## 7 Conclusions  
Optical networks have experienced tremendous growth over the past few decades and have moved beyond traditional applications, through undersea, terrestrial and submarine networks, to become the infrastructure of choice in new networks segments, such as 5G fronthaul and datacenter networks. The anticipated advancements over the next years, beyond the revolution that coherent systems based on DSP have enabled, will be based increasingly on the capabilities of photonic integrated circuit platforms and new approaches for optical signal processing and switching based on the unique properties of light. Those advancements will lead to a higher network capacity with reduced cost and energy consumption per bit transmitted/processed. The examination of the main advantages and drawbacks of these key elements will need to be addressed in the F6G, which will be the next era of optical networks and is expected to integrate various technologies, such as communication, sensing, computational functionalities and AI, into a single network entity.  
In this complex environment, the current article aims to serve as a comprehensive overview for the evolution of fixed networking, shedding light on the key enablers from various perspectives, such as the physical layer advances and limitations, the networking challenges from core to access as well as the evolution of the offered services. The motivation for this work is the fact that fixed networks lack consistent exploration of their  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Challenge** & **Future Direction** \\ \hline Low-cost transceivers & exploitation of direct digital to optical domain (D/O) conversion instead of using electronic digital to analog converters (eDAC). DSP modules can also be removed transmitting light in amplification bands beyond C as well as using bundles of SMFs. This can result in a 10 times capacity increase and connectivity introduction of oDACs while eliminating multiple power-hungry and costly electronic DACs \\ Remote sensing & employing OFDR-based distributed fiber optic sensors with a linewidth squeezed optical source and optically-amplified-recirculating phase gates \\ NFV and SDN introduction & transformation of the central office into a datacenter (CORD) can provide significant gains in terms of latency in order to support F6G services \\ \hline \hline \end{tabular}
\end{table}
Table 13: Summary of the main technical challenges and the proposed future directions.  
evolution over the past 50 years. For this purpose, this survey article aims to assist engineers and students of the telecom industry to understand, in a better way, (a) how broadband networks emerged and developed as a result of the third Industrial evolution; and (b) the main features of the current networking environment, in order to be prepared for the \"intelligent\" future, which will be marked by advancements of the fourth Industrial Revolution.  
All authors have contributed to the conceptualization, methodology approach development, editing, writing and reviewing of the article. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The data discussed in this article have been referenced at the corresponding points.  
The authors would like to acknowledge the assistance of [PERSON] for the review of the manuscript.  
The authors declare no conflict of interest.