AT&T built its original long distance network from copper wire strung on telephone poles. Telephone calls traveled down these wires as analog signals, electrical waves of analogous form to the original voices. Each call required two wires to form a complete electrical circuit. Telephone signals weaken from electrical resistance as they travel down the wires. But thicker wires have lower resistance, so wires as thick as 1/6 inch were used on circuits such as the original New York-Chicago line of 1892.
After 1904, properly spaced loading coils, which effectively reduce the resistance in the line, allowed for longer lines and thinner wires. Vacuum-tube repeaters, introduced in 1914, made it possible to amplify or strengthen the signals, allowing for still longer lines and still thinner wires. An electrical trick called a “phantom circuit” allowed two pairs of wires to carry three calls.
In the 1910s, AT&T developed several new technologies to meet the growing demand on major routes. Underground cables carried more wires in less space and provided protection from the weather. AT&T installed its first underground cable between Philadelphia and Washington, D.C., in 1912. Carrier-current systems sent several calls down a single pair of wires by superimposing the calls on higher frequency currents, rather than transmitting the signals on their natural voice frequencies. AT&T installed its first carrier system between Baltimore and Pittsburgh in 1918. That system carried four calls on a single pair of wires.
Still higher calling volume, and the beginning of work on television, fueled AT&T’s invention of the first broadband transmission medium, broadband copper coaxial cable. AT&T installed its first experimental coaxial cable between New York and Philadelphia in 1936. The first “regular” installation connected Minneapolis, Minn., and Stevens Point, Wis., in 1941. This L1 coaxial-cable system could carry 480 telephone conversations or one television program. Subsequent coaxial-cable systems had much higher capacity. The L5 systems of the 1970s could carry 132,000 calls or more than 200 television programs.
Microwave radio relay
Coaxial-cable systems developed in tandem with microwave radio relay, a broadband system by which conversations and television traveled via radio along a series of towers. The first such system, with seven towers on seven hilltops, connected New York and Boston in 1947. This system carried 2,400 conversations; later systems carried as many as 19,200.
Microwave relay had lower construction and maintenance costs than coaxial cable, particularly across difficult terrain. By the 1970s, radio-relay systems carried 70 percent of AT&T’s voice and 95 percent of its television traffic.
In the 1980s, both coaxial cable and microwave relay gave way to an entirely new system - fiber-optics. Fiber-optic systems use rapid pulses of light traveling on fibers of ultra-pure glass. It was a digital rather than an analog medium, and particularly well suited for transmitting data as well as voice.
Glass fibers, as thin as a human hair, make up the highways for modern fiber optic communication systems.
AT&T installed its first fiber-optic route between Washington, D.C., and New York in 1983. In 1989, AT&T announced that it would retire all its analog transmission facilities. Within a few years, the analog coaxial-cable and radio-relay systems were relegated to back-up duty. Meanwhile, continuing advances in fiber-optic technology greatly increased the capacity of the new systems, a process that continues today.