Date Added: Dec 2012
In recent years, the use of wireless sensor networks in real life applications has increased rapidly. The building block of WSNs comprises of hundreds or even thousands of sensor nodes. These sensor nodes are battery-powered with a limited lifetime and additional energy can be harvested from the external environment. They are directly connected to each other. The data exchange involves multi-hop communication. The route discovery and maintenance are governed by the routing protocols. This paper surveys the different hierarchical routing protocols used in the multi-hop communication in WSNs. Finally these papers summarize and conclude with the different hierarchical routing protocols.