Making WSN TDMA Practical: Stealing Slots Up and Down the Tree

Time Division Multiple Access (TDMA) communication protocols in wireless sensor networks provide collision-free communication that increases energy-efficiency while maintaining deterministic packet latencies. The TDMA-ASAP[1] protocol proposed stealing neighbor´s slots when networks are running at low-duty cycles to reduce the potentially large latencies of the TDMA cycle size. In this paper, we further reduce the end-to-end latencies by intelligently spreading slots across the TDMA cycle and by enhancing the stealing opportunities in the schedule, stealing slots scheduled for downstream (control) and upstream (data) messages. We also provide a practical time synchronization algorithm that operates within the TDMA schedule. In order to evaluate our new schemes, we carried out both simulation studies to show scalability and a test bed implementation of Fire Fly wireless sensor nodes to show feasibility. Our schemes provide higher peak throughput (nearly 2x) as compared to a common low-power-listen contention-based (LPL-CSMA) protocol and improves the average packet latency by as much as 5x as compared to existing TDMA protocols without slot-stealing and up to 2x as compared to TDMA-ASAP.