# An Energy-Efficient Fail Recovery Routing in TDMA MAC Protocol-Based Wireless Sensor Network
## 1 Introduction
Our goal is to restore the connection between isolated nodes and the functioning portion of the network. Many studies have shown that the occurrence of faults in WSN is largely classified into two groups: (i) transmission fault and (ii) node fault. The node fault is further classified into five categories: power fault (battery depletion), sensor circuit fault, microcontroller fault, transmitter circuit fault, and receiver circuit fault [10; 14]. In the cases of receiver or transmitter fault, the sensor node cannot receive nor send its sensed data as well as the data forwarded from the child nodes. The sensor circuit failure is considered as less critical, as the sensor node can still forward the data from its child nodes [15]. Figure 1: An example of isolation in WSN due to single parent fault. Depending on the hardware condition of the sensor node, they are categorized as a Normal node, Traffic node, End node, and Dead node [10]. According to the results of previous studies, the categorization helps in reducing the percentage of dead nodes in the network, therefore improving the network lifetime. As electronic components of wireless sensor nodes are becoming more reliable, battery depletion is considered as the most prominent source of failure according to recent reports [16]. For the WSNs in a harsh environment, a distributed fault detection (DFD) algorithm was proposed by [PERSON] [17]. The DFD algorithm does not incur additional transmission costs because they use existing network traffic to identify sensor failures. Due to the exchange of multiple enquiry messages, this method may consume more energy during the recovery process. Another common solution to providing fault tolerance (FT) is adding redundant hardware or software [18]. Highly stringent design constraints (e.g., limited battery power) of WSNs, however, make it difficult to add such redundancy due to the additional cost. In [19], the authors proposed a method of fault recovery during the routing process in a WSN. It classifies the fault recovery methods into two main classes based on the improvement in data transmission. The first technique is retransmission, in which the source retransmits the data through another path when the original path fails. The second technique is data replication which duplicates the data to multiple copies over multiple paths. Utilization of multiple paths for the same message may reduce the network efficiency and cause additional contention to the channel access. In [20], it is studied a temporal classification method that classifies fail recovery techniques as preventive and curative. Preventive techniques attempt to keep the network functioning without any interruption when any fault occurs. In contrast, curative techniques employ a reactive process that interrupts the network functions while it recovers an identified fault. The methods of [21] are examples of preventive techniques. They select in advance the second-best routing option as a redundant path to use when a fault appears in the shortest path. To meet the energy efficiency requirement, their algorithm utilizes the largest portion of the shortest path that can still forward the data to determine the redundant path. Since the nodes within the shortest path execute multiple transmissions, they may consume greater energy than the other nodes. An unequal distribution of network load may cause a failure of nodes in the shortest paths. Then, the system frequently executes a fail recovery procedure which makes the nodes consume additional energy. The authors of [22] proposed a routing protocol that allows real-time fault recovery. It uses the remaining time of each packet and the state of the forwarding candidate set of nodes, and chooses a path dynamically. Upon detection of a failure, sensor nodes change their status to the jump mode and dynamically adjust the probability of jump to increase the ratio of successful transmissions. Updating the state of the nodes requires additional control message exchange which can be costly in the network with limited energy.