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Fast development from last few years in wireless senor network has create great impact on researcher and motivated by various real time issues. There are huge numbers of sensors deploy in network which sense the neighbouring information and processed the gathered data to the base station, processing speed, storage and communication bandwidth. Because of these constraints fast transmission, fault tolerant, load balancing mechanism and energy efficient routing are critical requirements in WSNs to increase the network lifetime. By considering this limitation it is necessary to design novel energy aware and fault tolerant routing protocol for heterogeneous sensor network. In this survey paper various existing techniques are studies and list out the research challenges.
Routing, Energy Efficient, Fault Tolerant
Efficient Energy utilization and load balancing in densely deploy heterogeneous sensor network is a big challenge . Wireless Sensor Network (WSN) is consisting small sensors which is having very low battery power. These sensor nodes sense the information, performed data fusion at sink node and sent combined information to a base station (BS). Sensor is having limited power capacity and the battery replacement is not easily possible. Hence for long life functioning of wireless sensor network it is necessary to design energy efficient routing protocol. To achieve this goal, we required to design novel techniques which will helps to minimize total energy consumption and to balance the load of gateway/relay node or cluster head (CH). One important contribution to reduce energy consumption is to design a sensor that will operate on lower technologies and will consume less energy. Designing low power chip is the good technique but if sensor components are unable to operate properly then the complete sensor network will break. Wireless sensor network must work at least for a targeted area for longer period of time. Because of hardware failure sensor node might be damage or run away from transmission range, since the wireless communication between two nodes can be permanently interrupted. It is necessary to tolerant such faults otherwise it will affect complete network. To support long life of sensor, energy efficient operation is a key technique. Enhanced the network life time of a WSN becomes very challenging and this issue need to be consider.
In order to achieve the energy efficient operation in sensor network the main focus has to give on proper distribution of load to cluster head and balance the energy consumption of individual nodes to the highest possible extent. The existing techniques used to achieve these objectives are based on parameter such as load balancing, data aggregation, change the transition state of sensor (active and sleep mode) and efficient cluster formation with cluster head rotation as per balance energy  .
Various type of clustering technique is used for energy efficient routings. Sensor node are grouped in to cluster, each sensor node belongs to only one cluster. Each cluster has a Cluster Head (CH) which will collect the information from its sensor node, aggregate them and forwarded the sensing information to the base station (BS). Being a cluster head means taking over additional task. Hence the battery of cluster head will exhausted very soon. Therefore many researchers  have proposed that CH responsibility should be rotate. Clusters formation algorithm cannot do only once but it must execute after periodic time so that the load of cluster head will share and helps to save the energy. For selecting the cycle of cluster formation and cluster head we have to consider the average data transmission and reception speed of node, battery draining rate and residual energy etc. Many clustering techniques have been implemented so far. Authors  proposed the method the gateway or relay called as a cluster head. The cluster head provided a more energy and higher communication range than other sensor. However the gateways are also power constraints.
Another issue with the WSN is data aggregation. The huge number of sensor nodes gathered the surrounding information. Each sensor node required sufficient energy to transmit and receive the data generated by surrounding sensors. Mostly in densely distributed and heterogeneous network, the life of sensors is very short because of heavy load. In order to resolve these issues, we need energy-efficient method to aggregate huge data and load balancing mechanism to increase the network life time.
There are few documents which worked to tackle the challenges of distributed sensor network related to efficient load balancing and fault tolerant with less energy consumption.
We have presented a comprehensive analysis of existing techniques based on energy aware load balancing for WSNs. All analysed techniques are focus on performance parameter such as network lifetime, efficient data routing, quality of service with data accuracy and less energy consumption for homogeneous as well as heterogeneous network. It is observed that by considering the limitation of WSN it is very challenging to upgrade the quality of service of WSNs with respect to less energy consumption. This inspires to propose a novel energy efficient routing technique which will give more focus on dynamic energy management in case of fault tolerant in heterogeneous networks.
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