Frequency jamming is the disruption of radio signals through use of an over-powered signal in the same frequency range.
When most people think of frequency jamming, what comes to mind are radio, radar and cell phone jamming. However, any communication that uses radio frequencies can be jammed by a strong radio signal in the same frequency. In this manner, Wi-Fi may be attacked with a network jamming attack, reducing signal quality until it becomes unusable or disconnects occur. With very similar methods, a focused and aimed signal can actually break access point hardware, as with equipment destruction attacks.
Frequency jamming can be benign or malicious. Police and the military often use jamming devices to limit or disrupt communications during hostage situations, bomb threats or when military action is underway. Less benignly, jammers have also been used to disrupt First Amendment rights in peaceful protests.
Portable personal jammers are available to enable their owners to prevent others in their immediate vicinity (up to 60-80 feet away) from using cell phones. Similar equipment is manufactured to block signals in environments where Wi-Fi and cell phone and activity may not be desirable, such as theaters, churches, secure sever rooms and operating rooms.
There are ways to detect and help prevent these kinds of attacks. Wireless intrusion prevention systems (WIPS) monitor the radio spectrum 24/7, making a signal jammer apparent. Some radio-based devices support spread spectrum modulation so that hardware can cycle though different frequencies to make the devices harder to jam. In the case of outside of premises attacks, the use of paint with copper flakes can stop the signals from penetrating walls as a passive and permanent protection. For complete protection, a Faraday cage will stop any wireless signal from penetrating.
Commercial jamming devices are available, as are guides on how to make such devices. As this type of equipment violates FCC regulations, frequency jammers cannot legally be sold, built or used within the United States.