Abstract:The use of Wireless Sensor Network (WSN) technologies is an attractive option to support wide-scale monitoring applications, such as the ones that can be found in precision agriculture, environmental monitoring and industrial automation. The IEEE 802.15.4/ZigBee cluster-tree topology is a suitable topology to build wide-scale WSNs. Despite some of its known advantages, including timing synchronisation and duty-cycle operation, cluster-tree networks may suffer from severe network congestion problems due to the convergecast pattern of its communication traffic. Therefore, the careful adjustment of transmission opportunities (superframe durations) allocated to the cluster-heads is an important research issue. This paper proposes a set of proportional Superframe Duration Allocation (SDA) schemes, based on well-defined protocol and timing models, and on the message load imposed by child nodes (Load-SDA scheme), or by number of descendant nodes (Nodes-SDA scheme) of each cluster-head. The underlying reasoning is to adequately allocate transmission opportunities (superframe durations) and parametrize buffer sizes, in order to improve the network throughput and avoid typical problems, such as: network congestion, high end-to-end communication delays and discarded messages due to buffer overflows. Simulation assessments show how proposed allocation schemes may clearly improve the operation of wide-scale cluster-tree networks.
The IEEE 802.15.4/ZigBee set of standards is one of the most used wireless sensor network technologies. This set of standards supports cluster-tree networks, which are suitable topologies for wide-scale deployments. The design of wide-scale wireless sensor networks is a challenging task because it is difficult to test, analyse and validate new designs in real scenarios. Thus, simulation becomes a convenient and feasible method for its assessment before deployment. Within this context, we provide a set of simulation models for IEEE 802.15.4/ZigBee-based networks, which are able to deal with wide-scale cluster-tree wireless sensor networks and to address their major challenges. The provided simulation models implement important mechanisms for the assessment of wide-scale cluster-tree networks and associated data communication mechanisms, enabling an easier design and test of wide-scale wireless sensor network implementations.
As normas IEEE 802.15.4 e ZigBee definem a pilha de protocolos amplamente utilizada na operação de Redes de Sensores sem Fio (RSSF). A topologia cluster-tree, suportada por estas normas, oferece recursos como escalabilidade, sendo apontada pela literatura como a mais adequada para a implementação de aplicações de RSSFs em larga escala. Entretanto, essas redes podem sofrer com congestionamentos e atrasos devido aos seus diferentes padrões de tráfego. Por um lado, são usualmente priorizadas as mensagens do tráfego de monitoramento geradas por nós sensores, que seguem em direção ao coordenador da rede (tráfego upstream); por outro, sofrem maiores atrasos as mensagens de controle que seguem o tráfego inverso (tráfego downstream). Este artigo propõe um mecanismo eficiente para a rápida disseminação do tráfego de controle em RSSFs cluster- tree. A ideia principal consiste em combinar um esquema de escalonamento híbrido periódico, com a definição de janelas de oportunidade Top-Down, com um esquema de configuração adequada dos parâmetros CSMA-CA (Carrier Sense Multiple Access with Collision Avoidance) para os nós coordenadores, a fim de possibilitar uma rápida disseminação das mensagens de controle, sem gerar impactos relevantes para o típico tráfego de monitoramento. Resultados de simulações mostram que a definição de janelas de oportunidades Top-Down, aliada com uma adequada configuração de valores macMinBE e macMaxBE para nós coordenadores, podem reduzir significativamente os atrasos de comunicação e aumentar a taxa de entrega de mensagens de controle, sem interferir de forma relevante nas métricas do tráfego de monitoramento, mesmo em cenários de grande densidade de nós sensores.
The IEEE 802.15.4/ZigBee cluster-tree topology is a suitable technology to deploy wide-scale Wireless Sensor Networks (WSNs). These networks are usually designed to support convergecast traffic, where all communication paths go through the PAN (Personal Area Network) coordinator. Nevertheless, peer-to-peer communication relationships may be also required for different types of WSN applications. That is the typical case of sensor and actuator networks, where local control loops must be closed using a reduced number of communication hops. The use of communication schemes optimised just for the support of convergecast traffic may result in higher network congestion and in a potentially higher number of communication hops. Within this context, this paper proposes an Alternative-Route Definition (ARounD) communication scheme for WSNs. The underlying idea of ARounD is to setup alternative communication paths between specific source and destination nodes, avoiding congested cluster-tree paths. These alternative paths consider shorter inter-cluster paths, using a set of intermediate nodes to relay messages during their inactive periods in the cluster-tree network. Simulation results show that the ARounD communication scheme can significantly decrease the end-to-end communication delay, when compared to the use of standard cluster-tree communication schemes. Moreover, the ARounD communication scheme is able to reduce the network congestion around the PAN coordinator, enabling the reduction of the number of message drops due to queue overflows in the cluster-tree network.
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