# Efficient Actor Recovery Paradigm for Wireless Sensor and Actor Networks
## 4 Problem Statement and Optimized Deterministic Actor Recovery System Model  
### Problem Formulation  
WSANs comprise actors with powerful resources and sensor nodes with limited computation, power, and communication capabilities. The sensors and actors in WSANs collaborate together to monitor and respond to the surrounding world. WSANs can be applied to a wide range of applications, like health or environmental monitoring, chemical attack detection, battlefield surveillance, space missions, intrusion detection, etc. However, WSANs are greatly affected due to environmental changes, frequent changes in event-detection and actor failure processes. The failure of an actor node can result in partitioning of the WSAN and may limit event detection and handling. Actors may fail due to hardware failure, attacks, energy depletion, or communication link issues. Sensor node failure may cause loss of event detection of the assigned environment covered by the sensor. The probability of the actor failure is less than that of sensor failure and can be controlled through the relocation of mobile nodes due to their powerful characteristics; however, actor failure can cause more damage than sensor failure. Actor failure can cause a loss of coordination and connectivity between nodes, limitation in event handling, and can lead to a disjoint WSAN. The actor failure occurrence is very critical as it degrades the network performance. The failure of a critical actor may cause high impact to the whole network. Critical actor nodes refer to actors which their failure cause network partitioning. Figure 2 illustrates the concept of critical actor nodes. Assume that actor A3 failed. Its failure will cause the network to disjoint. Thus, A3 is a critical node. Actor nodes A2, A6, and A7 are critical nodes as well. Most of the existing approaches attempted to replace the critical node with another backup node, but they failed to maintain the QoS parameters and energy consumption. For instance RNF manage to handle failure by moving a small block of neighbor actors toward the failed node in order to recover the communication among them. Even though this manages the recovery of the network but it enlarges the recovery scope and cascade relocation. Such behavior should be eliminated in recovery algorithms. In addition, due to the fact that WSANs are deployed in harsh areas and require long term monitoring/acting process, proposed methods should offer robust self-healing failure detection/recovery techniques which ensure that the network lifetime is maximized as much as possible while maintaining QoS. In WSANs, the nodes track their neighbors by using heart beat messages to avoid any possible interruption. Moreover, algorithms are used to define the critical nodes using 1-hop or 2-hop message exchange information [8; 36; 40]. In addition, they identify the actor node failure by the interaction of those heartbeat messages with this particular actor node. Thus, there is possibility of interruption due to losing the trail of heart beat messages. Monitoring actor failure detection using 2-hop neighbor list  
Figure 2: Critical actor node in WSAN. is efficient once it is combined with QoS measurement capabilities, i.e., packet delivery and forwarding techniques should support efficient packet handling and forwarding. Also, we should minimize the through critical actor nodes. Thus, in our proposed model, we assume that each actor node stores the information up to 2-hops to keep the extended trail information. This helps determine the forwarding capability of the actor nodes. The model aims to ensure the contention-free forwarding capability that minimizes the loss of packets in case of node failure. To determine the actor's forwarding capability, each actor conveys the group of beacon messages using different power strengths. Furthermore, the neighbor of each actor listens and returns the value in response. After the neighbor actor receives the message, it starts calculating its RSSI value and sends it back to the sender actor.