Lighting Up Dark Fiber

By Jim Hayes | Apr 15, 2015




Last month, we looked at how dark fiber is tested to determine its capability of supporting newer, faster transmission networks. Once dark fiber has been tested and its usability confirmed, communications systems can be connected.

Generally, metropolitan and long-­distance networks using dark fiber are quite different from the systems common to customer-owned premises cabling. Premises cabling used for local-area networks (LANs) or security systems generally are based on multimode fiber and require two fibers dedicated to every link. Two fibers are used to provide full duplex operation, with each fiber transmitting in opposite directions.

Most multimode transmission systems use 850 nanometers (nm) VCSEL sources for short links at speeds up to 10 gigabits per second (Gbps). Beyond 10 Gbps, multimode fiber uses parallel links with multiple fibers transmitting 10 Gbps each or a new simple wavelength division multiplexing scheme with four different wavelengths per fiber.

Outside plant (OSP) dark fiber is always single-mode and may even be different types of single-mode. Most short links, such as metropolitan networks or fiber to the home, use the standard single-mode fiber, the same fiber that is also used for campus backbones, optical LANs (OLANs) and distributed antenna systems for cellular coverage inside buildings. Long distance links, such as intercity networks, will generally use a version of dispersion-shifted fiber that is optimized for 1,550 nm transmission and dense wavelength division multiplexing (DWDM).

These OSP links typically carry systems operating at 10–100 Gbps. The speed of the link depends on the fiber’s bandwidth capacity or the customer’s traffic requirements along the fiber route. Each fiber may carry many different signals using DWDM. Since fibers can carry up to 128 wavelengths, at 100 Gbps, that equals almost 13 terabits per second per fiber pair.

This much signal capacity is hard to comprehend. What could require such capacity? How about Internet traffic? Over the last 15 years, the data transmitted over the Internet has grown about 25,000 times. Certainly the Internet has more users these days, but an even bigger factor in that growth is streaming video, especially sites such as Netflix. On any given evening in the United States, Netflix represents about one-half of all Internet traffic. YouTube is the second-largest video source representing about 15 percent of all Internet traffic. Add Apple TV, Hulu and other video services, and Internet traffic clearly is congested in the evening while people are home.

People using video services—or reading email, web browsing, or playing online video games—connect to service providers (ISPs). Those ISPs need to connect to higher level providers who connect to the worldwide network we call the Internet. Much of that travels over dark fiber.

Dark-fiber suppliers rarely install a fiber optic cable in a facility or present a pair of fiber optic connectors for the user to plug in their equipment, unless the customer is a major telecom or cable TV (CATV) service provider. Mostly, users get a fiber cable into their facilities that connects to the fiber supplier’s equipment and provides the user with Ethernet connections to the Internet. The customer pays for transport services to their Internet provider unless the dark fiber supplier is also their Internet service provider. Either way, they are paying for a service that gives them bandwidth, not just a fiber connection.

Really big users, such as a telecom or CATV service provider, may actually lease a wavelength on a fiber pair and install their own equipment. Several dark fiber providers also offer low latency connections for financial services companies who are willing to pay high prices for connections that give them a timing advantage for high frequency stock or commodity trading.

Fiber providers often provide co-­location services in a large facility serving many users, where customers place their communications equipment and plug it into the dark fiber network or connect to other users at the same facility. Many users take advantage of these facilities to centrally locate their data centers where they can get connectivity and more reliability, including backup power, equipment service and security. Users of co-location services for data centers then require high-speed connections to their own place of business to use the services.

There are numerous opportunities for experienced electrical contractors (ECs) to work with dark fiber providers and users. Familiarity with the installation and testing of both OSP and premises single-mode fiber links is mandatory. The installation of communications equipment requires installing power, grounding, cooling and hardware for pathways and spaces just like any other premises network.

Dark fiber provides excellent opportunities for ECs who have expertise in fiber optics and premises cabling. However, it is necessary to ensure installers are properly trained and equipped. Next month, we will look at several applications that depend on dark fiber and discuss how ECs can be involved.

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

About The Author

HAYES is a VDV writer and educator and the president of the Fiber Optic Association. Find him at

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