Aasia Riasat

Minimizing the Null Message Exchange in Conservative Distributed Simulation

The performance of a conservative time management algorithm in a distributed simulation system degrade s significantly if a large number of null messages are exchanged across the logical processes in order to avoid deadlock. This situation gets more severe when the exchange of null messages is increased due to the poor selection of key parameters such as lookahead values. However, with a mathematical model that can approximate the optimal values of parameters that are directly involved in the performance of a time management algorithm, we can limit the exchange of null messages. The reduction in the exchange of null messages greatly improves the performance of the time management algorithm by both minimizing the transmission overhead and maintaining a consistent parallelization. This paper presents a generic mathematical model that can be effectively used to evaluate the performance of a conservative distributed simulation system that uses null messages to avoid deadlock. Since the proposed mathematical model is generic, the performance of any conservative synchronization algorithm can be approximated. In addition, we develop a performance model that demonstrates that how a conservative distributed simulation system performs with the null message algorithm (NMA). The simulation results show that the performance of a conservative distributed system degrades if the NMA generates an excessive number of null messages due to the improper selection of parameters. In addition, the proposed mathematical model presents the critical role of lookahead which may increase or decrease the amount of null messages across the logical processes. Furthermore, the proposed mathematical model is not limited to NMA. It can also be used with any conservative synchronization algorithm to approximate the optimal values of parameters.

Use of Self-Adaptive Methodology in Wireless Sensor Networks for Reducing Energy Consumption

A sensor network is made up of numerous small independent sensor nodes with sensing, processing and communicating capabilities. The sensor nodes have limited battery and a minimal amount of on-board computing power. A self-adaptive methodology that utilizes the source and path redundancy techniques to efficiently reduce the required energy consumption is proposed. The proposed methodology presents a self-adaptive strategy to optimize the number of active sensor nodes and assign equal time slots to each sensor nodes for sensing and communication with the BS. The self-adaptive property enables the proposed methodology to perform a global search for optimizing the position of active sensor nodes with respect to the location of the base station (BS). The global search performed by the proposed methodology is carried out in a complete top down manner. The proposed self-adaptive methodology, therefore, not only reduces the energy consumption of wireless sensor nodes but also effectively maximizes the lifetime of active sensor nodes. Simulation results demonstrate that the proposed methodology significantly minimizes the energy consumption and consequently increases the life time of active sensor nodes.

AN EFFICIENT OPTIMISTIC TIME MANAGEMENT ALGORITHM FOR DISCRETE-EVENT SIMULATION SYSTEM

Time Wrap algorithm is a well-known mechanism of optimistic synchronization in a parallel discrete-event simulation (PDES) system. It offers a run time recovery mechanism that deals with the causality errors. For an efficient use of rollback, the global virtual time (GVT) computation is performed to reclaim the memory, commit the output, detect the termination, and handle the errors. This paper presents a new unacknowledged message list (UML) scheme for an efficient and accurate GVT computation. The proposed UML scheme is based on the assumption that certain variables are accessible by all processors. In addition to GVT computation, the proposed UML scheme provides an effective solution for both simultaneous reporting and transient message problems in the context of synchronous algorithm. To support the proposed UML approach, two algorithms are presented in details, with a proof of its correctness. Empirical evidence from an experimental study of the proposed UML scheme on PHOLD benchmark fully confirms the theoretical outcomes of this paper. (Received in June 2009, accepted in April 2010. This paper was with the authors 5 months for 3 revisions.)

Reducing null message traffic in large parallel and distributed systems

Null message algorithm (NMA) is one of the efficient conservative time management algorithms that use null messages to provide synchronization between the logical processes (LPs) in a parallel discrete event simulation (PDES) system. However, the performance of a PDES system could be severely degraded if a large number of null messages need to be generated by LPs to avoid deadlock. In this paper, we present a mathematical model based on the quantitative criteria specified in (Rizvi et al., 2006) to optimize the performance of NMA by reducing the null message traffic. Moreover, the proposed mathematical model can be used to approximate the optimal values of some critical parameters such as frequency of transmission, Lookahead (L) values, and the variance of null message elimination. In addition, the performance analysis of the proposed mathematical model incorporates both uniform and non-uniform distribution of L values across multiple output lines of an LP. Our simulation and numerical analysis suggest that an optimal NMA offers better scalability in PDES system if it is used with the proper selection of critical parameters.

Transformation matrix algorithm for reducing the computational complexity of multiuser receivers for DS-CDMA wireless systems

Multiuser detection is an important technology in wireless CDMA systems for improving both data rate as well as user capacity. However, the computational complexity of multiuser detection prevents the widespread use of this technique. Most of the CDMA systems today and in the near future will continue to use the conventional matched filter with its comparatively low user capacity and a slow data rate. However, if we could lower the computational complexity of multiuser detectors, most of the CDMA systems would be likely to take advantage of this technique in order to gain increased system capacity and a better data rate. In this paper, a novel approach for reducing the computational complexity of multiuser receivers is proposed. It utilizes the transformation matrix technique to improve the performance of multiuser detectors. We show that the mathematical computations of the implementation complexity can turn in overall less complex system that has strong impact on the systems signal to noise ratio (SNR) and the bit error rate (BER). The performance measure adopted in this paper is the achievable bit rate for a fixed probability of error (10-7) and consistent values of SNR.

