Dark Fibre: what is it and who is it suitable for?

Your connection needs to be fast, secure and scalable? Dark Fibre is your solution! Let us explain exactly what fibre optics are and what Dark Fibre means.

What is Dark Fibre?

Dark Fibre is literally a lightless fibre optic connection. Without the activating equipment that is needed to send and receive data, the fibre optic cable remains dark.

So you could say that Dark Fibre is just a cable lying under the surface. Not very exciting, right?

In fact, the opposite is true! This innovative cable offers attractive opportunities for companies and organisations – a whole range of opportunities, as you will discover.

Dark Fibre makes it possible to establish a direct connection between two locations, for example between two branches of an organisation, two data centres or between a data centre and an office building.

Three examples of advantages:

  1. Unlimited bandwidth
  2. Almost immediate scalability
  3. Complete in-house control

But before we discuss these benefits in more detail, let us explain exactly what fibre optics are and what Dark Fibre means.

The background of Dark Fibre

Dark Fibre, i.e. a lightless fibre optic cable, is a term for a data communication cable to which no activating equipment has been connected as yet.

The Netherlands – and also Germany – has an excellent fibre optic infrastructure. These networks were originally installed by:

  • Network managers of large organisations, in areas where many companies are located;
  • Municipalities and non-profit organisations, or a collaboration between the two, sometimes with public money and sometimes without;
  • Preferred suppliers, like ProRail.

But the network is not used optimally. So why would you create a new network when you can use existing networks? This is where Relined Fiber Network steps in to help.

We have access to networks of both large and small operators, and make them available to the market on a commercial basis.

Our networks

Fibre optics – the theory A fibre optic strand is made from very clear glass and is extremely thin; between 125 to 250 microns in diameter. This is about the thickness of a single human hair. The fibre consists of a very thin core that has been fused with a thicker cladding.

To transmit data over long distances, light is sent through the fibre using a laser. The light enters the fibre at an acute angle relative to the outside surface, which causes internal reflection. As a result, the light stays in the fibre.

The speed at which the information can be sent depends on the laser. The faster it sends on and off signals, the faster the rate of data traffic.

Two types of fibre optics There are two types of fibre optic arrangements:

The single-mode

A single-mode (single-wave) fibre optic cable is suitable for long-distance transmission and high bit rates, or very fast data traffic. When we say very fast data traffic, we mean speeds of more than one terabit per second. This speed is achievable because the cable has a narrow core (about 9 microns). As a result, the light moves in a straight line through the cable and is not reflected back by the outside of the core. By contrast, the latter is the case with multi-mode fibre optic arrangements. Because a laser is used, the equipment is relatively more expensive.

The multi-mode

The second type is the multi-mode (multi-wave) fibre optic cable. This type is ideal for sending data over relatively short distances, at average speeds and with a high bandwidth. Because this type of fibre optic cable uses LED or VCSEL technology instead of laser technology, the equipment required is much cheaper. The core of this fibre optic cable has a larger diameter, which causes light scattering. This can cause bandwidth loss.

Possibilities with Dark Fibre

Dark Fibre makes it possible to establish a direct connection between two locations, for example between two branches of an organisation, two data centres or between a data centre and an office building.

Direct Connection

A direct connection between two locations is called a linked connection. Because you are not dependent on external fibre optic providers, you can set up your ‘own’ piece of Dark Fibre entirely according to your own wishes.

Redundant connection

Another possibility is a redundant connection, a so-called ‘ring structure’. Data can be sent via different routes in this structure so that the system as a whole continues to function correctly if one of the components fails.

Who benefits from a private Dark Fibre network?

Setting up your own network requires investment. For example, you need to procure routers, establish connections and, in some cases, extend the fibre optic cable to connect to a business building.

Even so, this is still more cost-efficient in the long term than opting for a broadband service. In the latter case, you rent capacity from a service provider and use a shared network. You are dependent on the range of services offered and also have to pay extra charges for additional services or bandwidths.

