• Typical multi-hop ( ) wireless sensor network architecture.• A wireless sensor network (WSN) consists of spatially ( ) distributed autonomous ( ) sensors to monitor physical or environmental conditions, such as temperature, sound, pressure, etc. and to cooperatively pass their data through the network to a main location.• The more modern networks are bi-directional ( ), also enabling control of sensor activity. -- -
• The WSN is built of "nodes" – from a few to several hundreds or even thousands.• Each node is connected to one (or sometimes several) sensors.
• A sensor node, also known as a mote (chiefly in North America), Sensor nodes can be imagined as small computers, extremely basic in terms of their interfaces and their components. They usually consist of a processing unit with limited computational power and limited memory, sensors or MEMS (micro-electro-mechanical- System)(including specific conditioning circuitry), a communication device (usually radio transceivers or alternatively optical), and a power source usually in the form of a battery.• A mote is a node but a node is not always a mote.
The microcontroller performs tasks,processes data and controls thefunctionality of other components inthe sensor node.Digital Signal Processors may bechosen for broadband wirelesscommunication applications, but inWireless Sensor Networks the wirelesscommunication is oftenmodest( ): i.e., simpler, easier toprocess modulation and the signalprocessing tasks of actual sensing ofdata is less complicated.
Sensor nodes often make use ofISM band, (Industrial, Scientificand Medical) which gives freeradio, spectrum allocation andglobal availability. WSNs tend touse license-free communicationfrequencies: 173, 433, 868, and915 MHz; and 2.4 GHz. Thefunctionality of both transmitterand receiver are combined into asingle device known as atransceiver. Transceivers often lackunique identifiers. The operationalstates are transmit, receive, idle,and sleep. Current generationtransceivers have built-in statemachines that perform someoperations automatically.
From an energy perspective ( ),the most relevant kinds of memoryare the on-chip memory of amicrocontroller and Flash memory—off-chip RAM is rarely, if ever, used.Flash memories are used due to theircost and storage capacity. Memoryrequirements are very muchapplication dependent. Twocategories of memory based on thepurpose of storage are: user memoryused for storing application related orpersonal data, and program memoryused for programming the device.Program memory also containsidentification data of the device ifpresent.
The sensor node consumes ( ) power for sensing, communicating and data processing. More energy is required for data communication thanany other process. The energy cost of transmitting 1 Kb adistance of 100 meters (330 ft) is approximately the same as thatused for the execution of 3 million instructions by a 100 millioninstructions per second/W processor. Power is stored either inbatteries or capacitors. Batteries, both rechargeable and non-rechargeable, are the main source of power supply for sensornodes. They are also classified according to electrochemicalmaterial used for the electrodes such as NiCd (nickel-cadmium),NiZn (nickel-zinc), NiMH (nickel-metal hydride), and lithium-ion.Current sensors are able to renew their energy from solarsources, temperature differences, or vibration . As wirelesssensor nodes are typically very small electronic devices, they canonly be equipped with a limited power source of less than 0.5-2ampere-hour and 1.2-3.7 volts.
Sensors are hardware devicesthat produce a measurableresponse ( ) to a change ina physical condition liketemperature or pressure.Sensors measure physical dataof the parameter ( )to be monitored. Thecontinual analog signalproduced by the sensors isdigitized by an analog-to-digital converter and sent tocontrollers for furtherprocessing.
• Power consumption constrains( ) for nodes using batteries or energy harvesting• Ability to cope ( )with node failures• Mobility( ) of nodes• Heterogeneity(the quality of being diverse( ) and not comparable in kind) of nodes• Scalability to large scale of deployment ( )• Ability to withstand harsh( ) environmental conditions• Ease of use
The development of wireless sensor networkswas motivated by military applications suchas battlefield surveillance; today suchnetworks are used in many industrial andconsumer applications, such as industrialprocess monitoring and control, machinehealth monitoring, and so on.