## 2 State of the Art  
Many researches are focusing on the integration of SDN in PONs. On the one hand, some studies integrate OpenFlow switching paradigms in the PON control layer. In this way, authors in [8] propose a SDN-OLT solution in GPONs using virtual switches and the OpenFlow standard in order to emulate a SDN layer in the OLT. In addition, authors in [9] propose to implement virtual switches inside ONUs able to select OFDM channels for TWDM-PONs. Other proposals regarding virtual switches for PONs were proposed in [10] to improve the efficiency and flexibility for data center networks, especially for intra-datacenter communications. The papers [11] describe an SDN-controller to simultaneously manage several SDN-OLTs in PON infrastructures. Other researches are working on mapping OpenFlow messages to native PON configuration commands [12] and defining extensions of the OpenFlow standard for PONs technologies [13]. In addition, the Open Source Virtual OLT Hardware Abstraction (VOLTHA) project [14] proposes to abstract PONs as programmable Ethernet switches that can be controlled by an SDN controller and therefore, to manage its network configuration and devices (OLT, ONTs). On the other hand, issues related to the Dynamic Bandwidth Allocation (DBA) or the registration and management of ONTs are difficult to cover with SDN in PONs, due to the latency between the SDN controller and the OLTs. In this line, authors in [15] propose the SIEPON architecture where the ONT registration process and some global DBA policies are moved to an external SDN controller, since they are not time-critical operations. Authors in this paper describe the simulation model and they analyze some network parameters such as the delay and the throughput performance by a simulation study. In addition, authors in [16] describe a novel SDN architecture for EPONs integrating a reprogrammable DBA module inside the SDN controller in contrast to the hardware DBA modules (inside OLTs) existing in traditional PONs. In this proposal, an SDN controller deals with several DBA algorithms and it deploys them in the OLT according to the network global status and the traffic demands of ONTs. This proposal and the results are supported with simulations. Authors in [17] experimentally demonstrate a novel SDN architecture for remote unified control with SA-FS to dynamically provision and adjust services according to the real-time Quality of Service (QoS) demand. Regarding TWDM-PONs, authors in [18] designed a novel software-defined solution to fulfil QoS requirements using a centralized SDN controller that dynamically allocates bandwidth and wavelengths to users. This model and the algorithm's performance are supported with simulations. In the same way, paper [19] describes a new SDN architecture and bandwidth allocation policy to guarantee QoS requirements in TWDM-PONs. Therefore, it can be noticed that most of the research related to DBA strategies or online service reconfiguration policies controlled by SDN are implemented using simulation models, but they have not been tested in real PON network environments with legacy equipment. Although we do not focus on protection and energy savings in PONs, it is worthy to mention that other research works on the use of SDN in PONs have considered those aims. For instance, authors in [20] developed an experimental SDN architecture to allow protection and dynamic service provisioning in a multi-wavelength PON in coordination with a core SDN-based network, and authors in [21] propose moving the power control of OLTs and ONTs to the SDN controller to achieve energy savings. Finally, SDN solutions are also applied in Virtual PONs (VPON), such as authors in [22] that carry out a simulation analysis of a novel software-defined Hybrid Passive Optical Network (HPON) architecture proposed over multiple VPONs managed by a priority-based DBA mechanism called SPB-DBA.