## 2 Related Work  
Since NFV and SDN are base technologies for establishing NSs, each of these technologies can be used for exploiting specific methods and approaches that can contribute to network energy savings improvements. The fundamental idea of SDN networks relies on a tangible division between the control and data plane of SDN networks. According to [11], the SDN concept can offer solutions for improving network E, including the energy-efficient plane and the data plane (Figure 1). This is in contrast with conventional networks, where each networking device incorporates both the data and the control planes. In the case of sliced 5G networks realized using NFV, the control plane can manage the data plane of multiple networking devices from a centralized location (Figure 1). This means of network realization sets the ground for a possible improvement in 5G network EE and energy savings, since networking devices only use power to perform those tasks that are directed by the control plane. Table 1 provides an overview of the main areas of research, which are based on the implementation of network slicing in 5G networks to improve 5G networks' energy efficiency. Solutions for improving network energy efficiency, realized through the implementation of SDN, can be classified into three models that are based on traffic-aware effects, end-host-aware effects, and rule placement effects (Table 1). The traffic-aware model implies that the SDN controller periodically updates the status of the router and switch states in the transmission network, and this information is used to optimize energy consumption through the decision to turn off certain devices in conditions of reduced traffic. The goal of the end-host-aware model is to find routes with a number of network nodes and connections linked to them, creating conditions for the implementation of the energy-efficient traffic-aware model. In this model, the SDN controller groups multiple connections over the same network nodes and links and thereby creates some network elements (NEs) that can be in idle (power-saving) mode. This contributes to a significant reduction in consumed energy, while ensuring quality of service (QoS). Considering that SDN switches use ternary content addressable memory (TCAM) to store the forwarding rules obtained from the SDN controller, and the access to TCAM directly impacts the increased energy consumption, the rule placement model attempts to reduce the number of TCAM entries by using wildcard entries or default routes that the SDN switch will follow. Since SDN is one of the key technologies for the realization of NSs, i.e., the establishment of NS is not possible without the use of SDN, this implies that these three models open space in the network slicing concept for significant energy consumption savings that can potentially be achieved in the transmission part of the network. In addition, there has been a proposal for core slicing using NFV technology (Table 1). This involves the separation of network functions from specialized hardware. A network function (NF) serves as a functional component in a network infrastructure, possessing clearly defined external interfaces and exhibiting precisely specified functional behavior. Instead of the traditional approach, where NFs are tied to specialized hardware, these functions, after the mentioned separation, operate as virtual NFs on standard servers located in data centers [12]. This means that multiple network functions can run on a single server, utilizing the resources of a piece of hardware, as opposed to the traditional setup where each network function had its own dedicated hardware, consuming energy individually. In NFV, virtual network functions are connected through virtual links, which also have lower energy requirements compared to network functions on dedicated hardware with physical connections. From [9], it is evident that, by implementing NFV, energy savings can be achieved without the need for specific energy-saving algorithms or methods.