**1. Introduction**

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A wireless sensor network (WSN) is a collection of spatially distributed, resourceconstrained sensor nodes, deployed within an application area, to monitor a specific event or set of events. These sensor nodes are standalone devices without access to a continuous energy source and are located either within or close to the phenomena they are observing. The nodes communicate with one or more central control point(s), generally called a sink or base station. A typical sensor node comprises a sensing unit, a small processing unit to perform simple computations, a transceiver unit to connect nodes to the network and a power unit. Some nodes are also equipped with a location finding system [1]. A WSN application contains hundreds to thousands of sensor nodes. These sensor nodes are designed for unattended operation and are generally stationary after deployment.

One of the main criteria in designing a WSN application is prolonging network lifetime and preventing connectivity degradation through aggressive energy management. There is a trade-off between a node's energy, node range, size and cost. Due to the need to conserve battery lifetime, the sensor nodes operate with low duty cycles and communicate sporadically, over short distances with low data rates. In WSNs the flow of data is predominantly unidirectional, from nodes to sink [2]. The limited resources, non-renewable power supply and short radio propagation distances, (and hence large number required for deployment), of sensor nodes impose constraints on WSN applications not found in wired networks. A WSN differs from local area networks in the following key areas [3, 4]:


© 2012 Chinnappen-Rimer and Hancke, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Chinnappen-Rimer and Hancke, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


A WSN also differs from other wireless networks, such as cellular networks and mobile ad hoc networks (MANETS) because these networks are linked to a wired or renewable energy supply. In cellular networks and MANETS, the organising, routing and mobility management tasks focus on optimizing quality of service (QoS) and ensuring high bandwidth efficiency. There is a large amount of network traffic and the data rate is high to cater for the demand for multimedia rich data. These networks are designed to provide good throughput/delay characteristics under high mobility conditions [2]. Energy consumption is of secondary importance as the battery packs can be replaced or re-charged as needed.

As the term ``wireless'' implies, there is no fixed physical connection between sensors to provide continuous energy and an enclosed communication medium. This creates two problems, firstly, the sensor has a finite amount of energy, which once depleted, disables the sensor and hence reduces network lifetime. Secondly, all transmitted messages will be detected by any listening device within receiving range, which then has to decide whether to accept, forward or ignore the message. This signal transmission and reception has a power cost. In addition, many WSN applications do not have a pre-planned network topology and nodes are only aware of their immediate neighbours. When routing a message to a sink, the nodes closest to the sink receive a disproportionate amount of messages, resulting in their energy being consumed earlier.

Initial message routing protocols assumed the sink or destination node was in a fixed location, and that network nodes had no or limited knowledge of the network topology [5]. An area of active research for a number of years has been how to notify the central sink (or monitoring hub) about an event in real-time by utilising the minimum amount of power of sensor nodes. Strategies to improve node energy efficiency include using multiple sinks in the application area and the use of mobile sinks to collect data from stationary sensor nodes to prevent nodes close to a sink from having their energy depleted and hence decreasing network lifetime.

A model for optimum path movement of mobile sinks to reduce the number of messages transmitted and received by an individual sensor node is proposed. An investigation is conducted into the optimum route a mobile sink can travel that will reduce the number of messages transmitted within a network, allow equitable usage of all nodes to transfer an event message and still allow an event to be reported in real-time.

In the following sections a brief discussion of the use of mobile elements in WSNs as well as current research using mobile sinks and/or nodes to improve the energy efficiency of routing protocols is provided. The algorithm to transmit data from a sensor node to a mobile sink is discussed and the results analysed.
