# 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
They finally studied a fiber granular routing as an alternative scheme for cost-effective switching. In [143] [PERSON], et al. experimentally demonstrated a high-throughput OXC architecture that employs spatially jointed flexible waveband routing. Their results confirmed a net OXC throughput of 2.15 Pb/s, an OXC port count of 84, and a hop count of 7 at a transmission distance of 700 km. Their structure employed 64-channel 400-Gb/s dual-carrier DP-16 QAM signals with 75 GHz spacing in 4.8 THz of the full C-band. Finally, the authors concluded that this OXC architecture can be applied efficiently to metro networks. Another way to speed up optical switching is by considering an OXC architecture that deploys spatially jointed flexible-waveband routing using a 37.5 GHz grid, for transmission distances over 1600 km, 700 km, and 600 km using 32 GBaud DP-QPSK, DP-8 QAM, and DP-16 QAM, respectively. This architecture was presented in [144] and follows a Joint Switching (J-Sw) strategy by deploying special switches (named Delivery and Coupling (DC) switches), which are inserted between J-Sw WSSs to face possible routing restrictions. Simulations of the proposed architecture showed similar performances to the non-restricted OXC architecture while, at the same time, complexity and costs were lower. The authors concluded that this architecture could suit metro networks and would be a cost-effective architecture that could overcome the barriers to the realization of SDM networks. A very Interesting two-layered architecture for ROADMs was presented by [PERSON] et al. in [145]. The two available layers provide a flexible and efficient management of MCF switching and, at the same time, support traffic grooming and the node's add/drop modules. This two-layer architecture achieved low values in average transmission loss, crosstalk, and BER, anticipating the somewhat higher construction complexity. Their findings showed that a multi-layer design can efficiently deal with the scalability issues of single-layered ROADMs, and also seems to be a promising architecture for future ROADM designs. The last alternative for higher bandwidth usage is the utilization of extra amplification bands, as analyzed in the previous section. Several recent proposals dictate the concurrent use of S, E, and C bands. Theoretically, UWB can use approximately 50 THz of optical bandwidth from 1260 nm to 1650 nm, thus providing the desired extra capacities. The price to be paid will be mainly the greater impact of fiber nonlinear effects in the smaller wavelengths. UWB switching can be built on multiple WSSs and a series of optical amplifiers. Therefore, UWB technology is a promising candidate in an effort to avoid higher sizes and costs and, therefore, to reduce complexity. [PERSON] et al. in [139] demonstrated a high port count (47 ports) Ultra-Wideband WSS that can support switching from 1300 nm to 1565 nm with low insertion loss. The switch provides a range of 36 THz (or 250 nm) and covers parts of the O band and S, E, and C bands. [PERSON], et al. in [146] proposed a flexibility measurement approach in SDM Elastic Optical Networks (EONs) to quantitatively evaluate the switching flexibility of node architectures. Their results showed that an architecture introducing lane changes (LCs), employing the Ind-Sw strategy, and maximizing the number of A/D ports and the spectral granularity can achieve the best switching flexibility performance. A new OXC structure ([147]), which can effectively reduce the hardware cost by saving the switch scale, was presented in [147]. Experimental results showed that the proposed layered m-OXC scheme can achieve similar blocking performance to fully connected OXCs with up to 76.1% hardware scale reductions and 93% searching time reductions. A new optical network architecture named the Spatial Channel Network (SCN) was presented by [PERSON] in [20].