Modern fiber cables can contain up to a thousand fibers in a single cable, with potential bandwidth in the terabytes per second. In some cases, only a small fraction of the fibers in a cable may be actually "lit". Companies can lease or sell the unused fiber to other providers who are looking for service in or through an area. Companies may "overbuild" their networks for the specific purpose of having a large network of dark fiber for sale, reducing the overall need for trenching and municipal permitting.