Patrick Loschmidt

White rabbit: Sub-nanosecond timing distribution over ethernet

White Rabbit (WR) is the project name for a ambiguous project that uses Ethernet as both, deterministic (synchronous) data transfer and timing network. The presented design aims for a general purpose, fieldbus like transmission system, which provides deterministic data and timing to approximately 1000 timing stations. The main advantage over conventional systems is the highly accurate timing (sub-nanosecond range) without restrictions on the traffic schedule and an upper bound for the delivery time of high priority messages. In addition, WR also automatically compensates for transmission delays in the fibre links, which are in the range of 10 km length. It takes advantage of the latest developments on synchronous Ethernet and IEEE 1588 to enable the distribution of accurate timing information to the nodes saving noticeable amounts of bandwidth.

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.

Improving Fault Tolerance in High-Precision Clock Synchronization

The very popular Precision Time Protocol (PTP or IEEE 1588) is widely used to synchronize distributed systems with high precision. The underlying principle is a master/slave concept based on the regular exchange of synchronization messages. This paper investigates an approach to enhance PTP with fault tolerance and to overcome the transient deterioration of synchronization accuracy during a recovery from a master failure. To this end, a concept is proposed where a group of masters negotiates a fault-tolerant agreement on the system-wide time and transparently synchronizes the associated IEEE 1588 slaves. Experimental verification on the basis of an Ethernet implementation shows that the approach is feasible and indeed improves the overall synchronization accuracy in terms of fault tolerance.

A novel approach for Flexible Wireless Automation in Real-Time Environments

To widen the control over a factory from purely wire based to the wireless domain, both secure and reliable communication infrastructure with real-time capabilities is needed. Based on wireless communication technologies, such infrastructures may be used to gain new flexibility within any step of a manufacturing process hence enabling the development of revolutionary new applications. The international FlexWARE (flexible wireless automation in real-time environments) initiative establishes such a new infrastructure in order to fill this technological gap in the market. This paper gives an overview of the main ideas and the targeted goals.

A novel, high-precision timestamping platform for wireless networks

The introduction of wireless networks in the factory floor offers many advantages. Besides a new flexibility for automation, also features like the localisation of wireless devices ease the use of this technology. However, for the application on the factory floor real-time guarantees have to be given, which can be ensured by schemes like TDMA, which is based on implicit or explicit clock synchronization. This is typically supported by a high accurate timestamping of incoming packets for the reduction of jitter effects introduced by the protocol layers. This paper introduces an open platform, which supports research on receiver and timestamper design. The receiver is implemented in a flexible fashion, in order to support simultaneous multi-channel monitoring as well as easy reconfiguration for technologies other than IEEE 802.11b/g for efficient deployment in automation systems.

Localisation of Wireless LAN Nodes Using Accurate TDoA Measurements

Time based localisation methods like GPS are widely used for outdoor navigation, whereas indoor navigation is typically performed only on a cell-basis or based on the Received Signal Strength Indicator. Since RSSI is not able to fulfil all current requirements, Time of Arrival and Time Difference of Arrival based approaches have recently gained focus. As time based localisation has high demands on the quality of the timestamps, we propose a special receiver architecture for IEEE802.11b capable of a timestamping accuracy in the sub-nanosecond range. The receivers operation is demonstrated by an FPGA based wireless physical layer device implementation. Experimental results show an improvement in terms of accuracy by a factor of 100 up to 1000 over COTS wireless LAN hardware. This enables accurate localisation in wireless LANs just by adding of timing receivers.

SmartFridge: Demand Side Management for the device level

With the evolving technologies smart metering, or generalized the smart grid, it becomes reasonable to think about integrating household devices into a Demand Side Management (DSM) network. This paper gives an overview about the state of the art in the area of DSM. Subsequently, it is shown what is needed to convert a standard, commercial-off-the-shelf refrigerator into a networked smart fridge. Following this, a simulation model and a basic algorithm for load control of a refrigerator is presented and analyzed. Finally, possible improvements of hardware and the algorithms are proposed.

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.

Compensation of asymmetrical latency for ethernet clock synchronization

Clock synchronization has become an indispensable service in most distributed systems as it allows to sort events on a common time scale and coordinate collaborative actions.With the demand for even higher synchronization accuracy, new challenges and barriers have to be tackled to fulfill these requirements. One of them, the inevitable signal propagation time between the devices, is compensated in many state-of-the-art synchronization protocols by round-trip measurements, neglecting any form of delay asymmetry of the communication link. In this paper, we analyze the impact of asymmetry in networks based on the physical layer of copper-based Ethernet and compare different approaches on how to mitigate the impact of asymmetry. We propose a non-invasive system performing asymmetry measurements on a link basis and show that such a system can integrate into existing synchronization solutions.

Software support for clock synchronization over IEEE 802.11 wireless LAN with open source drivers

Between the wired and the wireless world a synchronization gap in terms of accuracy obviously exists due to the different possibilities of the technologies. This paper investigates means to access WLAN functionality in order to gain system-wide synchronization between access points and clients in order to establish a common notion of time in IEEE 802.11 systems. For this, a novel approach is presented, which uses beacons to transparently transport timing information even in high-loaded wireless LAN networks. Using the presented approach jitter accuracies in the microsecond range can be reached using the open Unix WLAN driver interfaces like madwifi or ath5k.