## 2 Overview
### Energy Consumption in WSN  
Researchers consider various approaches to prolong network lifetime in WSNs because sensors become inaccessible after deployment. Hence, energy reduction is an essential criterion in the design process of a sensor network [54]. The protocol stack for WSN is  
Figure 4: Sensor node structure.  
given in Figure 5 with five distinct layers. Energy minimization approaches in WSN can be analyzed in each layer.  
Physical Layer: To receive and transfer data collected from the hardware, the physical layer must meet the needs of the receiving and transmitting device. The layer is responsible for generating and selecting the carrier frequency, signal detection, modulation and signal encryption, and signal reception. Due to the radio channel's usage for transmission and reception of data, the amount of energy consumed is significant. The channel can be operated in three distinct modes: Idle, Active, and Sleep. Consequently, the energy consumed can be minimized by shutting off the radio when the channel is idle [55].  
Datalink Layer: This layer is responsible for preventing neighboring signals from interfering with each other in a noisy environment. This layer should have the appropriate access, error control, multiplexing, and error detection and correction. TDMA-based protocols have been extensively used to avoid collisions of packets. However, [PERSON] et al. [56] reported that their deployment in multi-hop ad-hoc networks is very complex. Another method for efficient energy management is to reduce the time between transmission of a frame and idle listening.  
Network Layer: Several approaches, including topology control and routing schemes, have been adopted in this layer, increasing network lifetime. Selecting a suitable topology that could provide a well-connected network is often a difficult task. Routing plays a major role in lifetime enhancement by selecting the most energy-efficient path from sensing nodes to the base station (BS) [57]. Routing techniques can be categorized as location-based, data-centric, hierarchical, mobility-based, and quality of service (QoS)-based. However, hierarchical clustering routing algorithms have proved to be effective in enhancing lifetime and reducing power consumption by determining the optimal route.  
Transport Layer: Traffic flow regulation is provided by the transport layer, which distributes network traffic to the distant end. Additionally, traffic is provided with reliability measures. It is divided into sequential segments to forward upper layer application data, which are then reassembled into data packages. The transport layer can perform flow control, congestion control, and error checking at a higher level [58].  
Application Layer: The application layer serves as a connection point between users and the network services dedicated to electronic mail, file transfers, virtual terminals, and  
Figure 5: Protocol stack for WSN.  
file servers [59]. Extraction of energy from the environment in solar, thermal, and vibration have evolved as the latest technologies to deal with the energy problem. Energy harvesting is emerging as an efficient technique that recharges sensors after depletion.