Of course, all of the early undersea cables were laid for transmitting telegraphs, which could be easily sent over long distances via slow electrical impulses. When Alexander Graham Bell invented the telephone in 1876, he hoped that these same electrical signals could be used to transmit voices. Both overland and between continents using the established underwater network.
But Bell soon found that the electrical impulses that Morse code relied upon were too weak to carry voice signals. Voice signals, Bell concluded, needed much more power to travel over long distances. Dejected, he wrote of his findings to a magazine editor in 1911. Some thirty years after he had conducted his first experiments with voice signals and distance. "We are still a long way from establishing a telephone service across the Atlantic. I have no doubt however, that it will be done some day."
Scientists at Bell Laboratories were the first to develop a cable strong enough to transmit telephone signals. In 1921 they introduced a cable made of simple copper wire wrapped in Gutta Percha. a form of rubber insulation made from the sap of the Malaysian Gutta Percha tree. For a trial run, Bell laid its cable between Havana, Cuba and Key West, Florida. Equipped with three telephone circuits the cable enabled three phone calls to be placed at the same time. Soon Bell Laboratories began to work on developing a cable to run beneath the North Atlantic.
But with the end of WWI, radiotelephony, a new technology, sidelined the use of undersea cables for several years. It was both inexpensive and easy to use in comparison to the financially and physically cumbersome process of designing, producing, and laying underwater cables.
Then during the Second World War, a growing shortage of radio frequencies, and the fact that ocean-borne magnetic storms often disrupted overseas telephone calls renewed interest in undersea cables. Private companies as well military teams began researching new telecom technologies, which led to improvements in equipment used for undersea networking.
The discovery that wrapping copper cables in polyethylene and using vacuum tubes as repeaters could boost voice signals, allowing telephone calls to be transmitted over greater distances and at greater depths, altered the course of international communications forever.
It also led to the completion of the first transatlantic telephone cable in 1956. Built by AT&T, the cable known as TAT-1 stretched between Oban, Scotland and Sydney Mines, Nova Scotia.
TAT-1 was able transmit up to 32 calls at the same time.
Until the introduction of solid-state amplifiers in 1984, vacuum tubes remained the foundation of undersea cable technology.
The use of transistors as repeaters catapulted cable capacity to more than 10,000 simultaneous calls. Fiber optical cable boosted the speed and capacity of cables further by quickly converting voice signals into digital signals.
When TAT-8, the first cable to use fiber optic technology, was placed into service between the United States and Europe in 1988, more than 40,000 calls could be placed at the same time.
Within several years, technology had leapfrogged to capacities capable of transmitting four times the number of calls first sent via fiber optics. As an example TPC-5, a transpacific cable installed in 1996, can send the equivalent of 320,0000 simultaneous calls.