Definition and delimitation
The digital infrastructure enables the accessibility of the internet and thus the use of digital services. These are the basis for digital transformation and digitisation. The digital infrastructure is divided into the cable-based fixed network and the mobile network.
Within the telecommunications network as the foundation for the cable-based digital infrastructure, a differentiation is made between copper-based (conventional telephone cables) and light-wave-based (fibre optic cables) transmission media. These two technologies differ significantly in transmission speed, with lightwave-based transmission enabling significantly faster connections to digital applications. Furthermore, there is the possibility of connecting to the digital infrastructure via the television cable network. Each building can be connected to the different cable networks. With the help of routers, the respective households of the connected building tap into the network individually. [1, 2]
Devices with correspondingly integrated technology can also be connected to the mobile network. In this case, the device used establishes a connection to the nearest radio mast, which enables connections to other participants in the local and – via cable – supraregional network. The speed of data transmission depends on the available mobile radio standard of the device and the radio mast. A distinction is made here between generations (G), each of which provides different technologies in different network frequencies and thus transmission speeds. 
The use of the internet provided via this digital infrastructure is managed by private companies, just like the networks themselves. In Germany, the three largest network providers are currently Deutsche Telekom, Telefonica Deutschland and Vodafone. 
Occasionally, digital infrastructure also includes offerings accessible via the internet, such as cloud services or the World Wide Web, although these can be understood more as services for using the internet and not as basic digital infrastructure.
With the beginning commercialisation of the internet in the 1990s, the development of a digital infrastructure also became relevant. The technology was developed in the 1960s, building on the existing telecommunications infrastructure. Therefore, in the first years, the telephone line was the only access to the new communication medium. This dual use of the existing ISDN telephone line for telephone and Internet use, which was analogue at the beginning, meant that a household could either make telephone calls or use the internet. The introduction of a DSL modem and splitter, which separated the voice and data signal on the user side, meant that the existing telecommunications infrastructure could remain the main medium for both communication channels. However, the ever faster developing information and communication industry required significant increases in the performance of transmission speeds and computing capacities not only in the end devices, but especially in the infrastructure. 
The existing telecommunication infrastructure was based on a network of copper lines, which experienced large drops in performance over longer distances. Therefore, for some years now, the existing infrastructure has been supplemented by the more powerful fibre optic lines. Since this involves adding a new cable strand to the existing telecommunications infrastructure, the expansion is associated with high costs. Currently, the laying of fibre optic cables up to the distribution boxes is being pushed forward. From the distribution boxes, from where households in the respective street sections are supplied, the existing copper line is used. Consequently, the copper-based telephone cable is shorter and can be used for the increased power demand through further optimisations (vectoring) in the distribution boxes. [1, 2]
Parallel to this, different mobile radio standards developed, which are divided into generations (G). While until the early 2000s it was still possible to make a phone call or send an MMS with 2G in the GSM and EDGE network, the turn of the millennium heralded UMTS and thus 3G. The further development of this technology into HSPA and HSPA+ made it possible for the first time to send smaller data packets within seconds. Since 2015, there has been the so-called LTE network, which in its first form still belongs to 3G. LTE-Advanced, an optimised version of the original data transfer protocol, is equivalent to 4G, the fastest generation currently available on the market. Currently, the construction of the 5G network is being planned, with whose delay-free transmission autonomous vehicles, telemedicine and smart networks are to function without interference for the first time. 
Application and examples
The digital infrastructure is needed for internet-based services and products. Devices such as laptops and smartphones or even sensors are networked via the network. This enables the exchange of data packets that can contain applications or information.
Industry 4.0 developments are based on the networking of machines, suppliers and employees along, for example, production chains that operate beyond a single location. Without a digital infrastructure and the networking it provides with location-independent participants in the production chain, such a development would not be possible. 
Further examples can be found in almost every economic sector. Besides smart grids and smart metering in the energy sector, smart farming is becoming increasingly relevant for the agricultural sector. Smart Homes for society and Ambient Assistant Living for the older generation show that digital applications are already winning over every area of everyday life. 
Criticism and problems
The expansion and operation of digital infrastructure is regulated by the free market. The economic viability of expansion is far less guaranteed in rural and sparsely populated regions than in cities due to the lower population density. However, the digital infrastructure is nowadays perceived by a large part of the population as critical infrastructure and partly as a public good. 
The inaccessibility of the internet and the services it provides has posed major problems for businesses and private households, especially since the corona pandemic. Due to the pandemic, parts of the population are advised to use home offices, whereby a minimum speed of 50 Mbit/s is defined in order to be able to work fluently. In the education sector, pupils, students, teachers and lecturers are also dependent on a good digital infrastructure.
While 98% of households in urban regions have a connection with a bandwidth of 50 Mbit/s, every fifth household in rural regions has to cope with lower bandwidths. 
The federal government has therefore been developing various financing programmes for several years to enable the expansion of the digital infrastructure even for economically unviable regions.  However, the relevance of a trouble-free and fast connection is increasing faster than the expansion can be driven forward. As a result, the competitiveness and attractiveness of regions that are less well connected to the digital infrastructure is steadily decreasing, thus continuously exacerbating existing regional economic disparities.  At the same time, there are other voices that are critical of the expansion of the digital infrastructure through such infrastructure funding programmes. They argue that the uniform and nationwide expansion should be replaced by infrastructure funding that is state-run but demand-oriented. 
Currently, one research focus is on the application scenarios and infrastructure-related expansion possibilities of 5G. 
Other research projects deal with the use of the internet provided by the digital infrastructure, its possibilities and challenges on the technological, economic, legal and socio-political sides.
In the already completed project “Digital Transformation Strategies of the German Länder“, the digitisation strategies of the German Länder were examined. The final publication also considers the topic of digital infrastructure and the expansion of the mobile and broadband network in Germany.
The doctoral project “Analysis and Dynamic Optimisation of Vehicle Communication“ deals with the question of how optimal vehicle-to-vehicle communication can be achieved via the mobile network.
Further links and literature
Federal funding programmes for digital infrastructure:
Recommended reading in connection with the functioning of mobile radio:
Martin Sauter, From GSM to LTE-Advanced Pro and 5G: An Introduction to Mobile Networks and Mobile Broadband, 4th Edition, John Wiley & Sons, 2021 (ISBN: 978-1-119-71469-9)
 Martin Sauter, Grundkurs Mobile Kommunikationssysteme: UMTS, HSDPA und LTE, GSM, GPRS und Wireless LAN, Vieweg + Teubner, Wiesbaden, 2011.