A submarine communications cable is a cable laid on the sea bed between land-based stations to carry telecommunication signals across stretches of ocean.
The first submarine communications cables, laid in the 1850s, carried telegraphy traffic. Subsequent generations of cables carried telephone traffic, then data communications traffic. Modern cables use optical fiber technology to carry digital data, which includes telephone, Internet and private data traffic.
Transatlantic cables of the 19th century consisted of an outer layer of iron and later steel wire, wrapping India rubber, wrapping gutta-percha, which surrounded a multi-stranded copper wire at the core.
Early long-distance submarine telegraph cables exhibited formidable electrical problems. Unlike modern cables, the technology of the 19th century did not allow for in-line repeater amplifiers in the cable.
Large voltages were used to attempt to overcome the electrical resistance of their tremendous length but the cables’ distributed capacitance and inductance combined to distort the telegraph pulses in the line, reducing the cable’s bandwidth, severely limiting the data rate for telegraph operation to 10–12 words per minute.
In the 1980s, fiber optic cables were developed. A fiber-optic cable comprises multiple pairs of fibers. Each pair has one fiber in each direction.
The optic fiber used in undersea cables is chosen for its exceptional clarity, permitting runs of more than 100 kilometers between repeaters to minimize the number of amplifiers and the distortion they cause.