Friday, March 5, 2010

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.

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