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Duty cycling is a common mechanism for achieving energy efficiency in WSNs. Over the past several years a number of researchers have proposed low-power duty-cycle MAC protocols, but the tradeoff between energy efficiency and latency has been remained a challenging issue. We propose the Express-MAC protocol (EX-MAC) which can guarantee the requirement of the end-to-end transmission latency while conserving energy in a wireless sensor network using asynchronous duty cycling. To achieve low latency operation, EX-MAC employs a wakeup time reservation that utilizes a short preamble scheme to reduce end-to-end latency and to minimize energy consumption. Also, EX-MAC supports multi-hop applications through a cross-layering API, and it provides convergecast packets with unidirectional interfaces to optimize performance and to support reconfiguration routing. In this paper, we first introduce WSNs and the problems in existing MAC protocols. Then we propose the EXMAC protocol for low end-to-end latency and power efficiency in WSNs. From simulation results, it is proven that EX-MAC is able to adjust its wakeup time reservation for convergecast traffic and to guarantee the requirement of endto- end latency.