• Blog

Dark fibre: Connectivity, infrastructure and providers

Thousands of kilometres of unused fibre known as “dark fibre” lie buried in ducts across the UK. These cables can be leased to businesses to build private, high-performance networks. Companies bear the costs of operating the line, but have control over the specs.

Our guide explains what dark fibre is, how it works, and why large firms use it for critical tasks.

Content highlights:

• What is dark fibre?
• How to set up dark fibre?
• Benefits and drawbacks of dark fibre
• Which type of business is dark fibre for?
• UK dark fibre providers

What is dark fibre?

Dark fibre is the unused (or “unlit”) fibre optic cables that have been installed in the ground but are not currently active. It is passive infrastructure: physical fibre strands without any equipment attached or signals running through them.

It exists because it is standard practice for network providers to install surplus capacity during construction. Fibre is inexpensive compared to the high cost of digging. Laying extra strands upfront is a practical way to ensure resilience, redundancy and long-term scalability.

This overprovisioning was more extensive during periods of rapid infrastructure expansion (like the dot-com boom of the late 1990s) when providers overestimated future internet demand and accelerated laying cable. Estimates suggest there are hundreds of thousands of kilometres of unlit fibre across the UK.

Today, infrastructure owners lease these idle fibre strands to businesses that want to “light” them using their networking equipment.

In doing so, providers can monetise dormant assets and customers gain exclusive control over a dedicated fibre route, giving them complete autonomy over bandwidth, routing, performance, and security.

This level of control is ideal for organisations with high-capacity or sensitive data requirements, such as financial firms, NHS trusts, research institutions, and data-driven enterprises. It is a step up from any managed networking service available, even premium ones like cloud-native WANs or Metro Ethernet.

The caveat is that the customer is responsible for procuring and managing all active components, installation, maintenance, and monitoring, which requires a lot of resources.

How does a dark fibre network work?

Dark fibre is an infrastructure service where only the installed cables are offered. Once leasing conditions are agreed, the fibre link can be accessed and customised by the business.

Here is how it works:

1. Leased fibre strands

When an organisation leases dark fibre, they’re leasing specific fibre strands within a larger cable bundle. These bundles often contain dozens or even hundreds of strands, most of which remain unused (or “dark”).

These fibre strands usually run between fixed network nodes (e.g. Openreach exchanges, data centres, enterprise buildings, or metro network hubs)  along pre-built routes owned or operated by telecoms providers. In cities, routes may be just a few kilometres long, while regional or intercity links can span tens to hundreds of kilometres.

Organisations don’t lease arbitrary segments; they contract specific point-to-point links between nodes where providers have available capacity and can offer access. Multiple strands can be leased for extra capacity or redundancy, and in some cases from different providers using separate routes.

Once a contract is in place, that strand is theirs to use exclusively.

2. Lighting equipment and access points

Because dark fibre is passive, businesses must install active hardware at each end to transmit data. This is known as “lighting” the fibre, and the setup includes:

• Optical transceivers (e.g. SFP/QSFP) to convert electrical signals into light
• Routers or network switches to manage traffic and route data across the network
• DWDM equipment is used if multiple wavelengths are needed on a single strand for higher capacity or to have simultaneous channels running in parallel.

This kit is installed in rack space at each fibre endpoint, which might be in an on-site comms room, a data centre, or a telecoms facility. When this is a shared location, businesses typically lease rack space or arrange cross-connects to access the fibre handoff.

The exact setup depends on the use case and network design. Usually:

• Short metro links: A simple switch and transceiver may be sufficient
• Intercity links: May require optical amplifiers or dispersion compensation to maintain signal quality over distance
• Multi-site or high-capacity environments: DWDM systems are used to run multiple channels over a single fibre

3. Integration with the business network

Each leased fibre strand forms a dedicated, point-to-point connection between two sites, often from a telecom hub to a data centre, or between key business locations.

The leasing company or organisation is responsible for integrating this dark fibre link into its broader network. They decide what traffic will pass through it, under what routing and priority rules, and between which sites.

