Zhenghao Zhang Ming Ma Yuanyuan Yang Energy-Efficient Multihop Polling in Clusters of Two-Layered Heterogeneous Sensor Networks. 231-245 2008 57 IEEE Trans. Computers 2 http://doi.ieeecomputersociety.org/10.1109/TC.2007.70774 db/journals/tc/tc57.html#ZhangMY08