Research On OPC UA Deterministic Scheduling For Single-hop Industrial Wireless Networks

In recent years,the concept of industry 4.0 is rising gradually.Data sharing is an important feature of industry 4.0,which puts forward a high demand for industrial field information interconnection technology.For this reason,OPC foundation puts forward a new generation of OPC unified architecture(OPC UA),which is a safe,reliable,independent of manufacturers and can meet the high demand of industrial field information interconnection.However,OPC UA does not provide certainty for industrial real-time applications,especially in wireless networks vulnerable to electromagnetic interference.In order to solve this problem,this paper designs a certain real-time data transmission scheduling mechanism for industrial wireless network,and realizes cross layer optimization based on the demand of OPC UA for sampling period,which guarantees the real-time transmission of OPC UA data in wireless environment.(1)In order to ensure the certainty of data transmission in industrial wireless networks,the existing methods generally adopt centralized and synchronous scheduling in MAC layer,and use precise clock synchronization mechanism to control time slot to reduce the number of conflicts.As a result,there is a high synchronization overhead and a single point of failure may occur.To solve the above problems,this paper proposes a distributed,asynchronous scheduling method in MAC layer,which can ensure the real-time and deterministic data transmission.In this scheduling method,each wireless node has a fixed scheduling table,which sends the same data many times in a scheduling cycle,ensuring that at least one data can be sent successfully in the worst case,so as to ensure the certainty.According to this scheduling idea,this paper presents the necessary and sufficient conditions for n nodes to be successfully scheduled,and proposes a periodic optimal algorithm to find the node scheduling mode.Finally,the reliability of 4 nodes with delay in 26 time slots is achieved.However,this algorithm has high complexity and can calculate the number of successful scheduling nodes is less.In this paper,further optimization is carried out and a construction algorithm is designed.This algorithm uses the sufficient conditions of successful scheduling to construct the scheduling mode of nodes.This algorithm can achieve reliability of 10 nodes in one period.(2)There is no deterministic guarantee for OPC UA,this paper designs a cross layer optimization mechanism,which improves the real-time transmission of OPC UA periodic data stream by combining the OPC UA publish and subscribe mechanism and the MAC layer scheduling method realized in this paper.First,the MAC layer adjusts different scheduling methods according to the sampling period of OPC UA;then,the application layer samples data according to the MAC layer information and transmits it to the MAC layer for scheduling.This cross layer optimization mechanism ensures the real-time transmission of data streams of different sampling periods by OPC UA,so that OPC UA can be applied in industrial wireless environment.(3)In order to verify the Cross-Layer optimization mechanism of OPC UA in this paper,we implemented the OPC UA publish and subscribe mechanism and the MAC layer scheduling mechanism in this paper on the telosB wireless node using Contiki platform,and tested and verified them.At the same time,we compared the effect of adding ACK mechanism and not adding ACK mechanism.The results show that this Cross-Layer scheme can guarantee the boundedness and certainty of OPC UA to the transmission delay of periodic data stream.