An optical fiber network that is not actively used but is available as extra capacity.
Optical fiber left unlit in a fiber optic cable. Fiber optic cables often contain a great many fibers, some of which are lit, and others of which are left unlit, or dark. The dark fibers can be spares for backup purposes, can be held in reserve to accommodate future demand, or can be available for lease or sale to other carriers or user organizations with private line requirements. See also dim fiber and dry copper pair.