Fatih Senel

Distributed Recovery from Network Partitioning in Movable Sensor/Actor Networks via Controlled Mobility

Mobility has been introduced to sensor networks through the deployment of movable nodes. In movable wireless networks, network connectivity among the nodes is a crucial factor in order to relay data to the sink node, exchange data for collaboration, and perform data aggregation. However, such connectivity can be lost due to a failure of one or more nodes. Even a single node failure may partition the network, and thus, eventually reduce the quality and efficiency of the network operation. To handle this connectivity problem, we present PADRA to detect possible partitions, and then, restore the network connectivity through controlled relocation of movable nodes. The idea is to identify whether or not the failure of a node will cause partitioning in advance in a distributed manner. If a partitioning is to occur, PADRA designates a failure handler to initiate the connectivity restoration process. The overall goal in this process is to localize the scope of the recovery and minimize the overhead imposed on the nodes. We further extend PADRA to handle multiple node failures. The approach, namely, MDAPRA strives to provide a mutual exclusion mechanism in repositioning the nodes to restore connectivity. The effectiveness of the proposed approaches is validated through simulation experiments.

Distributed Recovery of Actor Failures in Wireless Sensor and Actor Networks

Wireless sensor and actor networks (WSANs) additionally employ actor nodes within the wireless sensor network (WSN) which can process the sensed data and perform certain actions based on this collected data. In most applications, inter-actor coordination is required to provide the best response. This suggests that the employed actors should form and maintain a connected inter-actor network at all times. However, WSANs often operate unattended in harsh environments where actors can easily fail or get damaged. Such failures can partition the inter-actor network and thus eventually make the network useless. In order to handle such failures, we present a connected dominating set (CDS) based partition detection and recovery algorithm. The idea is to identify whether the failure of a node causes partitioning or not in advance. If a partitioning is to occur, the algorithm designates one of the neighboring nodes to initiate the connectivity restoration process. This process involves repositioning of a set of actors in order to restore the connectivity. The overall goal in this restoration process is to localize the scope of the recovery and minimize the movement overhead imposed on the involved actors. The effectiveness of the approach is validated through simulation experiments.

Bio-Inspired Relay Node Placement Heuristics for Repairing Damaged Wireless Sensor Networks

Due to the harsh surroundings and violent nature of wireless sensor network (WSN) applications, the network sometimes suffers a large-scale damage that involves several nodes and would thus create multiple disjoint partitions. This paper investigates a strategy for recovering from such damage through the placement of relay nodes (RNs) and promotes a novel approach. The proposed approach opts to reestablish connectivity (i.e., 1-vertex connectivity) using the least number of relays while ensuring a certain quality in the formed topology. Unlike contemporary schemes that often form a minimum spanning tree among the isolated segments, the proposed approach establishes a topology that resembles a spider web, for which the segments are situated at the perimeter. Such a topology not only exhibits stronger connectivity than a minimum spanning tree but achieves better sensor coverage and enables balanced distribution of traffic load among the employed relays as well. To further increase the robustness of the formed topology, the SpiderWeb approach is further extended so that the final topology is guaranteed to be 2-vertex connected. Both centralized and distributed implementations of the SpiderWeb approach are discussed. The simulation results demonstrate the effectiveness of the proposed recovery algorithm.

Clustering of wireless sensor and actor networks based on sensor distribution and connectivity

Wireless Sensor and Actor Networks (WSANs) employ significantly more capable actor nodes that can collect data from sensors and perform application specific actions. To take these actions collaboratively at any spot in the monitored regions, maximal actor coverage along with inter-actor connectivity is desirable. In this paper, we propose a distributed actor positioning and clustering algorithm which employs actors as cluster-heads and places them in such a way that the coverage of actors is maximized and the data gathering and acting times are minimized. Such placement of actors is done by determining the k-hop Independent Dominating Set (IDS) of the underlying sensor network. Basically, before the actors are placed, the sensors pick the cluster-heads based on IDS. The actors are then placed at the locations of such cluster-heads. We further derive conditions to guarantee inter-actor connectivity after the clustering is performed. If inter-connectivity does not exist, the actors coordinate through the underlying sensors in their clusters to adjust their locations so that connectivity can be established. The performances of the proposed approaches are validated through simulations.

Relay node placement in structurally damaged wireless sensor networks via triangular steiner tree approximation

Wireless sensor networks (WSNs) have many applications which operate in hostile environments. Due to the harsh surroundings, WSNs may suffer from a large scale damage that causes many nodes to fail simultaneously and the network to get partitioned into multiple disjoint segments. In such a case, restoring the network connectivity is very important in order to avoid negative effects on the applications. In this paper, we pursue the placement of the least number of relay nodes to re-establish a strongly connected network topology. The problem of finding the minimum count and the position of relay nodes is NP-hard and hence we pursue heuristics. We present a novel three-step algorithm called FeSTA which is based on steinerizing appropriate triangles. Each segment is represented by a terminal. Each subset of 3 terminals forms a triangle. Finding the optimal solution for a triangle (i.e. connecting 3 segments) is a relatively easier problem. In the first step, FeSTA finds the best triangles and form islands of segments by establishing intra-triangle connectivity. Then in the second, disjoint islands of segment are federated. In the final step, the steinerized edges are optimized. The performance of FeSTA is validated through simulation.

