Quality Of Service Of Redundant Mechanism In Industrial Wireless Networks

With the rapid development of information technology, industrial wireless network technology has become the new hotspot in the field of automation after Fieldbus. Because of its high degree of flexibility, scalability, low-cost, low-power and other properties, wireless networking technology makes the investment costs and the use of cost of industrial control system reduction effectively, expands the application scope of industrial control system greatly. WIA-PA (Wireless Networks for Industrial Automation-Process Automation) has become a international standard documents, which was researched and developed by the Chinese Academy of Sciences, Zhejiang University, Southwest University, and more than 10 units under the support of National High Technology Research and Development Program of China(863 Program).Due to the harsh environment of industrial applications, the WIA-PA network which used for industrial real-time monitoring and controlling system needs to be strong real-time and high reliability in wireless communication. It makes the research of reliability is necessary. So how to improve the reliability of WIA-PA is an important issue. WIA-PA has a double-layer network topology which is consisted of Mesh and Star. The key nodes, such as the router and the cluster head, are on the duty of data processing and transmitting, data aggregation, allocation of communication resource, management of field devices and so on. The failure of the key node will lead to the lost of the critical data, even lead to the paralysis of the network. Therefore, the ability of node continuous service is particularly important. The researches of redundant mechanism of WIA-PA for the key node become very important significance.The author of this paper, under the participating in the research and development work of WIA-PA and the analyzing of the QoS requirement of WIA-PA'redundant mechanism, propose the QoS architecture of the redundant mechanism, based on which a heartbeat algorithm is developed to meet the requirement of QoS. Contrary to the network latency issues and the packet loss issues of industrial wireless network, the new algorithm proposes network state dynamic estimation and suspicion mechanism based on fresh sequence. As a result, the redundant mechanism has high detection accuracy under the maximum detection time upper bound. Thereby the requirement of the strong real-time and high reliability of WIA-PA for industrial monitoring and controlling system is satisfied well.This paper firstly introduces the principles and framework of WIA-PA'redundant mechanism, and then analyses the problems and points the drawbacks of AHB (Adaptive Heartbeat Protocol). These drawbacks lead to AHB does not available for industrial monitoring and controlling system. Therefore, the author study on the QoS specification of WIA-PA'redundant mechanism, and then propose the parameter framework and metrics of the QoS to constitute the QoS requirement of WIA-PA'redundant mechanism. Because of heartbeat algorithm is the core of the redundant mechanism, base on the probabilistic network model and stochastic process theory, the author then study on the heartbeat algorithm which satisfies the QoS requirement and proposes the DHB (Dynamic Heartbeat Algorithm). DHB uses the "n" most recently received heartbeat messages to reflect the current network status and uses suspicion mechanism based on fresh sequence to solve the data loss issues. Theoretical analysis proves that the QoS requirement of WIA-PA'redundant mechanism can be satisfied well using DHB.The process of simulation is shown at the end of this paper. Consistency simulation verified that the test results keep the pace with the theoretical analysis of DHB; thereby DHB can meet the demand of QoS. Value "n" simulation is to explore the affect for maximum detection time upper bound by the value "n". Under the same restrict of detection speed, Contrast simulation shows DHB improves much better detection accuracy than AHB. In summary, the algorithm DHB provides good QoS for WIA-PA'redundant mechanism, and promotes the reliability of WIA-PA network enormously.