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Modern Data Communications

A modern telephone. Source: Microsoft.

"Do you understand what I say?"

Alexander Graham Bell (1875): The first telephone message.

Modern data communications owes a great debt to many communication pioneers, including Alexander Graham Bell who is generally credited with inventing the telephone. This revolutionary device allowed speech to be converted to electrical signals and then to be transmitted over a wire to be transformed back into speech at the other end.

Early telephone systems used a single copper wire for signal transmission with an Earth ground to complete the circuit; however, later systems used two wires. These wires were known as open wire systems because they were connected in the open air between telephone poles. This exposure to the elements meant that transmission interference occurred because rain caused leakage paths for the signal to go the ground.

Each wire was able to carry up to 12 separate speech circuits with an optional telegraph signal on each. One of the problems that occurred is called attenuation, that is, as the radio signal moves down the wire part of it is absorbed by the wire so that a weaker signal than that sent is received. Copper wire was replaced by aluminium cables which were thicker, lighter and cheaper, although not as good an electrical conductor. Generally, alumimium was considered better than copper even although it suffers from oxidisation when exposed to the air which can cause signal problems.

Open wire systems were later replaced with underground multi-strand cable that is protected by strong plastic covering to prevent water damage. This type of cable is expensive and is used alongside special equipment so that each cable can be used efficiently and effectively.

Next: The Plain Old Telephone System (POTS)