# Future Scenarios for Software-Defined Metro and Access Networks and Software-Defined Photonics
## 3 Solutions for Software Defined Metro and Access Networks
### Higher Layer Re-Configurability  
The first proposal to introduce the ONF SDN paradigm in the access segment, called software-defined edge network (SDEN), has been advanced by [PERSON] and [PERSON] in 2013 and then developed by [PERSON] et al. in 2014 [23]. The purpose of that work is to extend OpenFlow protocol to gigabit-capable PONs (GPONs) through two main features: the flow mapping at the GEM ports,  
Figure 5: Main solutions for software-defined metro and access networks. i.e., the logical connection between optical network unit (ONU) and optical line terminal (OLT) in the GPON standard, each of them characterized by a specific class of service and a unique ID; the standard interface between the SDN controller and the OLTs through application program interfaces (APIs). In fact, the conventional PON management system allows manual configurations of the network by the administrator user, e.g., for the definition of the service profiles or the PON port attributes. SDEN approach, instead, allows real-time PON control by acting on SDEN agents, software modules placed on OLTs, through API [23]. A software defined access optical network (SDAON) architecture based on OpenFlow-enabled passive optical network has been proposed also by [PERSON] et al. [24] in order to ensure remote unified control and service-aware flow scheduling. Their proposal can enhance the resource utilization and QoS guarantee and reduce the OpEx by remote interaction and operation. Another method to allow programmability is to use the network configuration protocol (NETCONF), developed and standardized by the IETF in 2006. This protocol enables service orchestration for the IP layer through remote procedure calls and notifications to configure the network elements whose states, configurations, and parameter values are described through a data modeling language called YANG. NETCONF is gaining a growing industry acceptance and in the next years it is expected to enable end-to-end programmability through a description not only of network elements but also of links, adaptations, connections, communication paths, and bindings [6]. A recent proposal by [PERSON] et al. [25] in order to flexibly support advanced services, is based on SDN-enabled home gateways and SDN-enabled broadband remote access servers (BRASes). In actual network architectures, the BRAS--also called the broadband network gateway (BNG)--is the single aggregation point which is connected to the home gateways through a metro network using a point-to-point protocol in order to establish a session. The innovative proposal consists in a network composed of SDN-enabled home gateways, BRASes and OpenFlow switches in the metro network, under the supervision of a centralized SDN controller which manipulates traffic flows according to the operator needs and policies and frees the BRAS from many of the typical functions, such as policy enforcement and traffic monitoring. This solution enables flexible and granular control over traffic flows throughout the network, with the ability to implement QoS and traffic policies right at the edge of the network, without requiring a single aggregation point for policy enforcement [25]. Specific SDN solutions for optical access networks have been recently proposed by [PERSON] and [PERSON] [26]. This work takes its origin from an approach similar to the one described by [PERSON] et al. [25]: the control plane of the core network, based on multi-protocol label switching (MPLS), can be extended to the edge router--e.g., the BNG--using SDN concepts. In order to separate control and data planes inside the optical access networks, multiple virtual local area network (VLAN) control protocols are proposed to be used as the Ethernet control plane protocols in active optical networks. In this way, a centralized SDN controller can extend its control to both MPLS control plane for core optical networks and Ethernet control plane in optical access networks.