# Future Scenarios for Software-Defined Metro and Access Networks and Software-Defined Photonics
## 5 Architectures
### Unified Metro-Access Networks  
Unified all-optical metro-access networks represent one of the best possible solutions to realize re-configurability at the physical layer in these network segments [10]. Several converged metro-access architectures have been proposed in recent years, where a certain number of PON access stress is merged with a metro ring through ROADMs. Today, the large-scale deployment of optical fiber in the access segment provides a common transmission medium to both access and metro networks [81]. Moreover, the WDM-PON architectures can even extend the application range of PONs when compared to conventional GPONs. Higher per-ONU bandwidth and splitting ratios in conjunction with extended power budgets enable the use of PON technologies for a unified metro-access network [10]. Nevertheless, in a short-term perspective, a cost-effective and scalable solution could be to increase the transmission capacity deploying WDM in the former metro part, while TDM could be kept in the former access part [81]. These TDM/WDM metro-access networks will face some technical challenges: the backward compatibility with existing PON systems have to be preserved; a higher network reach must be accomplished, thus amplification in the RNs will be needed; burst-mode upstream transmission has to be managed, due to the TDM operation in this network segment. One of the first proposals to merge the metro and access segments has been the metro and access ring integrated network (MARIN) in 2007 [82]. MARIN is based on several interconnected DWDM rings, each one with its own CO to manage the traffic related to the WDM-PONs linked to the ring. Two kinds of nodes are considered: (i) the MARIN gateways, which manage add/drop functionalities towards PON trees, as shown in Figure 12; (ii) the MARIN switches, which manage metro traffic routing. The nodes are equipped with tunable transmitters and use OBS. The scaled advanced ring-based passive dense access network architecture (SARDANA) architecture has been proposed in 2011 [83]. This metro-access architecture is based on a bidirectional WDM ring with 32 wavelength channels; the ring is merged with 10 Gbit/s TDM PON trees through RNs capable of channel add/drop functionalities, as shown in Figure 13. All the network nodes are  
Figure 12: The MARIN architecture [82]. passive except to the OLT placed in the CO of the WDM ring. The RNs are equipped with splitters and remotely pumped EDFAs. The CNTs are equipped with colorless transceivers based on reflective semiconductor optical amplifiers (RSOAs), which reflect and modulate the downstream traffic signal to generate the upstream signal. The ring topology enhances network resiliency and ensures double links between OLT and CNTs, with a recovery time of less than 50 ms in case of failure. A proposal for the application of the OBS technique in a metro-access network has recently been advanced in order to ensure all-optical operations in a SDN-like optical architecture which offers the possibility to manage traffic data thanks to re-configurability in the physical layer and a centralized controller [49]. A SDN-based metro-access architecture has also been proposed by [PERSON] et al. [84]. The network is based on cost-effective, energy-efficient, and flexible nodes and transceivers, remotely managed by a SDN controller, able to support 5G services. The following proposals for converged metro-access networks have been reviewed by [PERSON] and [PERSON] [85]. Super passive optical networks (SuperPON) have been developed based on the old G.983 broadband PON (B-PON) architecture, upgradable through a larger splitting factor and longer reach. [PERSON] and [PERSON] [86] have proposed a hybrid DWDM-TDM long-reach PON. The architecture proposed by [PERSON] et al. [50] leverages the concept of an OBS multiplexer to transparently interface to distant metro routers in an all-optical manner.