Energy is the most crucial parameter in Wireless Sensor Networks (WSNs) because the life time of network is directly related to residual energy of nodes. And nodes are mostly battery operated. Therefore, data aggregation is the best effective and smart way to maximize the lifetime of wireless sensor networks. Large volume of data is sent from source nodes to sink in WSN and data collection in WSN consumes large amount of energy. So, there are various data aggregation models designed for WSN, mainly cluster based and tree based.