## 1 Introduction  
Telecommunications hugely impact important aspects of our lives, such as social and economic activities. Modern telecommunications are the offspring of the second Industrial Revolution, which emerged with the deployment of the Public Switched Telephone Network (PSTN) in the second half of the 19 th century. During the past century, the main service that the PSTN could underpin was telephony. Only after the third Industrial Revolution (that marked the dawn of the Internet) with the great advances in electronics and informatics, the introduction of new services could become a reality. Today, we are at the dawn of a new era, the fourth Industrial Revolution, which will fundamentally change the way we live, work, and communicate. Unlike in the previous three, in the fourth revolution era, various processes, such as fixed/wireless communication, computing and sensing, will converge into a single network entity. This transformed network, and especially its access part, will perform simultaneously both as a sensor and as a supercomputer, enabling new ways in which the physical world will be represented through a digital counterpart (i.e., \"digital twin\". This digital representation will provide additional features to further optimize network operation [1].  
This work aims to examine and review the key technological advances in fixed telecommunication networks over the past 50 years. In particular, the starting point of this survey is 1970 and the ending point is 2020, examining the impact of the third Industrial Revolution on the various layers of the telecommunications ecosystem. The motivation for our work is the lack of a comprehensive overview of the evolution of fixed networking in a concise way that takes into consideration the key enablers, the physical layer advances and limitations, the networking challenges from core to access, as well as the evolution of the offered services. As the wireless networks and their generations have been extensively investigated in the literature, the fixed networks lack a consistent exploration of their evolution over the past 50 years.  
Our work follows the F5G timeline of evolution [2] for the five generations of fixed networks. First, each generation is segregated into three layers, namely physical, networking (both core and access part) and service. In this work, the adopted taxonomy of layers is different from the OSI-compliant taxonomy. Next, the key achievements and limitations of each generation and for each different layer are discussed, designating the most important bottlenecks of the most widely adopted technologies and standards. Finally, this analysis helps to reveal the technical challenges posed to the network operators of the fixed telecom industry, such as energy efficiency, capacity scaling, cost-efficiency, etc., revealing potential future directions.  
This survey is organized as follows. Section 2 analyzes the layered approach, which is adopted to study the fixed network from various viewpoints. In Section 3, the key services are examined along with their main requirements for each generation, showing that the network development is service driven as the needs from a user's perspective are always increasing with time. In Section 4, both parts of the network layer, namely access and backbone, are studied. In Section 4.1, the evolution of the various access standards is examined, and next an outlook on future approaches is presented. In Section 4.2, the backbone part of the network is presented, and the main achievements are classified into the five fixed network generations. In Section 5, the advances of the physical layer (both in access and backbone) are categorized and analyzed per generation, along with the most important achievements. In Section 6, the main opportunities and technical challenges for network operators are briefly presented, and potential future directions are discussed. Finally, concluding remarks are found in Section 7.