No abstract

Network dynamics, such as mobility and increase in network load, can influence the performance of a Wireless Sensor Network (WSN). In this paper, we introduce a method which exploits design-time knowledge of the application scenario dynamics to construct a proactive run-time reconfiguration approach. The approach anticipates for the impact that predefined dynamic events can have on the performance of the WSN by switching between various modes of operation defined at design-time. A mode defines the values for the controllable parameters of the network protocol stack. Our approach explicitly differentiates between parameters that can be adapted locally, per node, and those that should be considered globally for the whole WSN. Design-time definition of modes results in a very low run-time overhead as we only require detection of the mode to use and a low overhead synchronization to change global parameters. The approach is made robust by using a recovery approach for nodes unaware of their global mode after, for example, (re-)joining the network. Experiments with an office monitoring deployment and extensive simulations of a cow-health monitoring scenario show that our approach can easily be adopted by practical WSN deployments and results in a significant reduction in resource usage, e.g., power consumption in our examples, at a very low run-time overhead cost.

The quality of the communication links in a Wireless Sensor Network often shows significant asymmetry and variation over time, due to, for example, heterogeneous settings of the transmission power, moving nodes or changing external interference. This makes it difficult for nodes to accurately maintain system-level properties, such as the minimum-energy path from the node to a given reference node, as required by many protocols. In this paper, we introduce a distributed service that allows nodes to maintain accurate information related to the minimum-cost path, such as its cost or parent on that path. Using controlled n-hop forwarding, to deal with asymmetric links, every node disseminates minimum-cost path and connectivity information allowing every connected node in the network to iteratively derive minimum-cost path information. This controlled nhop forwarding is repeated to avoid stale information due to dynamic changes in link qualities. The parameters of the service allow a trade-off between the accuracy and overhead. We study the characteristics of a deployment that impact this trade-off and how the service should be parameterized accordingly. Extensive simulations and experiments for an actual deployment show a significant increase in the accuracy of the maintained minimum-cost path information, compared to the typically used local broadcasting approach.

Abstract-The inherently unreliable communication infrastructure compel WSN protocols to employ error control mechanisms. Traditionally, error control is achieved by a retransmission scheme using acknowledgment mechanisms. WSN architectures are severely resource constrained and the additional energy expense of transmitting error control messages can seriously degrade network lifetime.In this paper, we analyze performance of error control schemes for the case of point-to-multipoint communication. An explicit acknowledgment mechanism may provide for reliable communication, but has two major drawbacks: 1) the overhead is significant for small data messages, and 2) in case of asymmetrical communication links, multi-hop dissemination of acknowledgments is required. As an alternative to such explicit acknowledgment schemes we propose the use of probabilistic acknowledgments. In this probabilistic scheme, a sender estimates the probability that a message has been successfully delivered, based on information about the quality of the radio channel. A message is then retransmitted until the probability of successful delivery reaches a defined threshold value. Network capacity available for error control can be distributed prudently among all information items to be disseminated, possibly taking into account different application requirements. We formulate a retransmission control strategy which results in minimal latency and maximal message delivery ratio.

Wireless sensor networks are typically operating in a dynamic context where events, such as moving sensor nodes and changing external interference, constantly impact the qualityof-service of the network. We present a distributed feedback control mechanism that actively balances multiple conflicting network-wide quality metrics, such as power consumption and end-to-end packet latency, for a heterogeneous wireless sensor network operating in a dynamic context. Nodes constantly decide if and how to adapt controllable parameters of the entire protocol stack, using sufficient information of the current network state. Using experiments with an actual deployment we show that our controller allows to maintain the required network-wide qualityof-service, with up to 30% less power consumed, compared to the most applicable (re-)configuration approaches. I. MOTIVATIONControllable parameters of individual sensor nodes in a Wireless Sensor Network (WSN), such as the radio transmission power, MAC protocol duty cycle and selected routing parent(s), determine the behaviour of the network. The values of these parameters, referred to as the configuration, determine to what extent expectations on quality metrics, i.e., the required Quality-of-Service (QoS), are satisfied. WSNs typically operate in a dynamic environment or exhibit dynamic behaviour themselves causing the configuration required to achieve sufficient QoS to vary over time. Statically configured nodes [5], [6] cannot cope with dynamics; run-time adaptation of parameters, or reconfiguration, is needed. Current approaches focus either on a single metric [4], [16], typically energy consumption, or on optimizing local metrics [13], [18] and often consider a homogenous network. In practice, the requirements of the applications are typically expressed by multiple conflicting quality metrics expressed on a networkwide scale [3], [14], such as a maximum end-to-end latency of 100 milliseconds, a packet delivery ratio of 90% or maximal network lifetime. There is only limited existing work on runtime adaptation techniques that consider multiple QoS metrics expressed on a network-wide scale. The recent work of [19] proposes a centralized approach which determines how to adapt MAC protocol parameters based on centrally collected network QoS information. In practice, centralized approaches might be too costly, non-scalable or not able to respond quick enough to dynamic events. By focusing on only the MAC protocol, it does not exploit the fact that the parameters from all protocols may influence the behaviour of the network [7].As a homogenous configuration is assumed where all nodes use the same MAC parameters, it furthermore ignores the typical heterogeneity in WSNs and resulting variation of the impact of nodes on the trade-offs involved.In this extended abstract, we introduce a distributed feedback control mechanism to maintain a required QoS, defined by multiple quality metrics, for a WSN in a dynamic context. The approach separates the adaptivity from protocol operation and lets...