A wireless sensor node, also known as mote, is a device in Wireless Sensor Network, which consists of inbuilt sensors for monitoring various environmental as well as physical conditions. These devices are capable of sensing information, processing that sensed information and sending that processed data to neighbor nodes or some remote station(s).
Structure of a Wireless Sensor Node (Mote)
A wireless sensor node, also called mote, will consist of five subsystems, which are listed below.
- Sensing Subsystem
- Processing Subsystem
- Memory Subsystem (optional)
- Communication Subsystem
- Power Subsystem
Processing Subsystem- It is the CPU of a wireless sensor node. It is also known as controller unit. The most commonly used controller in wireless sensor node is the micro-controller because its features like small size, low power consumption and ease of programming. And, most of the microcontrollers have an embedded Analog to Digital Converter(ADC) in them.Other alternatives are micro-processor, DSP(Digital Signal Processor) and FPGA(Field Programmable Gate Array). Microprocessors are rarely used in motes and is not considered as desirable choice for making the processor units of nodes. DSPs are mostly used when the nodes require to do some high processing like processing of captured images or complex processing of wireless signals. FPGAs are sometimes preferred as the control units in wireless sensor node due to it’s unique features of reprogramming and reconfiguration.
Communication Subsystem- It consists of the wireless antenna. It which makes use of RF(Radio Frequency), Optical Communication or Infrared waves as the transmission media. The wireless radio unit, which is capable of both transmitting and receiving wireless signals, is called as transceiver. The transceiver often operates in the ISM band(the radio band reserved for Industrial, Scientific and Medical purposes). The various communication technologies used in WSN are ZigBee(IEEE 802.15.4), WiFi, WiMax and Bluetooth. The following table gives a quick view on these connectivity technologies.
Sensing Subsystem- This unit consists of sensors, which are usually tiny devices which are capable producing some measurable response(often a change in voltage). These response is usually fed to an Analog to Digital Converter (ADC). The digitized signal is then sent to the processing unit for further processing. The sensors can be sometimes big in size like a rain gauge or a geophone which measures vibrations.
Power Subsystem- Batteries are the main source of power supply for a wireless sensor node. Due to the restriction in size, batteries used too have to be in small size, which implies that the lifetime of a wireless sensor node will be relatively low. Some motes are powered by solar source and other renewable forms of energy.