Dark fiber is a lease of unlit fiber-optic strands between two or more locations. There is no carrier equipment or service on it until you install your own optical gear at each end, so your capacity depends on the equipment you attach, not on the contract. It suits organisations with large, steady bandwidth needs between fixed sites and the engineering staff to run optical networks. For most other businesses, a managed wavelength or Ethernet service delivers most of the benefit without the work.
What you are actually leasing
A fiber cable contains many strands of glass. Carriers light some of them to run their own services and lease others unlit. With dark fiber, you get exclusive use of specific strands, usually in pairs (one to send, one to receive), along a defined route between specific points: two of your buildings, a building and a data centre, or two data centres.
Nothing else is included. No internet access, no carrier-managed service, no per-megabit billing. The strands are simply yours to light.
How it is usually sold
- An IRU (indefeasible right of use). A long-term right to use the strands, often 10 to 20 years, commonly paid largely up front, plus an annual charge for maintaining the cable.
- A lease. A shorter term with monthly payments, closer to how lit services are bought.
Either way, expect the provider to maintain the physical cable and repair cuts, under terms set out in the contract. What it does not do is monitor or guarantee the light on your strands, because it is not providing any.
What changes when you light it yourself
On a normal circuit you buy capacity and pay more for more of it. On dark fiber you buy the path, and capacity becomes a function of the equipment you attach. Moving from 10G to 100G can mean changing optics rather than renegotiating a contract. With DWDM equipment, which carries many wavelengths on the same strands, one fiber pair can carry many separate high-capacity channels.
You also get complete control of what runs over it: your own encryption, your own protocols, and no carrier equipment in the middle of your traffic. For some regulated or latency-sensitive workloads, that is the main attraction.
What you take on
Everything the carrier used to do on a lit service:
- Equipment. Optics and transport gear at both ends, and the capital to buy and eventually replace them.
- Engineering. Staff who can design the optical path, work out the loss budget over the distance, and choose the right optics.
- Monitoring and troubleshooting. Knowing within minutes that a path has degraded, and telling a strand problem from an equipment problem.
- Resilience. One dark fiber route is one path. Diversity means a second route that does not share conduit, bridges or building entries.
There is no SLA on the light, because you are the one lighting it.
Dark fiber vs lit alternatives
| Service | What you get | Who runs the optics | Good fit |
|---|---|---|---|
| Dark fiber | Unlit strands on a defined route | You | Very large, long-term capacity between fixed sites, with in-house engineers |
| Wavelength | A dedicated optical channel (for example 10G or 100G) between two points | The carrier | High, predictable capacity without running optical gear |
| Ethernet transport | A private Layer 2 connection between sites at a set bandwidth | The carrier | Connecting offices or data centres privately at moderate speeds |
| DIA | Dedicated internet access with an SLA | The carrier | Reaching the internet and cloud, not linking private sites |
See our wavelength services and Ethernet transport pages for the lit options.
Distance and routes
Dark fiber is most common within a metro area, between buildings a few kilometres to a few tens of kilometres apart. Standard optics are rated for specific distances, and longer spans need higher-grade optics and, eventually, amplification. The route also matters more than the straight-line distance, because fiber follows streets, rail lines and conduit. Ask for the route in detail, often supplied as a map file under a non-disclosure agreement, so you can check diversity against any other path you rely on.
Who it suits
Organisations with sustained, large, predictable capacity needs and the in-house capability to run transport:
- Hospitals and health systems moving imaging between campuses.
- Universities and research institutions.
- Trading firms where microseconds matter and the shortest route is worth paying for.
- Carriers, service providers and hosting companies.
- Enterprises linking their own data centres or reaching a colocation facility.
Who it does not suit
If you do not have network engineering staff, or do not want them, dark fiber converts a solved problem into your problem. A managed wavelength gives you most of the dedicated-performance benefit while leaving the optical layer someone else's job. That is the right answer far more often than dark fiber is. And if what you need is internet access for an office, dark fiber is the wrong category entirely; see DIA vs shared fiber vs coax.
Building resilience on dark fiber
A single dark fiber route between two sites is a single point of failure. Organisations that depend on it usually do one of three things:
- Lease a second, diverse route between the same two points, and have their equipment switch between the two when one fails.
- Build a ring linking three or more sites, so every site can be reached in two directions.
- Back the dark fiber with a lit service from a different provider on a different path, used only if the fiber is cut.
Protection switching on optical equipment can move traffic to the surviving path quickly, but only if the second path genuinely does not share conduit, bridges or building entries with the first. The route detail discussed above is how you check.
Comparing the real cost
Dark fiber and lit services are priced so differently that a monthly comparison misleads. Compare the total over the same period, usually the length of the IRU or lease:
- Dark fiber: the IRU or lease payments, annual maintenance, optical equipment at both ends (and its replacement partway through a long term), space and power for that equipment, cross-connects at each end, and the share of engineering staff time it takes.
- Lit service: the monthly charge for the term, any installation charge, and the price of upgrading capacity if needs grow.
Dark fiber tends to win when capacity needs are large and growing and the term is long, because each upgrade is an equipment purchase rather than a higher monthly bill. Lit services tend to win when needs are moderate, uncertain or short-term, or when the staff cost of running optics would be new.
Getting into the buildings
A dark fiber route has to reach real equipment at each end. In a data centre, that usually means a cross-connect from the provider's meet point to your cage or rack, with its own monthly charge. In an office building, it means a building entry, riser space and the landlord's agreement, the same issues any fiber install faces. Confirm who arranges and pays for each end before you sign, because these costs are easy to leave out of a comparison.
A worked example
A hypothetical organisation, used to show how the decision is framed.
A health system runs two data centres about 20 km apart and expects the traffic between them to grow from 10G to several 100G channels over the next decade. It already employs network engineers who run its core routers. For this organisation, a long-term dark fiber IRU on two diverse routes, lit with its own DWDM equipment, turns future capacity growth into an equipment purchase rather than a new contract each time. Compare a 40-person law firm linking two offices across town: a managed wavelength or Ethernet circuit gives it private, fast connectivity with an SLA and nobody to hire.
Questions to ask a dark fiber provider
- What is the exact route, and can I see it in detail to check diversity?
- How many strands, and what fiber type?
- What are the measured loss figures for the span?
- Who maintains the cable, and what are the repair commitments for a cut?
- How are building entries and any cross-connects at each end handled?
- For an IRU, what is the term, the up-front cost and the annual maintenance charge?
- What happens at the end of the term: can it be renewed, and on what basis?
- How much notice do you give before maintenance work on the cable, and can it be scheduled around us?
The honest test
If the section on optics and monitoring above read like a description of your team's ordinary work, dark fiber may be excellent value. If it read like a list of new hires, it is not. Either way, we can price dark fiber and the lit alternatives on the same route so you can compare them directly. It is free with no obligation, and we usually respond the same day. See dark fiber, send us your two endpoints, or call 478-758-8091 or text (347) 870-0965.
