How does a mobile network jammer work?

A mobile phone jammer is an instrument used to prevent cellular phones from receiving signals from base stations. When used, the jammer effectively disables cellular phones. These devices can be used in practically any location, but are found primarily in places where a phone call would be particularly disruptive because silence is expected. A cell phone signal jammer is used to stop nearby cell phones from sending or receiving signals to and from the cell phone tower. 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.
Cell phone jammers block cell phone use by sending out radio waves along the same frequencies that cellular phones use. This causes enough interference with the communication between cell phones and towers to render the phones unusable. On most retail phones, the network would simply appear out of range. Most cell phones use different bands to send and receive communications from towers. Jammers can work by either disrupting phone to tower frequencies or tower to phone frequencies or both.
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. The jammer also stops Internet use through Wi-Fi network and even blocks Bluetooth connections. The cell phone signal jammer also puts an end to text messaging, so students cannot SMS each other, while the teacher is trying to teach.
Interestingly, jammers are life saving devices for military. Terrorists use cell phones to trigger explosives. Army can use a jammer to make mobile phones go dead, so that army technicians can deactivate the bomb.