WIRELESS SENSOR NETWORKS: GENERALITIES AND APPLICATIONS
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10.1.1 Motivating the Use of Mobility in WSNs Most of the research in WSNs concerns networks whose nodes do not move and cannot be replaced. Nodes sense events of interest, and some energy-ef cient routing protocol is used for delivering the sensed data to static sinks. In this scenario, it has been observed that the nodes that more than all the others have their energy drained from data communication are those closer to the sinks. These nodes relay data for all the other nodes in the network as well as packets from the sinks to the sensors. As a consequence, nodes that are closer to the sinks soon die from energy depletion resulting in the disconnection of the sinks from the rest of the network. The problem of energy drainage at the sink neighbors is referred to as the sink neighbors problem. The consequences of this problem are illustrated in Figure 10.1, which shows the average nodal residual energy when the nodes closest to the sink die. The gure refers to networks with 400 homogeneous nodes, each with a short transmission range (25 m), placed on a 20 20 grid. The static sink is (optimally) located at the center of the deployment area. Packets are periodically sent to the sink by using a shortest path-like multihop routing protocol [9, 10]. The picture shows the quite uneven node average residual energy (percentage of the nodal initial energy) at the time the four sensor nodes that relay packets to the sink die, leaving the sink unable to receive any more data from the network. The remarkably high variance among the residual energies is due to the different distance of each node from the sink and, in general, to the different number of sensor-to-sink routes to which a node belongs, which implies different number of packets to relay. Nodes along the cross centered at the sink tend to be the preferred data relays. The closer these nodes are to the sink, the higher the number of packets they receive and transmit, and consequently the higher their energy consumption. These are the nodes with the lowest residual energy in the gure. In particular, when the nodes around the sink die, the energy at the nodes along the cross arms averages at 71.07% of their initial energy, while 42.75% of the
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Figure 10.1. Node residual energy (%).
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MOBILITY IN WIRELESS SENSOR NETWORKS
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network nodes have more than 95% of their initial energy available! This incapacity of balancing node energy consumption results in short network lifetime (the time until packets are no longer deliverable to the sink) and inef cient use of available resources. The sink is soon disconnected from the network, while a large number of the deployed nodes are still fully operational! This chapter explores solutions for mitigating the sink neighbors problem illustrated above, by exploiting the mobility of some of the network components, thus improving network performance. Given the typical mobile nature of general ad hoc networks, the impact of mobility on their performance has been extensively explored [11 19]. The primary objective of these works is that of exploiting the mobility of some of the nodes for message delivery in disconnected ad hoc networks, for improving network throughput, and for studying mobility-assisted routing in general. This chapter concerns approaches designed speci cally for WSNs, and in particular those solutions where the mobility of some network elements brings considerable improvement in key performance metrics that are typical of sensor networking. In the rest of the chapter we describe research on mobility in WSNs proposed recently. In particular, we will survey works where (a) the sensor nodes can move, so that batches of fresher nodes are kept close to the sink for providing uninterrupted data forwarding; and (b) relays are sent throughout the network to collect data and bringing them to the sink. Finally, (c) we will describe works where the sink itself moves to collect sensed data. The last two approaches appear to be the more promising for energy ef ciency and longer network lifetime, since sink and relays are usually considered resource-rich. Therefore, energy consumption and network lifetime are not impacted by the energy needed to move them. In the case where the sensor nodes move, a great deal of the nodes energy is spent on the movement itself, thus having a detrimental impact on the lifetime of the nodes. The nal part of the chapter concerns a quite thorough discussion on how to compare different approaches to mobility in WSNs, focusing on sink and on relay mobility. In particular, we show how, by using simulations, one can effectively assess pros and cons of different solutions and gain useful insights for de ning and testing new ones.
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10.2 SENSOR NODES MOBILITY Mobility of the sensor nodes has been exploited for improving, or enabling altogether, sensing and communication coverage [20 22]. The idea presented in reference 20 is to have the sensors move into positions that minimize the energy cost of reporting streams of data to the sink, which is statically placed. The protocols proposed by Wang et al.  aim at moving mobile sensors from densely deployed areas to areas with coverage holes, where for some reasons a limited number of sensors have been deployed. The three protocols are proven by simulations to be effective in terms of coverage, deployment time, and moving distance. Minimization of the energy consumption of moving nodes has been subsequently addressed by the authors in reference 22 by letting the node move logically that is, only after they can decide
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