# 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  
In the last decade, a wide spectrum of proposals has been advanced to implement network re-configurability at the physical layer (or photonic layer). Most of them have been advanced firstly for core networks but can be applied to metro and access segments as well, provided that technologies for access and metro networks should have low cost and complexity. In fact, in recent years these technological solutions have been quite widely debated for the application in core networks but, in the opinion of the authors, they will be the enabling technologies for the application of the SDN principles at the physical layer in the metro and/or access segments as well. Therefore, we can currently define them as technologies designed for the core networks, but extendible to metro and/or access networks, even if this is still an open research area. Great advances are desirable in this field in order to design low power, high-speed, and small area footprint and compact solutions, which would be suitable for metro and/or access applications, which represent very cost-sensitive contexts. In addition to this, in most cases, these solutions have been already proposed for metro and/or access networks very recently, even if their concept is still in an embryonic stage and their application in these contexts is not yet commercially available or still very rare.  
Four main trends can be outlined: flexible transceivers; flexible WDM grids and super-channels; optical routing at the physical layer; and optical switching at the physical layer [10]. In Table 1, these technological trends and solutions are outlined with reference to several recent proposals which will be detailed in the following paragraphs. The table explicitly reports whether the specific proposals refer to the metro segment or to the access segment.  
All these trends and solutions would enable re-configurability and programmability in metro and access segments, allowing these networks to address the challenges raised in the Section 2.  
In this paragraph, only the trends are described, since several examples of their building blocks are reviewed in the next paragraph.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Technological Trends**} & \multirow{2}{*}{**Technological Solutions**} & **References for Application in Metro/Access Segments** \\ \hline \multirow{2}{*}{Flexible transceivers} & Flexible modulations and distance-adaptive transceivers & Metro: [30] \\ \cline{2-3}  & & Access: [31; 32; 33] \\ \hline \multirow{3}{*}{Super-channels and flexible WDM grids} & Flexible grid & Access: [34; 35] \\ \cline{2-3}  & & Metro: [30; 35] \\ \cline{2-3}  & Spatial super-channels & Access: [36] \\ \hline \multirow{3}{*}{Optical routing at the physical layer} & Flow routing & Access: [37; 38; 39] \\ \cline{2-3}  & ROADM & Metro: [40; 41; 42; 43; 44; 45; 46; 47] \\ \cline{2-3}  & & Access: [41] \\ \hline \multirow{3}{*}{Optical switching at the physical layer} & \multirow{3}{*}{OPS, OLS, OBS} & Metro: [48; 49; 50; 51] \\ \cline{2-3}  & & Access: [49; 52] \\ \cline{1-1} \cline{2-3}  & Flexible modulations and distance-adaptive transceivers & Metro: [30] \\ \cline{1-1} \cline{2-3}  & & Access: [31; 32; 33] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Technologies designed for the core networks and extendible to metro and/or access networks.