Wireless Ad Hoc and Sensor Networks

386 Wireless Ad Hoc and Sensor NetworksNodeTHFaultSub-networkFIGURE 8.18Selection of temporary subnetwork head.CHs is calculated, the suitable nodes have to be identified, based on theCH_selection_ factor. This factor is calculated as the product of the energyavailable and the value of the sensed attribute from the event. For everynode in the subnetwork, the TH calculates the CH_selection_ factor asCH_selection_ factor = Er n × S(n), (8.3)where Er n is the energy available per node obtained from the batterymonitor, and S(n) is the sensed magnitude attribute from the event.An ideal CH should have both maximum energy available and thesensed attribute value from the event. However, the nodes near the faultyevent are more prone to failure because they will exhaust their energymore quickly by detecting the events subsequent to its occurrence. Also,as a node gets farther away from the event, the sensed attribute from theevent decreases, and the nodes will not have accurate information. Therefore,the CHs are selected such that the CH_selection_ factor is closer tothe signal strength value, which is the median of the collected signalstrength values, so that the CHs are distributed evenly. In other words,the TH arranges all the CH_selection_ factors in ascending order andchooses the node whose CH_selection_ factor is closer to the median ofthe collection as the CHs. The median is considered instead of takingthe average because it works better where extreme sensed attributes arepresent. It also ensures that the selected CHs are at an optimum distancefrom the event and also have maximum energy available. Once the CHsare selected, the TH broadcasts this information to all the nodes in thesubnetwork by sending a cluster head selection (CH_SELECT) packet.Subsequently, the TH becomes a regular node.All the sensors use the received signal strength indicator (RSSI) toidentify the radio frequency (RF) signal strength used for communication