# 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  
To further understand the current needs for ultra-high-capacity switching, we pictorially describe the evolution of switching over time in Figure 35. As it is evident, in the early years (before 2000), it was the Synchronous Digital Hierarchy Digital Cross-Connect (SDH DXC) architecture that was first used to switch SDH-based signals. Electronic switches (SDH DXC) were mainly used in backbone networks operating with WDM technology. During these years, the network topology was entirely fixed with preassigned wavelengths being dropped/added at certain sites with the assistance of OADMs (optical add/drop multiplexers). Later on, OADMs were replaced by ROADMs that brought a level of flexibility and adaptability to the network, as a response to changing needs. Afterward, Planar lightwave circuit (PLC)-based ROADM node architecture was initially used to support the Add/Drop function of wavelength channels to colored ports (fixed port/wavelength assignment) for delivering traffic from/to local premises. This Add/Drop ROADM part used a thermo-optic switch (TO-SW), which was implemented in a PLC. PLC TO-SWs (made from SiO\({}_{2}\), which is a rather stable material) have no moving parts and, therefore  
Figure 34: High-level representation of the industrial switching network for inter-(OXC-XF) and intra-factory (OXC-IF) connectivity. provide enhanced reliability to the ROADM architecture. Another approach comprising the same architecture is the wavelength blocker (WB)-based ROADM node architecture. In this method, the Add/Drop traffic is switched using a liquid crystal or micro electromechanical system (MEMS). This approach has good filtering characteristics, yet the dropped (and through) wavelength channels remain wavelength multiplexed. However, the filtering spectrum is broader and sharper than that of a PLC-based ROADM, which is based on wavelength demultiplexing. The Add/Drop switching part (in a WSS-based ROADM) is implemented using MEMS, a liquid crystal, or a liquid crystal on silicon (LCOS). Advanced filtering (as in WB-based ROADM) and excellent multi-degree capability (using multi-port WSSs) are now feasible. The size and complexity of the WSS scale are associated with the number of addressable spectral points (=operating bandwidth/optical resolution) and the number of output fiber ports. Increases in these factors lead to more capable ROADMs but with larger and more expensive WSSs. Therefore, we will likely need new optical node-switching architectures to support the needs for UWB/SDM switching. A few years later, the flex grid concept was introduced as a revolutionary technology that allowed network and service providers to maximize the capacity of their premises by tailoring the channel bandwidth to user demands. Flex grid is a flexible solution that can support channel spacing from 50 to 200 GHz. A key feature is its software configurability, which means that the channel spacing can be dynamically tuned according to specific user demands, offering 6.25 GHz or 12.5 GHz granularity. Figure 36 depicts a proposed next-generation optical switching node architecture, which can cross-connect communication channels among spatially parallel links that support UWB/WDM transmission (covering, e.g., at first the three distinct bands: S, C, L) and can serve as a part of a Multi-Granular (MG) switch. This architecture will be able to switch traffic from single channels, super-channels, bands, and entire fibers. Therefore, it will be a promising candidate for the future optical switching era. Figure 35: Evolution of optical switching over time (from SDH to SDM/UWB switching functionality). This switch is usually placed on intermediate nodes (OADMs/ROADMs) in a backbone optical network in order to switch traffic from/to the node. At first, the introduction of WDM in optical transmission required switches with wavelength switching capability.