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 nondeterministic 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.

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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 " flex WARE -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 flex WARE 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. 1

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.

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.