Optical fiber consists of a and a layer, selected for due to the difference in the between the two. In practical fibers, the cladding is usually coated with a layer of or. This coating protects the fiber from damage but does not contribute to its properties. Individual coated fibers (or fibers formed into ribbons or bundles) then ha. The (DF) IRU "shall mean the exclusive, unrestricted, and indefeasible right to use one, a pair, or more strands of fiber of a fiber cable for any legal purpose". With an IRU contractual arrangement, the buyer of the IRU can unconditionally, and exclusively, use the fibers of the IRU for a long time period, around 25–30 years. In this case is called "dark" since it has to be lit by the IRU owner, not the cable's owner. The.
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