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This paper proposes isochronous-MAC (I-MAC), which utilizes low-frequency radio waves time synchronization for sensor networks. Using IMAC, based on the low power listening (LPL), all sensor nodes wake and listen channel periodically and synchronously. Since a sender can easily predict wakeup time of an intended receiver, it can shorten the length of preamble to make the receiver prepare for reception of the following data packet. This saves power consumption for the sender to rendezvous with the receiver. In the paper, we use an analytical model to investigate the impact of the data transmission frequency, the number of neighboring nodes, the wakeup period, the clock drift, and the time-synchronization frequency on the power consumption for consideration of the power overhead to perform the time synchronization. Those results demonstrate that I-MAC allows determination of any arbitrary wakeup period without much difficulty, whereas LPL requires a much more careful setting of the wakeup period because its optimum wakeup period is sensitive to the frequency of data transmission as well as to the number of neighboring nodes. Therefore, IMAC has a great potential to reduce the power consumption in most situations compared with LPL, in spite of the overhead to perform time synchronization.