Izzet F. Senturk

Survey Topology management techniques for tolerating node failures in wireless sensor networks: A survey

In wireless sensor networks (WSNs) nodes often operate unattended in a collaborative manner to perform some tasks. In many applications, the network is deployed in harsh environments such as battlefield where the nodes are susceptible to damage. In addition, nodes may fail due to energy depletion and breakdown in the onboard electronics. The failure of nodes may leave some areas uncovered and degrade the fidelity of the collected data. However, the most serious consequence is when the network gets partitioned into disjoint segments. Losing network connectivity has a very negative effect on the applications since it prevents data exchange and hinders coordination among some nodes. Therefore, restoring the overall network connectivity is very crucial. Given the resource-constrained setup, the recovery should impose the least overhead and performance impact. This paper focuses on network topology management techniques for tolerating/handling node failures in WSNs. Two broad categories based on reactive and proactive methods have been identified for classifying the existing techniques. Considering these categories, a thorough analysis and comparison of all the recent works have been provided. Finally, the paper is concluded by outlining open issues that warrant additional research.

Tracing Origins of the Salmonella Bareilly Strain Causing a Food-borne Outbreak in the United States.

BACKGROUND Using a novel combination of whole-genome sequencing (WGS) analysis and geographic metadata, we traced the origins of Salmonella Bareilly isolates collected in 2012 during a widespread food-borne outbreak in the United States associated with scraped tuna imported from India. METHODS Using next-generation sequencing, we sequenced the complete genome of 100 Salmonella Bareilly isolates obtained from patients who consumed contaminated product, from natural sources, and from unrelated historically and geographically disparate foods. Pathogen genomes were linked to geography by projecting the phylogeny on a virtual globe and produced a transmission network. RESULTS Phylogenetic analysis of WGS data revealed a common origin for outbreak strains, indicating that patients in Maryland and New York were infected from sources originating at a facility in India. CONCLUSIONS These data represent the first report fully integrating WGS analysis with geographic mapping and a novel use of transmission networks. Results showed that WGS vastly improves our ability to delimit the scope and source of bacterial food-borne contamination events. Furthermore, these findings reinforce the extraordinary utility that WGS brings to global outbreak investigation as a greatly enhanced approach to protecting the human food supply chain as well as public health in general.

An optimization-based approach for connecting partitioned mobile sensor/Actuator Networks

Wireless Sensor and Actuator Networks (WSANs) employ mobile nodes in addition to stationary tiny sensors. Similarly, mobile sensors make it possible to have the flexibility of mobility in mobile sensor network (MSN) applications. Mobility can be exploited to connect partitioned WSANs and MSNs due to large scale damages or deployment problems. However, since mobility consume significant energy and it can be limited due to terrain constraints, the travel distance for the mobile nodes should be minimized in such a recovery effort. In this paper, we present a mathematical model which minimizes the total travel distance for connecting a given number of partitions. The idea is based on network flows and the problem is modeled as a mixed integer nonlinear program. The nonlinear terms in the model are linearized using a polygon approximation for computational efficiency. We evaluated the performance of the proposed approach in terms of total distance as well as the time to reconnect the partitions. The results show that our approach outperforms the heuristic approach in terms of total distance and delay and reveals various trade-offs involved in connecting multiple partitions.

Relay placement for restoring connectivity in partitioned wireless sensor networks under limited information

Several factors such as initial deployment, battery depletion or hardware failures can cause partition wireless sensor networks (WSNs). This results in most of the sensors losing connectivity with the sink node and thus creating disruption of the delivery of the data. To restore connectivity, one possible solution is populating relay nodes to connect the partitions. However, this solution requires information regarding the availability of the damaged area, number of partitions in the network and the location of the remaining nodes which may not be obtained for all applications. Thus, a distributed self-deployment strategy may better fit the application requirements. In this paper, we propose two distributed relay node positioning approaches to guarantee network recovery for partitioned WSNs by minimizing the movement cost of the relay nodes. The first approach is based on virtual force-based movements of relays while the second exploits Game Theory among the leaders of the partitions. Force-based approach stretches the network gradually with the deployment of additional relays. In the game-theoretic approach, the partition to be connected with is determined by the leader relay nodes based on the probability distribution function (pdf) of the partitions. Partitions with a higher pdf have priority over other partitions for recovery. Once the partition is connected with the relay nodes, it becomes the part of the connected network. Recovery proceeds with the partition having the next highest priority until network is completely recovered by reaching the system-wide unique Nash equilibrium. Both approaches are analyzed and evaluated extensively through simulation. Game-theoretic approach has been shown to outperform force-based approach as well as a centralized approach under most of the conditions.

Connectivity restoration in delay-tolerant sensor networks using game theory

Due to limited resources and harsh environments, partitioning can be inevitable in wireless sensor networks (WSNs). To re-establish network connectivity, healthy mobile sensor nodes can be repositioned in an on-demand basis. However, the nodes are equipped with limited batteries and their movement requires excessive energy consumption. In this paper, we address the problem of minimising the movement cost when re-establishing network connectivity in delay tolerant WSNs by presenting a distributed heuristic approach based on Game Theory. Compared to centralised heuristics which assume complete knowledge of the network and failures, this approach is self-healing that can work with uncertain network/failure information. Under limited knowledge on partitions and failure locations, Game Theory is used to facilitate decision making on the selection of the nodes to be moved and their movement directions. Extensive simulations have revealed the efficiency of the proposed approach in terms of movement distance and network coverage.

