LEFT: A Latency and Energy Efficient Flexible TDMA Protocol for Wireless Sensor Networks

This paper proposes latency and energy efficient flexible TDMA (LEFT), a medium access control (MAC) combined with routing protocol for data gathering from number of source nodes to a master station (MS) in a wireless sensor network (WSN). TDMA provides fairness, collision-free communication and reduces idle listening, which saves network energy. Data latency is reduced by allocating same transmission slots to nodes falling out of interference range of each other. Unlike a conventional TDMA, LEFT provides flexibility through slot seizing, wherein a non-holder of a slot can use slot when holder does not have data to send. This increases channel utilization and adaption to dynamic traffic patterns of WSN applications. Further, a node on a multi-hop path towards MS decides to participate in routing based on (i) its location with respect to MS, to forward data in correct direction, (ii) its current status of residual energy, to uniformly distribute energy across network, (iii) its transit traffic load, to prevent local congestion, (iv) its communication link quality, to guarantee reliable data delivery. This decision requires simple comparisons against thresholds, and thus is very simple to implement on energy, storage and computationally constrained nodes. LEFT also encompasses techniques to cater to link and node breakdowns. Experimental analysis of LEFT; Advertisement-based TDMA; Data gathering MAC; Energy Efficient Fast Forwarding and Cross layer MAC protocols using TI's EZ430-RF2500T nodes shows that LEFT is 65% more energy efficient compared to Cross layer MAC. Data latency of LEFT is 27 % less, delivery ratio is 17 % more and goodput is 11 % more compared to Cross layer MAC.