An Efficient Mechanism for Establishing Connectivity in Wireless Sensor and Actor Networks

Wireless sensor and actor networks (WSANs) employ powerful and mobile actor nodes that can perform application specific actions based on the received data from the sensors. As most of these actions are performed collaboratively among the actors, inter-actor connectivity is one of the desirable features of WSANs. In this paper, we propose a novel distributed algorithm for establishing a connected inter-actor network topology. Considering an initially partitioned actor network with intra-connected sub-networks, our algorithm pursues a coordinated actor movement in order to connect the sub-networks. The goal of this movement is to both minimize the total and maximum travel distances of the individual actors. Our algorithm considers the minimum connected dominating set of each sub-network when picking the appropriate actor to move so that the connectivity of each sub-network is not violated. We analytically study the performance of our algorithm. Extensive simulation experiments validate the analytical results and confirm the effectiveness of our approach.

Detecting and connecting disjoint sub-networks in wireless sensor and actor networks

Wireless sensor and actor networks (WSANs) can be considered as a combination of a sensor network and an actor network in which powerful and mobile actor nodes can perform application specific actions based on the received data from the sensors. As most of these actions are performed collaboratively among the actors, inter-actor connectivity is one of the desirable features of WSANs. In this paper, we propose a novel distributed algorithm for establishing a connected inter-actor network topology. Considering initially disjoint sets of actors, our algorithm first initiates a search process by using the underlying sensor network in order to detect the possible sub-networks of actors in the region. After these sub-networks are detected, our algorithm pursues a coordinated actor movement in order to connect the sub-networks and thus achieve inter-actor connectivity for all the actors. This coordinated movement approach exploits the minimum connected dominating set of each sub-network when picking the appropriate actor to move so that the connectivity of each sub-network is not violated. In addition, the approach strives to minimize the total travel distance of actors and the messaging cost on both sensors and actors in order to extend the lifetime of WSAN. We analytically study the performance of our algorithm. Extensive simulation experiments validate the analytical results and confirm the effectiveness of our approach.

Optimized interconnection of disjoint wireless sensor network segments using K mobile data collectors

Due to harsh environmental conditions a Wireless Sensor Network (WSN) may suffer from large scale damage where many nodes fail simultaneously and thus the network gets partitioned into several disjoint network segments. Restoring intersegment connectivity is essential to avoid negative effects on the application. Employing mobile data collectors (MDCs), which by repositioning-facilitate the establishment of communication links between segments, may provide flexible solution to this problem. However the problem of finding shortest tours for MDCs is NP-Hard. In this paper we study the problem under constrained number of MDCs which makes the problem more challenging. We present a polynomial time heuristic for Interconnecting Disjoint Segments with k MDCs (IDM-kMDC). IDM-kMDC opts to minimize the tour lengths and balance the load on the k available MDCs. We model each segment by a representative. The IDM-kMDC heuristic finds k-subsets of representatives, computes an optimized tour for each subset and assigns one MDC for each tour. The performance of the algorithm is validated through simulation.

Optimized Connectivity Restoration in a Partitioned Wireless Sensor Network

Due to the harsh operation conditions a wireless sensor network (WSN) may suffer large scale damage where many nodes fail simultaneously causing the network to get partitioned into multiple disjoint segments. Restoring network connectivity in such a case is very crucial to avoid negative effects on the application. This paper investigates a relay node (RN) placement strategy to establish inter-segment connectivity and proposes CIST, an algorithm for forming a Connected Inter-Segment Topology. CIST uses segment representation as a means of optimization for minimizing the required number of RNs. The idea behind the algorithm is to find the best subsets of three segments and form a triangular Steiner Minimum Tree with minimum Steiner Points and federate the remaining segment through populating the RNs along mst edges. The performance of CIST is validated through simulation.

A robust relay node placement heuristic for structurally damaged wireless sensor networks

Wireless sensor networks (WSN) can increase the efficiency of many real-life applications through the collaboration of thousands of miniaturized sensors which can be deployed unattended in inhospitable environments. Due to the harsh surroundings and violent nature of the applications, the network sometimes suffers a large scale damage that involves many nodes and would thus create multiple disjoint partitions. This paper investigates a strategy for recovering from such damage through the placement of relay nodes and promotes a novel approach. The proposed approach opts to re-establish connectivity using the least number of relays while ensuring certain quality in the formed topology. Unlike contemporary schemes that form a minimum spanning tree among the isolated segments, the proposed approach establishes a topology that resembles a spider web, for which the segments are situated at the perimeter. Such a topology not only exhibits stronger connectivity than a minimum spanning tree but also achieves better sensor coverage and enables balanced distribution of traffic load among the employed relays. The simulation results demonstrate the effectiveness of the proposed recovery algorithm.