# Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends
## 5 Internal Architectures of Submarine Cable Systems and BUs
PFE is provided by stations on each side (in the order of 15-20 kVs) and may limit the total capacity due to long distances. For regional networks, the options are a trunk to many branches, a mesh topology with nested branches, and the connection of a submarine network to terrestrial bypasses (like some roads in northern Norway heading for the Nord Cape). Power can be supplied from many spots and must be carefully designed to provide the necessary redundancy. As already mentioned, the only options for connecting relatively small distances are point-to  
Figure 28: Underwater connecting options. (**a**) Point to Point topology. (**b**) Trunk and Branch topology. (**c**) Ring topology. (**d**) Mesh topology. point links. In this case, power is coming from land stations and there is no need for tens of kVs as the distances are not demanding. For special-purpose submarine networks (like oil/gas mining companies' networks), a trunk to many connections' architecture is suitable. Power feeding is straightforward for the stations and has to be easy to reconfigure in order to cope with failures or programmed maintenance. The author of [70] summarized the main designing decisions of a submarine cable system: electrical (PFEs), optical (fibers, repeaters, amplifiers, span length), architecture (type of branching), route selection, and economics. As the wet plant of submarine networks is usually owned by several companies, [PERSON] in [19] noted all system design issues that affect the total cost. A general rule regarding route selection (for transoceanic systems) is to follow a direct, great-circle path. Of course, many other constraints may apply. Ocean trenches, countries specific restrictions, military prohibitions, and/or danger areas also affect the design. As noted earlier, sometimes the power scheme needs to be reconfigured for various reasons. A solution to that is the power-switched bus, which can select the power providing the line based on the distance from PFEs, network needs, possible failures, or scheduled maintenance. An example of such power-switched BUs is shown in Figure 29.  
In Figure 29, all BUs are powered by both the land trunk stations' PFEs, and branch cables are powered by branches' PFEs. In Figure 29, Trunk A provides power to BU1 and Branch station 2 (through BU2). Trunk B provides power to BU3. The power of Branches 1 and 3's power scheme remains as in Figure 29. This feature of reconfiguring power schemes can be utilized to provide power (in case power is lost in a network section) or cut off power (in case of scheduled hardware maintenance or upgrades). Although BUs contribute to the formulation of the submerged architecture, their \"static connection\" nature does not provide any flexibility. Reconfiguration of submarine networks is needed not only when traffic patterns change but also when programmed maintenance or upgrades need to deactivate specific devices. As noted in [70], two products can enable submarine network reconfiguration: the enhanced BUs (eBUs) and the recently announced submarine ROADMs. Figure 29: Reconfigurable power switched schemes for BUs. (**a**) Dual-end power feeding on trunk route (**b**) Reconfiguration of dual-end power feeding from one trunk station to the Branch 2 station. The eBUs can support both dynamic routing (by implementing optical switching in the FPs), and selective power feeding to temporarily enable device deactivation. After submarine specs, WSSs were announced in 2015 [19], and WSS ROADMs became waterproof and were submerged, usually in the same housing, with BUs in 2019. A submerged ROADM was deployed by SubComin 2019 [70] and can support up to four trunk FPs. ROADMs and eBUs can achieve bidirectional, unidirectional, or 3D shared branches depending on the traffic patterns and the clients or places they have to serve (Figure 30).