Idle-Slot Recycling in a Collision-Free Real-Time MAC Protocol

In wireless sensor networks (WSNs), timeliness is one of the most challenging problems for critical applications. Caccamo et al. designed a collision-free real-time MAC protocol for sensor networks by adopting the "cellular structure" in traditional telecommunication networks. They assumed that sensors are organized into many rigid hexagon cells and a router node is at the center of each cell to transmit inter-cell packets. By exploiting FDMA and TDMA, transmission collisions are avoided and real-time guarantees are provided. However, we observe that for inter-cell communication, idle-slots caused by the TDMA-based scheduling will degrade the throughput and delay performance of the entire network, especially for some real applications in which data flows have traffic-direction partiality characteristics. In the worst case, five sixths of inter-cell bandwidth will be wasted. In this paper, we propose four idle-slot recycling algorithms to improve channel utilization. Simulation results show that our proposed algorithms can greatly increase network throughput and decrease packet transmission delay, while the collision-free and real-time qualities of Caccamo\´s protocol are still retained.