Abstract :
Disrupting a
cell phone is the same as jamming any other type of radio communication A cell phone
works by communicating with its service network through a cell tower or base station.
Cell towers divide a city into small areas, or cells. As a cell-phone user drives
down the street, the signal is handed from tower to tower.
A jamming device
transmits on the same radio frequencies as the cell phone, disrupting the communication
between the phone and the cell-phone base station in the tower.
HOW IT WORKS
Jamming devices
overpower the cell phone by transmitting a signal on the same frequency and at a
high enough power that the two signals collide and cancel each other out. Cell phones
are designed to add power if they experience low-level interference, so the jammer
must recognize and match the power increase from the phone.
Cell phones
are full-duplex devices, which means they use two separate frequencies, one for
talking and one for listening simultaneously. Some jammers block only one of the
frequencies used by cell phones, which has the effect of blocking both. The phone
is tricked into thinking there is no service because it can receive only one of
the frequencies.
Less complex
devices block only one group of frequencies, while sophisticated jammers can block
several types of networks at once to head off dual-mode or tri-mode phones that
automatically switch among different network types to find an open signal. Some
of the high-end devices block all frequencies at once, and others can be tuned to
specific frequencies.
To jam a cell
phone, all you need is a device that broadcasts on the correct frequencies. Although
different cellular systems process signals differently, all cell-phone networks
use radio signals that can be interrupted. GSM, used in digital cellular and PCS-based
systems, operates in the 900-MHz and 1800-MHz bands in Europe and Asia and in the
1900-MHz (sometimes referred to as 1.9-GHz) band in
the United States. Jammers can broadcast on any frequency and are effective against AMPS, CDMA,
TDMA, GSM, PCS, DCS, iDEN and
Nextel systems. Old- fashioned analog cell phones and today's
digital devices are equally susceptible to jamming.
The actual range
of the jammer depends on its power and the local environment, which may include
hills or walls of a building that block the jamming signal. Low-powered jammers
block calls in a range of about 30 feet (9 m). Higher-powered units create a cell-free
zone as large as a football
field. Units used by law enforcement can shut down service up to 1 mile (1.6 km)
from the device
INSIDE A CELL-PHONE
JAMMER
Electronically
speaking, cell-phone jammers are very basic devices. The simplest just have an on/off
switch and a light that indicates it's on. More complex devices have switches to
activate jamming at different frequencies. Components of a jammer include:
Antenna:-
Every jamming
device has an antenna to send the signal. Some are contained within an electrical
cabinet. On stronger devices, antennas are external to provide longer range and
may be tuned for individual frequencies.
Circuitry :-
The main electronic
components of a jammer are:
Voltage-controlled
oscillator - Generates the radio signal that will interfere with the cell phone
signal
Tuning
circuit - Controls the frequency at which the jammer broadcasts its signal by sending
a particular voltage to the oscillator
Noise
generator - Produces random electronic output in a specified frequency range to
jam the cell-phone network signal (part of the tuning circuit)
RF
amplification (gain stage) - Boosts the power of the radio frequency output to high
enough levels to jam a signal
Power supply :-
Smaller jamming
devices are battery operated. Some look like cell phone and use cell- phone batteries.
Stronger devices can be plugged into a standard outlet or wired into a vehicle's
electrical system.
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