Towards the design of efficient nonbeacon-enabled ZigBee networks

This paper presents experimental results of the communication performance evaluation of a prototype ZigBee-based patient monitoring system commissioned in an in-patient floor of a Portuguese hospital (HPG – Hospital Privado de Guimar~aes). Besides, it revisits relevant problems that affect the performance of nonbeacon-enabled ZigBee networks. Initially, the presence of hidden-nodes and the impact of sensor node mobility are discussed. It was observed, for instance, that the message delivery ratio in a star network consisting of six wireless electrocardiogram sensor devices may decrease from 100% when no hidden-nodes are present to 83.96% when half of the sensor devices are unable to detect the transmissions made by the other half. An additional aspect which affects the communication reliability is a deadlock condition that can occur if routers are unable to process incoming packets during the backoff part of the CSMA-CA mechanism. A simple approach to increase the message delivery ratio in this case is proposed and its effectiveness is verified. The discussion and results presented in this paper aim to contribute to the design of efficient networks,and are valid to other scenarios and environments rather than hospitals.

Authors would like to thank Dr. João Paulo Carmo for the valuable comments and Mr. Pedro Macedo for programming the simulations. They also want to thank Mr.Nélson Brito, Mr. Teófilo Leite and Ms. Teresa Moura, from Hospital Privado de Guimarães, for the encouragement and support. This work has been supported by the Portuguese Foundation for Science and Technology (FCT), Lisbon, through the 3o Quadro Comunitário de Apoio, the MITPortugal,POCTI and FEDER programs, project PEst-C/ CTM/LA0025/2011, and the FCT grant SFRH/BD/39408/2007. Clinical and financial support for the case-study has been provided by Grupo AMI – Assistência Médica Integral (Casa de Saúde Guimar~aes, SA), Portugal, under the partnership established between this healthcare company and the University of Minho.