## 1 Introduction
From these factors, FT techniques in WSN are categorized into two types: one is 'proactive', and the other is'reactive'. Energy-based and data-based FT are two subcategories of node-based proactive methods. FT based on energy improves network MTTF and its lifespan. This approach relies on the clustering of deployed sensing nodes, network backbone scheduling, and node hibernation. Data recovery is made easier with the help of proactive data FT methods. One of the effective ways to combat data errors is via duplicate transmissions with the same information and comparing them to detect faults. Reactive FT strategies are activated by reactive methods when faults occur. This method waits for errors and subsequently starts the restoration/recovery procedures as an appropriate countermeasure. These techniques are further classified into three broad categories: network-based, node-based, and holistic, which are all based on the cause of the fault. Node-based reactive techniques are used to recover from node failures. They consist of strategies such as switching to the sleeping backup node on the occurrence of node failure. Network-based reactive techniques use multiple paths, backup paths, and path recalculation in case of network/link failure. Moreover, for restoring the connectivity, extra nodes are deployed or the existing nodes are repositioned. These are the techniques that can deal with and recover from both network and node-based faults. They provide complete fault tolerance for various faults. An effective WSN method makes use of existing resources to prolong the network lifespan or avert faults. Furthermore, these preemptive tactics are used to prevent any future failures that happen due to potential faults. Thus, all the associative methods consist of two primary techniques to overcome FT issues during base operations: maintaining network connectivity and multi-path routing. Here, the first technique is used to prolong the life of a WSN through an improved connectivity level. The bridge safeguarding method is a perfect illustration of a connection maintenance technique that extends the lifespan of WSNs that includes sufficient bridged units. In addition, the use of numerous routes inside the network increases data transmission reliability and provides a backup in the event of a network failure. Table 1 offers notations, terms, and abbreviations for the various variables used throughout the research. \begin{table}
\begin{tabular}{c c} \hline \hline
**Notations** & **Definitions** \\ \hline \(\xi\) & Overall energy consumption of the smart network \\ \(e_{sel}\) & Energy consumption for CH selection \\ \(e_{adv}\) & Energy consumption for advertising the selection \\ \(e_{B}\) & Energy consumption for the processing beacon signal \\ \(e_{SD}\) & Energy consumption to sense the data \\ \(e_{T_{x}}^{A}\) & Energy consumption to transmit aggregated data \\ \(e_{T_{y}}^{A}\) & Energy consumption for aggregating various service data \\ \(\Psi\) & Free space \\ \(\Phi\) & Multi-path \\ \(T_{x}/R_{x}\) & Transmission/reception \\ \(\varphi\) & Size of control packet \\ \(\eta\) & Size of data packets \\ \(h\) & Hop count \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Notations and its Definition.