Abstract—The Industrial Internet of Things (IIoT) is actively used for remote monitoring and control of parameters affecting the efficiency of product release. The use of IIoT technologies allows to identify the equipment malfunctions before they significantly affect production processes. For this purpose, the sensors data are transmitted in real time to the production control center in the form of telemetry messages. Many modern IIoT systems operate based on the Message Queuing Telemetry Transport for Sensor Networks (MQTT-SN) protocol. This protocol provides three levels of data transfer quality. The highest level is Quality of Service 2 (QoS 2). It allows messages to be delivered exactly once and guarantees the absence of duplication of delivered messages, but uses double acknowledgements that require the transmission of an excess number of service packets. Numerous studies of the MQTT-SN protocol are focused on security, performance, and delays reduction in data transmission. However, there is insufficient attention to the message delivery probability estimation when using this protocol. The paper proposes to fill this gap. A model is presented that was developed using the probability graph methodology. The model takes into account all possible states of the analyzed message transmission process when implementing QoS level 2. Using the model, it is possible to estimate the message delivery probability depending on the current bit error rate and the set retransmission counter. The results of calculations and experiments based on the model showed the possibility of using it to select the QoS level and the number of retransmissions that ensure the required reliability of data delivery, and minimize the packet traffic and energy consumption of network devices.
 
Keywords—Industrial Internet of Things (IIoT), Message Queuing Telemetry Transport for Sensor Networks (MQTT-SN) protocol, Quality of Service (QoS), telemetry message delivery, bit error rate, retransmissions, sensor device, probability graph


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