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10.5 Case Study > 10.5.1 Modeling Search-Based Physical Attacks - Pg. 259

CHAPTER 10 Defending Against Physical Attacks in Wireless Sensor Networks from the attacker's antenna, vibration due to the attacker's motion, etc.). A simple and straight- forward defense strategy against physical attacks here is for sensors to detect the attacker and prop- agate attacker information further to other sen- sors. Sensors within an area that receive attacker information simply shut down. A sensor is shut down (neither senses nor transmits) when it turns itself off completely. Thus, attackers will not be able to detect sensor signals, and the sensors avoid destruction. The conflict here is that the conservativeness of the above strategy compro- mises coverage and connectivity in a portion of the network. When one considers tracking sys- tems, there is an inherent conflict of sensing an intruder, while simultaneously hiding from a physical attacker. This conflict becomes more pronounced when the intruder (to be tracked) is itself a physical attacker. In this situation, when sensors shut down, they will not be able to sense 259 hand, active signals include communication mes- sages, beacons, and query messages that are part of normal network communication paradigms. These two are quite different from the perspec- tive of attacker detection. Passive signals are very small in range, and their detection can enable the attacker to accurately detect their source (the sen- sor emitting the passive signal). Active signals can propagate longer distance, but the attacker can only isolate the source of an active signal within an area. In case the attacker detects multiple sen- sors, if the attacker is equipped with memory, the attacker can store the locations of multiple sensors that it detects. Thus, if we use R ps and R as to denote the maximal distances from where the attacker can detect passive and active signals, respectively, R ps is smaller than R as . The ability of the attacker to detect a sensor also depends on the state of the sensor. A sen- sor is dead if it has been physically destroyed by