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July 26, 1994

Telephone calls are transmitted over land, under sea and across the skies. An overseas call is likely to be handled by an undersea fiber-optic cable made up of tiny, hair-thin glass fibers. This modern fiber, also used for regular overland service, has revolutionized the telephone system because it has allowed cables to carry more calls.


* Undersea cable: Through the years

These cables peeled open to reveal their inner components, have increased dramatically in call-carrying capacity while their size has shrunk. Modern fiber-optic cable is at right.

1950s: Carried 36 calls simultaneously

1960s: 138 calls

1970s: 845 calls

1970s: 4,200 calls

Today (fiber optic): 40,000 calls

Fall, 1994 (fiber optic): 225,000 calls

* Preparing a message for transmission

Words begin as sound waves, called analog signals because the wave shapes are analogous to the rising and falling tones of a voice. But now, voices, as well as data and images, are transmitted as digital signals, coded in binary 1s and 0s. To travel in digital form through fiber-optic cable, an analog voice signal is converted to energy pulses. A pulse stands for a binary 1, a missing pulse for 0. When the message reaches its destination, it is reconverted to analog and heard as the human voice.

How calls are handled

1. International long distance phone call originates at a traditional telephone.

2. Digital signal is transmitted via telephone lines, some along overhead cable, some underground. Cellular phone path would skip this step and go directly to computerized switching station.

3. AT switching station, electronic "decisions" are made. These switching stations decide which is the most convenient and efficient route available.

4. If the undersea cable route is selected, signal travels along one of several fiber-optic cable routes, depending on the destination.

5. If satellite is used, the signal is then transmitted from a satellite dish on the ground up 22,000 miles to an orbiting satellite.

6. Satellite traveling in a geo-synchronous orbit acts as a repeater and reroutes signal back down to its destination on the other side of the world.

Source: AT&T

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