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NYLON COATED TELEPHONE CABLE, 1960's
Data cable with armour protection, coated in hemp which has been saturated with a mixture for under ground use.
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The Transcontinental Cable System or L-carrier system, was developed by AT&T to create a hardened Telecom network using coaxial cable for long distance communications. There were five phases of development of the system, designated by the Bell System as L-1 through L-5. Later versions were hardened against the dangers of the cold war.
The initial systems in the 1930s had 600 voice channels, far more than could be carried by balanced pair carrier systems, and cheaper per channel for high-usage routes. This version was standardized as "L-1" in 1941. Each successive version had at least twice as many channels as the previous version, culminating in the L5 design in the 1970s, which used the then-novel error-control method of feed-forward. AT&T Long Lines built two coast to coast systems of L3 as well as shorter ones connecting major cities, especially the big cities of the eastern United States, as a supplement to the mainstay microwave radio relay systems. Many were later upgraded to L4. L-carrier systems were loaded by multiplexing and super multiplexing Single side-band channels.