Machine-to-Machine (M2M) networks must be energy-ef¿cient to operate autonomously for years, or even decades. In this paper, we consider a synchronized duty-cycled M2M network composed of a huge number of dormant devices that periodically wake up to transmit data to a coordinator. We propose the use of Distributed Queuing (DQ) tree-splitting algorithms to optimize the shared access to the channel among the high number of devices, in order to improve the energy ef¿ciency and thus extend the network lifetime. We evaluate the energy performance of DQ access in this kind of dense M2M networks, and we compare it to traditional access schemes based on variations of Frame Slotted-ALOHA (FSA) and the Contention Tree Algorithm (CTA). Computer-based simulations show that DQ can reduce the energy consumption in more than a 50% with respect to FSA and CTA. Results show that there is an optimum number of contention slots which maximizes the energy ef¿ciency of DQ regardless of the number of devices. The performance evaluation presented in this paper also compares the energy consumption of DQ using low power Wi-Fi and IEEE 802.15.4 devices

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