## 2 Related Work  
### Component Reliability Models  
A WSN is a collection of a large number of sensor nodes that are deployed over an area or inside a target that needs to be detected, monitored, or tracked [22]. These nodes self-organize into a cooperative network [23], communicate in an ad hoc manner, and transmit sensor measurements to the end-user. A typical sensor node contains four major components: a sensing component for data acquisition, a processing component for local data computing, a radio or communication component for data transformation, and a power supply component [24; 25]. Other components, such as the locating component or actuator (for sensor adjustment and movement), may also be part of sensor nodes in specific applications. Several issues can affect the reliability of WSN by degrading its functionality in terms of coverage and/or connectivity. These concerns are mainly power supply failure, subsystem failure, and software failure. These issues can be summarized as follows:  
Power supply failure: most of the industrial and commercial SNs are battery-powered. The latest development of battery manufacturing has recently introduced high durability batteries (e.g., lithium thionyl chloride batteries) for SNs that can be used for years under certain conditions. Although these batteries can support the operation of SNs for an extended period, premature battery failures may still occur in practice. The failure can be attributed to various reasons, such as the deployment of SNs in harsh environmental conditions (e.g., extreme temperatures or rain), incorrect handling, or random failures caused by defective hardware. Subsystem failure: SNs are affected by random hardware failure for two main reasons. First, most commercial SNs are cost-sensitive, which means they are not always made up of the highest quality subsystems. Second, SNs are often subjected to adverse environmental conditions, which will affect the regular operation of their components [26]. Software failure: SNs are prone to random permanent software failures, rendering them inactive, i.e., unable to sense or communicate [22]. The power supply sources divide the sensor nodes into two categories, battery-powered sensor node (BPSN) and energy-harvesting sensor node (EHSN), that can convert ambient energy into electrical energy [27]. [PERSON] et al. [9] modeled the reliability of a BPSN as its battery lifetime. Because the limited battery lifetime essentially determines the lifetime of BPSN in many practical scenarios, it is impossible to charge or replace the sensor battery during the mission [28]. [PERSON] et al. [13] modeled both BPSN and EHSN as multi-subsystem components and evaluated their reliabilities based on the reliability of each subsystem's energy flow and reliability. [PERSON] et al. [10] assessed the BPSN reliability by considering failures and dependencies of its four major components. The fault-tolerant design of nodes is also critical for reliability. To make WSN resilient to sensor node failures, [PERSON] et al. [11] investigated and compared several reliability designs using hot or cold standby sparing techniques. [PERSON] and [PERSON] [12] considered the hot standby sparing design to achieve a highly reliable sink node subsystem. Path level reliability design is usually realized by the multi-path routing protocol [14; 15], and link reliability/path reliability is considered in routing algorithm design. Routing algorithms and protocols play a significant role in the reliability of the communication/networking layer. The routing algorithm used in the protocol determines the information transmission path from the source node to the destination node. When a node or link fails on the selected path, a reliable routing protocol is responsible for detecting the failure and finding an alternative path to complete the required information transmission. System-level reliability pays attention to the execution of actual functions, usually related to connectivity [29].