Common Time Reference Research Articles

Study of seismic signals of artificially released snow avalanches for monitoring purposes

Seismic signals generated by artificially released avalanches were studied from the seismological viewpoint in the time and frequency domains in an attempt to characterise them for monitoring purposes. The avalanches were artificially released and the corresponding seismic signals were recorded at different sites within a radius of up to 3 km together with the video images with a common time reference. The results obtained indicate that avalanches following the same path and recorded at the same site present similar seismic signals. A relationship between the avalanche seismic signals (amplitude, frequency content and wave trains) and the avalanche path was observed. The sources of the recorded seismic signals corresponds to avalanche path slope changes, interaction with obstacles and phenomena associated with the stopping stage of the avalanche. Local site effects were also present. These results lend support to the use of seismic signals as an efficient method of monitoring areas where avalanches are frequent and human observation is difficult.

Adaptive group multicast with time-driven priority

This paper shows how to provide an adaptive real-time group multicast (many-to-many) communication service. Adaptive means that the number of nodes that transmit to the multicast group is continuously changing. In order to meet deterministic quality-of-service (QoS) requirements of a real-time group multicast, some communication resources are reserved. We show (1) how bandwidth is reserved for each multicast group and (2) how an active source can dynamically share the bandwidth allocated to this multicast group with other active group members. Quality-of-service support for a real-time multicast group is based on time-driven priority. In this scheme the time is divided into time frames of fixed duration, and all the time frames are aligned by using a common global time reference, which can be obtained from the Global Positioning System. Bandwidth is allocated to a multicast group as a whole, rather than individually to each user. The allocation is done by reserving time intervals within time frames in a periodic fashion. This type of allocation raises two problems that are studied in this paper: (1) scheduling: how time intervals are reserved to each multicast group and (2) adaptive sharing: how the active (transmitting) participants can dynamically share the time intervals that have been reserved for their multicast group. The proposed approach is based on the embedding of multiple virtual rings, one for each multicast group. By using the virtual rings, it is simple to route messages to all the participants while minimizing the bound on the buffer sizes and queueing delays. The final part of this paper introduces a scalable growth of the multicast group by adding multiple subtrees to the virtual ring.

End-to-end delay analysis of videoconferencing over packet-switched networks

Videoconferencing is an important global application-it enables people around the globe to interact when distance separates them. In order for the participants in a videoconference call to interact naturally, the end-to-end delay should be below human perception; even though an objective and unique figure cannot be set, 100 ms is widely recognized as the desired one-way delay requirement for interaction. Since the global propagation delay can be about 100 ms, the actual end-to-end delay budget available to the system designer (excluding propagation delay) can be no more than 10 ms. We identify the components of the end-to-end delay in various configurations with the objective of understanding how it can be kept below the desired 10-ms bound. We analyze these components step-by-step through six system configurations obtained by combining three generic network architectures with two video encoding schemes. We study the transmission of raw video and variable bit rate (VBR) MPEG video encoding over (1) circuit switching; (2) synchronous packet switching; and (3) asynchronous packet switching. In addition, we show that constant bit rate (CBR) MPEG encoding delivers unacceptable delay-on the order of the group of pictures (GOP) time interval-when maximizing the quality for static scenes. This study aims at showing that having a global common time reference, together with time-driven priority (TDP) and VBR MPEG video encoding, provides adequate end-to-end delay, which is (1) below 10 ms; (2) independent of the network instant load; and (3) independent of the connection rate. The resulting end-to-end delay (excluding propagation delay) can be smaller than the video frame period, which is better than what can be obtained with circuit switching.

