Abstract
The Industrial Internet of Things (IIoT) is considered a key enabler for Industry 4.0. Modern wireless industrial protocols such as the IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) deliver high reliability to fulfill the requirements in IIoT by following strict schedules computed in a Scheduling Function (SF) to avoid collisions and to provide determinism. The standard does not define how such schedules are built. The SF plays an essential role in 6TiSCH networks since it dictates when and where the nodes are communicating according to the application requirements, thus directly influencing the reliability of the network. Moreover, typical industrial environments consist of heavy machinery and complementary wireless communication systems that can create interference. Hence, we propose a distributed SF, namely the Channel Ranking Scheduling Function (CRSF), for IIoT networks supporting IPv6 over the IEEE 802.15.4e TSCH mode. CRSF computes the number of cells required for each node using a buffer-based bandwidth allocation mechanism with a Kalman filtering technique to avoid sudden allocation/deallocation of cells. CRSF also ranks channel quality using Exponential Weighted Moving Averages (EWMAs) based on the Received Signal Strength Indicator (RSSI), Background Noise (BN) level measurements, and the Packet Delivery Rate (PDR) metrics to select the best available channel to communicate. We compare the performance of CRSF with Orchestra and the Minimal Scheduling Function (MSF), in scenarios resembling industrial environmental characteristics. Performance is evaluated in terms of PDR, end-to-end latency, Radio Duty Cycle (RDC), and the elapsed time of first packet arrival. Results show that CRSF achieves high PDR and low RDC across all scenarios with periodic and burst traffic patterns at the cost of increased end-to-end latency. Moreover, CRSF delivers the first packet earlier than Orchestra and MSF in all scenarios. We conclude that CRSF is a viable option for IIoT networks with a large number of nodes and interference. The main contributions of our paper are threefold: (i) a bandwidth allocation mechanism that uses Kalman filtering techniques to effectively calculate the number of cells required for a given time, (ii) a channel ranking mechanism that combines metrics such as the PDR, RSSI, and BN to select channels with the best performance, and (iii) a new Key Performance Indicator (KPI) that measures the elapsed time from network formation until the first packet reception at the root.
Highlights
Industrial environments demand high reliability for safety-critical messages, low latency, often demanding real-time communication guarantees, resistance to background noise produced by large machinery, wireless network coexistence in the Industrial-ScientificMedical (ISM) band, fault-tolerance to allow networks to continue functioning in case of 4.0/).node failure, link reliability to avoid high packet loss and high delays, and scalability [1]
The Channel Ranking Scheduling Function (CRSF) is mainly comprised of two processes: the bandwidth allocation, in which a number of required cells based on the current occupancy of the sending buffer is computed; and the channel selection, in which the mathematical model described in Equations (2) and (3) is employed to generate a list of best channels based on the Packet Delivery Rate (PDR), Received Signal Strength Indicator (RSSI), and Background Noise (BN)
The Scheduling Function (SF) is composed of a buffer-based bandwidth allocation algorithm based on the Kalman filter and a channel selection algorithm that incorporates several metrics such as the PDR, RSSI, and BN with an Exponential Weighted Moving Averages (EWMAs) mechanism in order to select the best channel available
Summary
Industrial environments demand high reliability for safety-critical messages, low latency, often demanding real-time communication guarantees, resistance to background noise produced by large machinery, wireless network coexistence in the Industrial-ScientificMedical (ISM) band, fault-tolerance to allow networks to continue functioning in case of. In the TSCH mode of the IEEE 802.15.4e amendment, nodes communicate by following a Time Division Multiple Access (TDMA) schedule combined with frequency hopping, which improves network reliability by mitigating the effects of interference and multi-path fading. A robust SF for 6TiSCH industrial networks must define efficient bandwidth estimation and channel selection mechanisms. Current approaches in Scheduling Functions (SFs) provide efficient bandwidth estimation mechanisms with weak random or sequential channel selection. It is important to define an SF that provides efficient channel selection based on several metrics such as PDR, RSSI, and Background Noise (BN); and robust bandwidth estimation mechanisms that can adapt dynamically to different traffic patterns and topologies. The intention of this paper is to present a new scheduling mechanism that effectively builds distributed TSCH schedules using 6TiSCH and IEEE 802.15.4e networks by defining efficient bandwidth estimation and channel selection mechanisms.
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