Transmission Time Scheduling for Stations with Different Packet Generation Periods in Smart Factory Environment: System Implementation and Experimental Evaluation

In smart IoT environments such as smart factory ones, periodically generated data packets as well as aperiodically generated ones with various sizes and generation time intervals must be transferred at low costs from the viewpoints of network installation and operation. Using Wireless Sensor Networks (WSNs) based on IEEE 802.11 is desirable for the low cost nature, but the channel access mechanism with Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) suffers from packet collision as packet transfer load increases. In this paper, we establish a new transmission time scheduling method for avoiding packet collision among wireless stations (STAs) that generate and transmit packets to their access point at various time periods, give the detailed system design, and verify its operation through prototype implementation and experimental studies. In the proposed method, transmission durations during which each STA is permitted to transmit packets are assigned in a discrete-time manner on the time axis, so that the overlap of transmission durations from different STAs is minimized based on the periodicity of packet generation. The transmission permission control is realized with a software buffer provided just before packets are passed from the upper IP layer to the MAC layer. This has the advantage that no modifications and replacements of existing IEEE 802.11 devices are required. The discrete-time assignment of the transmission durations ensures the low cost nature and feasibility of the transmission timing control for STAs whose computation resource is assumed to be limited as wireless sensor nodes. We implement the proposed method as a prototype system and conduct several experiments to verify its operation and evaluate its performance.