Dark fiber is the popular denomination that is attributed to fiber optic circuits , which have been deployed by some telecommunications operator , but they are not being used. The connectivity of the fiber is marketed raw, so it is the customer who applies transmission technology that best suits their needs, thereby improving the yield obtained since unnecessary conversions are avoided protocols .   When a telecommunications operator deploys its fiber optic network, it has to make a large investment to build the pipelines and lay the fiber optic cables. If in the future it were necessary to expand the capacity of an existing network, it would be necessary to lay and connect additional cables. Given the cost of this operation, it is more attractive to oversize the initial network by installing more fiber optic cables than are necessary.   Fiber cables can contain different numbers of fibers: 8,16,32,64,128,256 etc. Due to oversizing, not all fibers are used. Those that remain unused are called dark fibers. Ultimately, some of these cables never come to be used, that is, infrared radiation is never transmitted through them. Hence the name dark fiber.