# Future Scenarios for Software-Defined Metro and Access Networks and Software-Defined Photonics
## 3 Solutions for Software Defined Metro and Access Networks
### Physical Layer Re-Configurability
#### 3.2.2 Super-Channels and Flexible WDM Grids  
Super-channels can be defined as a group of separated signals which are digitally combined to create an aggregate channel of a higher data rate. This approach is originated from the need of overcoming the limitation of DSP techniques in the elaboration of higher-order modulation schemes, which become increasingly sophisticated in order to maximize the total throughput of the optical transport system. Two main categories of super-channels have been proposed: spatial super-channels and spectral super-channels (sketched in Figure 6a). Spatial super-channels are a form of space-divisionmultiplexing (SDM), since multiple spatial paths are used in parallel. They could be distinct light paths belonging to multiple parallel physical fibers, multi-core fibers, or different optical modes inside a multimode fiber. Spectral super-channels use multiple parallel sub-carriers (at different frequencies or wavelengths) with lower-order modulation to achieve a more efficient use of the spectrum [6].  
The elastic combination of sub-carriers allows major flexibility in the spectrum management and enables flexible WDM grids (flex-grid), which are already available for core network applications. The advantages are well known and sketched in Figure 6b: if wavelength spacing among optical WDM carriers is not strictly anchored to a fixed grid (e.g., conventional 50 GHz or 100 GHz DWDM grid), the available spectrum for each carrier can be managed dynamically by allocating a minimum required bandwidth. Therefore, spectral efficiency can be optimized and, indirectly, energy efficiency can be enhanced as well; as a consequence, OpEx saving can be pursued. Naturally, the nonlinear dynamics of various high bit-rate super-channel configurations have to be accurately investigated when upgrading the current static network structure to a flex-grid network [57].  
While spatial super-channels cannot be considered a viable solution for metro and access networks in the short term (even if some proposals for SDM in the access segment have already been presented [36]), spectral super-channels can be considered a more realistic possibility. In fact, since WDM techniques are expected to be extended also in the domain of access networks (WDM-PONs have been widely proposed in recent years), it can be envisioned that flexible WDM grids will be applied in the edge segment in order to adapt link capacity to the user needs [10] even if, to the best of our knowledge, specific applications of spectral super-channels in metro and access networks have not yet been proposed.  
On the other hand, the application of flex-grid in optical ring metro networks, in combination with distance-adaptive optical coherent transceivers, have been discussed and analyzed by [PERSON] et al. [30], with a detailed formalization of the routing, modulation level, and spectrum assignment (RMLSA) optimization problem. In the access segment, an optical network paradigm achieving low latency, high throughput, and energy efficiency, based also on the concepts of software defined networks and a flexible grid, has been proposed by [PERSON] and [PERSON] [34].  
Figure 6: (**a**) Comparison of spatial and spectral super-channels. (**b**) Schematic representation of the advantages of flexible grids in terms of spectral efficiency.  
[PERSON] et al. [58] have proposed the use of flexible grids in a reconfigurable flexible optical network (RFON) and have demonstrated that concept on a test-bed constituted by four nodes equipped with wavelength selective switches (WSSs), optical amplifiers, and a system for supervision and monitoring of optical channels. [PERSON] et al. [35] have proposed the use of flexible grids in an access network based on OpenFlow protocol.