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Due to the characteristics of the underwater acoustic channel,such as long propagation delay and low available bandwidth,the media access control (MAC) protocol designed for underwater acoustic sensor networks (UWASNs) is quite different from that for terrestrial wireless sensor networks.In this paper,we propose a MAC protocol for the UWASNs,named the funneling MAC (FMAC-U),which is a contention-based MAC protocol with a three-way handshake.The FMAC-U protocol uses an improved three-way handshake mechanism and code division multiple access (CDMA) based technology for request-to-send (RTS) signals transmitting to the sink in order that the sink can receive packets from multiple neighbors in a fixed order during each round of handshakes.The mechanism reduces the packet collisions and alleviates the funneling effect,especially alleviating the choke point of the UWASNs.Simulation results show that the proposed FMAC-U protocol achieves higher throughput,smaller packet drop ratio,lower end-to-end delay,and lower overhead of the control packet compared to the existing MAC protocols for UWASNs.