# Advances in Improving Energy Efficiency of Fiber-Wireless Access Networks: A Comprehensive Overview
## 6 Implementation of SDN-Based Energy Conservation Concepts in the FiWi Networks

Table 5: Summary of the related works on PS methods in SDN-based networks. According to Figure 11, which presents a visualization of the SDN PON architecture, the separation of the data and control planes are the main features of SDN. Such separation enables centralized network management which is executed in the centralized controller. Two controller interfaces, namely, a northbound and southbound interface, are defined within the SDN architecture. They enable the centralized controller to communicate with applications and network equipment (Figure 11). The northbound interface is used to communicate with the application layer, which enables the application of SDN control management functions. On the other hand, the southbound interface is used for communication between a centralized controller located within the control plane and the data plane (Figure 11). The data plane consists of network equipment and it is responsible for packet-forwarding decisions. OpenFlow is a commonly used protocol at the southbound interface (Figure 11). It provides access to the data plane and is considered the enabler of the SDN concept [104]. Due to the constant network infrastructure growth, the power consumption of access networks has become one of the main concerns for companies that own larger networks and telecom operators. The SDN-based approach has emerged as a viable and effective solution for reducing power consumption of access networks [10]. Table 1 summarizes the related works on PS methods in SDN-based networks. The authors in ref. [99] showed through simulation that the proposed SDN-based energy management scheme in the EPON system can reduce the overall energy consumption of optical access networks (Table 5). Although PON is considered an energy-efficient fixed network access technology, due to its mass deployment worldwide, PON systems need to be even more energy efficient to meet today's green policy requirements. Due to modern consensual requirements related to  
Figure 11: Software-defined PON architecture. ensuring more sustainable and energy-efficient operation of access networks, PON devices need to be enhanced in order to more easily adapt to such an environment. Having this as a goal, the authors in ref. [100] proposed an energy-conservation scheme through adaptive SDN-based TDMA-PON system architecture (Table 5). They introduced software-defined (SD)-, OLT- and SD-ONU architecture and employed an energy-saving scheme using an OpenFlow-based SD controller (Figure 11). To enable PS operation, the SD-OLT and SD-ONU devices were enhanced with SD agents connected to the SD controller via the OpenFlow protocol. The approach uses flow tables for packet classification and forwarding and an energy-saving table to store SD-ONU receiver sleeping periods. The transceiver of the SD-ONU was decoupled and thus the independent working state of the transmitter and receiver was enabled in such an architecture. Hence, depending on the traffic characteristics, the energy conservation of the SD-ONU was orchestrated via the SD controller through the assignment of the SD-ONU receiver's sleep period and the SD-ONU transmitter's wake-up threshold. The simulation results performed in low-traffic conditions showed a significant reduction in energy consumption, while maintaining QoS requirements. In addition, an energy-efficient SD optical access architecture based on TWDM-PON was proposed in ref. [98] (Table 5). In this architecture, the authors enhanced existing fiber-optic network equipment with SD capabilities. SD orchestration was proposed to manage the use of different wavelengths and link speeds in the network. Through this management, saving energy could be achieved by turning off or decreasing the data rate of the OLT/ONU transceivers when traffic was low, while still maintaining the required quality of service. The authors in ref. [98] also evaluated the performance and EE of the proposed system in various scenarios which consider factors such as packet delay, link capacity and data loss in the network.