APPLICATION DRIVEN SECURITY IN WIRELESS SENSOR NETWORKS -thesis
While there is no universal de¯nition of life, it can be argued that one of the charac- teristics that can de¯ne any living being is the ability to respond to changes in their environment and inside themselves . The environment is full of physical events, which can be felt using our senses. In fact, humans are known to have sensory cells that fall into the following categories: photoreceptors (for light), mechanore- ceptors (for touch, sound, and equilibrium), chemoreceptors (for smell and taste), thermoreceptors (for heat), and nociceptors (for pain). Through these senses, we can maintain our internal biological equilibrium (i.e. homeostasis), but also we can picture the physical world that surround us. In fact, we need to perceive the physical world in order to survive. There are some the basic needs that, as human beings, we need to ful¯ll: ¯nd food and water to stay alive, create a shelter where we can be safe from the weather and other menaces, communicate with other beings to socialize and perpetuate the species . But without our senses, it would not be possible to decide whether food is edible or not, to choose an adequate place to sleep, to participate in a social environment. Beyond survival, we also need our senses to satisfy more complex needs such as identity, creation, leisure, and freedom . Although we can live without some of our senses, it is di±cult to imagine life if we were completely cut o® from the physical environment.
In a certain degree, most computer systems do live that kind of life. Computer systems by themselves cannot perceive the physical world directly, like living beings do. they are capable of acquiring information from external and internal sources, such as storage devices, human-machine interfaces, operative systems, and so on. They can also communicate with other computer systems in order to exchange infor- mation. But the ability to perceive the physical world is not inherent to their nature: Computer systems are tightly tied to the realm of the abstract. The existence of sensor hardware tries to build a bridge between the abstract world and the physical world. Sensors are devices that can measure a physical quan- tity and convert it into a digital signal. For example, speci¯c sensors such as thermis- tors, hygrometers, and photodetectors, allow a computer to \feel” the temperature, humidity, and light intensity of its surroundings, respectively. Using these sensors, computer systems ranging from the simplest washing machines to the Large Hadron Collider (a particle accelerator located at the European Organization for Nuclear Research (CERN) ) can acquire and process information coming from the physical world. The ability to \feel” the environment using the sensors is usually an inte- gral part of the design of the computer system. Besides, the physical information
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