“Dark fiber” is a term often heard in conversations about fiber optic communications. Perhaps this is because it has a name that sounds evil or nefarious. But dark fiber is just fiber that has been installed and is not currently in use; instead, it is reserved for spares or future use.


Having fibers for later use certainly makes a lot of sense, and it’s a no-brainer when installing fiber optic cables, especially outside plant (OSP) cables. Fiber is cheap; one industry analyst likes to say that fiber today is cheaper than kite string or fishing line. When you look at the cost of installing a cable, especially a buried or aerial OSP cable, the cost of installation dwarfs the cost of the fiber. In fact, the cable material itself is generally much more expensive than the fiber inside it.


I know of telecom backbone cables that were installed on routes where only two to four fiber cables, plus spares, were needed, but 144 cables were installed. 


Around 2002, it was estimated that the flurry of Internet installation activity of the late 1990s had left the United States with a fiber optic network in which 90 percent of the fibers were dark. That situation did not last long. The meteoric growth of Internet traffic rapidly lit up all of those dark fibers and called for more. The advent of video over the Internet makes the demand for more fiber even stronger today. Data centers are being built all around the world, and companies, such as Google, Amazon and Facebook, use fiber to “mirror,” or duplicate, their data centers. Duplicating data centers is done to keep most Internet traffic local and reduce Internet backbone traffic. Even the growth of data over cellular networks is driving the expanded use of fiber. AT&T has reported that the data traffic over its cellular network has grown 50,000 percent since the introduction of the iPhone in 2007.


In the beginning, telcos—AT&T, Verizon, Sprint and some others no longer with us—installed most of the fiber. Now there are communications infrastructure companies, such as Level 3 and Zayo, that own fiber infrastructure that they lease to communications users. Zayo, with 81,000 miles of routes, is “a global provider of bandwidth infrastructure services, including dark fiber, wavelengths, Ethernet, IP services, SONET, carrier-neutral colocation, and technical services.”


Many cities have installed their own fiber networks and lease out fibers to companies for use in their area. The city of Santa Monica, Calif., for example, built Santa Monica City Net to connect schools and libraries and public services like police and fire departments; it helps build networks of security cameras and smart traffic lights. The city then leased dark fibers to Verizon for its FiOS fiber-to-the-home network, Time Warner Cable for its hybrid fiber-coaxial cable TV network, and tech companies, such as Google and Yahoo, which have local facilities.


Besides communications companies, others want to lease dark fibers. The U.S. government leases fiber as well as private companies to connect facilities. Utilities use a lot of fiber and own much of their own cables but may lease along routes where they do not have cables. There is even a group in Los Angeles that leases dark fiber to connect movie studios to their sound stages to allow direct transmittal of the daily shoots (which are now mostly digital video) back to the studios.


Does it make sense to install a lot of extra fibers that may not get used for years? Doesn’t the fiber have a problem with obsolescence or a limited lifetime? Not so much. There is plenty of fiber in use today that is more than 20 years old. The design of single-mode fiber has changed little in that time, and fiber companies design cables for more than 40 years of service. But since some of that fiber was installed for use at speeds under 1 gigabit per second (Gbps) while today’s long distance networks are operating at 40–100 Gbps, the fiber is usually tested to ensure its ability to carry higher speed signals.


There are some quirks in the dark fiber business. Owners of the fibers are sometimes reluctant to lease a complete “dark fiber” itself. A pair of fibers can be used to carry signals from low speeds to more than 100 Gbps. You can multiply many times by using wavelength division multiplexing at up to 128 different wavelengths. Some owners, therefore, prefer to provide the electronics to transmit your data or lease you a “wavelength” in the fiber. Then they can charge you by the amount of data you transmit and maximize the return on their investment in the fiber.


Over the next few months, this column will look at how dark fiber is managed, monitored and used for new and expanding communications applications.

Editor's Note: This is part one of a four-part series on dark fiber. For the other parts in the series, click the following links: part two, part three, and part four.