ARQ with Implicit and Explicit ACKs in Sensor Networks

A common application of unattended sensor networks (WSN) is low data rate streaming from many scattered sensors to one or more sink nodes, where packet error rate per link vary within 1-70% and path length could be up to tens of hops. To cope with the stringent requirement of battery lifetime, a new notion of statistically reliable transport protocol is introduced and the energy-efficiency of a several variants of stop-and-wait hop-by-hop ARQ with explicit and implicit ACKs are proposed. The energy-efficiency of the protocols are precisely analyzed and compared numerically. The analysis reveals that implicit ACKs should be applied with caution so as to prevent an “avalanche” of implicit ACK transmissions. It is further shown that a simple combined implicit/explicit ACK resolves the “avalanche” problem. The mathematical analysis is verified by simulation using “TOSSIM” a detailed simulator of the entire “TinyOS” system.