# 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
Nowadays, optical switches have super-channel switching capabilities in order to provide the required traffic from/to different nodes using super-channels, which can provide flexible data rates as the exploited bandwidth is in multiples of 12.5 GHz or even 6.25 GHz. In the coming decades, SDM is expected to be a strong candidate to increase capacity and connectivity, as was analyzed in Section 4, and as a consequence, the development and introduction of full-fiber switches will be a must. However, there will be a strong need for this switch to be MG in order to provide all capabilities between wavelength switching and full-fiber switching. This switch needs to incorporate both WSSs, which can distinguish every wavelength of each incoming fiber and then switch it to the appropriate output port, and WaveBand Selective Switches (WBSSs), which can switch entire amplification bands to the appropriate output ports. The MG-switch will be able to perform switching at the level of (i) wavelength, (ii) super-channel, (iii) band, and (iv) fully populated with channels fiber, alongside spatial lane switching of (i) independent switched, (ii) fractionally joint switched, and (iii) joint switched spatial super-channels. The development of an WBSS is an important intermediate step between current super-channel switches and future full-fiber switches. Several subsystems and technologies are available to implement such a novel optical switching node, but the \"missing-link\" is a subsystem that can support flexible reconfigurable switching of UWB, bridging the gap from today's super-channel switching to tomorrow's full-spectrum fiber switching. Also, since F6G network nodes will become denser, more plentiful, and located in more space-limited sites, the switching technology must move away from bulk optics towards more compact, planar technologies. Therefore, the development of a WBSS in PIC format is a must, being able to complement the vital WSSs of today's state-of-the-art network architectures (thus providing the \"missing-link\" to the evolution of optical networks). The general concept of the WBSS is pictorially described in Figure 36.  
The need for a WBSS switch is further highlighted by Bell Labs, which predicted that by 2040, we may need a 250 spatial channels-based network operating over the full  
Figure 36: Multi-Granular Wave Band Selective Switch. (C + L + S)-bands bandwidth, supporting 66 wavelengths with a possible symbol rate of 300 Gbaud and a spectral efficiency of 20 b/s/Hz. This system must realize a capacity per spatial channel of 400 Tb/s and a total throughput of 100 Pb/s. In this scenario, UWB and SDM need to be combined to transport such amounts of traffic. However, as both techniques introduce new switching challenges at network nodes, the switching scalability is much greater, and operation without blocking and contention needs to be ensured. While the available optical fiber switches can be modified to support wider operating wavelength ranges and larger wavelength channel counts, there is no elegant, cost-effective way to do so. Currently, available WSSs can be modified to support multi-band transmission but become physically large and costly [139]. Further, OXCs can support wideband operation, but access to wavelength channels necessitates the introduction of demultiplexers, making this option cumbersome with the need for large up-front investments. Moreover, both available WSSs and OXCs utilize slow switching mechanisms based on beam-steering to transit from one state to the other, and the induced delays are significant. This makes them less suitable for F6G applications, which have very stringent latency requirements and need significantly denser networks compared with F5G ones. In parallel, continuous progress in optical transceivers by way of greater integration of novel photonics and optoelectronic devices and advancements in DSP are leading to higher-capacity optical interfaces (at the desired lower cost/bit and energy/bit metrics).