Wireless Ad Hoc and Sensor Networks

36 Wireless Ad Hoc and Sensor Networksand guerrilla warfare countermeasures require distributed networks ofsensors that can be deployed using aircraft and have self-organizing capabilities.In such applications, running wires or cabling is usually impractical.A sensor network is required that is fast and easy to install and maintain.The challenges in the hierarchy of detecting the relevant quantities, monitoring,collecting and aggregating the data, assessing and evaluating theinformation, formulating meaningful user displays, and performing decisionmakingand alarm functions are enormous. The information needed bysmart environments is provided by distributed WSNs, which are responsiblefor sensing as well as for the first stages of the processing hierarchy.The study of WSNs is challenging in that it requires an enormousbreadth of knowledge from a variety of disciplines. In this chapter, weoutline communication networks, WSNs and smart sensors, commerciallyavailable wireless sensor systems, self-organization, and finally, some conceptsfor home automation.Routing tables for distributed networks increase exponentially as nodesare added. An n × m mesh network has nm links, and there are multiplepaths from each source to each destination. Hierarchical network structuressimplify routing, and also are amenable to distributed signal processing anddecision making, because some processing can be done at each hierarchicallayer. It is important to note that a fully connected network has NP-hardcomplexity, while imposing routing protocols by restricting the allowedpaths to obtain a reentrant flow topology results in polynomial complexity.Such streamlined protocols are natural for hierarchical networks.As nodes are added, the number of links increases exponentially. Thismakes for NP-complexity problems in routing and failure recovery. Tosimplify network structure, we can use hierarchical clustering techniques.The hierarchical structure must be consistent, that is, it must have the samestructure at each level. Hierarchical structure is quite common in WSNs,where geographically located nodes are clustered, and a cluster head iselected during self-organization (see Chapter 8). The data from the nodesare aggregated at each cluster head and transmitted to a central base stationover the multihop network constructed from different cluster heads.There are many fundamental differences between ad hoc and sensornetworks. Ad hoc networks may not have stringent memory, power andprocessing constraints as much as a sensor network. Also, WSN may nothave an IP address, whereas an ad hoc network node has an IP address.Typically, an ad hoc network has mobility, whereas a WSN can havestationary or mobile nodes. A WSN has to transmit large quantities ofdata and, therefore, hierarchical structure is imposed and data aggregationis performed. By contrast, in MANET, data is not always abundant, andit is information that is transmitted. Moreover, the density of nodes in aWSN is significantly higher compared to an ad hoc network. Sensor nodesare prone to failure in comparison with an ad hoc network. Finally, a