Operating a Dark Fiber Network: Use Cases, Complexity, Cost vs. Lit Fiber

If you’re seeking more bandwidth and lower latency to improve the performance of your applications and services, then dark fiber could be the silver bullet

dark fiber
Matt Pinto

May 16, 2023


Are you afraid of the dark? There’s no need because Lightyear’s here to shine a light on dark- fiber network connectivity.

If you’re investigating high-speed fiber options and looking for greater bandwidth and lower latency to improve the performance of your applications and services, then dark fiber or lit fiber could be the silver bullet. In this post, we’ll help you think through considerations on when to go with dark fiber.

What Is Dark Fiber?

Fiber-optic cables carry pulses of light which communicate data. So, if you’re procuring 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.

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 (billing for lit fiber services is also slightly different, with the cost increasing depending on tiered bandwidth usage).

At this point, you might be wondering why there’s all this fiber lying around unused, anyway? Surely, it’s expensive to lay this stuff in the first place?

It’s a valid question, and there are two main reasons.

  • Technological improvements. When the Incumbent Local Exchange Carriers (ILECs) first started laying down fiber, back in the late 80s/early 90s, a single fiber channel could transmit data at around 2.5 Gbps. Then the lab geeks figured out they could use Dense Wavelength Division Multiplexing (DWDM) to send multiple data streams down a single channel. Multiple streams = more bandwidth. How much more? Well, you can squeeze 32 Tbs down one channel now, which is about 12,800 times more data. So, modern fiber-optic usage leaves plenty of spare fiber for everyone.

  • Long-term planners laid extra capacity to save on future costs. Labor and project management makes up much of the cost of a fiber install. Those crazy 20th century ILECs figured they’d save a few bucks and installed additional fiber, far more than their requirements at the time – not anticipating the huge efficiencies DWDM would bring.

So, what’s the net result of these two factors? Depending on your geographic location, there could be plenty of dark fiber going unused that your service provider would be more than happy to lease to you.

Who Uses Dark Fiber?

Lots of folk. We’ve seen dark fiber deployed in many ways by a range of businesses who tend to have the following things in common.

  • Multiple, well-established static locations

  • A qualified team of in-house transport technicians capable of lighting a fiber network

  • A strong need for a secure, low latency network, with scalable network connections

These are some common users of dark-fiber networks.

Wireless Internet Service Providers (WISPs). Cellular providers often use a dark-fiber network to connect their cell towers to the data center. They’ll install and manage their own Dense Wavelength Division Multiplexing (DWDM) equipment, configured either as a point-to-point service or as an any-to-any topology, to best suit local requirements.

Municipalities. Critical infrastructure such as police stations, fire departments, and hospitals needs to be connected with as much security, reliability, and permanence as possible, making dark-fiber networks a great fit for public services.

Schools and colleges. Large, multi-campus institutions like to connect their locations securely, and enjoy the level of control a dark-fiber network provides.

Connecting two data centers. More of a business need than a use case or typical user – we’re often contacted by companies that, for one reason or another, want dark fiber to run between their data centers. Again, as a secure, predictable low-latency solution, it’s a useful addition to a wide area network (WAN).

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.

Which Should I Choose – Dark Fiber or Lit Fiber?

When it comes to performance metrics, there’s little difference between these two dedicated internet services. The factors to be considered are scalability, security, and cost.

Scalability. Dark fiber isn’t infinitely scalable, due to the physics involved and the mounting cost of equipment, but it's not that far off! Also, any upgrades you carry out on your existing bandwidth capabilities are entirely your own business – no need to contact the carrier, and there are no additional monthly costs.

Lit fiber upgrades will require some negotiation with the service provider, who is likely to charge you a tiered rate according to your bandwidth requirements.

Security. Neither dark fiber nor lit fiber services provide any visibility into traffic through that part of the network – any security provision must take place after the fiber hands off to the customer premises equipment (CPE).

Cost. When comparing like-for-like network locations and provision, lit-fiber services are generally more expensive. The provider is obviously going to charge more for their leased equipment and technicians than you’d pay if you were procuring these things in-house.

However, the relatively lower cost of dark fiber is only available for customers with considerable connectivity needs – you’ll need to have fairly hefty throughput requirements to justify the outlay, although there’s often wildly unpredictable regional variations in how much providers charge for dark fiber. To run a dark fiber network, you'll need to invest in hardware as well as smart hands to run your network and deal with issues, so there is a breakeven bandwidth point where this makes sense.

Subletting fibers on your dark-fiber network can be a handy way to offset the expense of this long-term investment – although this comes with its own set of legal and contractual hassles and responsibilities with which not every business is prepared to engage.

There are several other variables to consider when gauging dark-fiber costs. These include distance from the fiber splice point, and the links used to reach the fiber line, as well as the distance between your connected locations.

This last cost can quickly mount up, depending on the range of your transmitters – should your transmitter only be effective for up to 500 miles, and your locations are several thousand miles apart, then you’ll need to install and maintain repeaters every 500 miles, increasing your project management and labor costs.

While this list of pros and cons will help you get a sense of what both these services can offer, it might not be so simple to apply this knowledge to the complexities of your business and connectivity procurement needs.

The Lightyear telecom operating system is designed to solve problems like these. With ease, you can configure a dark fiber or lit fiber network and compare options digitally, or arrange a real-time meeting to discuss your options with an experienced industry professional.

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