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Thus energy represents one of the greater constraints in designing algorithms for mobile devices [37]. It is interesting to notice that energy conservation is related to all network layers [38, 39], including MAC [40], routing [41], and application [42] protocols for MANETs. Power-aware protocols are often based on the following techniques: active and standby modes switching, power setting, and retransmission avoidance. Mode switching between active and standby aims to avoid spending energy during system idle periods. Furthermore, power transmission must be set to the minimum level for the correct message reception at the destination. Retransmissions should be avoided since they waste energy by sending messages that will not be processed by the destination nodes. Power awareness is achieved using power management or power control mechanisms [40]. A power management mechanism alternates the state of a mobile device wake and sleep periods. Furthermore, the wireless data interface consumes nearly the same amount of energy in the receive, transmit, and idle states, whereas in the sleep state, a data interface cannot transmit or receive, and thus its power consumption is highly reduced [43]. However, it is not possible to have a mobile device most of the time in power-saving mode (sleep state), which will extend its battery lifetime but comprise the network lifetime, because ad hoc networks rely on cooperative efforts among participating nodes to deliver messages. A possible strategy is to allow the network data interface to enter a power-saving mode while trying to achieve a minimum impact to the process of sending and receiving messages. In general, these algorithms depend on data collected from the physical and MAC layers. For instance, an algorithm can monitor the transmission error rates to avoid useless transmissions when the channel noise reduces the probability of a successful transmission [44, 45]. At the MAC layer, an algorithm can save energy by determining intervals during which the network data interface does not need to be listening [46]. This is the case, for instance, whenever a node transmits a message and the other nodes within the same interference and carrier sensing range must remain silent. During this period, these nodes can sleep with little or no impact on system behavior. Related to this strategy is to have a density control algorithm controlling the operational mode of mobile devices so only those needed to forward the data traf c are awake and the overall network lifetime is optimized [47]. The strategies of controlling the transmission power and the node density in a MANET must be performed very carefully. Reducing the transmission power and keeping the node density to a minimum level may lead to a smaller number of available data communication links among nodes and, hence, a lower connectivity that can increase the number of messages not transmitted. 1.3.7 Network Security Mobile ad hoc networks are generally more prone to physical security threats than are xed-wired networks [48, 49]. The broadcast nature of the wireless channels, the absence of a xed infrastructure, the dynamic network topology, the collaborative multihop communication among nodes, and the self-organizing characteristic of the network increase the vulnerabilities of a mobile ad hoc network.
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The starting point to provide a proper security solution for a mobile ad hoc network is to understand the possible forms an attack can happen. In a MANET, a security problem may happen at any network layer and include: data integrity attacks, by accessing, modifying, or injecting traf c; denial-of-service attacks; ow-disruption attacks, by delaying, dropping, or corrupting data passing through, but leaving routing traf c unmodi ed; passive eavesdropping; resource depletion attacks, by sending data with the objective of congesting a network or draining batteries; signaling attacks, by injecting erroneous routing information to divert network traf c, or making routing inef cient; and stolen device attacks. Given the variety of possible attacks to a mobile ad hoc network, different solutions have been proposed to address them [49]. The rst step is to protect the wireless network infrastructure against malicious attacks. Digital signatures can be used to authenticate a message and prevent attackers from injecting erroneous routing information and data traf c inside the network [50]. This scheme requires a certi cation authority function to manage the private public keys and to distribute keys via certi cates, which needs to be distributed over multiple nodes in the MANET [51]. A strategy is to use a threshold cryptographic model to distribute trust among the MANET nodes [52, 53]. This model tolerates a threshold t of corruptions/collusions in the network, whereas it allows any set of t + 1 nodes to make distributed decisions such as regarding admission of new nodes to the network. These proposals require that each node must receive a certi cate and a secret share in a distributed manner. However, as long as each node is able to obtain an updated VSS information (Feldman s Veri able Secret Sharing mechanism [54]), there is no need for node-speci c certi cates, and it is possible to create new secret shares in a distributed manner [55]. In many mobile ad hoc network applications, such as emergency disaster relief and information sharing in a meeting, it is important to guard against attacks such as malicious routing misdirection [56]. The problem is that ad hoc routing protocols were designed to trust all participants, are cooperative by nature, and depend on neighboring nodes to route packets. This naive trust model allows malicious nodes to attack a MANET by inserting erroneous routing updates, replaying old messages, changing routing updates, or advertising incorrect routing information. Furthermore, a mobile ad hoc environment makes the detection of these problems more dif cult [57]. Some of the proposed solutions to the problem of secure routing in a MANET involve the use of a pre-deployed security infrastructure [58], concealing the network topology or structure as in the Zone Routing Protocol [59], and introducing mechanisms in the network to mitigate routing misbehavior such as the SAR (Security-Aware Ad Hoc Routing) technique [60] to add security attributes to the route discovery [61].
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1.4 APPLICATIONS Mobile ad hoc networks have been employed in scenarios where an infrastructure is unavailable, the cost to deploy a wired networking is not worth it, or there is no time to set up a xed infrastructure. In all these cases, there is often a need for collaborative computing and communication among the mobile users who typically work
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