Cross-layer energy model for beacon-enabled 802.15.4 networks

A transmitted frame in a wireless sensor network (WSN) may get corrupted due to erroneous bits in the frame, an event characterized at the physical (PHY) layer. The frame may also get dropped at medium access control (MAC) layer due to collision with other frames. As energy is wasted due to both, a combined cross-layer (PHY + MAC) model is needed for evaluating the energy for every successful transmission at frame/bit level. The model would also help in assessing how green a technique is, which might be proposed for PHY/ MAC layer of any WSN, by computing the improvement in energy efficiency. In this paper, we present a cross-layer energy model for communication in beacon-enabled 802.15.4 networks operating in star topology. IEEE 802.15.4 is the worldwide accepted WSN standard. Further, for status monitoring and event detection, which are the most common low data rate WSN applications, sensor nodes form a star topology where a central node sends periodic beacons to access data from all other nodes. Our model includes energy consumption for both source and destination node during the actual frame transmission as well as during the operations that precede frame transmission such as backoff and clear channel assessment (CCA). A two dimensional Markov model has been adopted for the purpose and parameters like average number of backoffs and average number of CCAs have been computed. Finally, the energy consumption per successful bit transmission is found by combining the collision probability in MAC layer and error probability in PHY layer. The results revealed that while the number of nodes in a WSN does not affect the energy consumption much, the frame length has a huge influence. Also, energy consumption rises considerably when acknowledgement is opted for.