## 3 Network Slicing in 5G Networks
### Energy Efficiency in NS Lifecycle Management  
The NSs implemented within operators' networks must undergo comprehensive management throughout their entire lifecycle. According to [16], the 3 GPP lifecycle management of an NS is composed of four phases: preparation, commissioning, operation, and decommissioning. Elements of these four phases are shown in Figure 4. From Figure 4, the **preparation phase** is the initial stage in the lifecycle of an NS and during this phase, there is no existing NS instance in the operator network. This stage of the NS lifecycle involves the design of the NS, capacity planning, onboarding, an evaluation of network functions, and organizing the network environment. Although not specified in the 3 GPP NS lifecycle management specifications, NS energy consumption should be also considered as one of the factors in the NS planning phase. This is because energy-efficient NS planning can have a positive impact on the remaining phases of the NS lifecycle and can contribute to the reduction in NS energy consumption.  
Furthermore, during the **commissioning phase**, the NS instance is created, and in this process, all necessary resources are configured and allocated with the aim of establishing the required capacities of the NS (Figure 4). In the **commissioning phase**, establishing the NS instance can involve both creating NS components and modifying them. To have as many energy-efficient NSs as possible, in the commissioning phase, the implementation of energy-efficient resource allocation techniques should take place.  
The **operational phase** of an NS lifecycle encompasses NS commissioning, monitoring, generating feedback on performance quality and KPIs, utilizing adequate resources as needed, and modifying the NS instance according to users' needs (Figure 4). Although the initial practical realizations of NSs energy consumption have not been considered as a relevant KPI, the energy consumption of NSs should be considered as an important KPI for the operation of the network and should be one of the KPIs that need to be monitored. This phase also includes deactivating and re-activating the NS when necessary. Activation puts the NS instance into operation for various communication services. This phase also engages the NS resources for supervision and reporting on the operation of the NS instance. Based on this supervision and reporting, modifications are made to the instance during NS operation. These modifications can involve changes to the capacity or topology of the NS, as well as the creation or modification of its components. Modifications may be initiated in response to new requirements for the NS or based on information obtained through monitoring and reporting. In this phase, deactivation involves temporarily terminating communication services and functions, representing the opposite action to the activation of the instance. From the perspective of network energy efficiency, the activation and deactivation of NSs according to the demand for network services or functions can contribute to the optimization of NS energy consumption. Therefore, significant network energy savings can be achieved during the operation phase of the NS lifecycle if appropriate operation approaches for optimizing NS energy consumption take place during the operational phase.  
Finally, during the **decommissioning phase**, the provisioning of the instance involves deleting the configuration that was specifically created for this instance using the network element components (Figure 4). After this phase, the NSE instance ceases to exist [16]. From the perspective of the network energy footprint, the decommissioning phase should result in the full termination of resources that will not contribute to the future energy consumption of the network.