Reliability, Resources, and Acknowledgments in Low-Power Wireless Sensor Networks
Kristofer Pister and George Shaw
Berkeley Sensor and Actuator Center
All wireless-sensor networks with greater than 0% packet delivery rate (PDR) can be made 100% reliable when given an unbounded amount of time to achieve successful packet delivery. Real systems, however, don’t have unbounded time or resources.
The delivery reliability requirement and time bound is determined from the application–dependent MTBF at which exceptional reliability measures, such as human intervention, can be tolerated. Reliability is generally achieved through packet delivery acknowledgments, and conventional wisdom says that reliable operation requires these acknowledgments to be end-to-end. However, the acknowledgment protocol can greatly affect network resources and thus the ability of the network to achieve the desired reliability.
While end-to-end acknowledgments can supply the required time-bounded reliability, various link-level alternatives can supply equivalent reliability at far lower resource cost. While end-to-end acknowledgments can appear better as PDRs near 100%, this level of PDR is not realistic in wireless systems.
Link-level acknowledgments perform better at a 99% PDR on paths exceeding three hops, and increasingly better as PDRs decline to realistic values at paths as short as one hop.
Further, end-to-end acknowledgments combined with link-layer acknowledgments can achieve overall path reliability at far less cost than end-to-end acknowledgments alone under the previously given conditions. Various acknowledgment protocols are evaluated with respect to path and network energy, latency, connectivity, bandwidth, resources, and node reliability. Graphs are supplied to aid in designing for reliability.