Distributed and Efficient Slot Assignment–Alignment Protocol for Resource-Constrained Wireless IoT Devices

In distributed wireless Internet of Things (IoT) networks, time-slotted medium access control (MAC) protocols incorporating transmission scheduling mechanisms are needed for supporting contention-free real-time data flows. To achieve this, nodes need to be synchronized and know about each other’s notion of time. However, distributed time synchronization protocols rely on frequent message exchanges which often incur high overheads. The proposed distributed and efficient slot assignment-alignment (DESAA) protocol is a decentralized MAC protocol incorporating algorithms which minimize collisions incurred during slot requests and assignments, detect hidden terminals, and eliminate message overheads of global time synchronization by implementing time slot synchronization and continuous time slot realignment. DESAA leverages preassigned IP addresses of nodes for channel access arbitration and time slot assignment during its setup phase. Furthermore, the time slot alignment process maintains the time slot synchronization throughout the multihop network hierarchy during the data phase. The analysis showed a significant reduction in medium access contention, while the final testbed and simulation results for DESAA confirm a very fast total convergence time of 3.62 s per hop for both slot assignment and slot synchronization. Packet loss was minimized in comparison with the carrier sense multiple access protocol used by existing time synchronization and slot assignment algorithms. An average time slot synchronization error of 0
$+/- 30.51 ~\mu \text{s}$
for a 4-node line topology with three hops, and 0
$+/- 67.81 ~\mu \text{s}$
for a 25-node grid topology with eight hops was achieved, outperforming most time synchronization approaches in terms of convergence time and message overhead while maintaining comparable or better accuracy.