The following is a basic description of the main fiber optic systems used in ISDN architecture. Today many communications systems use electromagnetic propagation. Radio, television, and satellite systems are all designed around the principles of antennas. Increasingly, the industry is adopting optical fiber channels for the transmission of data and audio-video information.
Optical fibers utilize light as a signal source, instead of electrical signals.
A laser or a light-emitting diode generates the light which is transmitted through an extremely thin reflective glasslike cable. The light is switched on and off by a transmitter to obtain a a binary code that represents the user data. Since the core's refractive index is higher than the cladding's refrective index, the light signal cannot escape the otpic cable. So the light travels down the cable in a reflecting path. Laser signal sources provide for a greater bandwidth and yield significantly greater capacity than other methods.