An effective and scalable connectivity restoration heuristic for Mobile Sensor/Actor Networks

Due to inhospitable environments, the actors/sensors in Mobile Sensor/Actor Networks are subject to various damages which can disrupt the data delivery and cooperation. Typically, the damages affect the existing routes and may even cause network partitioning. In such a case, the set of actors/sensors disconnected from the rest of the network, namely a partition, can be re-connected with the network through topology adjustment by exploiting node mobility. However, movement of the nodes consumes significant energy which needs to be minimized. In this paper, we propose an effective yet scalable heuristic approach for restoring network connectivity while minimizing the total movement distance of the nodes. Given that the nodes can move to infinitely many locations in the damaged area, the basic motivation of the heuristic is to reduce the number of locations where the nodes can move. For this purpose, a minimum set of relay points is identified first by running a relay node placement heuristic that can ensure connectivity. Existing nodes in the partitions are then used to fill the relay points based on a greedy heuristic. When selecting nodes from the partitions, the nodes that do not cause further disconnectivity in the partition are picked. This is done by determining the connected dominating set of the partition and identifying the dominatee nodes. The experiment results show that the proposed approach not only performs very close to a near-optimal solution but also scales well when the number of nodes or partitions are increased.

Distributed relay node positioning for connectivity restoration in partitioned Wireless Sensor Networks

Due to limited battery life of sensors and harsh deployment environments where they are deployed, Wireless Sensor Networks (WSNs) can be subjected to node failures. This can split the network into partitions containing healthy but unreachable nodes by the rest of the network including the sink node. One possible solution to this problem is deploying relay nodes assuming that the damaged area, the number of partitions and the location of the partitions are known to a centralized party. However, depending on the application, some of this information may not always be available, requiring a distributed self-deployment placement strategy. Such a strategy should not only guarantee the network connectivity but also strive to minimize the movement overhead on the relay nodes assuming that they are also battery-operated. In this paper, we present a distributed relay node positioning approach to address the problem of connectivity restoration in partitioned WSNs. The approach exploits Game Theory among the relay nodes and the partitions. Relay nodes determine the partitions to connect based on the probability distribution function (pdf) of the partitions. If the partition has a higher pdf, it is recovered earlier and becomes the part of the connected network. The recovery process takes place until reaching the system-wide unique Nash equilibrium. Game Theoretic approach has been shown to outperform baseline approaches under all conditions.

Handling large-scale node failures in mobile sensor/robot networks

In Wireless Sensor Networks (WSNs), maintaining connectivity with the sink node is a crucial issue to collect data from sensors without any interruption. While sensors are typically deployed in abundance to tolerate possible node failures, a large number of simultaneous node failures within the same region may result in partitioning the network which may disrupt the network operation significantly. Given that WSNs are deployed in inhospitable environments, such node failures are very likely due to storms, fires, floods, etc. The self-recovery of the network from these large-scale node failures is challenging since the nodes will not have any information about the location and span of the damage. In this paper, we first present a distributed partition detection algorithm which quickly makes the sensors aware of the partitioning in the network. This process is led by the sensors whose upstream nodes fail due to damages. Upon partition detection, sensors federate the partitions and restore data communication by utilizing the former routing information stored at each sensor to the sink node and exploiting sensor mobility. Specifically, the locations of failed sensors on former routes are used to assess the span of the damage and some of the sensors are relocated to such locations to re-establish the routes with the sink node. Relocation on such former routes is performed in such a way that the movement overhead on sensors is also minimized. Our proposed approach solely depends on the local information to ensure autonomicity, timeliness and scalability. The effectiveness of the proposed federation approach is validated through realistic simulation experiments and has been shown to provide the mentioned features.

Towards realistic connectivity restoration in partitioned mobile sensor networks

The connectivity of a disjoint mobile sensor network can be restored by moving a set of nodes to certain destinations. However, all of the existing works have assumed that the selected destinations can be reached via direct path movement, which may not be the case in real-world applications because of obstacles or terrain elevation. In addition, even if direct path movement is successful, optimal energy efficiency cannot be attained by neglecting the elevation or friction of the terrain when determining the movement path of the nodes. Thus, in the recovery efforts, terrain type, elevation, obstacles, and possible localization errors should be considered in order to guarantee the connectivity restoration while minimizing the recovery cost in terms of energy. In this paper, we pick two sample distributed and centralized connectivity restoration approaches from the literature to show that these approaches fail to restore connectivity in many cases due to the lack of considering realistic issues. These approaches are re-designed in order to determine movement trajectory based on a path planning algorithm, which considers the risk and elevation of the terrain sections. Experiment results reveal several issues regarding the performance in terms of energy consumption and recovery delay. Copyright

Topology Management Techniques for Tolerating Node Failure

In Wireless Sensor Networks (WSNs) sensor nodes often operate unattended in a collaborative manner to perform some tasks. In many applications, the network is deployed in harsh environments such as battlefield where the nodes are susceptible to damage. In addition, nodes may fail due to energy depletion and breakdown in the onboard electronics. The failure of nodes may have major consequences. First, some areas may be left uncovered. Second, the fidelity of the collected data gets degraded. And finally, the network may get partitioned into disjoint segments. In particular, losing network connectivity has a very negative effect on the applications since it prevents data exchange and hinders coordination among some nodes. Therefore, restoring the overall network connectivity with the least resource overhead and performance impact is very crucial. This chapter focuses on network topology management techniques for tolerating node failures. It analyzes the effects of node failure on network connectivity in WSNs, categorizes recently published recovery schemes, and outlines related open issues.