Time (or clock) synchronization is a large and vital field of research, as synchronization is a precondition for many applications. A few example applications are distributed data acquisition, distributed databases, and real-time communication. First, this survey paper introduces the research area of time synchronization and emphasizes its relation to other research areas. Second, we give an overview of the state-of-the-art of time synchronization. Herein, we discuss both established protocol and research approaches. We analyze all techniques according to three criteria: used estimation algorithm, achievable synchronization accuracy, and the experimental conditions. In our opinion, this analysis highlights potential improvements. The most important question in this survey is as follows: which estimation method can be used to achieve which accuracies under which conditions? The intention behind this is to identify estimation methods that are particularly worth considering, as these already achieve good results in the wireless area but have not yet been examined in the wired area (and vice versa). This survey paper differs from other surveys in particular through the consideration of wireless and wired synchronization and the focus on estimation algorithms and their achievable accuracy.

Recently, cooperative autonomous underwater vehicles (AUVs) have been deployed in application areas such as surveillance and protection of maritime infrastructures for inspection and monitoring purposes. These cooperative methodologies require wireless transmission of data between the different AUVs operating in the underwater environment. Communication over ranges exceeding 100 m exclusively relies on underwater acoustic communication. However, the propagating acoustic waves suffer from several challenges due to the presence of path loss, multi-path propagation, the slow and variant propagation speed, background noise, and Doppler distortion. Since the power supply of the AUVs is limited, communication must be very energy efficient and energy constraints have to be known to be able to plan the mission of AUVs. Due to the difficulties of real experiments, the modeling and simulation of the energy consumption and underwater acoustic communication play an essential role in studying and developing these systems. We provide a modular simulation model for the energy consumption and acoustic underwater communication of AUVs implemented in the network simulator OMNeT++ using the INET framework. More specifically, we extend several INET modules in such a way as to reflect the characteristics of AUVs and underwater communication. We study and analyze the AUVs’ energy consumption and dependence of the message quality on different properties such as those mentioned above.

Currently, Open Platform Communication Unified Architecture (OPC UA) is nominated as a reference standard to meet all Industry 4.0 demands with one protocol, enabling an efficient communication between devices in industrial systems. Recently, OPC UA has been extended by a publish/subscribe pattern to support multicast communication since there is a need for an efficient data aggregation and distribution to cope with the increasing number of controllers, sensors, and measured values in industries. Message Queuing Telemetry Transport (MQTT) is the most widely used publish/subscribe communication protocol and is based on a central broker for message exchange. In this paper, the communication latency of OPC UA end devices exchanging data over MQTT as a broker-based middleware is investigated and compared to the client/server-based communication. In particular, we analyze the use case of one-to-many communication for a crane model from the material handling domain. We provide a prototype implementation of the OPC UA MQTT extension and our evaluation results show the reduced traffic overhead and communication latency compared to client/server-based communication.

This paper addresses the growing need for efficient autonomous underwater vehicle (AUV) path planning, used in environmental monitoring, underwater surveillance, and search and rescue operations. The aim was to develop robust path planning strategies that ensure safe and efficient AUV navigation in complex and unpredictable underwater environments. Our approach combines path planning algorithms with the OMNeT++ network simulator. The procedures for the implementation of these algorithms and AUV motion simulation have been outlined. The algorithms were applied not only to single AUV missions but also to scenarios involving multiple AUVs, allowing exploration of cooperative and coordinated mission planning in diverse underwater settings. The results of the study demonstrate the potential of our approach to address real-world challenges encountered by AUVs. AUV behavior was observed in different simulated mission scenarios and environmental conditions. Our findings shed light on the adaptability of AUVs in the face of unexpected obstacles and dynamic ocean currents. In conclusion, this study contributes to the field of AUV applications, offering a path planning and simulation strategy adaptable to diverse AUV mission requirements. By utilizing simulation models, we illustrate AUVs' ability to autonomously adapt their mission plan and avoid obstacles during missions, improving operational efficiency and safety.

The broad availability of WLAN-capable off-the-shelf hardware lets WLAN mesh networks appear as promising technology for future distributed wireless applications. Featuring automatic device discovery, interconnection and routing, they provide a higher scalability, flexibility, and robustness compared to common centralized WLAN infrastructures. Besides these advantages, characteristics such as variable network topologies and link qualities imply new technical challenges for administration and real-world operation. Adopted in late 2011, IEEE 802.11s appears as new WLAN standard amendment, enabling vendor-independent mesh networks based on the widespread WLAN technology. However, network monitoring and management fall out of the standardization scope and are therefore not specified. In this paper, we present a novel 802.11s management solution based on the SNMP protocol. It covers dynamic mesh bootstrapping, error recovery, status monitoring and remote configuration. The presented solution was implemented and evaluated in a real-world testbed comprising more than 10 mesh nodes.