Tech Primer: Dark Fiber, Lit and Wavelengths

Jerry Gilreath

Some IT colleagues have asked me, “What is dark fiber and what’s the difference between lit and wavelengths?” Let’s begin by understanding the basic concepts of fiber optics and the difference between dark and lit fiber.

Difference between dark fiber and lit fiber

Unlike wire, which passes electricity through a metal conductor, fiber optic cables use a specialized glass or plastic allowing for data to be transmitted great distances by passing light through the glass. Fiber that isn't currently being used and has no light passing through it is called dark fiber.

Utilizing this fiber, telecommunications carriers can provide something called “wavelength” services, also known as “waves.” This works by splitting the light into various wavelength groups called colors or “lambdas”. Carriers sell these wavelengths to separate customers and then recombine the colors and transmit it across fiber. Therefore, lit fiber is fiber that has been lit with light by a carrier.

Dark and lit fiber explainedTo better understand lit fiber’s wavelengths, think of a rainbow where each color is a channel of light. Remember Mr. "ROY G. BIV" from middle school – Red, Orange, Yellow, Green, Blue, Indigo, and Violet?

Wavelengths essentially split a single fiber into channels. Unlike copper wire, which uses an electrical signal, fiber optic communications utilize either a LASER or a LED operating at a very high frequency. Fiber optic cables have the ability to carry much higher frequencies than copper cables. Traditional bandwidth throughput (1Gb/10Gb/40Gb/100Gb) will easily fit into a single color channel. Each fiber can be split into hundreds of colors, but a typical lit fiber is split into sixteen colors or lambdas.

The business of fiber optics

In the late 1990's, there was an uptick in the number of carriers building out dark fiber networks. In addition, there was a high degree of inter-carrier trades – a practice where carriers would swap dark fiber assets with other carriers in order to gain a foothold in markets where they were not present or had limited capacity. Inter-carrier trades coupled with mergers and acquisitions allowed even the smallest of carriers to offer competitive data transport agreements around the world.

However, a significant portion of this built capacity remained untapped for years. Carriers wanted to avoid potential long-term lost telecommunications revenues and were reluctant to enable competitors in their high margin wavelength services market. In addition, carriers did not want to cannibalize their often oversubscribed and lucrative Ethernet services market with inexpensive high-capacity fiber. For these reasons, many carriers today still do not sell dedicated fiber assets directly to customers.

New demand for bandwidth

Technology needs have changed over time. Enterprises have become more dependent upon cloud services, interconnected infrastructures have grown in number, and a massive growth in the Internet of Things (IoT) all require a large data communications infrastructure that can scale rapidly, predictably, and on demand.

To fulfill these needs, dark fiber providers have entered the market and are working to provide massive bandwidth, low latency, and high quality connectivity to the end customer in the form of raw glass: dark fiber.

For additional information on the pros and cons of dark fiber versus lit services from carriers, read my blog post titled, “Shedding Light on Dark Fiber for Data Centers.”

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