John C. Eidson

An Implementation of IEEE 1588 Over IEEE 802.11b for Synchronization of Wireless Local Area Network Nodes

IEEE 1588 is a new standard to synchronize independent clocks running on separate nodes of a distributed measurement and control system. It is intended for high-accuracy implementations on compact systems such as a single subnet. This paper examines potential accuracy limitations introduced by the physical layer of the IEEE 802.11b wireless local area network. Experimental results are presented that show that these limitations do not preclude clock-synchronization accuracy of several hundred nanoseconds.

Spider transparent clock

This paper discusses a device, the spider transparent clock, which can be used to retrofit existing bridges and routers to allow them to deliver highly accurate time using the IEEE 1588 protocol. The spider transparent clock corrects for the internal queuing jitter and asymmetry introduced by these bridges and routers.

Provision of Precise Timing via IEEE 1588 Application Interfaces

The protocol specified in IEEE 1588, together with a profile, define a timing system that may be used to supply precise timing to applications. However, IEEE 1588 does not say anything about the interface to the applications. In designing this interface, the application performance requirements (e.g., jitter, wander, time synchronization) must be considered. For example, an application that requires microsecond or better time synchronization needs a hardware or firmware interface; a software interface can result in exceeding the synchronization requirement by a factor of 1000 or more. This paper describes the performance requirements of example applications. It then describes a general application interface in abstract terms. Finally, it describes realizations of the interface that can meet the performance requirements for selected applications.

The design of distributed measurement systems based on IEEE1451 standards and distributed time services

This paper discusses the design challenges in creating effective and robust distributed measurement and control systems. The design of distributed measurement systems is greatly facilitated by recent standards such as IEEE1451.1 and IEEE1451.2, and the existence of network infrastructure features such as distributed time services. The point-to-point connection between the central controller and a peripheral is usually accomplished via a serial protocol such as RS-232, HART/sup TM/, or a specialized bus such as IEEE488. A distributed design allows additional degrees of freedom to meet real-time requirements and an opportunity to build more flexible yet more robust systems. A variety of infrastructure services, such as TDMA protocols typified by the Sercos protocol and the MARS/sup TM/ system, exist to aid in this coordination. Another approach is to use services built on a global sense of time established by synchronizing real-time clocks contained in each component. These services are discussed both theoretically and in terms of an example system. To illustrate these points, a simple distributed measurement system is examined both in terms of the design issues and the performance. This system has been constructed out of a mixture of commercially available and prototype components that make use of Ethernet, the IEEE1451 standards, synchronized clocks, and commonly available commercial infrastructure services and components such as Web browsers, databases and other common application tools.

LAN-based LXI Instrument Systems- the Next Step in the Evolution of Measurement System Technology

In the field of measurement, many of the leading test and measurement, T&M, companies have formed the LXI Consortium with the explicit goal of specifying an Ethernet-based architecture for T&M instrument systems. This paper explores the LXI architecture and gives performance figures illustrating the benefits of using this architecture.

Learning from Randomly-Distributed Inaccurate Measurements

Traditional measurement systems are designed with tight control over the time and place of measurement of the device or environment under test. This is true whether the measurement system uses a centralized or a distributed architecture. Currently there is considerable interest in using mobile consumer devices as measurement platforms for testing large dispersed systems. There is also growing activity in developing concepts of ubiquitous measurement, such as “smart dust.” Under these conditions the times and places of measurement are random, which raises the question of the validity and interpretation of the acquired data. This paper presents a mathematical analysis that shows it is possible under certain conditions to establish dependence between error bounds and confidence probability on models built using data acquired in this manner.