What Is Dark Fiber?

Dark fiber, which also goes by the terms “unlit fiber” and “black fiber”, is fiber optical cabling that a telecommunications service provider has laid but does not use. In the 1980s and 1990s, communications companies installed miles of fiber optic cable underground. Their strategy when doing so was often to dig down and bury more fiber than they needed at the time. The rationale was that if the lion’s share of the cost associated with underground fiber installation was due to the labor, equipment, and operations necessary to bury cable — not the cable itself — it made sense to install extra cable to save money in the future when more communications and data transfer capacity would be required. Now, these unused, unlit fiber cables are leased and sold as core network services!

Fiber-optic cables carry pulses of light which communicate data. So, if you’re leasing a dark-fiber line, you’re basically looking to rent an unused (or “dark”) set of fiber lines, without any of the additional transmitters usually included. To use technical parlance (according to the OSI model used to describe telecoms infrastructure), dark fiber is an unlit, layer-1 service. You’re expected to set up and maintain electronics to “light” the fiber yourself to create a WAN link as you’d like.

Lit fiber is an adjacent service to dark fiber. With a lit-fiber service, the customer rents a dedicated fiber line, along with all the transmission equipment, from the service provider. In this case, the ISP operates the network and electronics required to “light” the fiber and you’re just leasing the actual network service itself. Billing for lit fiber services is also slightly different, with the cost increasing depending on tiered bandwidth usage).

ISPs often sell the lines to other carriers and enterprises in need of high bandwidth connections. The businesses light up the cabling by deploying network equipment on both ends of the line and establishing their own high-speed, private network.

Such deployments represent a significant and growing portion of the wide area network (WAN) market. The global dark fiber network market generated $5.49 billion in revenue in 2022 and is expected to reach $13.45 billion in 2030.

Dark Fiber Network Uses vs. Lit Fiber

Reliable, high-capacity connectivity is more important than ever before. With internet service providers (ISP) handling more and more bandwidth, companies in need of greater volumes are often finding themselves paying more and more for premium access or services, such as very high bandwidth point-to-point or wave circuits (what you’d call “lit fiber” products).

Dark fiber represents an alternative. For the most part, these cables are reserved by and leased out by large telecoms and ISPs, but businesses can take advantage of them as well by leasing the fibers and “lighting” them with their own electronics and network gear, dictating the network’s architecture, speed and security.

Here are a few instances where dark fiber may be better to use than lit fiber:

Your Bandwidth Needs Are High and Continue to Grow

With best-effort services, most ISPs are rife with bottlenecks built into their network architecture at various points to monetize the carrier's services. That isn’t the case with dedicated WAN services, such as dark fiber.

Dark fiber networks ensure that your data travels the most direct path between point A and point B, quickening data speeds and lowering latency. Your IT team manages your dark fiber network, identifying performance issues and addressing them without waiting for ISP techs to intervene. When buying dark fiber, a good question for the provider is to ask how far the distance of the link is between both sites — the closer to a straight line, the lower the latency.

You Want to Set Up a Private, Highly Customized, Secure Network

One of the main selling points of using dark fiber for your business is that it allows you almost complete control of your network infrastructure. Universities and other government institutions often prefer setting up their own networks because of the increased security and ability to directly manage their networks. Individual users of dark fiber can set and control the speed and latency of their network, providing fine-tuned control not always available from an ISP.

Standard WAN Use Cases

Need to connect two buildings with a high bandwidth connection? Need a high bandwidth data center link? If you’re in a major metro area with lots of fiber, dark fiber may be an attractive, futureproof option to replace what you’d typically use a wave circuit or other point-to-point fiber option for. Dark fiber pricing can vary dramatically depending on regional competition.

What Are the Potential Pitfalls of a Dark Fiber Network?

Savvy readers will probably have already guessed the main limitation of a dark fiber network, which is that you’re in charge of everything. From architecting the network itself to maintenance and troubleshooting, any business that elects to go with dark fiber will want to have a high level of technical expertise and be certain that it’s worth the expense and effort. 

Additionally, companies must invest in premise equipment to light the fiber and make it usable. This equipment can be costly and complex to maintain. It’s also worth remembering that even being able to connect to a dark fiber strand requires there to be unused cable in your area — this may not be an option, depending on where you are.

Another main concern, of course, is cost — most strand leases are for infinite capacity over a long period, which can require a hefty investment. Using dark fiber can be cost-effective for many businesses, but it’s especially beneficial when businesses with larger networks compare costs to their monthly ISP bill, which are often ISPs themselves. The most frequent buyers of dark fiber services are ISPs and WISPS, along with municipalities, schools / colleges, and highly network-centric enterprises. 

Large organizations can reach a tipping point where they’re transmitting so much data that charges for purchasing bandwidth from an ISP grow higher than a flat rate for leasing or buying dark fiber. Sometimes the other advantages make dark fiber worth the investment, but at a certain point, it’s a dollars game.

Where Can I Get Dark Fiber?

The nearer your location to a major city, the greater the chance that dark-fiber services are available to you. There are fiber lines running between the major cities, too.Outside of major cities and their connective infrastructure, it becomes trickier for carriers to make a business case for fiber installations. As we mentioned previously, the CapEx and project management costs make it harder to recoup fiber network investment from major population centers.

Even if your location is close to a major fiber line, it’s by no means a given that you can tap directly into their line to build your dark-fiber network. Connections to a fiber line are made via splice points – and most carriers have no desire to create additional splices on the line.

Once you know where the splice points are, you can work out whether a service provider is able to offer you a fiber buildout to your locations. Think of splice points as highway onramps – even though your house might be underneath the flyover, you could still have several miles to drive before you can merge onto the highway.

To determine whether a service provider is able to offer you dark fiber, you’d work with the same set of culprits you’d deal with for lit fiber service (Zayo, Crown Castle, Lumen, and the like) and go through similar bidding processes (sounds fun, right?).

What Kind of Contracts Are Available for Dark Fiber?

If you find a carrier that owns fiber between the locations you’re looking to connect, the most common kind of dark-fiber agreement is an Indefeasible Right of Use (IRU) lease. These are usually long-term leases – typically between 20 and 30 years.

If that’s more of a bind than you’re looking for, it’s worth investigating whether potential vendors would be willing to offer an International Private Leased Circuit (IPLC). As the name suggests, IPLCs are designed for international networks, and offer shorter terms – some IPLCs even offer monthly rolling contracts. 

One disadvantage of IPLCs, however, is the prohibition of subletting – you can’t rent the dark fiber out as if it was your own, which removes the possibility of offsetting the costs of your network.

If you’re looking for a lit-fiber deal, with leased fiber transmitters providing DWDM services, a term length of between three and five years is more commonplace. The costs involved for the provider are subject to change over time, so it makes more financial sense for them to ensure they can adjust equipment rental costs at more regular intervals.

The Network Potential Under Your Feet

For sophisticated users with high bandwidth needs, dark fiber is a viable and attractive option. It’s important to understand your needs and available options to decide the best option for you. Work with Lightyear today to see what WAN or dark fiber options are available for you!