Being a fully private, premium connection without bandwidth contention or throttling, dark fibre is normally reserved for high-priority traffic between key sites, such as real-time data backups, private cloud access, high-frequency trading and voice and video communications.

To do this, businesses integrate dark fibre by using one or a combination of:

• MPLS: Dark fibre routers can be MPLS-enabled, allowing the link to become a label-switched path that is part of a larger MPLS network.
• SD-WAN: SD-WAN can be layered on top to route traffic intelligently across multiple links, to arrange auto-failover along secondary broadband lines and to encrypt traffic.
• Secure Access Service Edge: To enforce zero-trust security models and cloud-hosted firewalls across the private network and beyond.

4. Dark fibre operation

Leasing dark fibre makes the business its operator, meaning they are responsible for installation, maintenance, and support. Businesses can opt for:

• A fully in-house setup where their team handles all operations.
• A hybrid approach with third-party support for specific tasks.
• A fully managed dark fibre service, where an external provider handles operations entirely.

For more details, see our dark fibre vs managed service section.

In any case, key aspects of dark fibre operation include:

Performance monitoring

Because the business controls the equipment, it’s responsible for monitoring traffic flow and link quality. Network monitoring tools such as SNMP-based platforms and optical performance monitors help track latency, jitter, packet loss, and light levels, and flag anomalies in real time.

Equipment and fibre maintenance

All active components at either end of the link, including transceivers, switches, routers, and DWDM equipment, must be maintained by the organisation. This involves regular firmware updates, configuration checks, and on-site hardware servicing.

When signal issues arise, tools like OTDR (Optical Time-Domain Reflectometers) are used to identify faults or degradation along the fibre strand, even though the business doesn’t own the fibre itself.

Provider responsibilities

The infrastructure provider retains responsibility for the passive layer, including the ducts, fibre cables, joints, and physical routing. If a break occurs due to construction or environmental damage, the provider will handle repairs.

Some contracts include service level agreements (SLAs) for fault response and fibre restoration times, though the specifics vary by supplier and route.

How to set up dark fibre

Setting up dark fibre is very different from getting a managed service. It involves working closely with the network provider during planning, procurement, testing and commissioning.

Here are more details on each:

1. Define objectives and requirements

What is the dark fibre network for? e.g., low-latency trading, secure data centre replication, or high-throughput inter-site communication.

This determines the bandwidth, endpoints and redundancy required to support the application, as well as whether the link will form part of a larger SD-WAN or MPLS architecture.

This directly affects the physical segments needed and the equipment specification.

2. Identify available dark fibre routes

Engage with dark fibre providers to explore existing capacity between your chosen locations. They normally offer pre-built route maps, including:

• The data centres, exchanges, or metro nodes can be connected
• Diverse routing options are available for redundancy
• The types of handoff that are supported at each end (e.g. ODF, patch panel, cross-connect)

If no route exists between your endpoints, providers may offer to build it as a custom project, but this involves laying new fibre, like in a leased line installation, requiring significantly longer lead times and higher costs.

You can also combine routes offered by different providers to add redundancy and reach, and integrate them later via SD-WAN.

💡 If you want to quickly engage with multiple dark fibre providers at once, leave us your postcode on our business broadband comparison page, and we’ll find which providers operate in your area.

3. Finalise the lease agreement

Once a suitable route is identified, negotiate a dark fibre lease with your provider(s). Contracts usually include:

• Number of fibre pairs (or strands)
• Route details and handoff locations
• SLA terms for restoration or fault resolution
• Optional cross-connect or rack space charges (if needed)

Strands are usually leased on a 1, 5, or 10-year basis, with costs reflecting route length, location, exclusivity and contract length.

4. Procuring equipment

Since businesses are responsible for “lighting” the fibre,  the next step is designing the equipment setup. Typical components include:

• Optical transceivers matched to fibre type and distance
• Network switches or routers at each endpoint
• Optical mux/demux gear (DWDM/CWDM) if multiple wavelengths are required for max output
• Fibre patching hardware for rack integration’
• Optional optical amplifiers or dispersion compensators for long-haul links

Ensure compatibility across both ends and factor in rack space, power, and cooling needs.

