Fiber optic technology converts electrical signals carrying data to light and sends the light through transparent glass fibers about the diameter of a human hair. Fiber transmits data at speeds far exceeding current DSL or cable modem speeds, typically by tens or even hundreds of Mbps.
Lit Fiber is optical fiber that is regularly being used to transmit data.
Dark Fiber, also known as “unlit fiber,” is optical fiber that is laid in the ground, but not used. During the late 1990s, so much fiber had been installed worldwide in anticipation of Internet demand that there is still a huge amount of fiber that has never been lit. Dark fiber is also optical fibers that are installed and sold to carriers and large businesses without any optical or electronic signaling in their path. The customer is responsible for adding the transmission system at both ends.