In this paper, we consider a Machine-to-Machine (M2M) wireless network composed of a group of devices which duty cycle to save energy. These devices operate in low-power sleeping mode for most of the time and, periodically, they wake-up to listen to a poll packet transmitted by a data collector. Upon this broadcast poll, all devices try to get access to the uplink channel to transmit a burst of data packets. Therefore, the idle network is suddenly set into saturation conditions when all devices wake up and attempt to get access to the channel simultaneously. The Medium Access Control (MAC) protocol used to coordinate these transmissions has a strong influence on the energy efficiency of the network, and thus the lifetime of the devices. Frame Slotted ALOHA (FSA) has been identified in the literature as a simple yet efficient MAC protocol for such kind of communications. However, when the devices have to transmit more than one data packet per channel invocation, the Reservation Frame Slotted- ALOHA (RFSA) may be more efficient, since it guarantees the collision-free transmission of data for a device once it succeeds for the first time. Existing analyzes of both FSA and RFSA are valid for steady traffic conditions and not for abrupt idle-to-saturation traffic patterns. Motivated by this fact, in this paper we evaluate the energy efficiency of RFSA through computerbased simulations to show its better performance compared to FSA. Results show that RFSA can attain up to 48% energy gains compared to FSA, thus extending the lifetime of data-collection M2M networks.
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