Crown Castle International owns and leases roughly 40,000 cell towers in the United States. It also owns more than 85,000 route miles of fiber. It leases space on its towers to wireless service providers, which install equipment on the towers to support their wireless networks. The company's fiber is primarily leased by wireless service providers to set up small-cell network infrastructure and by enterprises for their internal connection needs. Crown Castle's towers and fiber are predominantly located in the largest U.S. cities. The company has a very concentrated customer base, with about 75% of its revenue coming from the big three U.S. mobile carriers. Crown Castle operates as a real estate investment trust.