How long is the lifetime of a wireless sensor network?

Wireless sensor networks (WSNs) are known to be highly energy-constrained and each network's lifetime has a strong dependence on the nodes' battery capacity. As such, the network lifetime has been a critical concern in WSN research. While numerous energy-efficient protocols have been proposed to prolong the network lifetime, various definitions of network lifetime have also been used for the different scenarios and protocols. The lifetime of a sensor network is most commonly defined as the time to the first sensor node failure - seemingly over-pessimistic in many envisaged deployment scenarios. While other definitions exist, there has not been any consensus on which quantitative lifetime definition is most useful. In this paper, we aim to provide as objectively as possible, a comparative study of WSN protocols based on various network lifetime definitions. We also discuss the implications of these metrics and their applicability in evaluating the effectiveness of WSN data delivery schemes.