Dark fiber or a leased line: which is right for your business?
Dark fiber and dedicated leased lines (DIA) compared on cost, control, capacity and management, with a simple framework for choosing the right option for your sites.
- A leased line (DIA) is a managed, ready-to-use service; dark fiber is raw fiber you light and run yourself.
- Dark fiber suits organisations that need very high or fast-growing capacity between fixed sites and have network engineers.
- For most offices and branches, a managed leased line gives the best balance of cost, speed and simplicity.
When your business outgrows broadband, two options come up again and again: a dedicated leased line and dark fiber. Both run over fiber optic cable, both are far more reliable than shared connections, and both are used by banks, governments and large enterprises. But they are very different products, and choosing the wrong one can cost you money or control.
This guide explains how they differ and how to decide.
What is a leased line (DIA)?
A leased line, often sold as Dedicated Internet Access (DIA), is a private, uncontended fiber connection between your site and your provider’s network. The provider lights the fiber, supplies and manages the equipment at your site, and guarantees a fixed speed, typically from 10 Mbps up to 10 Gbps or more.
Key characteristics:
- Symmetrical speeds: upload is as fast as download.
- Uncontended: the bandwidth is reserved for you, not shared with neighbours.
- Service level agreement (SLA): uptime, latency and fault-repair times are guaranteed in the contract.
- Fully managed: the provider monitors the line and fixes faults.
What is dark fiber?
Dark fiber is an unlit strand of fiber optic cable leased to you on its own. “Dark” means no light is travelling through it until you connect your own optical equipment at each end. You decide the technology, the speed and how the capacity is used.
Key characteristics:
- Virtually unlimited capacity: upgrade speed by upgrading equipment, not the contract.
- Full control: your network, your encryption, your routing.
- Long-term contracts: often leased for years or bought on an IRU (indefeasible right of use).
- Self-managed: your team runs the equipment and network layer.
Cost, control and capacity compared
| Leased line (DIA) | Dark fiber | |
|---|---|---|
| What you get | A managed service at a set speed | Raw fiber you light yourself |
| Capacity | Fixed per contract | Limited only by your equipment |
| Equipment | Supplied by the provider | Bought and run by you |
| Skills needed | Minimal | Network and optical engineering |
| Best for | Offices, branches, most sites | Data centres, campuses, carriers |
| Time to scale | Change the contract | Upgrade the optics |
When to choose a leased line
A leased line is the right choice when you want dependable, fast internet or site-to-site connectivity without running the network yourself. It is ideal for head offices, branches, retail stores, clinics and schools, and it pairs well with SD-WAN to connect many sites into one managed network.
When to choose dark fiber
Dark fiber makes sense when:
- You connect a small number of fixed, high-traffic sites, such as two data centres or a large campus.
- Your capacity needs are large or growing fast, where paying per gigabit would become expensive.
- You have, or want to build, an in-house network team.
- You need complete control over encryption and routing for regulatory or security reasons.
A simple way to decide
Ask three questions:
- How much capacity will this route need in three years? If the answer is tens of gigabits or more, look at dark fiber.
- Do we have the skills to run optical equipment? If not, choose a managed leased line.
- How many sites? Many sites favour managed services; a few large sites can favour dark fiber.
Many organisations use both: dark fiber between their core sites, and managed leased lines for everywhere else.
How Telxer can help
Telxer Fiber provides dedicated internet access, private Ethernet links, wavelengths and dark fiber on one owned network, with a 24/7 network operations centre. Our engineers can survey your sites and recommend the right mix for your traffic, budget and team.
Frequently asked questions
Is dark fiber faster than a leased line?
Dark fiber has no fixed speed: capacity depends on the equipment you connect to it, from 1 Gbps to 400 Gbps and beyond. A leased line is sold at an agreed, guaranteed speed.
Who maintains dark fiber?
The provider maintains the physical fiber path. You own and run the optical equipment at each end, and the network on top of it.
Can I start with a leased line and move to dark fiber later?
Yes. Many businesses start with a managed leased line and move busy routes, such as head office to data centre, onto dark fiber as their traffic grows.
Want help applying this? Our team can review your setup and recommend next steps.
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