Somewhere under the street outside your office there may be fiber-optic cable that nobody is using. It's called dark fiber: glass that is already in the ground but carries no light until someone connects equipment on both ends. For a growing number of Dutch businesses, that unused glass is the cheapest route to a fast, private link between their own sites.
What dark fiber actually is
When you buy a managed line from a carrier, you rent a service: the carrier owns the fiber, the equipment at each end and the bandwidth you're allowed to use. With dark fiber you rent (or buy the right to use) only the glass. You supply the switches and optics yourself, so you decide the speed. The same pair of fibers can run 1G today and 10G or 100G tomorrow, just by changing the optics.
Dark fiber vs a managed line at a glance
- Speed: with a managed line you pay for a fixed bandwidth. With dark fiber the only limit is the optics you plug in.
- Upgrades: more bandwidth on a managed line means a new contract. On dark fiber it means swapping two optics.
- Privacy: dark fiber is a physical point-to-point link. Your traffic doesn't share equipment with other customers of a carrier.
- Monitoring: the carrier watches a managed line for you. On dark fiber that job is yours, or your IT partner's.
Why this matters in the Netherlands
Over the past two decades, municipalities, regional development agencies and grid operators have laid their own fiber networks, often to connect town halls, schools and business parks. A well-known example is TReNT in the east of the country: founded in 1999 by the University of Twente and OostNL, now part of grid operator Alliander, with more than 2,000 km of fiber. Many of these networks offer dark fiber to companies as well as to public bodies.
For an SME with two or three sites, this changes the question. It's no longer "which carrier line do we buy?" but "can we light this ourselves?"
What you need to light it
- A switch or router at each end with the right cage type: SFP for 1G, SFP+ for 10G, SFP28 for 25G or QSFP28 for 100G.
- Optics rated for the distance. This is where most mistakes happen. Short-reach multimode optics (SR) are for inside a building. Dark fiber between sites is almost always single-mode, so you need LR optics (around 10 km), ER (around 40 km) or ZR (around 80 km), depending on the length of the route.
- The fiber details from the owner: route length, fiber type and the measured loss in dB. Match the power budget of your optics to that loss, not just to the distance on the map.
- A plan for growth. If you only have one fiber pair, CWDM, DWDM or BiDi optics can run several channels, or both directions, over the same glass.
A worked example
Say you run an office and a warehouse 14 km apart, and the fiber owner reports a route loss of 6 dB. A 10 km LR optic is not the right pick: the route is longer than its rating and its power budget would be tight. A 40 km ER optic covers the route with room to spare for ageing connectors and future splices. In practice we'd put a 10G SFP+ ER module on both ends, keep one spare on the shelf, and the link is ready.
The opposite mistake happens too. A long-reach optic on a very short route can send so much light that it overloads the receiver on the other side. A small inline attenuator solves that. It's the reason the loss figure from the owner matters more than the distance alone.
Is it always cheaper?
Not always, but often over the life of the contract. You trade a monthly service fee for a one-off investment in hardware plus a lease for the fiber. The trade-off is responsibility: there's no carrier NOC watching your link, so you need someone who can monitor it and keep a spare optic on the shelf. For most organisations with an IT partner, that's a small price for a link they fully control.
Questions to ask the fiber owner
- Is the fiber single-mode (OS2), and how many fibers are free on the route?
- What is the measured end-to-end loss, and is there an OTDR report?
- Which connector is used at the handover point, LC or SC?
- How long is the lease, and who repairs the cable if it is damaged during roadworks?
- Where exactly is the handover point in your building, and who brings the fiber to your rack?
Our tip
Before your next connectivity contract renews, find out who owns the fiber in your street or business park. Ask the municipality, the regional development agency or the local fiber operator whether dark fiber is available and what the route loss is. With that one number, choosing the right optics becomes straightforward.
How we can help
Networkser Group supplies the hardware for exactly this: transceivers and optics from short reach to long-haul, 10G SFP+ modules, CWDM, DWDM and BiDi optics for single-pair routes, and the switches to put them in. New to optics? Read our guide on how to choose the right SFP transceiver, or get in touch and we'll help you match the optics to your route.