Radio jamming
From Wikipedia, the free encyclopedia
Radio jamming is the transmission of radio signals that disrupt communications by decreasing the signal to noise ratio. Unintentional jamming occurs when an operator transmits on a busy frequency without checking that it is in use first, or without being able to hear distant stations on the same frequency. Another form of unintentional jamming occurs when equipment accidentally radiates a signal, such as a cable TV plant that accidentally emits on an aircraft emergency frequency.
Intentional communications jamming is usually aimed at radio signals to disrupt control of a battle. A transmitter, tuned to the same frequency as the opponents' receiving equipment and with the same type of modulation, can with enough power override any signal at the receiver. The most common types of this form of signal jamming are random noise, random pulse, stepped tones, warbler, random keyed modulated CW, tone, rotary, pulse, spark, recorded sounds, gulls, and sweep-through. These can be divided into two groups – obvious and subtle.
Obvious jamming is easy to detect as it can be heard on the receiving equipment. It is some type of noise such as stepped tones (bagpipes), random-keyed code, pulses, erratically warbling tones, random noise (hiss) and recorded sounds. The purpose of this type of jamming is to block out reception of transmitted signals and to cause a nuisance to the receiving operator. One early Soviet attempt at jamming western broadcasters used the noise from the diesel generator that was powering the jamming transmitter.
Subtle jamming is that during which no sound is heard on the receiving equipment. The radio does not receive incoming signals yet everything seems superficially normal to the operator. These are often technical attacks on modern equipment, such as "SQUELCH capture".
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[edit] History of Jamming
During World War II a variation of radio jamming was used where ground operators would attempt to mislead pilots by false instructions in their own language. Radar jamming is also important to disrupt use of radar used to guide an enemy's missiles or aircraft. Modern secure communication techniques use such methods as spread spectrum modulation to resist the deleterious effects of jamming.
Jamming of foreign radio broadcast stations has often been used in wartime (and during periods of tense international relations) to prevent or deter citizens from listening to broadcasts from enemy countries. However such jamming is usually of limited effectiveness because the affected stations usually change frequencies, put on additional frequencies and/or increase transmission power. During the Cold War Soviet jamming of Western broadcasters led to a "power race" in which broadcasters and jammers alike repeatedly increased their transmission power and added extra frequencies to the already overcrowded shortwave bands to such an extent that many broadcasters not directly targeted by the jammers (including pro-Soviet stations) suffered from the rising levels of noise and interference. Meanwhile some listeners in the Soviet union and Eastern bloc devised ingenious methods (such as homemade directional loop antennas) to hear the Western stations through the noise.
Jamming has also occasionally been used by the Governments of Germany (during WW2), Cuba, Iran, China, South Korea and several Latin American countries.
Jamming has also occasionally been attempted by the authorities against pirate radio stations including Radio Nova in Ireland and Radio Northsea International off the coast of Britain.
In 2004, China acquired radio jamming technology and technical support from French state-owned company, Thales Group. It is used for jamming foreign radio stations broadcasting to China.
[edit] Wifi jamming
It's possible to jam 802.11b signals. Some cards have a CSMA/CA mode, and it is written on the specs that these will send out a jamming signal when a collision is detected. They then try to send their signal. So when you write garble data with three wifi cards on channels 1, 6 and 11 it's possible that you disrupt communication in the entire wifi spectrum. The cards possibly need to be in an ad-hoc mode. At the least the jammee will see slowdowns of his spectrum.
