Optical fiber: Internet with gigabit speeds

Optical fiber is playing a crucial role in network build-outs and digitalization in Germany. As Internet usage patterns of both consumers and businesses have evolved over time, with a sharp increase in data traffic caused by complex, data-intensive online applications, demand for bandwidth has been growing for years.

A bundle of fiber optic cable

A bundle of fiber optic cable

Copper cables, which were the main channel for transmitting digital information until just a few years ago, could not keep up with this demand. That makes the fiber optic Internet the technology of the future. The optical fiber build-out is in full swing. Lines based on optical fibers are already available to many businesses and private homes, with new ones being added every day.

What is optical fiber?

Optical fiber is a thin glass filament, as thin as a human hair. In telecommunications, optical fibers are used as optical waveguides (OW). Although they are so thin, they can transport data at nearly the speed of light. Modern optical fibers are made of three components: the core, the cladding, and the coating. The inner core consists of pure glass. It transports the information as light pulses, while the cladding prevents the light from leaking. The coating is made of plastic and serves as a shell to protect the sensitive fiber optic lines.

How fiber optic technology works

Optical fiber is ideal for transporting very large amounts of data in very short times, making them ideal for the high-speed Internet. In contrast to the previously used copper cables, fiber optic cables transfer data optically, not electrically. This means much higher bandwidth is available for data transmission.

The transfer of large amounts of data at nearly the speed of light has become commonplace. Optical transmission involves sending light pulses through a fiber optic cable between two end points. These light pulses are created using lasers. To bridge very large distances, however, optical amplifiers are used to send data over thousands of kilometers, such as submarine cables that cross the world’s oceans.

Many single optical fibers are grouped together in a fiber optic cable. In turn, these fiber optical cables are laid to create a fiber optic network – even over long distances.. A fiber optic network makes it possible for many people to use data-intensive services in the Internet simultaneously. Examples include streaming videos and gaming, and in particular the upload and download of large datasets for business customers, for industrial applications and medicine, for instance.

Downloads at gigabit speeds – how fast is optical fiber?

Data transmission via optical fiber differs from transmission via DSL and VDSL as a result of its much higher upload and download speeds. DSL and VDSL run on copper-based phone lines. Due to the physical dampening properties of copper wire, the potential data transmission rate decreases as distance increases. Data lines based on optical fibers support much higher bandwidths than copper wires for both upload and download, because the physical dampening effect here is extremely low.

A fiber-to-the-home connection usually gives customers transmission speeds of up to one gigabit per second. Even higher bandwidths are possible from a technical perspective, but 1,000 megabits per second is enough for all current usage scenarios in the Internet. In comparison, copper-based technology has hit its limits: a copper line with VDSL supervectoring can achieve a maximum of 250 megabits per second. VDSL without vectoring can reach transmission speeds of up to 50 megabits per second, while DSL is only capable of up to 16 megabits per second.

Overview of data upload/download speeds





16 Mbit/s

2,4 Mbit/s


50 Mbit/s

10 Mbit/s


100 Mbit/s

40 Mbit/s


250 Mbit/s

40 Mbit/s

Glasfaser - FTTH

1,000 Mbit/s

100 Mbit/s

Variants for fiber-to-the-home connections

Fiber optic links make high-speed Internet possible. Many buildings, both residential and office, have a “building connection room”, which contains all utility lines and facilities for the building. In addition to water and energy supply, this also involves telecommunications technology – that is, the phone and Internet line.

The lines laid by Deutsche Telekom normally end at the demarcation point. When a property is connected with optical fibers, a fiber optic building entry point is installed where the optical fiber enters the building – usually in the basement. For FTTH connections, this location is the interface between the incoming fiber optic signal and the customer’s local network. The optical network termination (ONT) converts the optical fiber signal into an electrical signal. 

Fiber optic network architectures have different names, depending on how close the fiber optic cables are installed to the computers in the building:

  • FTTC – Fiber to the curb: to the curb or property boundary
  • FTTB – Fiber to the building: into the building or to the building entry point 
  • FTTH – Fiber to the home: to the house or apartment

The last letter indicates the network architecture involved. FTTx (fiber to the X) indicates the specific position up to which fiber optic cables have been installed. In existing buildings, the inner wiring is often still copper; in new construction, optical fibers can be installed in the internal areas from the start.



DT invests several billion euros every year in building networks.