Medium Access Control protocols for reliable communication in low-power industrial applications
Saleem Raza · Digitalen Hochschulbibliothek Sachsen-Anhalt (Universitäts- und Landesbibliothek Sachsen-Anhalt) · 2019
A Wireless Sensor and Actuator Network (WSAN) is a network of sensors and actuators connected through wireless medium in which sensors transmit the sensed phenomenon to the actuators and actuators act on it. These networks have appealed many of the industrial applications, particularly wireless networked control systems where the whole control loop can be closed through WSAN. These applications require reliable communication together with deterministic delay from sensor to actuators so as to timely control industrial processes and systems. In this way, they impose severe constraints on reliability, latency, scalability, adaptivity, and energy consumption. Mostly, such constraints can be addressed by the Medium Access Control (MAC) layer, because it can directly control many of the radio related activities such as collisions, retransmissions, interference, and idle-listening. Therefore, this thesis explores MAC layer and analyzes existing MAC protocols so as to determine to what extent they can satisfy the requirements of low-power industrial applications. In this direction, first we provide a critical analysis of Time-Slotted Channel Hopping (TSCH) MAC protocol, which is considered the most suitable protocol for the Industrial Internet of Things (IIoT) because of its low energy consumption and high reliability claims. Notably, we study the impact of inter-network interference, intra-network interference, and behavior of interaction among closely co-located TSCH networks. To achieve this, we investigate the impact of inter-network interference among networks when introducing a clock drift between them. We analyze the impact of channel hopping on interference by varying the number of available channels in each network. Our results denote that, since TSCH networks are time-synchronized, co-located networks interfere periodically and this period depends on the clock drift among the networks. Second, we critically analyze TSCH under node mobility, where we denote how mobility can impact reliability of the protocol. Our analysis reveals that mobility can cause significant network downtime where nodes are unable to associate to the network for long period of time because of synchronization loss, especially if the space is not fully covered by enough nodes. As per our analysis, TSCH can handle mobility well if the network space in which mobile nodes are evolving is fully covered by static nodes or there are enough mobile nodes to maintain a consistent coverage. However, this increases message overhead which causes more delay and energy consumption and thus impacts reliability of the protocol. Based on our analysis, we propose Dual-Mode Time-Slotted (DMTS)-MAC protocol for industrial process control applications. The DMTS-MAC protocol uses time-slotted structure in a dual-mode together with frequency diversity to satisfy high reliability, low latency, high scalability, and low energy consumption requirements of industrial process control applications. It takes into account the dynamics of process controller and it is adaptive to its varying traffic requirements. The protocol is evaluated through simulations and the results denote that DMTS outperforms existing MAC protocols under several performance metrics.