(3) + 1(7) - 111= 526

Converted to binary: 1000001110

The remainder of the number 8687, is processed further by taking the first digit, eight(8) and converting it directly to binary.

8 = 1000(binary)

The last three digits are processed as the other two sets of three numbers were processed.

100(6) + 10(8) + 1(7) - 111 = 576

Converted to binary: 1001000000

So the completed MIN number would look like this:

--637--- 8- ---687-- ---617--

1000001110100010010000000111111010

It may also be found in a ' computer on a chip ', which are the new microcontrollers which have rom/ram/microprocessor all in the same package. This type of setup usually has the ESN and the software to drive the unit all in the same chip. This makes is significantly harder to dump, modify and replace. But it is far from impossible.

The ESN is a 4 byte hex or 11-digit octal number. I have encountered mostly 11-digit octal numbers on the casing of most cellular phones. The first three digits represent the manufacturer and the remaining eight digits are the units ESN.

## Frequency Division Multiple Access (FDMA)

FDMA is a single channel per carrier analog method of transmitting signals.

## Time Division Multiple Access (TDMA)

With TDMA calls are simultaneously held on the same channels, but are multiplexed between pauses in the conversation.

## Code Division Multiple Access

This system has been used in mobile military communications for the past 35 years. This system is digital and breaks up the digitized conversation into bundles, compressed, sent, then decompressed and converted back into analog.

## Time- Division/ Frequency Division Multiple Access TD/FDMA

a relatively new system which is an obvious hybrid of FDMA and TDMA. This system is mainly geared towards the increase of digital transmission over the cellular network. TD/FDMA make it possible to transmit signals from base to mobile without disturbing the conversation.

With FDMA there are significant disturbances during handoff which prevent continual data transmission from site to site. Cellular Frequency Usage and Channel Allocation There are 832 cellular phone channels which are split into two separate bands. Band A consists of 416 channels for non-wireline services. Band B consists equally of 416 channels for wireline services. Each of these channels are split into two frequencies to provide duplex operation.