Dark Fibre vs Lit Fibre

What is Dark Fibre?

Dark Fibre, also known as unlit fibre, is fibre optic cable that’s installed but not transmitting data. It’s called “dark” because there’s no light signal running through it.

A network provider supplies and maintains the passive fibre infrastructure, such as fibre cables and ducts. You provide your own optical equipment to “light” the fibre and create your own high capacity network.

As a result, the bandwidth and performance depend on the equipment you use and the characteristics of the fibre. You’re responsible for deploying, monitoring, maintaining and securing your network, which gives you maximum control over how it operates.

That’s why Dark Fibre is often used to connect data centres, network exchanges and large sites where high capacity, low latency and long‑term scalability are critical.

What is Lit Fibre?

Lit Fibre is managed optical connectivity where the provider delivers a service over fibre they operate. That means they provide the passive fibre infrastructure and the optical equipment to transmit data.

Lit Fibre is delivered with defined bandwidths and transmission speeds. For example, you can order Neos Networks Optical Wavelength services in bandwidths of 10Gbps, 100Gbps or 400Gbps+. As an end-to-end service, the provider manages configuration, monitoring and fault resolution, with performance governed by SLAs.

Lit Fibre is commonly used by organisations that want high capacity connectivity with predictable performance but don’t need full control over how the network operates.

Dark Fibre vs Lit Fibre: key differences

Here’s an overview of how Dark Fibre differs from lit optical services like Optical Wavelengths:

Capacity, scalability and latency

Key deciding factors when choosing between Dark Fibre and Lit Fibre include capacity, scalability and latency.

First, with Dark Fibre, capacity is determined by the optical equipment and wavelengths you deploy, rather than predefined service limits. In contrast, Lit Fibre is delivered in predefined service tiers (10/100/400Gbps+ for Optical Wavelengths). So Dark Fibre is ideal if you want very high bandwidth that you directly control.

Second, while lit optical services scale in increments within a provider’s service tiers, Dark Fibre is almost infinitely scalable. You can boost capacity as much as you like by upgrading the equipment lighting the fibre, subject to the fibre’s physical limits.

Finally, both Dark Fibre and Lit Fibre can support very low latency connections when routes are optimised, as they use the same fibre infrastructure. However, Dark Fibre gives you greater control to optimise the routing and optical equipment to meet specific low latency requirements.

Control, management and security

Control and operational responsibility differ significantly between Dark Fibre and Lit Fibre, and those differences affect how security is implemented.

With Dark Fibre, you select and operate the optical equipment yourself. You control the network at the Physical Layer (Layer 1), deciding how capacity is delivered and scaled. With Lit Fibre, your provider controls the network design and active optical systems.

That control brings responsibility. With Dark Fibre, you manage day‑to‑day operation, monitoring and fault management, while the provider maintains only the passive fibre infrastructure. With Lit Fibre, the provider delivers and operates the service end to end under agreed SLAs, shifting operational risk away from you.

These differences also shape your security approach. Neither Dark Fibre nor Lit Fibre is inherently more secure, but Dark Fibre allows you to implement bespoke encryption and security controls, which can be important in regulated environments. Lit Fibre relies on provider‑defined security built into the service, which may well be sufficient if you prefer a managed model.

Dark Fibre and Lit Fibre costs compared

Dark Fibre and Lit Fibre have different cost structures because they follow different contractual models.

Commercial model

With Dark Fibre, you typically secure long‑term rights to use the fibre, often through an Indefeasible Right of Use (IRU). You then provide your own optical equipment to light and operate the network. In contrast, with Lit Fibre, the provider owns, operates and prices the network as an ongoing service.

Capex vs opex

Dark Fibre involves higher upfront capex. You need to invest in optical equipment, fixed fibre lease costs and, in some cases, fibre build. Lit Fibre is primarily an opex model, with costs charged as recurring service fees for capacity, network operation and support.

Fibre tax and telecoms licence

Unlike Lit Fibre, Dark Fibre attracts business rates, also known as “fibre tax”, once the fibre is lit. These annual charges, levied by local authorities per connection, add to your ongoing network costs. You may also need to pay for a telecoms licence, depending on how you use the fibre.

Total cost over time

All these differences impact the cost of scaling over time. With Dark Fibre, you can usually increase capacity by upgrading your own equipment without increasing the fibre lease cost. With Lit Fibre, scaling usually involves moving to higher service tiers, which increases monthly charges.

So Dark Fibre can deliver a lower cost per gigabit at higher capacities or over longer timeframes. For shorter commitments or where bandwidth requirements are lower or stable, a managed Lit Fibre service is often more cost-effective.

Dark Fibre vs Lit Fibre: which is right for your business?

Ultimately, choosing between Dark Fibre and Lit Fibre depends on how much control you want over your network, how you plan to scale capacity, and how much operational responsibility you’re ready to take on.

Do you want full control over your network? What are your long-term bandwidth needs? Do you have the internal IT resource and expertise to deploy and maintain your network? What about upfront and long-term costs?

Consider Dark Fibre if you:

Consider Lit Fibre if you:

Not sure which is right for you?

At Neos Networks, we specialise in delivering high capacity, low latency connectivity across our B2B-only nationwide network built for Critical National Infrastructure. Discover our UK-wide network.

If you’re looking for Dark Fibre, our experts work with you to map the optimal route, overcome challenges and plan for growth. Whether you need a flexible, short-term lease or a 25-year IRU, we design the best solution for you and your budget.

If you prefer managed Optical Wavelengths up to 400Gbps+, we specialise in serving hard-to-reach areas with our extensive nationwide network and a wide range of third-party tails.

Want to discuss your options? Get in touch.