Regimental Division,
Office Chief of Signal

United States Army Signal Center, Fort Gordon, GA
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Cellphone animated GIF"Hello, operator? Give me information!" go the words to an old song.
Army people say "give me information" to Signal soldiers. One of the ways information is transmitted is by way of mobile telephones, like the cellular phones many people have. If you would like to know how you are able to hear the voice on the other end of the telephone line, read below.

How telephones work

A non-cellular telephone sends and receives sound, especially the human voice, over wires in electric circuits.

When you dial a phone number, this causes interruptions in the phone’s electric current. These interruptions create a pattern representing numbers. Special electric switches called relays count those interruptions, or pulses. When you dial a sequence of numbers, the automatic relays ring the person you’re calling.

To talk to the person, this is what happens. A telephone’s transmitter is basically a microphone. It has loosely packed grains of carbon in it. When someone talks in the telephone, the microphone vibrates a diaphragm in the transmitter, causing the carbon grains to be squeezed and released. This motion charges the electric current flow in the electric circuit.

Electric current is transmitted over wires to another telephone, and the diaphragm in that telephone vibrates in response to changes in the electric circuit’s magnetic field. So when the person you’re calling picks up the phone, he or she can hear your voice.

Long-distance telephone calls can also use radio waves or microwaves. Microwaves can be sent to an orbiting communications satellite and relayed back to the earth.

Microwaves are electromagnetic radiation in the radio frequency range of 1,000 to 300,000 megahertz (wavelengths of 300 to one millimeter, or about 12 to about .04 inches). Microwaves are used in microwave ovens, of course, but in radar and communications links as well.

Computers can communicate over telephone circuits via a modem.

The cellular telephone doesn’t need wires, so you can take it almost anywhere. A cellular phone is a small radio transmitter. The phone’s signals are too weak to be heard at long distances. The phone sends and receives microwave signals to a base transmitter and receiving station. The receiving station retransmits the signals to the place you’re calling, connecting to a conventional (with wires) telephone network.

A cellular telephone is called that because a geographic region serving the cellular system is divided into areas called cells. Each cell has a central base station and two sets of assigned transmission frequencies. The base station uses one set of frequencies. The other frequency is used by mobile telephones.

To prevent radio interference, each cell uses frequencies different from cells around it. (Cells far enough away from each other can use the same frequencies, since signals won’t travel far enough to cause interference.) When a mobile telephone leaves one call and enters another, the telephone call is transferred from one base station to the one in the new cell. Special circuits in the base station detect the cellular phone signal’s strength. Cells next to other cells can also detect a phone’s signal strength. So when the signal in one cell drops to where the call may break up because the signal is too weak, the next cell with the strongest signal takes over.

A computerized switching system changes transmission frequencies. This station switching continues automatically until you take your phone out of range.

Each cellular phone is unique. Each has its own code or serial number so base stations know whose phone is where. That same computer system that switches transmission frequencies keeps track of all code numbers and where the phones are within the cell, as well as who people call and how much they will be charged to make their calls.

Since cellular phones send microwave radio signals to make a call, anyone with the right receiver can listen in to your call. Although cellular telephones are very handy because they are mobile, cellular telephone calls are not private.

For more information on telephones and phone networks, link to HowStuffWorks.com/telephone.htm.

A note about external links

Parts of a telephone
When someone talks into a telephone, the transmitter charges the electric flow and electric current is transmitted over wires to another telephone.

Long-distance telephone calls use satellites
Microwave signals are sent from a terminal on earth to an orbiting communications satellite and relayed back to earth to connect many long-distance telephone calls.


Last modified on:
April 04, 2012

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