# Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends
## 5 Internal Architectures of Submarine Cable Systems and BUs  
Regarding submarine architectures and topologies, we have four options which were shown in [9, 25]. If the distance between two stations is small or there is not a nearby third land station that has to be connected, a point-to-point topology is used (Figure 28a). The trunk-and-branch topology is used when we split a point-to-point link to support other neighboring land nodes (Figure 28b). The ring topology (Figure 28c), as its name suggests, makes use of a ring and serves the obvious: flexibility, connectivity, and redundancy. For example, in closed seas like the Mediterranean Sea or the Aegean Sea (in Greece) [13], the relatively short distances between land stations not only favor point-to-point topology but also can easily use rings needed for redundancy. On the other hand, transatlantic connections (i.e., from the UK to the US), with no intermediate stations to connect, there is no other choice than the point-to-point topology. In cases where unfriendly neighboring countries must be connected to a submarine cable (and do not want to use direct links between each other), the trunk-and-branch option is the most appropriate. Finally, the mesh topology (Figure 28d) can be applied in closed seas or nearby islands. Although CDC-ROADMs are already used in terrestrial optical networks, their use in submarine systems was not (until recently) straightforward due to demanding underwater specifications. However, ROADMs can be used only in submarine systems in closed seas, where they can be placed and operated from land. Recently announcements [68] include submarine ROADMs and reconfigurable BUs (RBUs) based on passive mux/de-mux, and submarine specs WSSs. The former suffers from limited reconfiguration capabilities whereas the latter questions system reliability and increases in cost and complexity. Moreover, as RBUs will use additional filtering devices, they will probably need more amplifiers and power, which may limit their applications. To cope with all these problems, [PERSON]. [PERSON] et al. in [69] studied the possible savings from the use of recently proposed filterless optical network architecture. \"Filterless\" refers to submerged components. Its main idea was to avoid active switching and filtering in submerged devices in an effort to avoid the above-mentioned issues. Therefore, the architecture is based on agile edge nodes and coherent transponders in the terminal dry plant and passive colorless combiners and splitters in the submerged BUs. The main objective of this architecture is to perform the light path reconfiguration from the land nodes. This architecture was simulated under time-varying offered traffic and proved to be a resource and cost-saving alternative. Figure 27: Up-to-date long-haul transmission experiments at transoceanic distances implementing different types of EDFA amplifiers. As expected, the branching architecture and the BU's layout affect the number and use of other submarine network components. [PERSON] et al. in [25] studied various Bus' potential architectures and investigated the number of network components required for an MCF-based submarine network. Moreover, they focused on the crosstalk effects in long-haul submarine systems and proposed a trunk-and-branch architecture using passive OADM BUs for an ultra-long-haul submarine network. However, the authors noted that this architecture did not support submerged BUs' reconfiguration. Finally, they studied the effect of crosstalk as is seems that it can affect the benefits of MCFs. After conducting various experiments, they concluded that selecting the proper number of FPs in an MCF cable is crucial to keep crosstalk effects at low levels. [PERSON] in [70] reported on various branching architectures depending on a network's geographical limits. Regarding transoceanic networks the usual is to use one MCF per connection and if needed split it to several branches near the shore.