A hybrid MAC protocol for emergency response wireless sensor networks

We introduce ER-MAC, a novel hybrid MAC protocol for emergency response wireless sensor networks. It tackles the most important emergency response requirements, such as autonomous switching from energy-efficient normal monitoring to emergency monitoring to cope with heavy traffic, robust adaptation to changes in the topology, packet prioritisation and fairness support. ER-MAC is designed as a hybrid of the TDMA and CSMA approaches, giving it the flexibility to adapt to traffic and topology changes. It adopts a TDMA approach to schedule collision-free slots. Nodes wake up for their scheduled slots, but otherwise switch into power-saving sleep mode. When an emergency occurs, nodes that participate in the emergency monitoring change their MAC behaviour by allowing contention in TDMA slots to achieve high delivery ratio and low latency. In its operation, ER-MAC prioritises high priority packets and sacrifices the delivery ratio and latency of the low priority ones. ER-MAC also guarantees fairness over the packets' sources and offers a synchronised and loose slot structure to allow nodes to join or leave the network. Simulations in ns-2 show the superiority of ER-MAC over Z-MAC, a state-of-the art hybrid MAC protocol, with higher delivery ratio, lower latency, and lower energy consumption. When a cluster of nodes in the network detects fire, nodes with ER-MAC deliver twice as many high priority emergency packets and four times faster than Z-MAC. This is achieved by ER-MAC with only one fifth as much energy as Z-MAC.