"ARCHITECTURE OF WIRELESS SENSOR NETWROKWireless sensor networks (WSNs) consist of small, resource constrained sensor nodes applied tomonitoring physical phenomena, such as temperature, acceleration, lighting level, humidity,pressure, movement, etc. Typically, a sensor node includes four basic components: ? Energy source.? Sensing block.? Processing block.? Communication block.A typical architecture of a wireless sensor network node is presented in Figure 1.EXPLANATION:? Energy source:The energy source provides the node with the energy for the other blocks and it is typicallyassumed to be either a battery or a capacitor. These non-renewable energy sources have largelymotivated the WSN research carried out in recent years. The energy source may also berenewable, either mains powered or based on energy harvesting. The former is a commonassumption in wireless sensor networks for the sink or the controller nodes, as they are oftencentral devices for the operation of the network and will exhaust a limited energy source much 6 faster than a typical sensor node would. The latter is based on the ability to collect energy fromthe surrounding environment, e.g. by solar energy or mechanical vibrations. The renewableenergy in this case is limited by the ability to harvest energy and a power budget for operation iscreated.? Sensing block:The sensing block is the main driver of a sensor node, as it contains the sensors withwhich the node gains information of its surroundings. The energy consumption of the sensingblock depends heavily on the nature of the sensors, as well as how often samples are taken. Fortemperature sensors, the energy consumed is rather low, since the sampling interval may be ofthe order of hours, whereas for wireless multimedia sensor networks, when active, samples withhigh energy consumption have to be taken at least once per second in cases such as using chargecoupled device (CCD) cameras. In the latter case, the sampling may become the most energyconsuming activity of the sensor node.? Processing block:The processing block typically holds the micro-controller unit (MCU) of the node, aswell as the memory to support the possible operating system, data storage and processing, andthe communication protocol stack. It has been shown that computation consumes much lessenergy than wireless communication [132]. Therefore, the research community has largelyconcentrated on reducing the communication requirements by data processing and increasingcommunication efficiency by fine-tuning the protocol stack at the expense of computation. Themedium access control (MAC) is especially important for improving communication energy- efficiency, as it is the protocol entity directly controlling the communication block.? Communication block:The communication block consists of the wireless transceiver and it defines many of thecommunications constraints apart from the actual energy source. Data rate, legislated duty cycle(according to the frequency band used), transmit:receive:sleep energy consumption ratios, biterror ratio, and communications range among others are the characteristics that influence MAC 7 layer protocol design. Strictly speaking, the communication block should not influence higherlayer protocol design. 8 "