5. Installation and commissioning

Work with the provider and internal teams (or a systems integrator) to:

• Arrange access to endpoint facilities (data centres, comms rooms, etc.)
• Install and patch the fibre into your active equipment
• Perform fibre acceptance testing (e.g. OTDR trace, light-level checks)
• Configure transceivers, routing protocols, and monitoring systems

Many businesses opt for a phased rollout, lighting a single pair first as a pilot, then expanding as demand grows.

6. Testing, monitoring, and go live

Before switching production traffic over, the link(s) need to be validated through:

• Throughput and latency tests
• Optical signal quality checks
• Failover behaviour (if part of a redundant design)

Once confirmed, bring the link online and integrate it with your routing policies and network monitoring tools.

7. Ongoing maintenance

Businesses are responsible for monitoring, hardware upkeep, and coordinating with their provider on physical issues.

Key tasks include:

• Optical monitoring: Regularly check light levels and error rates to catch signal degradation early.
• Hardware checks: Replace ageing transceivers, update firmware, and ensure cooling and power remain stable.
• Provider coordination: Stay in touch with your fibre provider for fault repairs, planned works, or SLA updates.
• Capacity planning: Track spare strands and test backups to support growth or failover when needed.

Some businesses opt for third-party support to simplify ongoing management and ensure rapid response to faults.

What’s the difference between dark fibre and normal fibre?

Here’s what each means:

Dark fibre

Dark fibre is unused fibre optic cable that businesses lease to build their high-performance, high-security private networks.

Leaseholders need to supply the hardware, manage the connection, and have full control over performance and security.

It is used for specialised applications (e.g. high frequency trading, research and higher education facilities) and is typically expensive to rent and operate.

Normal fibre

Normal fibre is a group of managed internet services where the provider handles everything, from equipment to internet access, and businesses pay for a specific level of service (speed, latency, and uptime). Examples of “normal fibre” services include:

• SoGEA (FTTC without the landline)
• Fibre-to-the-Premises (FTTP),
• leased line business broadband
• Business Ethernet

Which type of business is dark fibre for?

Dark fibre is exclusively used by large and specialist businesses, as well as public institutions that need to move massive amounts of data quickly, securely, and without relying on shared networks.

At its core, dark fibre is about superfast, private, multi-site connectivity, giving organisations full control over performance, routing, bandwidth, and security. The most common use cases include connecting:

• Corporate headquarters to data centres
• Hospitals to digital imaging or records systems
• Broadcast studios to live production facilities
• Research labs to high-performance compute clusters
• Government buildings to secure private networks

While public sector organisations (like the NHS or universities) often disclose dark fibre usage due to transparency rules, private businesses rarely do to maintain security (confidential network architecture is part of their security posture), respect non-disclosure agreements, and maintain competitive advantages.

However, based on our industry knowledge, we can confidently outline the main types of use cases and notable UK organisations associated with them:

Logistics & retail

Companies like Tesco, Sainsbury’s, and Ocado likely use high-speed fibre links between HQs, data centres, and fulfilment hubs. Ocado’s robotics and automation systems rely on real-time data and low-latency cloud connectivity.

Media production & broadcast

Broadcasters such as BBC, ITV, and Sky handle huge video files and live feeds between studios, edit suites, and transmission hubs. Private fibre ensures smooth post-production and real-time broadcasting.

Healthcare

NHS Trusts like Newcastle Hospitals and regions such as Swansea Bay use dark fibre to connect hospitals and data centres. This supports medical imaging, records systems, and secure clinical data exchange.

Finance & trading

Firms like LSEG, Barclays, and HSBC require ultra-low latency connections for high-frequency trading and secure data replication. Private dark fibre is certainly used to reduce delay between trading hubs as not doing so would be a competitive disadvantage.

Research & education

Universities such as Cambridge, Oxford, and Cardiff rely on dark fibre to link campuses and HPC clusters. It enables fast scientific data transfer and secure academic collaboration.

