Monday, June 3, 2013

CELL PHONE JAMMERS

It's great to be able to call anyone at anytime. Unfortunately, restaurants, movie theaters,
concerts, shopping malls and churches all suffer from the spread of cell phones because not all
cell-phone users know when to stop talking. Who hasn't seethed through one side of a
conversation about an incredibly personal situation as the talker shares intimate details with his
friend as well as everyone else in the area?
Disrupting a cell phone is the same as jamming any other type of radio communciation. 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. It's a called a
denial-of-service attack. The jammer denies service of the radio spectrum to the cell-phone
users within range of the jamming device.




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.
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.
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.


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:
1. 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.
2. Circuitry : The main electronic components of a jammer are
a) Voltage-controlled oscillator - Generates the radio signal that will
interfere with the cell phone signa
b) Tuning circuit - Controls the frequency at which the jammer broadcasts its
signal by sending a particular voltage to the oscillator
c) Noise generator - Produces random electronic output in a specified
frequency range to jam the cell-phone network signal (part of the tuning
circuit)
d) RF amplification (gain stage) - Boosts the power of the radio frequency
output to high enough levels to jam a signal
1. 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.
During a hostage situation, police can control when and where a captor can make a phone call.
Police can block phone calls during a drug raid so suspects can't communicate outside the area.
Cell-phone jammers can be used in areas where radio transmissions are dangerous, (areas with a
potentially explosive atmosphere), such as chemical storage facilities or grain elevators. The
TRJ-89 jammer from Antenna System & Supplies Inc. carries its own electrical generator and
can block cellular communications in a 5-mile (8-km) radius.


Corporations use jammers to stop corporate espionage by blocking voice transmissions and
photo transmissions from camera phones. On the more questionable end of the legitimacy
spectrum, there are rumors that hotel chains install jammers to block guests' cell-phone usage
and force them to use in-room phones at high rates.

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