In the early 1980s fiber-optic cables connected most of the major cities in North America. Today fiber-optic cables circle the globe caring voice, video and data signals over these glass fibers. There are many different types of fiber-optic cables dedicated to many different applications, but they all share the same common configuration.
The very center of the fiber-optic cable is a thin piece of glass that provides the path that the light signals follow. This part of a fiber-optic cable is called the core. The core is very small in diameter. Both single-mode and multimode fibers (cores) are 125 microns outside diameter. To put this in perspective, a micron is one 1 millionth of a meter, or 125 microns, and is equal to 0.005 inch.