# 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 currently deployed backbone networks, intermediate nodes (OADMs/ ROADMs) have to switch traffic to their successor nodes in an efficient way. A common practice is to use network elements such as WSSs, which can distinguish every wavelength of each incoming fiber and then can switch it to the appropriate output port. So, in this way, there are two main approaches: (a) The spatial domain and (b) the frequency domain. The spatial domain consists of three main switching strategies: Independent Switching (Ind-Sw), Fractional Joint Switching (Fr-J-Sw) and Joint Switching (J-Sw). The Independent Switching (Ind-Sw) strategy uses wavelengths that can be switched independently in a non-blocking way. The Joint Switching (J-Sw) and Fractional Joint Switching (Fr-J-Sw) strategies switch wavelengths either altogether or in small groups, respectively. However, if the number of fibers and, consequently, the total number of wavelengths is increased, a ROADM must deploy many WSSs, resulting in large size and cost. On the other hand, the frequency domain consists of WSS, WSS, and, finally, Full Spectrum Spatial Switching (FSSS). The Wavelength Switching (WS) of the uncoupled spatial super-channels approach in [140] switches specific wavelengths of all incoming spatial channels together, an approach similar to J-Sw. The trade-offs are obvious: The Ind-Sw strategy switches all data carriers with no limits but with large sizes and costs. J-Sw, Fr-J-Sw, and WS strategies do not provide a high degree of switching freedom (as Ind-Sw does) but can save money and complexity. In the FSSS approach [140], some pre-selected fibers or cores are committed to serving specific source-destination pairs. In FSSS, no wavelength distinction is required in the intermediate nodes, resulting in simpler backbone network architectures. On the other hand, if the traffic demands are much lower than the dedicated fibers' capacity, the total capacity can be under-utilized. Of course, depending on traffic conditions, hybrid approaches can be deployed. There has been a lot of scientific research on the above switching architectures so far. [PERSON], et al. in [140] proposed using the Ind-Sw. strategy when dealing with light loads and diverting to FSSS when heavy loads arise. This happens because high loads ensure that the capacity of the pre-selected fibers/cores will be fully used. Moreover, the authors reviewed several node architectures that can support the Ind-Sw, WS of uncoupled spatial super-channels, and FSSS switching strategies. As mentioned above, a fundamental element in optical network nodes is the OXC, which handles WDM signals without costly optical-to-electrical and electrical-to-optical conversion. It employs integrated interconnections to build an all-optical switching device, achieving high-level, integrated, fiber patching-free, and all-optical cross-connections, and manages to improve the switching efficiency, especially on large-granularity services. [PERSON] et al. and [PERSON] et al. in [141, 142], respectively, studied the scalability problems of large ROADMs/OXCs due to the rising number of required WSSs. More specifically, the authors in [141] proposed a two-level architecture approach to construct a modular large-scale OXC. Several small-scale OXCs are designed and then interconnected to each other in an effort to both ensure full-scale connectivity and reduce the number of devices and intra-node optical links. It is found that this modular OXC design achieves low insertion loss and does not become \"big and fat\" when input fiber numbers increase. A proposal for relaxing the full-mesh connectivity to resolve the issue of the large number of required WSSs was presented in [142]. The authors studied the trade-off between full-mesh connectivity and routing performance. They showed that this trade-off can be faced by splitting the inter-connection parts into small regular sub-systems. Therefore, they tried to save space and costs by adding a little more complexity.