On relative timing in substorm onset signatures

We investigate the timing of popular substorm onset signatures to understand their temporal relationship. Proxies for substorm onsets include auroral breakups, high‐latitude magnetic bays, low‐latitude Pi2 bursts, dispersionless injections at geostationary orbits, and auroral kilometric radiation. We use the auroral breakup, identified with Polar UVI images, as a common reference time frame to calibrate the others. Results, illustrated by two well‐defined auroral substorms, unambiguously indicate that none of the four frequently used substorm onset proxies can provide a consistent timing of substorm onset. This inconsistency in substorm onset timing is attributed as a consequence of temporal and spatial limitations on each observational technique. A delay between the proxy identifiers and the auroral breakup is found to be typical. It is therefore strongly suggested from this study result that a common reference time frame for substorm onset is necessary, and we propose it should be auroral breakups. We argue that there is a need for an intercalibration of magnetospheric substorm phenomenology by using a unified definition of the substorm onset.

Building a global clock for observing computations in distributed memory parallel computers

A common time reference (i.e. global clock) is needed for observing the behavior of a distributed algorithm on a distributed computing system. The paper presents a pragmatic algorithm to build a global clock on any distributed system, which is optimal for homogeneous distributed memory parallel computers (DMPCs). In order to observe and sort concurrent events in common DMPCs, we need a global clock with a resolution finer than the message transfer time variance, which is better than what deterministic and fault-tolerant algorithms can obtain. Thus a statistical method is chosen as a building block to derive an original algorithm valid for any topology. Its main originality over related approaches is to cope with the problem of clock granularity in computing frequency offsets between local clocks to achieve a resolution comparable with the resolution of the physical clocks. This algorithm is particularly well suited for debugging distributed algorithms by means of trace recordings because after its acquisition step it does not induce message overhead: the perturbation induced on the execution remains as small as possible. It has been implemented on various DMPCs: Intel iPSC/2 hypercube and Paragon XP/S, Transputer-based networks and Sun networks, so we can provide some data about its behavior and performances on these DMPCs.

Ground motion in Mexico City during the April 25, 1989, Guerrero earthquake

Instrumental observations of ground motion in Mexico City during the April 25, 1989, Guerrero earthquake were analyzed. Our aim was to understand various aspects of the seismic response of the valley that had not been completely resolved. Such understanding of the basic mechanisms that control the seismic behavior of the valley sediments is crucial in any modeling attempt. The study of vertical motion for this event, which was shown to be practically unaffected by site conditions, lead to the identification of a prominent long-period Rayleigh wave. This, together with the availability of absolute time for some stations, allowed the establishment of a common time reference for all recordings. Horizontal motion, in contrast, was significantly amplified, with large increases in duration, at lake bed sites. In order to interpret the observed complexity of ground motion we studied two simplified models of soft alluvial valleys. One of these is two-dimensional and it is excited by plane S waves with variable polarization and incidence angles. This model allows three-dimensional response. The other is a three-dimensional axi-symmetric flat valley with a rigid base. Computations were performed in the frequency domain by means of a boundary integral method for the two-dimensional model and using a collocation least-squares technique for the three-dimensional one. Seismograms were obtained through Fourier synthesis. It was found that the irregular soft layer response produces polarization patterns which are similar to the observations, suggesting that the latter are a consequence of three-dimensional effects.

Computer Based Instrumentation for Nuclear Power Station

Publisher Summary This chapter focuses on the computer-based instrumentation for nuclear power station. It describes a data acquisition system comprising of different intelligent subsystems for Rajasthan Atomic Power Station. This new system would consist of six functionally independent microprocessor-based sub-systems and totally caters to nearly 2000 analog and 1500 digital inputs. The subsystems for acquiring analog data are: (1) data logger, (2) disturbance analyser, and (3) channel temperature monitoring subsystem. A centralized Master-Slave Clock system caters to all the subsystems for their real time need. This common time reference is essential to correlate printouts from different subsystems in analyzing events. The present system is designed to minimize component count and maintenance efforts. Bridge circuits and high excitation voltage present in the earlier system are eliminated. This system is designed around Euro standard. 3080 CPU, memory, I/O controller and other allied circuits are on Euro cards of size 110 mm × 160 mm. and are all housed in one 19” Euro crate. Some of the important features available in the disturbance analyser system are: (1) self-calibration facility which corrects each of the scanned channels with respect to a standard channel and (2) signal conditioning is achieved through the software.