To physically transmit multiple telephone calls over a
large area is quite a task. There have been many methods over the
years to send phone calls over long distances.
Telephone Transmission Technologies
A brief overview of the history and operation of
telephone transmission technologies.
Open wire carrier was developed to carry multiple
calls over a pair of copper wires simultaneously. It uses a method of
frequency division multiplexing (FDM) where calls are sent on
different frequencies. The frequencies are around 100Khz at about 4
KHz per channel.
Coaxial cable was used for cross country
communications. Coax cable was also used in the transmission of radio
and television programming. It was also used for use by government and
defense department purposes. Many repeaters were involved and many
distribution facilities as well. Coaxial cable is no longer in use,
and most of it is abandoned in place.
Developed in the 1940s and 1950s, microwave
transmission became a widespread telephone call transmission medium.
Many microwave towers were erected in many countries worldwide.
Microwave again uses frequency division multiplexing.
Most people think of satellites for television transmission. But Bell Labs/AT&T invented communications satellites for long distance communications. The first satellite was Telstar in 1962. The major drawback was the 1/2 second delay because of the distance from the Earth to the geostationary orbit over 22,000 miles from Earth and return. Satellites are still used today for far remote places.
Ultra pure glass optical fibers using amplitude modulated infrared light, commonly known as Fiber (or "fibre") Optics, revolutionized telecommunications transmission techniques. Developed in the 1970s and implemented in the 1980s and beyond - fiber optics is the high bandwidth and high quality transmissions medium that is in widespread use in the telephone industry today. Almost all telephone companies in North America use fiber optics, as do many telephone companies in the industrialized world.
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