Dark fiber is fiber-optic cable that is already in the ground or in buildings but is leased to you without any provider equipment attached. You connect your own optical gear at each end and decide how much capacity to run over it. Organizations use dark fiber when they need very high or rapidly growing bandwidth between a few fixed locations, such as data centers, campuses or a colocation facility, and want control over the path instead of buying a managed circuit.
At a glance
- You lease specific fiber strands, usually a pair, between two or more points, often under a long-term agreement.
- You supply and operate the optics that light the fiber, so capacity grows by upgrading equipment, not by renegotiating a contract.
- It is most common within a metro area; long distances need amplification and more complex optical engineering.
- The provider typically maintains the cable; you maintain the electronics.
- Availability depends entirely on whether a provider has spare strands on the route you need.
What problem it solves
When an organization leases a circuit, such as an Ethernet private line or a wavelength, it pays for a fixed amount of bandwidth. Every upgrade means a new order, a new price and sometimes a new installation. For heavy, growing traffic between fixed points, such as replication between two data centers or a campus linking its buildings, that model gets expensive and slow.
Dark fiber turns bandwidth into an equipment decision. Once you have the strands, moving from 10 Gbps to 100 Gbps or adding multiple wavelengths over the same pair is a matter of buying and installing new optics. You also control the path end to end, with no shared provider equipment in the middle, which some organizations value for security and troubleshooting.
How it works
Leasing the strands. A provider with spare fiber on the route leases you specific strands. Agreements are often long, sometimes structured as an indefeasible right of use (IRU) paid largely up front, and sometimes as a monthly lease. You usually receive route maps, the locations of splice points and test results showing signal loss along each strand.
Lighting it. You install optical transceivers or a dense wavelength-division multiplexing (DWDM) system at each end. Short metro routes can often run with transceivers plugged directly into switches or routers. Longer routes, or many wavelengths on one pair, need DWDM equipment and sometimes amplifiers at intermediate sites, which adds cost and space requirements along the route.
Maintenance. The provider normally repairs cuts and damage and handles locate requests before digging near the route, under a maintenance agreement with defined response and repair targets. You monitor and maintain your optical equipment and the network running over it.
Diversity. Two dark fiber routes are only diverse if they follow physically separate paths and enter buildings separately. Route maps let you check this, which is harder to do with managed circuits.
For more on evaluating dark fiber routes and terms, see our Dark Fiber solution page.
When it matters for buyers
- Data center interconnect. Two or more data centers or colocation sites exchanging large volumes of data are the classic case.
- Campus and multi-building sites. Hospitals, universities and corporate campuses use dark fiber between their own buildings.
- Rapid growth in a fixed footprint. If bandwidth between the same points keeps doubling, the economics shift toward owning the capacity.
- Control and compliance. Some organizations want no shared provider equipment on a path carrying sensitive traffic.
- Long-term planning. Dark fiber commitments run for many years, so they fit sites you expect to keep.
Questions to ask vendors
- What is the exact route, and can we see maps showing where it runs and where it enters each building?
- What test results can you provide for each strand, and what loss do they show end to end?
- Is the agreement an IRU or a lease, for how long, and what are the renewal terms?
- What maintenance is included, and what are the response and repair-time commitments after a cut?
- How much notice do you give for planned work or route relocations?
- Does any part of this route share conduit or building entry with our other connections?
- Are spare strands available on the same route if we need more later?
How it differs from a wavelength
A wavelength is a managed optical service: the provider runs the optical equipment and sells you one dedicated channel of light at a fixed speed, such as 10 or 100 Gbps. With dark fiber you get the glass itself and run your own optics, so you can carry as many channels at whatever speeds your equipment supports. A wavelength is simpler and faster to buy; dark fiber gives more control and can cost less at high capacity but requires optical expertise. Dim fiber sits between the two.
Problems this helps with
- Bandwidth between our data centers keeps outgrowing the circuits we lease
- Monthly circuit costs rise every time we need more capacity
- Need full control over a network path between our own sites
