For Capacity Expansion
One reason that dark fibre exists in well-planned networks is that much of the cost of installing cables is in the civil engineering work required. This includes planning and routing, obtaining permissions, creating ducts and channels for the cables, and finally installation and connection. This work usually accounts for more than 60% of the cost of developing fibre networks. For example, in Amsterdam's city-wide installation of a fibre network, roughly 80% of the costs involved were labor, with only 10% being fibre. It therefore makes sense to plan for, and install, significantly more fibre than is needed for current demand, to provide for future expansion and provide for network redundancy in case any of the cables fail.
Many fibre optic cable owners such as railroads or power utilities have always added additional fibres for lease to other carriers.
In common vernacular, dark fibre may sometimes still be called "dark" if it has been lit by a fibre lessee and not the cable's owner.
Read more about this topic: Dark Fibre
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