Improving the Scalability of 6TiSCH Wireless Industrial Networks using Scalable Scheduling Reservation Protocol

Abstract

The Internet of Things is currently evolving. The demand for scalable and high throughput low-power wireless sensor networks is transforming the concept of automation of industrial processes. IPv6 is considered a potential solution allowing a large number of sensor devices connected to multi-hop low power wireless sensor networks to exchange information over a range of 1-2 km and without being dependent on infrastructure. TSCH over IEEE 802.15.4 standard is a proposal at the MAC layer in the low power IPv6 protocol suite called 6TiSCH. However, the implementation of TSCH has been subject to poor scalability in the 6TiSCH networks beyond 50 nodes where nodes frequently appear and disappear. The main cause is poor scheduling of link-layer resources. In 6TiSCH networks, the reservation of TSCH cells without a context leads to under-or-over estimation of actual bandwidth requirements, while monitoring the buffer condition for traffic adaption leads to a high level of additional overheads. Existing proposals employ an ‘On the fly’ reservation approach using a fixed threshold to tune up cell consumption and incur performance trade-offs. More generally, cell selection in TSCH-led scheduling has been a key challenge, as allocating a Tx cell closer to each other in TSCH slotframe reduces delay; however, this causes collisions during transmission to increase. Furthermore, delay is also increased by not allocating a sufficient volume of cells to a node probing the shortest path to the root. Existing approaches adapt cell selection based on the requirements of the application. Apart from poor traffic adaption, and inefficient cell selection, a fixed distribution of traffic in the network undermines the ability of nodes to adapt their behavior according to demand as some nodes may be able to transmit higher payload than the others depending on their distance to the root and volume of overprovisioned cells. This inability negatively affects propagation. Existing algorithms have not addressed this issue. This thesis introduces the Scalable Scheduling Reservation Protocol (SSR) to tackle these issues. SSR uses an analytical technique called cake-slicing for traffic adaptation, prioritizing higher resource allocation to nodes closer to the network root. It employs schedule compactness via cell selection and collision-free scheduling for faster, more reliable delivery, and integrates dynamic queue optimization to minimize congestion. While SSR delivers a strong proposal for medium-sized networks using lower consumption of TSCH resources, its performance in terms of reliability, a ratio between the number of data packets successfully received over the volume sent by a transmitter, declines in larger networks due to limited proliferation of information, causing fewer routes in the network. SSR is then implemented under hybrid scheduling design, aiming to further improve reliability. The results showed improved performance in terms of reliability within the network size of 100, compared to the minimal scheduling function, for all tested conditions. However, with more challenging traffic conditions, the performance still deteriorates. The main reason is the poor proliferation of information. To further improve the scalability, an increased penetration to shared resources (cells) is necessary. That is, any node under the common ancestry (in the topology) can negotiate for available cells. This functionality of distributed scheduling is incorporated in the proposed Decentralized and Broadcast-based Scalable Scheduling Reservation protocol (DeSSR), which exhibits high reliability under heavy traffic conditions, incurs low latency, and low consumption of TSCH cells compared to other solutions under this category.