Save Energy - The Buzz Word in Sensor Network

WSN(Wireless Sensor Network)applications use sensor nodes which are mostly powered using a 3 V battery.Sensor networks are supposed to last at least for six months ,hence the available energy needs to be used very prudently. This is similar to usage of available financial sources during recession,every penny needs to be spent after much thought. Lot of research has been done and is being done in the field of optimal usage of energy in sensor networks.

I will be briefing about some of these energy saving methods in WSN. All different procedures and methods given here are from papers or patents which I have come across.

Dynamic Power Management(DPM) specifies five modes of operations for each sensor node.Each mode consumes different amount of power. The mode of operation of sensor node varies from being fully active to deep sleep state. Maximum energy is consumed in fully active state and least is consumed in deep sleep state. A method called as Dynamic Voltage Scaling (DVS) is also proposed , the operating frequency and operating voltage is varied according to the sensor node's mode. The components in sensor node such as transceiver , processor, sensors are dynamically switched off or put to sleep in different modes. By these dynamic changes in the sensor nodes energy conservation achieved in the sensor nodes. The table below summarizes the different sates of the sensor node.

State Processor Memory Sensor Radio
S0 Active Active On Tx, Rx
S1 Idle Sleep On Rx
S2 Sleep Sleep On Rx
S3 Sleep Sleep On Off
S4 Sleep Sleep Off Off

After DPM lets see how a protocol called "LEACH" to uses energy optimally. LEACH is one of the well known protocol in WSN. In this protocol data aggregation is done at Cluster heads before further forwarding data , this in turn reduces the amount of data being transferred and saves lot of energy, which would be otherwise consumed for transmission and reception of the un-aggregated data. In WSN data transmission or data reception is the most energy consuming activity compared any other activity so data aggregation at various levels helps in conservation of energy. LEACH also saves energy by making all nodes(other than cluster head) go to sleep when they do not having anything to communicate. All nodes are allocated different time slots to communicate by the cluster head hence the nodes are awake only during their time slot. The figure below shows Cluster Heads, normal nodes and Sink in LEACH protocol.



In paper titled "Power Efficient Organization of Wireless Sensor Network" authors try to use only the optimal number of sensor nodes from all available sensor nodes to cover a given area. By this unnecessary sensor nodes are made to sleep and unnecessary redundant data is avoided and also one can rotate between sleeping and active sensor nodes.

In paper tilted " Power efficient Topologies for Wireless Sensor Networks" authors propose a method to find a sensor network topology which consumes least power to cover given area, in other words the sensor nodes are placed such that they cover the given area by utilizing minimum number of sensor nodes.

There are some more protocols such as S-MAC, STEM, SMACS which try use to available energy in best possible way , if interested one can find them in Google. As I told earlier the efficient usage of energy in one of the most important and one of most researched topics in Wireless Sensor Network, hence one can find many articles, papers and patents on the same.

Wireless Sensor Networks - A New Network Paradigm

Welcome to the world of sensor networks , where small is big and unity is strength. In this fascinating domain of sensors one needs to think at micro level rather than at the macro. The designer of this network needs to think one hundred times before he decides to introduce communication of few bytes of data. Even a three volt battery is considered to be considerably huge source of energy to a sensor node., and the sensor node itself is as big as a dollar. Wireless sensor network collects data utilizing a group of tiny sensor nodes and here the data as a whole is more important than the individual sensor node collecting the . Hence it is appropriate to say that "In sensor network small is big and unity is strength ".

Wireless sensor network consists of a group of tiny sensor nodes, distributed in a wide geographic area, forming an ad-hoc network, collecting and conveying information regarding the area under surveillance. The data collected by these sensor nodes is aggregated and analyzed at a more capable node called as sink or gateway or base station. Wireless sensor network combine simple wireless communication,minimal computation facilities and some sort of sensing of the physical environment and this leads to a new paradigm of networks that can be deeply embedded in our physical environment,fueled by the low cost and wireless communication facilities.

Wireless sensor networks have many wide variety of applications . Some of the examples being

• Military Applications : In battlefield sensor network can be used to monitor enemy movements, to monitor various defense equipments. to detect biological and chemical attacks and to evaluate damage caused by the war.


• Environmental Applications : Sensor networks can be used, to monitor habitats, to monitor forest fire, to monitor the pollution caused, to monitor weather conditions etc...


• Traffic Surveillance: WSN can be used to monitor traffic on land or water or in air.


• Industrial Applications: WSN can be used to monitor equipments, processes, conditions inside a factory and also can be used to detect leakage of gas/chemicals etc..


• Home Applications: The applications at home that can be built using WSN range form home automation to intruder detection.


• Health Applications: Another branch called Body Area Networks is carved specifically to concentrate on health applications using senor network.


• Precision Agriculture: Examples for this include monitoring of fertility and humidity in soil using sensor network.

Each wireless sensor network needs to be designed according to the applications for which it will be used. The number of nodes present in the network, the redundancy required, the precision necessary, the periodicity of collection of data all depends on the application to which the sensor network is used.Wireless sensor networks are self organizing, autonomous, fault tolerant,scalable, long lasting and are able to survive with less energy consumption and are generally low cost.

A typical sensor node in a WSN is made up of, a micro controller such as MSP430FG4618 which is specially designed for low power consumption , a transceiver such as CC2420 which works in ISM band, sufficient memory,power supply and sensors such as temperature sensor, photo sensor ,IR sensor .

If we see from software perspective then a sensor node has necessary drivers, a tiny operating system such as TinyOS, MAC layer modules such as 802.15.4 or S-MAC or STEM , Network Layer modules such as Zigbee Specification or LEACH or gossiping or SPIN , Security modules for data confidentiality, authentication , data integrity and Applications such as data aggregation apps, in node processing apps, senor collaboration module .

Sensor network was considered among top ten technologies in the world. Sensor network is also one of researched topic in many leading universities and companies. Research mainly concentrates on reducing power consumption, lighter protocols, self organization , autonomous operation , lighter OS. Time is not far when many of our day today applications will be built and powered using sensor networks.

Note: I have refered several books, articles , IEEE papers to write all the artciles on sensor networks . I have tried to explain my understanding of sensor networks through these articles in my blog. In some cases I have used the orginal author's examples and diagrams. I am thankful to all the orginal authors.