Syed Sajjad Rizvi

IEEE 802.11 & Bluetooth Interference: Simulation and Coexistence

IEEE 802.11 and Bluetooth, these two operating in the unlicensed 2.4Ghz frequency band are becoming more anymore popular in the mobile computing world. The number of devices equipped with IEEE 802.11 and Bluetooth is growing drastically. Result is the number of co-located devices , say within 10meters, grown to a limit, so that it may cause interference issues in the 2.4Ghz radio frequency spectrum. Bluetooth supports both voice(SCO) and data(ACL) packets. In this paper we investigate these interference issues and use a new Bluetooth voice packet named synchronous connection-oriented with Repeated Transmission (SCORT) to study the improvement in performance. For the sake of simulation results, we provide a comprehensive simulation results using MATLAB Simulink.

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.

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 novel approach for creating trust to reduce malicious behavior in MANET

This paper presents a Reputation-Trust (RT) system that can be used to stabilize the performance of the network for the working nodes even in the presence of malicious nodes which intentionally do not route and forward packets send by others correctly. The situation becomes even worst when nodes do not rely on any routing infrastructure but relay packets for each other. We introduce two extensions to the Dynamic Source Routing algorithm (DSR) [1] to mitigate the effects of routing misbehavior: the watchdog and the pathrater. Using these two approaches, the proposed RT system will update the RT table of each node with the ranked values of other nodes. The implementation of the proposed RT system is entirely based on the underlying proposed RT algorithm. Simulation results demonstrate that the RT system can be used to reduce the malicious behavior of mobile nodes and consequently improve the overall performance of MANET.

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.

Investigating the Effects of Trees and Butterfly Barriers on the Performance of Optimistic GVT Algorithm

There is two approaches for handling timing constraints in a heterogeneous network; conservatives and optimistic algorithms. In optimistic algorithms, time constraints are allowed to be violated with the help of a time wrap algorithm. Global Virtue Time (GVT) is a necessary mechanism for implementing time wrap algorithm. Mattern [2] has introduced an algorithm for GVT based computation using a ring structure. which showed high latency. The performance of this optimistic algorithm is optimal since it gives accurate GVT approximation. However, this accurate GVT approximation comes at the expense of high GVT latency. Since this resultant GVT latency is not only high but may vary, the multiple processors involve in communication remain idle during that period of time. Consequently, the overall throughput of a parallel and distributed simulation system degrades significantly In this paper, we discuss the potential use of trees and (or) butterflies structures instead of the ring structure. We present our analysis to show the effect of these new mechanisms on the latency of the system.