AI & compute infrastructure

Companies like DeepMind and Graphcore depend on high-throughput, low-latency links between compute clusters and research teams. Fibre is key for training large-scale AI models.

Manufacturing & engineering

Firms like Jaguar Land Rover, Rolls-Royce, and BAE Systems need private fibre to sync design, simulation, and telemetry systems across sites. It supports real-time diagnostics and R&D workflows.

Where is dark fibre available in the UK?

Dark fibre is most commonly found in urban centres, business parks, and data centre hubs, areas that have historically been prioritised for fibre rollouts (recall dark fibre is excess fibre capacity installed over the last 20-30 years).

However, as the UK’s fibre network continues to grow, the availability of unlit fibre continues to expand into regional towns and even rural areas, particularly where there’s strategic demand.

Here is how coverage varies across different parts of the country:

Major cities and metropolitan areas

Dark fibre is most readily available in large urban centres where fibre networks are dense and commercial demand is high. These cities typically host multiple data centres, financial institutions, universities, and media organisations, all of which benefit from high-capacity, low-latency links.

Common coverage areas include:

• London (especially Docklands, Shoreditch, and Southbank)
• Manchester (including MediaCity, the city centre, and science parks)
• Birmingham, Leeds, Glasgow, Bristol, Edinburgh, Cardiff, and Sheffield

Regional towns and mid-sized cities

Dark fibre is becoming increasingly available in regional hubs, particularly where:

• Public-sector networks (councils, NHS Trusts, universities) have driven demand
• Government-funded programmes (e.g. the Gigabit Broadband Voucher Scheme) have encouraged infrastructure build-out
• Secondary data centres or mid-sized enterprises require private connectivity

Examples of dark fibre reach in regional cities include:

York, Coventry, Milton Keynes, Peterborough, Aberdeen, Hull, Newcastle, Portsmouth, and Norwich

These towns are often part of regional fibre grids or near-edge data centres, making dark fibre both feasible and increasingly common.

Suburban and rural areas

While less common, dark fibre is not out of reach in suburban and rural settings, especially where one or more of the following apply:

• The area lies close to a major fibre route or national fibre corridor
• Local authorities or anchor tenants (like NHS Trusts or universities) have commissioned bespoke fibre builds
• Business parks sit near the edge of urban networks
• Fibre runs along transport corridors (motorways, rail lines)
• There is proximity to renewable energy sites, defence infrastructure, or enterprise zones

Notable examples include :

Bury St Edmunds (Suffolk), Forres / Rafford area (Northeast Scotland), Michaelston‑y‑Fedw (Wales), Oxfordshire villages

Even where fibre infrastructure was originally laid for full fibre or leased lines, the presence of physical fibre routes opens up the possibility for dark fibre access, often through bespoke agreements.

UK dark fibre providers

UK dark fibre providers include not only the usual retail broadband providers like BT and Virgin, which we’re so used to hearing about, but other “altnet” providers focused on high-performance business connectivity that have amassed strategic networks of dark fibre in the UK over time.

Here are the main dark fibre providers in the UK:

BT Openreach

• Network coverage: 170,000+ km of fibre covering urban and rural UK areas
• Points of Presence (PoPs): 5,500+
• Data centre integrations: Hundreds of data centres nationwide

BT Business is the UK’s largest dark fibre network provider and part of the BT Group. It builds and maintains the wholesale fibre infrastructure other providers and businesses use to deliver connectivity. Openreach’s extensive network ensures comprehensive coverage across urban and rural areas, making it the backbone of UK connectivity.

Virgin Media Business

• Network coverage: 190,000+ km of fibre; the 2nd largest network in the UK
• Points of Presence (PoPs): 336
• Data centre integrations: 160+ connected data centres

Virgin Media Business (Wholesale), part of Virgin Media O2, operates the UK’s second-largest dark fibre network.

