## 5 Results
### Performance Metrics (RQ4)  
Many performance parameters or metrics should be considered when deploying WSNs, starting with coverage and connectivity, which are the most important parameters that ensure high quality of service from WSNs, as discussed earlier in Section 2.6, as well as other parameters, such as the lifetime, energy consumption, latency, signal strength (RSSI), accuracy, scalability, reliability, and more [32]. A brief discussion of these parameters is explained below. \(\bullet\)**Lifetime**  
One of the most important metrics in WSNs is the network's lifetime. Through research, it is found that network lifetime depends on two dimensions, the first is network connectivity, which means that network lifetime can be defined as the period from the network deployment until one or more nodes lost connection to the sink node. While the second dimension depends on energy consumption, and this means network lifetime is the period from network deployment until one or multiple live nodes fall below a specific energy threshold [104]. Most conducted studies in Table 1 tried to maximize lifetime  
Figure 13: Optimization algorithms ratios. through optimal node deployment that maximizes network coverage and connectivity, hence minimizing the power consumption that leads to prolonged network lifetime. \(\bullet\)**Energy Consumption**  
Each node in WSN needs energy in three parts, i.e., collecting, processing, and communicating data. The amount of consumed energy depends not only on the energy capacity but also on the heterogenous functionalities of the sensor node. The efficient use of energy prolongs the network lifetime. This can be achieved through reducing the number of exchanged messages between nodes and scheduling sleep intervals for redundant nodes, while leaving the remaining nodes active to save network coverage and connectivity, as well as using efficient routing protocols; all these steps will minimize energy consumption [28]. \(\bullet\)**Latency and accuracy**  
Latency and accuracy are related to each other. Latency means the time required to send a message from the source node to the destination node across the network, and this represents the total delay of the sent message. Accuracy represents the efficient arrival of the sent message to the destination within the limited time required, so reducing delay ensures network accuracy, and this is an important performance metric in WSN [11]. \(\bullet\)**Signal strength (RSSI)**  
This metric represents a measure of link quality and depends on the distance between two nodes to calculate the reachability of the node through the communication process. RSSI stands for received signal strength indicator that can be determined from the following equation [76; 105]:  
\[\text{RSSI}=-10\times\text{n}\times\text{log}_{10}(\text{d})+\text{p} \tag{5}\]  
where,  
d: is the distance from the sensor node measured in meters;  
n: is the propagation constant or path-loss exponent;  
p: is the power in reception mode (Dbm) (decibel-milliwatts). \(\bullet\)**Scalability and Reliability**  
Scalability can be defined as the ability of the network to be extended by including more nodes in the network, while preserving network performance. The reliability of a WSN represents its ability to deliver sensed data to the sink node, while maintaining coverage and connectivity through the mission period. This period is application-dependent [90]. While using optimization algorithms in solving the WSN deployment problem, the performance metrics that have been presented in the literature to measure the qualities of the approximations collected from different optimization algorithms can be described as follows [95]. \(\bullet\)**The number of non-dominated solutions (NDS)**  
If Ps represents the set that contains all the non-dominated solutions generated by the proposed algorithm A, then the NDS-metric is defined by the size of Ps, as follows:  
\[\text{NDS}=|\text{P}_{\text{s}}| \tag{6}\]  
A higher value of NDS means that a sufficient number of choices exist [95].