Cell Phone Jamming

Cell phones are everywhere these days. According to the
Cellular Telecommunications and Internet Association, almost
195 million people in the United States had cell-phone
service in October 2005. And cell phones are even more
ubiquitous in Europe.

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?

While most of us just grumble and move on, some people are
actually going to extremes to retaliate. Cell phones are
basically handheld two-way radios. And like any radio, the
signal can be disrupted, or jammed.

Cell Phone Jamming Basics

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.

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.

Cell Phone 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.

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.