Active scanning: A better approach to reduce handover time at MAC layer for wireless networks

Extensive studies have been carried out for reducing the handover time of wireless network at medium access layer (MAC). However, none of them depicts the impact of reduced handover time on the overall performance of wireless networks. This paper presents the methodology that can be used to effectively reduce the handover time. Our proposed model incorporates many critical performance measurements to show the impact of reduced handover time on wireless networks. Our experiments verify that the active scanning can reduce the overall handover time at MAC layer if comparatively shorter beacon intervals are utilized for packet transmission.

A closed-form expression for BER to quantify MAI for synchronous DS-CDMA multi-user detector

A closed-form expression to determine an average error rate for synchronous DS-CDMA multi-user detector is derived based on the transformation matrix (TM) algorithm proposed in [1]. The derived expression for average bit error rate (BER) can be used to produce higher and consistent values of signal-to-noise ratio (SNR) that consequently results good quality signal. In addition, the closed-form expression can be used to quantify the multiple access interference (MAI) for a desirable BER performance by which unwanted signals or interference can be suppressed relative to the desired signal at the receiving end. The derived closed-form expression for BER is not only shown to substantially improve the performance of the multiuser detectors by means of higher values of SNR but also has much lower MAI. The performance measure adopted in this paper is the achievable bit rate for a fixed probability of bit error (10-7) and the consistent values of SNR.

Trees and Butterflies Barriers in Distributed Simulation System: A Better Approach to Improve Latency and the Processor Idle Time

Global virtual time (GVT) is used in parallel discrete event simulations to reclaim memory, commit output, detect termination, and handle errors. Mattern s [I] has proposed G VT approximation with distributed termination detection algorithm. This algorithm works fine and gives optimal performance in terms of accurate GVT computation at the expense of slower execution rate. This slower execution rate results a high GVT latency. Due to the high GVT latency, the processors involve in communication remain idle during that period of time. As a result, the overall throughput of a discrete event parallel simulation system degrades significantly. Thus, the high G VT latency prevents the widespread use of this algorithm in discrete event parallel simulation system. However, if we could improve the latency of GVT computation, most of the discrete event parallel simulation system would likely take advantage of this technique in terms of accurate G VT computation. In this paper, we examine the potential use of tress and butterflies barriers with the Matterns GVT structure using a ring. Simulation results demonstrate that the use of tree barriers with the Matterns GVT structure can significantly improve the latency time and thus increase the overall throughput of the parallel simulation system. The performance measure adopted in this paper is the achievable latency for a fixed number of processors and the number of message transmission during the G VT computation.

Implementation of Tree and Butterfly Barriers with Optimistic Time Management Algorithms for Discrete Event Simulation

The Time Wrap algorithm [3] offers a run time recovery mechanism that deals with the causality errors. These run time recovery mechanisms consists of rollback, anti-message, and Global Virtual Time (GVT) techniques. For rollback, there is a need to compute GVT which is used in discrete-event simulation to reclaim the memory, commit the output, detect the termination, and handle the errors. However, the computation of GVT requires dealing with transient message problem and the simultaneous reporting problem. These problems can be dealt in an efficient manner by the Samadi’s algorithm [8] which works fine in the presence of causality errors. However, the performance of both Time Wrap and Samadi’s algorithms depends on the latency involve in GVT computation. Both algorithms give poor latency for large simulation systems especially in the presence of causality errors. To improve the latency and reduce the processor ideal time, we implement tree and butterflies barriers with the optimistic algorithm. Our analysis shows that the use of synchronous barriers such as tree and butterfly with the optimistic algorithm not only minimizes the GVT latency but also minimizes the processor idle time.

Deterministic formulization of bandwidth efficiency for multicast systems

End-System multicasting (ESM) is a promising application-layer scheme that has been recently proposed for implementing multicast routing in the application layer as a practical alternative to the IP multicasting. Moreover, ESM is an efficient application layer solution where all the multicast functionality is shifted to the end users. However, the limitation in bandwidth and the fact that the message needs to be forwarded from host-to-host using unicast connection, and consequently incrementing the end-to-end delay of the transmission process, contribute to the price to pay for this new approach. Therefore, supporting high-speed real-time applications such as live streaming multimedia, videoconferencing, distributed simulations, and multiparty games require a sound understanding of these multicasting schemes such as IP multicast and ESM and the factors that might affect the end-user requirements. In this paper, we present an analytical model that formalizes the bandwidth efficiency of both IP and ESM multicast systems. Specifically, our proposed formalization of the bandwidth efficiency is based on the end-to-end delays proposed by [11] for both IP and ESM multicast systems.