Neos Networks

• Network coverage: 34,000+ km of fibre connecting major cities and business hubs
• Points of Presence (PoPs): 600+
• Data centre integrations: 90+ UK commercial data centres

Neos Networks, originally part of Scottish and Southern Energy (SSE), leverages its power infrastructure roots to deliver enterprise-grade fibre solutions. With a strong presence in Scotland and other key business hubs, it focuses on providing high-performance networks for enterprises, government, and public sector organisations.

CityFibre

• Network coverage: 26,000+ km of fibre infrastructure across 60+ towns and cities
• Points of Presence (PoPs): 100+
• Data centre integrations: Key UK data centres nationwide

CityFibre is a fast-growing challenger in the fibre market, focused on building full fibre networks in underserved towns and cities. Their wholesale division aims to take fibre to areas often overlooked by incumbents.

Zayo Group

• Network coverage: Extensive metro and long-haul fibre connections with Europe
• Points of Presence (PoPs): Multiple in major UK cities
• Data centre integrations: Key UK and European data centres

Zayo is a global bandwidth provider specialising in high-capacity metro and long-haul fibre. Zayo has a strong focus on data centre connectivity, which makes it a key player for enterprises with demanding needs.

euNetworks

• Network coverage: Metropolitan networks in London and Manchester, linked to a European backbone covering 53 cities
• Points of Presence (PoPs): Multiple in major UK cities
• Data centre integrations: 14+ cloud platforms and others

euNetworks is a pan-European provider offering metro and intercity fibre networks. In the UK, its networks in London and Manchester cater to high-demand industries needing bespoke, high-performance connectivity. Its integration with a European backbone makes it ideal for cross-border business operations.

Other Altnets

Here are some notable alternative network providers (altnets) offering dark fibre services in specific regions or for specialised applications:

• Wright Fibre: Offers dark fibre solutions tailored to businesses in select localised areas.
• Cambridge Fibre: Provides dark fibre connections to businesses within the Cambridge region, enabling custom networks.
• National Grid Telecoms: Utilises its extensive energy infrastructure to deliver dark fibre services across the East Midlands and South West England.
• Vorboss: Focuses on high-capacity dark fibre networks in London, connecting businesses and data centres.
• KCOM: Offers dark fibre services in the Hull area, providing businesses with dedicated fibre connections for custom network solutions.

Benefits and drawbacks of dark fibre

The benefits of dark fibre revolve around its unparalleled performance, security, control and scalability, with its drawbacks being its high costs, complex management, limited availability and high responsibility:

Benefits of dark fibre

• Scalability: Businesses can light the fibre to their specifications, accommodating growing bandwidth demands without relying on provider upgrades.
• Long-term control: Full control over the network allows custom configurations, traffic prioritisation, and certainty of continuity throughout the lease period.
• Security: Physically isolated connections with no shared infrastructure or third-party access. It’s ideal for zero-trust security models and environments requiring air-gapped or regulated data transport
• Performance: High-speed, low-latency, symmetrical connections tailored to critical applications, such as data centres and financial transactions. Provider-imposed bandwidth limits constrain performance; headroom speed is only limited by the optics used.
• Cost certainty: Dark fibre helps manage costs through one-off hardware investments (CAPEX) and flexible lease options, such as sales-type leases for long-term capitalisation or operational leases for short-term flexibility. These models align with cash flow needs and can improve EBITDA.

Drawbacks of dark fibre

• High costs: While predictable, costs are substantial. Leases, equipment, and operational expenses, such as fibre tax, can make it inaccessible for smaller businesses without a clear use case.
• Complex management: Requires technical expertise to install and maintain hardware, monitor the network, and quickly resolve issues.
• Limited availability: Dark fibre is unavailable in all areas, particularly rural locations, limiting accessibility.
• Long deployment times: Custom network builds can take weeks or months to set up, delaying operational use.
• Responsibility: Without Service Level Agreements (SLAs), businesses must self-manage uptime and reliability, increasing the operational burden.

Dark fibre vs managed services

If dark fibre is available, it is almost certain that the network operator or another business broadband provider also offers a managed service covering the same route.

There are pros and cons for each option; here is a summary:

Back to blog Back to broadband guides

• Share:
• 
• 
• 
•