Radio Frequency (RF) Jammers
The Basics of Cell Phone Jamming
Cell phones work by communicating with a service network through the utilization of cellular towers or base stations. Individual towers partition cities into small sections called cells. As a cell phone user traverses the cells in an area, the signal is passed from tower to tower.
Jamming devices take advantage of this fact by transmitting on the spectrum of radio frequencies used by cellular devices. Through its concurrent transmission, the jamming device is able to disrupt the two-way communication between the phone and the base station. This form of a denial-of-service attack inhibits all cellular communication within range of the device.
How It's Done
Through the transmission of a high power signal on the same frequency of a cell phone, the jamming device creates a competing signal that collides with, and, in effect, cancels out the cellular signal. Cell phones, which are designed to increase power in the case of low levels of interference, react to this interference. Consequently, jamming devices must be aware of any increases in power by the cellular device and match that power level accordingly.
As cellular telephones are full-duplex devices utilizing two separate frequencies (one for talking, one for listening where all parties to a call can talk at the same time as opposed to half-duplex walkie-talkies and CBs), any removal of one of these frequencies tricks the phone into thinking there is no cellular service. Consequently, the jammer need only block one of the frequencies.
The less complex jammers can only block a specific frequency group while the more complex jammers can block several different networks thus preventing dual- or tri-mode phones from switching to a different network with an open signal. Jammers are able to broadcast on any frequency and can interrupt AMPS, CDMA, TDMA, GSM, PCS, DCS, iDEN and Nextel systems. The effective range of a jammer is dependent upon the strength of its power source and the immediate pysical environment (hills or walls which may block the jamming signal). Lower powered jammers have a call-block range of about 30 feet while higher power units can create a cellular signal-free zone about the size of a football field. In addition, certain units applied by law enforcement have been known to shut down cellular service approximately 1 mile from the jamming device.
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