Modern telecommunication networks rely primarily on two types of cable. Older copper cables have been replaced in many cases by fibre optic "light pipes," first in undersea cables and major intercity trunks, and later directly to many homes and businesses.
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Copper cables carry information as an electrical signal, which may be digital, as in phone lines, or analogue, as in audio speaker cable. In optical fibres, however, data is carried digitally by light from a laser or LED. This travels along the cable by means of Total Internal Reflection, bouncing from one side of the glass's inner surface to another and therefore following the cable's path.
Although new techniques may allow copper cables to carry much more data than in the past, in most cases optical fibres provide much greater bandwidth. The first generation of submarine fibre optic cables carried 280 megabits per second, compared with only 2.5 megabits per second through copper cables. Modern optical fibres can carry terabits, or millions of megabits, per second.
Copper cables are inherently vulnerable to electromagnetic interference. Although fibre optic lines do have metallic components, these do not carry the signal, so interference is not a problem. Copper cables can also be "tapped" electromagnetically, allowing the signal to be covertly monitored. Optical fibres, on the other hand, cannot be tapped without disrupting the signal.
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