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Wireless Ad Hoc and Sensor Networks

Wireless Ad Hoc and Sensor Networks

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32 <strong>Wireless</strong> <strong>Ad</strong> <strong>Hoc</strong> <strong>and</strong> <strong>Sensor</strong> <strong>Networks</strong>for personnel/vehicle surveillance), there is a focus on increasing thelifetimes of sensor nodes through power generation, conservation, <strong>and</strong>management. Current research is in designing small MEMS (microelectromechanicalsystems), RF components for transceivers, includingcapacitors, inductors, etc. The limiting factor now is in fabricating microsizedinductors. Another thrust is in designing MEMS power generatorsusing technologies including solar, vibration (electromagnetic <strong>and</strong> electrostatic),thermal, etc.Meanwhile, software power management techniques can greatlydecrease the power consumed by RF sensor nodes. TDMA is especiallyuseful for power conservation, because a node can power down or “sleep”between its assigned time slots, waking up in time to receive <strong>and</strong> transmitmessages.The required transmission power increases as the square of the distancebetween source <strong>and</strong> destination. Therefore, multiple short message transmissionhops require less power than one long hop. In fact, if the distancebetween source <strong>and</strong> destination is R, the power required for single-hoptransmission is proportional to R 2 . If nodes between source <strong>and</strong> destinationare taken advantage of to transmit n short hops instead, the powerrequired by each node is proportional to R 2 /n 2 . This is a strong argumentin favor of distributed networks with multiple nodes, that is, nets of themesh variety.A current topic of research is active transmission power control,whereby each node cooperates with all other nodes in selecting its individualtransmission power level. This is a decentralized feedback controlproblem. Congestion is increased if any node uses too much power, buteach node must select a large-enough transmission range, so that thenetwork remains connected. For n nodes r<strong>and</strong>omly distributed in a disk,the network is asymptotically connected with probability one, if the transmissionrange r of all nodes is selected using (Kumar 2001)r ≥ log n+γ( n)π n(1.12)where,γ( n)is a function that goes to infinity as n becomes large.1.4.7 Network SimulatorNS-2 (Fall <strong>and</strong> Varadhan 2002) is a discrete event simulator targeted atnetworking research. NS provides substantial support for simulation ofwired <strong>and</strong> wireless networks, satellite networks, TCP, <strong>and</strong> routing. NSbegan as a variant of the REAL network simulator in 1989 <strong>and</strong> hasevolved substantially over the past few years. In 1995, NS development

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