Wireless Sensor Networks (WSN) have gained wide popularity and have increased tremendously in recent time due to growth in Micro-Electro-Mechanical Systems (MEMS) technology. WSN has the potentiality to connect the physical world with the virtual world by forming a network of sensor nodes. Energy saving of sensor nodes is a major design issue since sensor nodes are usually battery-operated devices. Sensor network‟s lifetime can be prolonged by minimizing the energy consumption at all layers of the network protocol stack starting from the physical to the application layer including cross-layer optimization. In this paper, clustering based routing protocols for WSNs have been discussed. In cluster-based routing, special nodes called cluster heads form a wireless backbone to the sink. Each cluster heads collects data from the sensors belonging to its cluster and forwards it to the sink. In heterogeneous networks, cluster heads have powerful energy devices in contrast to homogeneous networks where all nodes have uniform and limited resource energy. Hence it is essential to avoid quick depletion of cluster heads. This is done by the cluster head role rotation, i.e., each node works as a cluster head for a limited period of time. Energy saving in these approaches can be obtained by cluster formation, cluster-head election, data aggregation at the cluster-head nodes to reduce data redundancy and thus save energy.

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Routing protocols Energy efficiency WSN