| Summary: | 博士 === 國立成功大學 === 電機工程學系碩博士班 === 98 === In the monitoring and surveillance applications of wireless sensor networks (WSNs), such as environment monitoring, battlefield surveillance and diaster relief coordination, the sensor nodes could be deployed in hostile environments and operate untethered. Therefore, there might be various hazards occurred in the network and these hazards could wreak havoc on the functionality of the system. Two kinds of hazards, which are the existence of coverage holes and Sybil attacks, are discussed in this thesis. In the detection of coverage holes, the existing obstacles can be identified and a density control scheme is also developed to prolong the system time of WSNs.
It is likely that a deployed area will contain obstacles of some form in WSNs. These obstacles may potentially degrade the functionality of the WSN, e.g. the occurrence of deadends due to obstacles in geographic routing protocols, or objects might crash into obstacles when moving in WSNs. If the size and location of the obstacles can be detected, their influence can be reduced. Accordingly, this thesis describes a scheme for detecting obstacles in WSNs. The scheme identifies the obstacles by marking the sensor nodes around the obstacle boundaries. The scheme does not require the absolute position of individual nodes in the sensing field nor any additional hardware, and thus can significantly reduce the deployment costs. The efficiency of the scheme is demonstrated via simulations performed using the network simulator ns2. The results show that the detection scheme needs much less overhead compared to previous research while still marking the nodes close to the obstacles precisely.
WSNs are typically expected to work for a long time. However, the power supply of a sensor node is usually a battery that cannot provide long operation time or be replaced easily due to hostile environments. Therefore, energy is an important and scarce resource in WSNs. For prolonging the lifetime of WSNs, some research maintained adequate degree of node density instead of the higher degree of that. Only a partial set of the sensor nodes, active nodes, are required for providing the full coverage of the interested area while others are inoperative. Previous schemes for active node selection needed location information for each sensor node. This thesis presents an algorithm for density control without position information of sensors. The simulation results show that the scheme guaranteed the 100% coverage of the target area and the number of active nodes was competitive to the algorithms that require location data.
As the prevalence of WSNs grows in the military and civil domains, the need for network security has become a critical concern. In a Sybil attack, the WSN is subverted by a malicious node which forges a large number of fake identities in order to disrupt the network’s protocols. In attempting to protect WSNs against such an attack, this thesis develops a scheme in which the node identities are verified simply by analyzing the neighboring node relation of each node. The analytical results confirm the efficacy of the approach given a sufficient node density within the network. The simulation results demonstrate that for a network in which each node has an average of 9 neighbors, the detection rate is around 95% when multiple malicious nodes are considered.
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