SIMULATION: Transactions of The Society for Modeling and Simulation International

Vahidi, Behrooz; Ghaffarzadeh, Navid; Hosseinian, Sayed Hosein; Ahadi, Seyed Mohammad

doi: 10.1177/0037549709340823pmid: N/A

High impedance faults (HIF) are faults that are difficult to detect by conventional protection relays. In this paper, a new HIF model is introduced and a novel methodology is presented to detect HIF by means of discrete wavelet transform (DWT) and artificial neural network (ANN). The distorted waveforms (HIF, load switching, line switching, capacitor switching and non-linear loads that behave similar to HIF current) are generated using PSCAD/EMTDC, captured with a sampling rate of 20 kHz and de-noised using DWT to obtain signals with higher signal-to-noise ratio. DWT is used to decompose the distorted signal and to extract its useful information. Appropriate feature vectors are created and applied in training the ANN. The effectiveness of the proposed method was tested using a wide spectrum of disturbances. Simulations are carried out to confirm the suitability and capability of the proposed method in HIF detection.

Abdallah, Walid; Hamdi, Mohamed; Boudriga, Noureddine; Obaidat, Mohammad S.

doi: 10.1177/0037549709341583pmid: N/A

Optical burst switching (OBS) is a promising method for data transfer in photonic networks based on a Wavelength Division Multiplexing (WDM) technology. Transmission control protocol (TCP)-based applications generate the majority of data traffic in the Internet; thus, understanding and improving the performance of TCP over OBS networks is critical. In this paper, we develop a novel burst-dropping strategy to improve the quality of service provided by TCP over OBS networks. Our approach relies on random-segment dropping according to the capacity of a special optical component, called the optical virtual memory, which is used for buffering purposes within the optical switches. The core node predicts incipient congestion by computing the average blocking duration in the optical virtual memories. When this size exceeds a threshold, segments are randomly dropped. Simulation results show that the proposed method performs better than common techniques in terms of burst loss probability and transmission delay.

Marurngsith, Worawan; Ibbett, Roland N.

doi: 10.1177/0037549709349843pmid: N/A

The emergence of chip multiprocessors is leading to rapid advances in hardware and software systems to provide distributed shared memory (DSM) programming models, so-called DSM systems. A DSM system provides programming advantages within a scalable and cost-effective hardware solution. This benefit derives from the fact that a DSM system creates a shared-memory abstraction on top of a distributed-memory machine by caching data replicas locally. In this respect, a coherence protocol is a vital component responsible for assuring data consistency across all replicas. The design of coherence protocols impacts a DSM system in terms of both performance and accuracy. Performance is often measured via simulation and various verification techniques have been proposed to deal with protocol accuracy. Nevertheless, integrating accuracy verification into a DSM cluster simulation to ensure correct simulation results is still an open issue. In this paper, we address three properties of a coherence protocol (safety, liveness, and inclusion) without which errors may occur in the simulation results. We propose a specification-based parameter—model interaction (SPMI) technique to detect these cases in a particular DSM cluster simulator called DSiMCluster. Our experimental results demonstrate that with SPMI, DSiMCluster can ensure the coherence protocol properties and provides a correct reflection of memory characteristics in shared-memory and DSM multiprocessors.

Chakraborty, Anshuk; Bhatnagar, Shalabh

doi: 10.1177/0037549709349324pmid: N/A

In this paper, we study the behaviour of the slotted Aloha multiple access scheme with a finite number of users under different traffic loads and optimize the retransmission probability qr for various settings, cost objectives and policies. First, we formulate the problem as a parameter optimization problem and use certain efficient smoothed functional algorithms for finding the optimal retransmission probability parameter. Next, we propose two classes of multi-level closed-loop feedback policies (for finding in each case the retransmission probability qr that now depends on the current system state) and apply the above algorithms for finding an optimal policy within each class of policies. While one of the policy classes depends on the number of backlogged nodes in the system, the other depends on the number of time slots since the last successful transmission. The latter policies are more realistic as it is difficult to keep track of the number of backlogged nodes at each instant. We investigate the effect of increasing the number of levels in the feedback policies. We also investigate the effects of using different cost functions (with and without penalization) in our algorithms and the corresponding change in the throughput and delay using these. Both of our algorithms use two-timescale stochastic approximation. One of the algorithms uses one simulation while the other uses two simulations of the system. The two-simulation algorithm is seen to perform better than the other algorithm. Optimal multi-level closed-loop policies are seen to perform better than optimal open-loop policies. The performance further improves when more levels are used in the feedback policies.