Wireless Ad Hoc and Sensor Networks

38 Wireless Ad Hoc and Sensor NetworksTopology maintenance in a WSN is a challenging task due to the sheernumbers of inaccessible and unattended sensor nodes, which are proneto failures. Due to their sheer numbers, it may not be possible to placethe nodes according to a carefully engineered deployment plan; theschemes for initial deployment must eliminate the need for any preorganizationand preplanning and should promote self-organization andfault tolerance.Because a WSN employs a multihop routing protocol for transmittinginformation at the cluster head level, communicating nodes are linked bya wireless medium formed by either radio, infrared, or optical media.Wireless media uses a standard ISM frequency band. Besides radio links,nodes use infrared communication because it is license-free and robust tointerference from electrical devices.A sensor node expends more energy in communication, which normallyinvolves transmission of data and reception. In Shih et al. (2001), formulationof the radio power consumption ( ) isP cP = N ( P ( T + T ) + P T ) + N ( P ( R + R )c T T on st out on R R on st(1.16)where P T , P R is the power consumed by the transmitter/receiver; P out isthe output power of the transmitter; T on /R on is the transmitter/receiver ontime, T st /R st is the transmitter/receiver start-up time, and NT / R is thenumber of times transmitter/receiver is switched on per unit time, whichdepends on the task and MAC scheme used. The term T on can be furtherwritten as L/ R,where L is the packet size and R is the data rate.Energy expended in data processing is much less compared to datacommunication. Hence, data aggregation by local processing is veryimportant to minimize power consumption. Research is being done tominimize energy expended during processing as well. Computing energyconsumed, during data processing, is a difficult task because it is dependentupon the processor clock cycle, number of memory operations, andso on. However the power consumption in data processing is given by(Shih et al. 2001)2P = CV f + V I ed dd dd 0Vddζ′VT(1.17)where C is the total switching capacitance, is the voltage swing, andf is the switching frequency.Now the sensor network communication architecture is coveredbriefly. Data from the sensor node is routed to a base station via amultihop path. The protocol stack for communication used by the basestation and all sensor nodes is depicted in Figure 1.7 (Akyildiz et al. 2002).V dd