Cellular telephony
Cellular telephony is the set of technology that makes possible the modern range of portable telephones including smartphones. In the past, the most common assumption in mobile communications, such as "two-way radios" in police cars and taxicabs, is that the transmitter had to have the range and power to reach the ultimate receiver. To a limited extent, signal repeaters could extend their range by receiving and retransmitting their signals from high points, but cellular telephony takes a completely different approach.
Perhaps counterintuitively, cellular systems often reduce the range and power of the individual telephones, as there is no attempt to communicate with the distant destination. The only goal is communicating with the electronics in a local cell, which is a relatively small geographic area centered on an antenna tower, monitoring receivers that note when a given phone enters the cell (i.e., so it can be reached). Other receivers and transmitters have the same function as the end office in the "plain old telephone service" (POTS) of the Public Switched Telephone Network: they are the first "on ramps" to the POTS accessible to the subscriber.
The more cells, the more capacity there is for calls to enter and leave the cells, which are the gateway between mobile phones and the pots. By lowering power and cell size, one can increase the capacity by avoiding call collisions.
While there are fixed cellular services in areas where it was uneconomic to cable copper or fiber to the individual customers, the more common case is that the cellular phone is moving. During that process, there must be handoffs from one cell to the next, without dropping the call. The call information, once a connection is made, is internal to the POTS networks; the cells have to know what phones are reachable through, which of those phones are making calls and how to connect to the ongoing call in the POTS network. There has to be an intelligent interaction among two cells and a telphone to manage a handoff.
Security
It is relatively easy to intercept first-generation cellular telephony, although the problem becomes significantly harder with more advanced digital generations, which use frequency agility and advanced multiplexing.
Technologically, it is quite feasible to put strong encryption into a handheld cellular radio. The concerns are more legal and security; military and emergency forces routinely use such equipment.
Electronic warfare
In urban guerrilla warfare, cellular telephones have become cheap and effective means of communications for the irregular forces. They have also been used as remote detonators for improvised explosive devices.
Some countries, which have a problem with urban terrorism, have required their cellular providers to implement an override capability. For example, after a terrorist bombing in a civilian area, the civilian emergency response organization may send a command to the cellular repeaters in that area, making them temporarily unusable by other than preauthorized emergency responder telephones. This takes away the ability for insurgents to coordinate additional attacks or their escape.
In other approach, the United States Marine Corps is implementing the mobile ULQ-30|AN/ULQ-30 system, which allows moving troops to jam cellular communications in their immediate area.