A sensor network consists of tiny, low-powered and multifunctional sensor devices and is able to perform complex tasks through the collaborative efforts of a large number of sensor nodes that are densely deployed within the sensing field. Maintaining connectivity and maximizing the network lifetime are among the critical considerations in designing sensor networks and its protocols. Conservation of limited energy reserves at each sensor node is one of the greatest challenges in a sensor network. It has been suggested that mobility of some nodes/entities in a sensor network can be exploited to improve network performance in a number of areas, including coverage, lifetime, connectivityy, and fault-tolerance. In this context, techniques for effectively utilizing the unique capabilities of mobile nodes have been attracting increasing research attention in the past few years. In this chapter, the authors focus on some of the new and innovative techniques that have been recently proposed to handle a number of important problems in this field. It also presents a number of open problems and some developing trends and directions for future work in this emerging research area.
A sensor network is an interconnection of tiny, lightweight, energy-constrained devices, known as sensor nodes, and is usually deployed to monitor some kind of physical phenomena from the territory of its deployment. For example, a sensor network may be deployed to monitor the humidity or the temperature of a certain region, or it may be deployed to detect the presence or absence of some objects, as well as the movement of objects within the area being monitored. Recent technological advances in the field of micro-electro-mechanical systems (MEMS) have made the development of such tiny, low-cost, low-powered and multi-functional sensor devices technically and economically feasible (Akyildiz, 2002; Chong, 2003). These nodes are usually equipped with a sensing unit, a processing unit, a memory unit and a RF unit that is used for wireless data communication.
The data generated by each sensor is required to be sent to a central point, known as Base Station (BS) (also referred to as sink or access point). The base station is not power constrained and its location may or may not be fixed. A general layout of a sensor network, including the sensor nodes and a base station, is shown in Figure 1. Some researchers have also proposed the deployment of multiple sinks within a sensor network.
A general layout of sensor networkMultiple home agents
The nodes in a sensor network are deployed inside or very close to the phenomenon being monitored, in order to carry out the sensing task effectively. The placement of sensor nodes in a network can be pre-determined (e.g. the deployment of a sensor network in a factory or in the body of a human, an animal or a robot) or random (e.g. the deployment of nodes by dropping them from a helicopter/airplane or delivering them in an artillery shell or in a missile) (Akyildiz, 2002; Chong, 2003). All data from the sensor nodes must eventually be collected at the base station(s) or sink(s). The collected data may be aggregated and forwarded to the user, possibly using the Internet, where it can be further analyzed to extract useful information.
Although the capability of an individual sensor node is limited, a sensor network is able to perform bigger tasks through the collaborative efforts of a large number of sensor nodes (hundreds or even thousands) that are densely deployed within the sensing field (Akkaya, 2005; Akyildiz, 2002; Chong, 2003). There is a wide range of applications, for both military and civil purposes, where the use of sensor networks can be very useful that include medical, industrial, military, and environmental fields. For example, sensor networks can be used for target and/or movement detection, which is extremely important for military/battlefield applications as well as habitat monitoring and health monitoring.
Key Terms in this Chapter
Flat Sensor Network Architectures: In a sensor network with flat architecture, all sensor nodes are treated equally and each are responsible to send/route data towards the sink.
Wireless Sensor Networks (WSN): A sensor network is an interconnection of tiny, lightweight, energy-constrained devices, known as sensor nodes. These nodes are usually equipped with a sensing unit, a processing unit, a memory unit and a RF unit that is used for wireless data communication.
Mobile Observer (mobile data collector): A mobile observer is a special mobile entity that can be deployed into a sensor network, whose sole purpose is to move around and collect data from the sensor nodes within the network. Data are buffered in sensor nodes until they can be downloaded into a mobile observer.
Mobile Sink: In a sensor network with mobile sink, sinks are capable of movement and sensor nodes relay data to the mobile sinks with little or no buffering.
Coverage Hole: A coverage hole in a sensor network can be defined as the area within the network that is not covered by any sensor node, resulting in lack of monitoring of that area.
Hierarchical Sensor Network Architectures: In a sensor networks with hierarchical architecture, sensor nodes are partitioned into clusters and one node takes responsibility of being the cluster head of a cluster. Each sensor sends data to the respective cluster head, which is responsible to send/route data towards the sink
Base Station: The data generated by the sensor nodes in a sensor network is collected to one or more central points, known as Base Station(s). A base station is also referred to as access point or sink.
Multi-Hop Routing Scheme: In the multi-hop routing scheme, nodes located further away from the base station use some intermediate nodes to forward the data to the base station.