By Wayne Rash

Wayne Rash has been writing technical articles about computers and networking since the mid-1970s. He is a former columnist for Byte Magazine, a former Editor of InternetWeek, and currently performs technical reviews of networking, wireless, and data center products. He is the former Director of Network Integration for American Management Systems and is one of the founders of the Advanced Network Computing Laboratory at the University of Hawaii. He is based in Washington, DC, and can be reached at wayne@waynerash.com.

What is Dark Fiber?

Technically, dark fiber is optical fiber that’s not active. Most commonly it refers to optical fiber cables that were installed in urban or suburban areas in the hope that a demand would arise, allowing the company that owns the fiber to sell access. In some cases, the term may refer to fiber that was installed, used, and then abandoned; it may also refer to fiber that exists within a building or campus, but isn’t currently being used.

Dark fiber is a point-to-point solution. In addition, dark fiber is only available where the company installing it decided to put it, or in some cases only where they could get permission to lay the fiber cable. Dark fiber is not available everywhere, and a terminus for the dark fiber may not be available near you.

The Case for Dark Fiber

Dark fiber excels at providing extremely low-latency solutions with little added overhead. This makes it ideal for data center mirroring, for example, where primary considerations might include extremely low latency with very high throughput, where no network protocol is desired, and distances are relatively short. Dark fiber also works well in situations that require either specialized network protocols or a wide variety of network protocols.

Dark fiber also excels in situations where the fiber can be connected directly to the network router with no extra equipment, such as in a campus environment where connectivity between adjacent buildings or nearby data centers is necessary. In these circumstances, dark fiber may be able simply to be terminated and attached to the building router.

The Case Against Dark Fiber

Dark fiber installations exist in a variety of locations, and they can cover widely varying distances. While local and metropolitan dark fiber is the most common, especially in large metropolitan areas, dark fiber sometimes can run much longer distances. In fact, dark fiber is available on undersea cables and can cover transcontinental distances. However, long distances require amplifiers, and amplifiers require maintenance. “Maintenance” potentially translates to more dollars.

Likewise, when dark fiber crosses long distances, it’s also exposed to a greater chance of accidental damage. Those stories you hear about a construction backhoe cutting a fiber-optic cable are true; and if it happens to be your data connection, your downtime could be lengthy. Worse, repairs likely depend on the third-party cable owners, unless you have a second, diverse, dark fiber installation that’s available for backup.

Dark fiber also requires maintenance and management of the network equipment at each end when you take it over and light it up. Some dark fiber providers offer this service. They may also make available service-level agreements and even diverse routing. But those features and services cost money and aren’t optional purchases.

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