Collection of rare but delay-critical messages from a group of sensor nodes is a key process in many wireless sensor network\r\napplications. This is particularly important for security-related applications like intrusion detection and fire alarm systems. An\r\nevent sensed by multiple sensor nodes in the network can trigger many messages to be sent simultaneously. We present Alert, a\r\nMAC protocol for collecting event-triggered urgent messages from a group of sensor nodes with minimum latency and without\r\nrequiring any cooperation or prescheduling among the senders or between senders and receiver during protocol execution. Alert\r\nis designed to handle multiple simultaneous messages from different nodes efficiently and reliably, minimizing the overall delay\r\nto collect all messages along with the delay to get the first message. Moreover, the ability of the network to handle a large number\r\nof simultaneous messages does not come at the cost of excessive delays when only a few messages need to be handled. We analyze\r\nAlert and evaluate its feasibility and performance with an implementation on commodity hardware. We further compare Alert\r\nwith existing approaches through simulations and show the performance improvement possible through Alert.
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