The Internet of Things (IoT) facilitates mutual interaction by establishing a connection between humans and objects. The adoption of IoT is rapidly expanding across various aspects of human life. Industrial IoT (IIoT) is the intersection point between IoT and industry, serving as a key driver of the Fourth Industrial Revolution, also referred to as Industry 4.0 (I4.0). Low-Power and Lossy Networks (LLNs), which contain many resource- constrained nodes, as the basic block in IoT and IIoT, are an essential part of this revolution. Given the resource constraint of LLNs and the mission, safety, and business criticality of IIoT, enhancing Quality of Service (QoS) has become a significant challenge. Addressing this challenge requires the optimal utilization of resources and fully coordinated decision-making across different layer protocols, which can be achieved through cross-layer design. Routing Protocol for Low-Power and Lossy Networks (RPL) and Time Slotted Channel Hopping (TSCH) are two widely used standard protocols in the IIoT protocol stack. In this paper, we investigate the fundamental factors affecting QoS within the RPL and TSCH and propose an RPL-TSCH Cross-Layer (RTCL) design for LLNs in IIoT, motivated by equations derived from queuing theory. The equations and simulation results reveal that the inconsistency between the arrival rate and service rate of the node’s queue leads to increasing queue overflow and delay and decreasing reliability and throughput. The proposed RTCL design, structured as a six-step solution that integrates RPL and TSCH information, enhances QoS by employing two primary strategies: controlling the arrival rate and increasing the service rate of the nodes’ queues. Evaluation results from various simulation scenarios demonstrate that RTCL improves QoS parameters while also enhancing load balancing and network stability.

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