Anetta Nagy

Limits of synchronization accuracy using hardware support in IEEE 1588

Clock synchronization protocols for packet-oriented networks, like IEEE 1588, depend on time stamps drawn from a local clock at distinct points in time. Due to the fact that software-generated time stamps suffer from jitter caused by non-deterministic execution times, many implementations for high precision clock synchronization rely on hardware support. This allows time readings for packets with very low jitter close to the physical layer. Nevertheless, approaches using hardware support have to carefully consider influences on synchronization accuracy when it comes to the range of nanoseconds. Among others, limits come from the update interval, oscillator stability, or hardware clock frequency. This paper enlightens the limits for such implementations based on an analysis of the influences of the main factors for jitter. The conclusions give hints for efficiently optimizing current implementations.

Time-based localisation in unsynchronized wireless LAN for industrial automation systems

In the recent years, the term wireless factory automation began raising interest. Its probably most appreciated feature, mobility, is yet acknowledged as the key for new applications. Nevertheless, this apparent freedom comes with a palette of requirements, whereof one is localisation. Although locating systems have been an extensive research topic for years, industrial systems impose additional constraints, both in terms of accuracy and scalability. Typically, for state-of-the-art time-based locating systems one assumes that the network infrastructure is perfectly synchronized. However, in large-scale cellular wireless industrial networks this is not achievable without redesigning the complete infrastructure. This paper proposes a differential time difference of arrival based localisation system using IEEE802.11, which eliminates the need for clock synchronization.

A novel, wireless sensor/actuator network for the factory floor

One of the most promising technologies for tomorrows sensor networks are wireless approaches. Mesh networks using ad-hoc concepts are very popular, but have the drawback that real-time constraints cannot easily be fulfilled, because data transmission times are not fully deterministic. On the other hand, infrastructure networks based on central access points are limited in their range. For time-critical applications in automation environments, novel wireless sensor/actuator network approaches are desired which are able to support real-time data transmission without being restricted to single access point domains. This paper presents such an approach, addressing particular problems like clock synchronization and localization for support of seamless handover and real-time guarantees at the example of WLAN technology.

Achieving a Realistic Notion of Time in Discrete Event Simulation

Distributed sensor systems require clock synchronization between all sensor nodes to provide consistent view of the overall system. Owing the growing size of networks, the evaluation of the synchronization performance becomes difficult, if done by means of experiments. Simulation is another method to tackle this issue. Realistic simulation of synchronization schemes requires accurate modelling of oscillators which are the driving timers generating various events. One way to characterise oscillators is to utilize the Allan variance, which can be used to generate a phenomenological model based on power spectral density. Since discrete event simulation (DES) tools are widely used to model network protocols, models which combine accuracy and performance are needed. This paper presents a model that was optimised for use in DES. To verify that the simulation results sufficiently match measurements, an implementation in OMNeT++ was done. The results show that the behaviour of distributed sensor systems, resulting from imperfect timebases, can be accurately simulated.

Location-based handover in cellular IEEE 802.11 networks for Factory Automation

The use of wireless technologies in Factory Automation is attractive due to several advantages (mobility, cost, etc.); however, to satisfy the requirements of industrial applications, they have to be improved in terms of real-time performance. Handover is a particular weakness in cellular wireless systems, e. g., in IEEE 802.11, since it may introduce delay beyond acceptable bounds. The project “flexWARE - Flexible Wireless Automation in Real-Time Environments” aims at implementing such an infrastructure based on IEEE 802.11. To enhance overall system performance, it offers a localisation service. In this paper we present the flexWARE handover mechanism which exploits localisation to reduce the discovery phase. A performance evaluation, based on simulation and empirical measurements, shows that the mechanism results in a seamless handover for a class of industrial applications.

A novel, high resolution oscillator model for DES systems

High accurate synchronization of clocks in environments with a high number of nodes can take advantage of modern communication networks. However, with the large number of nodes another problem arises: systems of this size cannot any more be evaluated on experimental basis. Therefore, simulation tools and models are required. These simulator models have special requirements which are outlined in this paper. Moreover, a modeling strategy is proposed and demonstrated, which can be implemented for Discrete Event Simulation tools.

Localisation in wireless sensor networks

The traditional task of a sensor network is data collection and aggregation for further processing. In the case of mobile networks this is usually done via wireless technologies. However, with the establishment of mobility another problem occurs: The question of the actual location of the measured data. This paper proposes a localisation scheme for Wireless LAN, where an existing WLAN infrastructure can be used not only for the collection of sensor data but also for the localisation of the data source. In this investigation localisation in sensor networks is done via the Time Difference of Arrival scheme, which has the advantage that the sensor nodes can be left unmodified.

Clock synchronization simulation for wireless sensor networks

A lot of research is done in the field of wireless sensor networks, particularly in the field of ad-hoc networks. Large-scale real-time wireless networks for factory automation are hardly covered by existing research. Such networks usually have thousands of nodes, which makes simulation a necessity for system validation. Discrete Event Simulators (DES) are efficient tools to do so, but their event schedulers make it impossible to simulate distributed clocks, which is mandatory for validation of TDMA based protocols. This paper proposes an approach to enhance DES systems with models of local clocks. Further, it provides a realistic simulation of distributed clocks without losing the performance advantages of abstract modeling.

Distribution of control functionality in energy-aware industrial building environment

Modern trends in automation require flexible production with an onus on saving energy. Till recently, the focus of production systems has been only saving production costs, but the growing concern over environmental impact of such system has led to a partial change in paradigm. The key for the future generation production system is to successfully integrate the manufacturing environment with energy optimisation. This relies on transparent representation of the building from the point of the Manufacturing Execution System (MES) that is not feasible using conventional centralized approaches to building thermal modeling and optimization. This work focuses on distributing the building automation system in a set of generic cells that encapsulate thermal energy models and optimization algorithms to minimize computational resources required for the system implementation on embedded devices. Combined with a notion of the global overview of the energy optimization with functional aggregation and dynamic clustering concepts, the proposed architecture provides a flexible energy saving solution that can be easily integrated in majority of conventional manufacturing systems as well as related factory buildings without compromising production operations.

Distributed clock synchronization in discrete event simulators for wireless factory automation

One very interesting trend in factory automation is the introduction of wireless networks to the factory floor. This requires investigation of problems which are not present with wired technology. Tracking of mobile nodes, as well as seamless handover are such problems. Precise clock synchronization is needed to solve these new issues. For large-scale systems it is economically unreasonable to implement complete factory scenarios with prototypes in order to test for architectural, engineering, and design problems. It is mandatory to have a proper simulation environment at hand in order to study the influence of various design decisions. This paper discusses techniques of how to simulate such a large-scale system with special focus on proper modeling and simulation of distributed clocks in a discrete event simulation environment.