# A Vision of 6 th Generation of Fixed Networks (F6G): Challenges and Proposed Directions
## 5 Optical Switching: Key Challenges and Proposed Directions
### Ultra-High Capacity Switching
#### 5.2.2 Optical Node Switching Architectures
In SCNs, the optical layer \"is explicitly decoupled from the hierarchical SDM and WDM layers, and an optical node evolves into a Spatial Cross Connect (SXC) and a Wavelength Cross Connect (WXC) to achieve a Hierarchical Optical Cross Connect (HOXC)\", in an effort to simplify optical switching procedures and protocols. Moreover, the author presented a Routing and SDM/WDM Multilayer Resource Assignment (RSWA) heuristic protocol in order to minimize the number of needed Spatial Lanes (SLs). An interesting and useful characteristic of SCNs is that they separate traffic demands and network nodes into two kinds. Traffic demands are classified as express and local ones. Express traffic has an SL filling ratio higher than a relatively high threshold. This means that this demand can almost fulfill an SL, and as so, an entire SL is dedicated to it. Although this technique may not use the whole SL's bandwidth in some cases, it does, however, result in simpler and faster RSWA protocols. Local traffic demands, as they cannot fill an SL, are directed to adjacent specific nodes and multiplexed with other similar traffic. So, SCNs are another way to relax switching demands from the physical layer and thus the need for large OXCs and move them to an upper layer. In this way, SCNs seem to be able to face ROADMs' scalability issues. We predict that SCNs will be widely used in future optical networks as they combine significantly less hardware in ROADMs (less WSSs) and faster switching for the express traffic class at the small cost of handling two types of traffic and a different (but not necessarily slower) Routing and Wavelength Assignment (RWA) algorithm. We are currently working on deploying the SCN architecture in a recently proposed submarine optical network for the Mediterranean Sea [148] to analytically exploit their potential. A recent EU-funded project that aims to develop optical switches with UWB capabilities is the \"Flexible Scalable Energy Efficient Networking\" [149]. FLEX-SCALE has the ambition of developing an UWB+SDM optical node (ON) supporting capacities \(\geq\)10 Pb/s, facing the scaling challenges and requirements posed by 6G applications on the backbone and x-haul networks. This node is based on innovative optical switching technologies such as the novel WBSS and state-of-the-art energy efficient optical transceivers, the oDACs. The main aim of the WBSS is to switch traffic by flexibly defining distinct bands carved from the UWB window of operation (spanning the wavelength range of 1460-1625 nm, designated today as the S, C, and L bands) and switching the information-bearing flexible bands (wide spectral super-channels) about ingress-egress fibers using the proven R&S topology. Introducing OXCs and WSS beneath the WBSS completes the MG-ON, which can then switch communication traffic at full-fiber, flexible-band, or individual wavelength channel granularity, whilst supporting reconfigurable add-drop in color-/direction-/contention-less (C/D/C) fashion. We predict that architectures overcoming the scalability limitations of ROADMs will be more than essential in the near future. The first future direction is the multi-layered OXC architectures (two-layered or even three-layered), as these architectures will need fewer optical components and fibers, relax the full connectivity OXC demands, and provide acceptable fiber/wavelength routing performance. A second direction will be the use of specialized routing and wavelength assignment algorithms, which will meet the upcoming excessive routing/switching needs in the OXCs and, at the same time, allow for the use of a more efficient J-Sw strategy. Last, but not least, a third future direction will be the use of both WBSSs and WSSs, preferably within the same node in order to efficiently support Multi-Band switching.