Reliability and Acknowledgements in Low-Power Wireless Sensor Networks

All wireless sensor networks with greater than 0% packet delivery rate (PDR) can be made 100% reliable—when given unbounded time to achieve successful packet delivery. Real systems, however, don't have unbounded time or resources. Reliability is the probability that a packet will be successfully delivered within the required time bound. The delivery reliability requirement and time bound are determined from the application-dependent delivery MTBF at which exceptional reliability measures can be tolerated. Reliability is generally achieved through packet delivery acknowledgements, and conventional wisdom says that reliable operation requires these acknowledgements be end-to-end. However, reliability meeting the application requirement can often be achieved without the overhead of an end-to-end acknowledgment. This report contributes a fundamental comparison of the common underlying mechanisms used to acknowledge packet delivery, their behaviors under changing conditions, and the environments in which they perform best. It suggests a method for evaluating actual packet delivery reliability to estimate the packet delivery rates used for planning the network, and a method for determining the time bounds that must be accommodated based on the estimated packet delivery rates so as to achieve the desired delivery reliability. It finds that by changing acknowledgment protocols, the reliability of the system can be increased by several orders of magnitude (to the limit of the inherent network reliability), that immediate acknowledgments perform generally best overall, and that if an end-to-end acknowledgment must be used, implementing it with a link-layer acknowledgment is the most efficient.