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As wireless sensor networks are used more widely, the design of medium access control protocol will be largely various. The paper reviews Sensor MAC (S-MAC), which is the basic MAC protocol for wireless sensor networks, and points its advantages and disadvantages. Several improving S-MAC protocols that focus on different problems in S-MAC are introduced. The Fragment Indication Message Passing (FIMP), a new energy-efficient message passing approach, is proposed to solve the turnaround consumption problem in S-MAC. It exploits a set of special binary codes to indicate each fragment. Thus, the receiver can easily check the failed fragments. Moreover two ACK packets are used to reduce the possible of retransmission.