A slender strand of extremely pure glass, specially constructed to serve as a conductor, or waveguide, for infrared (IR) light signals in a fiber optic transmission system (FOTS).There are a large number of GOF types, all of which support very high signaling rates over considerable distances with very low attenuation and, therefore, excellent error performance.The fiber comprises a core of pure silica doped with germanium or some other substance to alter its index of refraction (IOR) to slow the velocity of propagation (Vp), i.e., to slow down the speed of light, by approximately one-third to about 200,000 kilometers per second, or 124,000 miles per second. Surrounding the inner core is the cladding, which consists of pure silica doped in such a way as to have a lower refractive index, i.e., index of refraction (IOR), which translates into increased purity and, therefore, enhanced propagation speed, as compared to the core. A step-index fiber is characterized by a sharp step in the IOR at the core/cladding interface, which serves to reflect errant signal components back into the core. A graded-index fiber is characterized by cladding comprising many layers of doped silica that are gradually and successively lower in IOR, which construction serves to gradually refract, or bend, any errant signal components back into the core. In either case, the ultimate effect is that the light signal is essentially confined to propagate through the core through a process known as total internal reflection. Graded-index multimode fiber (MMF) generally is used in short haul applications requiring bandwidth of 1 Gbps or less, such as local area networks (LANs). Step-index single-mode fiber (SMF) generally is used in long haul, high-bandwidth applications such as wide area networks (WANs). Plastic optical fiber (POF) sometimes is used over short distances in low bandwidth applications where its flexibility, general durability, and low cost are advantageous. See also diffraction, graded index fiber, IOR, IR, MMF, POF, propagation, SMF, step-index fiber, and Vp.