A fiber-optic transmitter must emit light that can be received efficiently by the optical fiber; two types of light sources -- the light-emitting diode and the semiconductor diode laser -- can be used in transmission. The fiber consists of the thin strand of optically pure glass surrounded by another layer of less pure glass. Fibers are classified by their width or the number of modes or paths they support. Optical fibers are packed inside cables for strength and protection against breakage and environmental hazards. The receiver in the system converts the optical signal into an electrical signal compatible with conventional equipment and communications networks. Connectors and couplers are used to link the various components of the fiber-optic system. Fiber- optic transmissions operate in several types of systems: one- way video system; two-way video and data system; and two-way video, audio, and data system.