Object tracking is an important application of WSNs and location management is one of the key steps involved in object tracking. In this dissertation, we pro-pose several tree-based location management schemes to reduce the communica-tion cost. We also address the contencommunica-tion and collision problems for event-driven WSNs (e.g., object tracking sensor networks). The significant results with future works are summarized as follows.
In Chapter 3, we have developed several efficient ways to construct a logical object tracking tree for a single-sink sensor network. We have shown how to or-ganize sensor nodes as a logical tree so as to facilitate in-network data processing and to reduce the total communication cost incurred by object tracking. For the location update part, our work can be viewed as the extension of the work in [14], and we enhance the work by exploiting the physical structure of the sensor net-work and the concept of deviation avoidance. In addition, we also consider the query operation and formulate the query cost of an object tracking tree given the query rates of sensors. In particular, our approach tries to strike a balance be-tween the update cost and query cost. Performance analyses are presented with respect to factors such as moving rates and query rates. Simulation results show that by exploiting the deviation-avoidance trees, algorithms DAT and Z-DAT are able to reduce the update cost. By adjusting the deviation-avoidance trees, algo-rithm QCR is able to significantly reduce the total cost when the aggregate query
rates is high, thus leading to efficient object tracking solutions.
Chapter 4 further explores the possibility of having multiple sinks in the net-work. One advantage of having multiple sinks is to reduce the response time of queries. In addition, using multiple sinks can also relieve the traffic congestion problem associated with a single-sink system. We extend the single-sink loca-tion management scheme proposed in Chapter 3 by constructing multiple trees to support multi-sink WSNs. The corresponding update cost is formulated for-mally. Based on the formulation, we have presented two distributed algorithms to construct multiple trees. We have verifies the benefits of a multi-sink WSN from different aspects, including the total (update plus query) cost, the number of sinks, query response time, query success rate, and load balance factor.
By exploiting the inherent property of imprecision of sensor data, Chapter 5 presents an imprecision-tolerant location management model for object tracking sensor networks. The proposed model consists of imprecision-tolerant update and query mechanisms that can be used to support imprecision-tolerant queries. By exploiting the feature of the tree-based location management schemes, the pro-posed model can provide multiple imprecision levels and ensure that the quest cost will be proportional to the imprecision level. In addition, we develop a tree con-struction algorithm to facilitate the proposed location management model, which can reduce the query cost while minimize the increment of update cost.
By simulation, we observe that packet loss may make the location informa-tion incorrect in object tracking sensor networks. Thus, a protocol is proposed in Chapter 6 to support the location management schemes from the link layer.
We have shown how to exploit the spatial correlation of sensor data on the link layer for event-driven WSNs. A hybrid TDMA/CSMA protocol is proposed. The protocol has three features that makes it very efficient. First, the TDMA part is triggered only when sensors detect an event. By doing so, the protocol enjoys the benefits of collision-free transmission of TDMA and low latency transmission of CSMA. Second, the slot assignment strategy associated with the TDMA part takes the spatial correlation of sensor data into consideration. By intentionally
allow-ing one-hop neighbors to share the same time slot, the number of slots required per frame is significantly reduced. Thus, the transmission latency is also reduced.
Third, by enlarging the slot size on purpose, an interesting effect of pipeline trans-mission is formed, and thus the interference problem in the non-event area is al-leviated. In addition, redundant reports are significantly reduced by our proposed report reduction scheme. We also discuss how to combine our scheme with LPL to achieve energy efficiency. Simulation results have demonstrated the efficiency of our scheme.
The future work includes two aspects. First, due to the fault-prone property of sensor nodes, developing a fault-tolerant mechanism for the tree-based loca-tion management is required. We will investigate a virtual-tree system, in which an implicit tree is still used. However, because no explicit tree exists, the fault-tolerant can be done easily. Second, we have proposed a link-layer protocol to support event-driven sensor networks in this dissertation. In the future, we will further develop a link-layer protocol to support hybrid time-driven/event-driven sensor networks.
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