  Machine-to-machine (M2M) communication's severe power limitations challenge
the interconnectivity, access management, and reliable communication of data.
In densely deployed M2M networks, controlling and aggregating the generated
data is critical. We propose an energy efficient data aggregation scheme for a
hierarchical M2M network. We develop a coverage probability-based optimal data
aggregation scheme for M2M devices to minimize the average total energy
expenditure per unit area per unit time or simply the {\em energy density} of
an M2M communication network. Our analysis exposes the key tradeoffs between
the energy density of the M2M network and the coverage characteristics for
successive and parallel transmission schemes that can be either half-duplex or
full-duplex. Comparing the rate and energy performances of the transmission
models, we observe that successive mode and half-duplex parallel mode have
better coverage characteristics compared to full-duplex parallel scheme.
Simulation results show that the uplink coverage characteristics dominate the
trend of the energy consumption for both successive and parallel schemes.
