Energy efficiency is of utmost importance in current generation resource-constrained but very useful wireless sensor networks. Energy efficiency is usually achieved by optimizing both the hardware and software of the network. This work focusses on reviewing software-based optimization for energy efficiency specifically in the network layer of the communication stack. The taxonomy of the routing protocols is derived based on (a) topology (b) reliability (c) network structure and (d) communication model. Due to the page constraints, only the topology and reliability-based routing protocols are reviewed from the energy efficiency perspective. Besides, the strength and weakness of the protocols are presented along with the ideas that can be used to improve the protocols further. Finally, the paper concludes with ideas to propel further research in this field.
