Dark Fibre: the Unlit Strand, Under Your Control
Dark fibre is the physical fibre itself, leased unlit: you put your own optical equipment on both ends and run whatever capacity the physics allows. Enormous power, real obligations — and honestly not what most businesses asking about it actually need. Here's the straight version.
Dark fibre is unlit optical fibre leased as the physical strand: the provider supplies the glass between two points and you supply the equipment that lights it, so capacity is limited only by your optics. It is priced per route (distance, carrier and scarcity — not bandwidth), typically on multi-year terms, and it genuinely suits data-centre interconnect, large campuses and organisations with in-house optical skills. For most businesses, a lit ethernet circuit — 10Gbps from £349/month — delivers the capacity without the engineering obligations, and Ethernet First Mile (EFM), the old copper-bonding alternative, is now a legacy footnote.
What dark fibre actually is
Every fibre service you can buy is the same physical thing — glass strands in the ground — differing only in who lights them. On a leased line, the carrier lights the fibre and sells you a service at a speed. On dark fibre, nobody lights it until you do: you lease the strand itself, put optical equipment on both ends, and the capacity is whatever your optics can drive — 10Gb today, 100Gb by swapping transceivers, multiple wavelengths if you run DWDM. That's the whole product. The power and the obligations both follow directly from it.
How dark fibre is priced in the UK
Per route, not per megabit. The carrier prices the strand on distance, the route's construction cost, term length and — bluntly — scarcity, since spare strands between two useful points are a finite resource. That's why no honest provider publishes a dark fibre rate card, and why AMVIA quotes dark fibre per route — bespoke to the endpoints, with multi-year terms the norm. The discipline that keeps the decision honest: always price the equivalent lit service alongside. If your requirement is 10Gbps between two sites, the number to beat is a managed 10Gbps circuit from £349/month — dark fibre wins that comparison at scale and with growth, and loses it for a single steady-state link more often than the marketing admits.
Dark fibre vs a lit leased line
Three trade-offs decide it. Capacity economics: lit services charge again each time you upgrade; dark fibre absorbs growth by changing your own optics — decisive when you can already see the growth curve. Control: with dark fibre nothing of the provider's sits in your transmission path — attractive for encryption and latency reasons, if you have the team to own it. Responsibility: when a lit circuit fails, the carrier's SLA clock runs; when dark fibre fails, diagnosing whether it's your optics or the strand is your first job. Our business ethernet page covers the lit product line this gets compared against, and the pricing page publishes the lit floors.
Where EFM fits (historically)
Ethernet First Mile deserves a paragraph because procurement documents still mention it: EFM delivered ethernet over bonded copper pairs — a clever answer to fibre scarcity that gave real SLAs at sub-fibre prices in the 2010s. With full fibre now widespread and the copper network being retired, EFM is a legacy product: speed-capped, on borrowed infrastructure, and rarely quotable for new supply. If EFM is the only thing reachable at a site, that's a signal to check the fibre build timeline — and in the meantime SoGEA or 4G/5G usually bridges better than new copper commitments.
The honest recommendation
Ask two questions. Do you have — or want to build — the optical engineering capability to own a transmission layer? And can you already see growth that makes re-buying lit bandwidth expensive? Two yeses: dark fibre is a genuinely good product, and we'll route-check it properly. Anything else: take the lit circuit, keep the SLA, and spend the difference on the parts of your IT that actually differentiate you. We make the same margin either way, which is what lets us tell you straight.
Dark fibre in one honest table
Who dark fibre genuinely suits
If you recognise your organisation below, it's a real conversation. If not, a lit ethernet circuit will serve you better.
Data centre interconnect
Two sites, your own wavelengths, latency you control, capacity you upgrade by swapping optics — the classic dark fibre case.
Networks at scale
Campuses, healthcare estates, local authorities and ISPs whose traffic growth outruns lit-service economics.
Absolute control requirements
Organisations that need the transmission layer in their own hands — encryption end to end on kit they own, no provider equipment in the path.
Known heavy growth
When you can already see 10Gb becoming 100Gb, paying per-route once beats repeatedly re-buying lit bandwidth.
In-house optical skills
Dark fibre transfers the engineering to you: optics, monitoring, fault diagnosis to the strand. Without the team, 'control' becomes 'liability'.
Everyone else: lit services
A managed 10Gbps ethernet circuit from £349/month gives most organisations the capacity without the obligations.
How a dark fibre engagement runs
Route first — everything else follows the map.
Route feasibility
Dark fibre only exists where a carrier has spare strands on a route between your points — we check availability across the networks that publish dark fibre products.
Commercials
Pricing is per route: distance, carrier, term and scarcity set it. Expect annual charges and multi-year terms; we benchmark against the equivalent lit service so the premium is a decision, not a surprise.
Light it
Your optical equipment at both ends — chosen, installed and configured by your team or ours, with the demarcation documented.
Run it
You own the transmission layer. If you'd rather not carry that pager, our managed option monitors the link and the optics for you.
Why run this through AMVIA
Independent advice on a product most resellers either can't source or oversell.
Dark vs lit, benchmarked
We price the dark route against the equivalent lit circuit every time — because for most requirements, that comparison ends the conversation honestly.
Managed option
Take the strand with monitoring and optical management from our team if you want control without the 3am pager.
Security-first
The transmission layer is only as good as the edge around it — firewalls, segmentation and monitoring from a Cyber Essentials Plus provider.
UK team, published SLAs
Sheffield engineers, defined commitments, one phone number for the whole network.
Client testimonial coming soon. AMVIA protects over 1,200 UK businesses.
AMVIA Client
Not sure dark fibre is the right call?
That's the most common outcome of this conversation, and we'd rather tell you early. Give us the two endpoints and the growth picture — we'll price dark against lit honestly.
Dark fibre questions
Optical fibre leased as the unlit physical strand: the provider supplies the glass between two points, you supply and run the equipment that lights it. Capacity is set by your optics rather than a service tier — which is both the entire appeal and the entire obligation.
It's priced per route — distance, construction cost, term and strand scarcity — not per megabit, so there is no honest rate card and every quote is bespoke, typically on multi-year terms with annual charges. The benchmark to hold any dark fibre quote against is the equivalent lit service: a managed 10Gbps circuit from £349/month.
It's not inherently faster — it's uncapped. A lit leased line runs at the speed you bought; dark fibre runs at whatever your optical equipment drives, and upgrades by changing optics rather than re-contracting. For a fixed, steady requirement that flexibility may be worth little; for compounding growth it's the whole point.
Organisations connecting data centres or large sites, ISPs and public-sector estates with heavy growth, and anyone whose control or encryption requirements demand owning the transmission layer — provided they have the optical engineering to run it. Most SMEs are better served by a lit ethernet circuit with an SLA, and we'll say so when that's the honest answer.
EFM delivered ethernet over bonded copper pairs — a respectable pre-full-fibre product that is now legacy: speed-capped, running on infrastructure being retired, and rarely available for new orders. In 2026 the practical alternatives are fibre ethernet where built, and SoGEA or 4G/5G as bridges where it isn't.
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Serious capacity, one accountable provider
Dark fibre, 10Gb and 100Gb lit circuits, and the managed network edge around them — engineered by the same Sheffield team.
Related connectivity
Business Ethernet
The lit product line dark fibre gets compared against — from £69/month.
Dedicated Internet Access
Uncontended internet-facing circuits, 100Mbps to 100Gbps.
Leased line pricing
The lit-tier floors: 10Gb from £349, 100Gb from £1,999.
Multi-site connectivity
WANs across every site, with SD-WAN over the top.
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