Furthermore, a private network is a much more appropriate solution for organisations where data security is very important. They have full control and are the only network users.

This also answers our question:

Large companies and organisations that need stable, reliable and secure connections benefit most from Dark Fibre. In addition, these connections are easily scalable.

These companies or organisations often also need access to a large bandwidth and would like to have full control over their own network. For example, data centres, network suppliers, ISPs, hosting companies and system integrators. And companies and organisations in the wholesale, healthcare and public sectors also frequently use Dark Fibre.

Dark Fibre segments, connecting the dots

The advantages of Dark Fibre

Dark Fibre is the basic platform for a private, future-oriented network. Because fibre optic networks are widespread in our country, you do not have to construct your own network. In many cases, an existing network can be used and only the last mile to the location needs to be excavated. Based on the equipment you have chosen, you can construct a private network that meets all your requirements using existing fibre optic infrastructure.

Let us clarify the benefits for you:

  1. Unlimited bandwidth
    The bandwidth that an organisation can access with Dark Fibre is huge – more or less unlimited, in fact. This results in fast transmission of data and optimal connectivity for the user. You only pay for the fibre optic connection and the equipment, not based on Mbits or Gbits. The latter is the case when you opt for a bandwidth service.
  2. Scalability
    Scalability is a further advantage. Because you have full control over the equipment, you can scale up based on your own connectivity needs. So if you want to assign extra bandwidth to a service or add extra services to the network, you can do so in a very short space of time. You do not need to buy any additional bandwidth to do this.
  3. Full control
    This immediately brings us to the next advantage: full control. An organisation builds its own network and can develop its own network strategy. You make your own strategic choices about how you set up, manage and secure the network. You decide for yourself which service providers you want to work with; e.g. where you purchase your internet services. In short, as an organisation you are not dependent on a third party’s partners in any way. Not only do you retain full control over how you manage and maintain the network, you also control project costs.
  4. Encryption
    Organisations – particularly institutions in the healthcare, public and financial sectors – are increasingly required to demonstrate that they are in control and that they handle personal data and other confidential information responsibly. The General Data Protection Regulation (GDPR), which applies to all companies from 25 May 2018, is an important development in this area. So a secure connection is a hot topic at the moment.Dark Fibre has an edge here. Leased connections and Dark Fibre cables both run through publicly accessible areas, which makes them vulnerable to physical ‘taps’ on the line.

However, because Dark Fibre has its own light-generation equipment, you have (greater) control over a connection’s security.

You can also apply extra encryption to the signal. In other words: additional security because you put your safeguards in place based on wavelength (colour), i.e. per individual connection. One advantage of encryption at the level of the fibre optic infrastructure is that it causes much less delay than encryption at higher levels.

Implementing encryption is mainly relevant for organisations that work with sensitive data. Dark Fibre also offers enhanced cloud and backup capabilities. This also helps make your data more secure.

Data centres

Security and continuity considerations mean that many of our customers want to house their business-critical ICT equipment in properly protected data centres.

Relined now has over 110 carrier-neutral data centres in the Netherlands and Germany at its disposal that are (redundantly) connected to its Dark Fibre network.

Possible locations for this are in almost all cases to be found on securely protected premises, such as at TenneT’s 380 kV network stations, at various of Relined’s own regeneration locations and at many of our data centre partners. This means the best possible security is guaranteed at almost any location you choose.

How is a network created?

Although our country already has an extensive fibre optic network, fibre optic cables are still lacking in some spots. This often requires excavation of the last mile to the location.


This process starts with the excavation of trenches that are 70 centimetres deep and about 30 centimetres wide.

We choose to excavate down to 70 centimetres, because our pipes are normally 4 to 5 centimetres in diameter. So when they are laid at the bottom of the trench, there is about 65 cm of soil between them and the surface. The covering soil layer must be at least 60 centimetres deep. So we always have a margin of 5 centimetres.

