Wireless sensor networks (WSNs) heavily rely on a dense deployment of sensor nodes in order to ease deployment, increase fault-tolerance and network coverage, so that events do not go undetected. A dense deployment however results in several sensor nodes close to each other detecting and transmitting event reports at almost the same time, resulting in severe contention for channel access. Channel contention is a serious problem in WSNs resulting in collisions, re-transmissions, energy depletion, and ultimately loss of event reports. TDMA-based protocols prevent contention, but require tight synchronization and may lead to severe wastage of bandwidth especially in event-based applications where the traffic is bursty in nature. Other approaches that handle spatially correlated contention are fairly complex and contradict the reason for dense deployment, by selecting only a subset of nodes that generate and transmit event reports, affecting the fault-tolerance and confidence of event detection.Motivated by the challenge to reduce contention and improve performance, we propose a protocol that dynamically schedules transmissions in the network. The protocol exploits the broadcast nature of a wireless medium, which allows nodes to overhear transmissions of neighboring nodes and establish a cooperative transmission schedule dynamically, without the need for synchronization or explicit message exchange. To further mitigate contention, we propose a heuristic to reduce the number of active forwarder nodes in the network, by increasing the overlap of forwarder nodes used while routing packets. This forwarding mechanism can isolate the areas prone to interference, within which the dynamic transmission scheduling mechanism works better to mitigate contention. We evaluate the performance of our protocol using the NS2 simulator. Results show that our protocol significantly reduces collisions and re-transmissions, thereby improving the performance of the network.
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