Abstract
Leaving aside the physical transportation of written messages there are three main ways of transmitting information between widely separated locations. These are communication over wire lines—that is, telegraphy and telephony; wireless communication, that is radiotelephony, radiotelegraphy, sound and vision broadcasting; and optical communication. Of these, by far the oldest is optical communication. It includes the semaphore telegraph, heliograph and smoke signals for daylight use, and beacon fires or Aldis lamps for use after dark. However, these are all examples of broadcast light signals even if, as with the heliograph and Aldis lamp, there is an important element of directionality. But optical communication is also the newest form, thanks to high-grade optical fibres or light pipes and electro-optic transducers such as light emitting diodes (LEDs), lasers and photo detectors. These potentially offer huge bandwidth systems and, in conjunction with digital voice communications, are rapidly emerging as the new backbone of national trunk telephone networks. These carry both voice and data traffic in digital form over an integrated services digital network (ISDN).
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