When we encounter obstacles such as driveway exits, sewers, roads and water pipes during excavation, we use alternative methods such as:

  1. microtunnelling,
  2. directional drilling,
  3. impact ramming.

1. Microtunnelling

This method allows us to literally dig underneath the obstacles listed above. And under subsurface obstacles such as sewer conduits, tree roots, cables and pipes. Depths of up to 10 metres are no problem at all here and special calculation methods and soil surveys make greater depths achievable as well.This method is also used to excavate past car parks/yards and narrow pavements with many entrances. An unobstructed area is chosen and a hole bored down to a depth of about 2 metres. We maintain this depth until we reach the desired location.

In microtunnelling, a casing tube is installed to create a passage through which cables and pipes can be pulled, without causing nuisance in the surrounding area. Distances up to hundreds of meters can be bridged in a single drilling operation.

2. Horizontal directional drilling

Directional drilling is a second trenchless technique. Ductile pipes are forced into the ground in an arc. These ductile pipes are inserted at a specific angle to avoid obstacles such as other pipes, expanses of water and roads. This method is mainly used to bridge short distances of up to about 25 metres, especially when crossing underneath ditches.

3. Impact ramming

This trenchless technique is used to bridge short distances of up to 15 metres. It is a fairly simple method, which involves using a torpedo-shaped pneumatic hammer to ram a steel tube horizontally into the ground.

Two pits are dug first: the launch pit (approximately 6 metres long or along the length of the trench) and the reception pit. The metal tube is rammed from one pit to the other.

Horizontal auger drilling
A fourth method is horizontal auger drilling. This method does not push the soil aside. A hole is drilled and then immediately filled with a casing tube.

The advantage of this method is that no subsidence occurs and it can be applied when little space is available for the launch and reception pits, e.g. in narrow streets where pipes cannot be installed using a pneumatic hammer. The disadvantage of this method is that it is time-consuming and expensive.

Fibre optic cable blowing
An HDPE tube is rolled out in the trenches. To make optimal use of the space in this tube, four sub-ducts (guide tubes) are blown through the tube. Using several of these guide tubes means that more fibre optic cable can be blown through, which in turn means that more cable operators can use the fibre optic network.

The blowing process requires an air pressure of about 10 bars and a capacity of about 17 cubic metres per hour. The ‘whirlwind’ created in this way causes the small tubes and the fibre optic cables to hover, allowing them to be blown through the HDPE tube. Shallow grooves in the tube steer this process in the right direction.

The tubes can be blown over a distance of 400 to 500 metres, the fibre optic cable over a distance of at least 600 metres and around 1000 metres in most practical situations.

Blowing usually takes place from a manhole or handhole. We install them every 6 kilometres along the entire route (depending on the size of the reel). A manhole or handhole is a plastic connection box, measuring half a meter by a meter, where one or more HDPE tubes exit. The cables are connected together in these handholes (using a fusion splicer). This infrastructure can also accommodate branches. In that case, a third cable, which ultimately runs to the customer, is inserted into the handhole.

Final assembly

The customer location is approached from underneath the surface. We bring the HDPE pipe with the fibre optic cables into the building, often in the basement, by cutting a hole through the façade (in some cases an existing hole is used, but in general we have to drill a hole ourselves using a diamond-tipped core drill and then seal it to prevent the ingress of gas and water).

We then run the fibre optic cable to the desired location from the basement. But an organisation cannot do much with the fibre optic cable on its own.

That is why the cables are also assembled, usually in an FO drawer where the end of the cable is fused to pigtails. You can connect the equipment to these pigtails.


Organisations and companies can benefit greatly from Dark Fibre. Why? Because this lightless fibre optic infrastructure offers an extremely secure, stable and fast connection and the ability to set up, secure and scale up a network according to your own needs.