Mid Sweden University

Industrial Wireless Sensor Networks (IWSNs) designedfor mission- and time-critical applications require timelyand deterministic data delivery within stringent deadline bounds.Exceeding delay limits for such applications can lead to system malfunction or ultimately dangerous situations that can threaten human safety. In this paper, we propose SS-MAC, an efficient slot stealing MAC protocol to guarantee predictable and timely channel access for time-critical data in IWSNs. In the proposed SS-MAC, aperiodic time-critical traffic opportunistically steals time slots assigned to periodic non-critical traffic. Additionally, a dynamic deadline-based scheduling is introduced to provide guaranteed channel access in emergency and event-based situations where multiple sensor nodes are triggered simultaneously to transmit time-critical data to the controller. The proposed protocol is evaluated mathematically to provide the worst-case delay bound for the time-critical traffic. Performance comparisons are carried out between the proposed SS-MAC and WirelessHARTstandard and they show that, for the time-critical traffic, theproposed SS-MAC can achieve, at least, a reduction of almost 30% in the worst-case delay with a significant channel utilization efficiency.

Industrial wireless sensor networks are becoming popular for critical monitoring and control applications in industrial automation systems. For such type of applications, providing reliable real-time performance regarding data delivery is considered as a fundamental challenge. The problem becomes more prominent with mixed-criticality systems, where different data flow with different levels of criticality (importance) coexist and characterized by different requirements regarding delay and reliability. In this paper, we propose a wireless fieldbus protocol to enable real-time communication and service differentiation for cluster-based mixed-criticality networks. A process monitoring scenario of plastic extrusion is used to define the protocol requirements and elaborate the working principle of the proposed work. In our proposed protocol, each data flow is scheduled for channel access based on its criticality level using a distributed prioritized medium access mechanism that ensures a guaranteed channel access for the most critical traffic over other traffic types. The performance of the proposed protocol is analyzed analytically using a discrete-time Markov chain model to evaluate the performance in terms of delay and throughput. Moreover, the extensive simulations are conducted to prove the analytical claims and different performance assessments are provided, which also demonstrate the effectiveness of the proposed approach compared with the related existing work.

In the perspective of the emerging Industrial Internet of things (IIoT), the 6TiSCH working group has been created with the main goal to integrate the capabilities of the IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) with the IPv6 protocol stack. In order to support time-critical applications in IIoT, reliable real-time communication is a key requirement. Specifically, aperiodic critical traffic, such as emergency alarms, must be reliably delivered to the DODAG root within strict deadline bounds to avoid system failure or safety-critical situations. Currently, there is no mechanism defined in the 6TiSCH architecture for timely and reliably handling of such traffic and its prioritization over the non-critical one. In this paper, we introduce REA-6TiSCH, a reliable emergency-aware communication scheme to support real-time communications of emergency alarms in 6TiSCH networks. In REA-6TiSCH, the aperiodic emergency traffic is opportunistically enabled to hijack transmission cells pre-assigned for the regular periodic traffic in the TSCH schedule. Moreover, we introduce a distributed optimization scheme to improve the probability that an emergency flow is delivered successfully within its deadline bound. To the best of our knowledge, this is the first approach to incorporate emergency alarms in 6TiSCH networks. We evaluate the performance of REA-6TiSCH through extensive simulations and the results show the effectiveness of our proposed method in handling emergency traffic compared to Orchestra scheme. Additionally, we discuss the applicability of REA-6TiSCH and provide guidelines for real implementation in 6TiSCH networks.

Many emerging Industrial Internet of Things (IIoT) applications involve the use of mobile devices, such as sensors and robots in industrial automation scenarios. Movement of mobile sensor nodes causes intermittent connectivity which in turn deteriorates the network performance in terms of packet loss and delay. The Routing Protocol for Low Power and Lossy Networks (RPL) is introduced as the standard routing protocol for IIoT networks. Although RPL constitutes a reliable and energy-efficient solution for static networks, there is no mechanism defined in the RPL standard of how to support routing in mobile IIoT networks. This paper introduces RMA-RP, a reliable mobility-aware routing protocol to support mobile IIoT networks. RMA-RP utilizes a dynamic motion detection mechanism based on the link quality to cope with topology changes by updating next-hop nodes. Moreover, an adaptive timer is introduced to manage the transmission rate of control messages in order to decrease the network overhead and in turn the energy consumption. We evaluate the performance of RMA-RP through extensive simulations in comparison to existing works and the results demonstrate that RMA-RP has at least 17% higher packet delivery ratio and achieves a reduction of 34% and 51% in terms of delay and network overhead, respectively.

The Routing Protocol for Low power and lossy networks (RPL) has been introduced as the de-facto routing protocol for the Industrial Internet of Things (IIoT). In heavy load scenarios, particular parent nodes are likely prone to congestion, which in turn degrades the network performance, in terms of packet delivery and delay. Moreover, there is no explicit strategy in RPL to prioritize the transmission of different traffic types in heterogeneous 6TiSCH networks, each according to its criticality. In this paper, we address the aforementioned issues by introducing a congestion control and service differentiation strategies to support heterogeneous 6TiSCH networks in IIoT applications. First, we introduce a congestion control mechanism to achieve load balancing under heavy traffic scenarios. The congestion is detected through monitoring and sharing the status of the queue backlog among neighbor nodes. We define a new routing metric that considers the queue occupancy when selecting the new parent node in congestion situations. In addition, we design a multi-queue model to provide prioritized data transmission for critical data over the non-critical ones. Each traffic type is placed in a separate queue and scheduled for transmission based on the assigned queue priority, where critical data are always transmitted first. The performance of the proposed work is evaluated through extensive simulations and compared with existing work to demonstrate its effectiveness. The results show that our proposal achieves improved packet delivery and low queue losses under heavy load scenarios, as well as improved delay performance of critical traffic. 

The Routing Protocol for Low power and lossy network (RPL) is designed to support communication requirements in 6TiSCH networks in Industrial Internet of Things (IIoT) applications. RPL is mostly optimized for uplink communication, however, less attention is given to maintain connectivity for downlink communications. Supporting downlink communications is non-trivial task in process automation and control scenarios within the IIoT. RPL in its current definition is inefficient to support reliable downlink communications in terms of scalability and memory requirements leading to significant degradation in network performance. This paper introduces HyS-R, a Hybrid Subscription-Recovery method to maintain downlink connectivity and mitigate memory limitations in large-scale 6TiSCH networks. The proposed method is based on a relief group that is used as alternative route to unreachable destinations in the network. An intermediate node subscribes to the relief group when it fails to advertise a destination to its next-hop node. In addition, members of the relief group keep searching for alternative forwarders to keep the communication traffic to a minimum. Performance evaluations are carried out and the results demonstrate that the proposed HyS-R attains significant improvements in downlink communications compared to RPL storing and non-storing modes with a margin of energy cost.