Mansoor Alam

Reliability analysis of phased-mission systems: a practical approach

This paper presents a novel and simple approach to reliability analysis of phased-mission systems. The technique presented in this paper transforms a phased-mission system into several non-phased-mission systems, and thus allows each phase to be modeled separately. The mission succeeds as long as every phase successively executes its requirements, and failed component(s) of any phase is(are) repaired so that the system is in a successful state before as well as after it begins the next phase. Alternative mission success scenarios are also readily definable within this context. The benefit of this simplified approach is that it provides a practical implementation for the reliability analysis of phased-mission systems. The different phases of the system are computed independently, and the duration and the structure of the phases are left to the designer or manager. If the requirements of the mission change during execution of the mission (duration of a phase or the sequence of phases), the impact of the change in mission objectives is easily found without recomputing the applicable or operative Markov model. This is due to the fact that all of the reliability analysis data are presented over the entire mission duration consisting for all phases, and the initial and final probabilities can easily be found by examining the output reliability data instead of recomputing the model

Performance analysis of large multicast switches with multicast virtual output queues

In multicast switches, the accommodation of multicast traffic in multiple queues per input buffer reduces the throughput degradation caused by head-of-line (HOL) blocking. Such an arrangement, called multicast virtual output queuing (MC-VOQ), is very promising in theory but can only be implemented in practice with heavy approximation. Complete avoidance of the HOL blocking problem would in fact require a distinct queue for each fanout set possible, leading to an exponential growth of the number of queues needed with the switch size. If only a limited number of queues can be used per input buffer, criteria must be identified for setting the number of queues, for associating the fanout sets with the individual queues, and for scheduling the transmission of packets out of the queues. This paper presents an analytical model for the investigation of saturation throughput and packet delay in MC-VOQ multicast switches. The model relies on the assumption of Poisson-distributed uniform input traffic and random queuing and scheduling policies. Extensive simulation experiments validate the results of the analysis for large switch sizes.

Alleviating path correlation using directional antenna in node-disjoint multipath routing for mobile ad hoc networks

Identifying independent node-disjoint paths is a challenging task in an ad hoc environment. In this paper, we study the impact of employing directional antennas on identifying independent node-disjoint paths in mobile ad hoc networks. We propose a protocol to identify multiple node-disjoint paths using semi-directional and directional antennas. To evaluate the protocol, we focus on correlation among multiple node-disjoint paths and overheads incurred in routing. We propose schemes to mitigate path correlation and routing overheads. We also show analytically that path correlation as well as routing overheads are reduced using proposed schemes which utilize directional antennas in comparison to existing schemes where omni-directional antennas are used.

Measuring the reliability of existing web servers

As the usage of the Internet increases and more critical functions rely on the World Wide Web for operation, it is important that network researchers have current and accurate information from a real-world standpoint of the achieved reliability of the Internet. Anecdotal evidence suggests that Web outages are a serious issue from a cost standpoint. However, there have been few mainstream studies showing the reliability of the existing web infrastructure. As the Internet has changed, so to has its capabilities and its reliability. Yet, there have been few independent studies which have provided quantitative measurement of actual Web reliability. This article provides results from a controlled experiment intended to measure the reliability of the existing web infrastructure.

Data Mining of Bayesian Network Structure Using a Semantic Genetic Algorithm-Based Approach

A Bayesian network model is a popular formalism for data mining due to its intuitive interpretation. This chapter presents a semantic genetic algorithm (SGA) to learn the best Bayesian network structure from a database. SGA builds on recent advances in the field and focuses on the generation of initial population, crossover, and mutation operators. In SGA, we introduce semantic crossover and mutation operators to aid in obtaining accurate solutions. The crossover and mutation operators incorporate the semantic of Bayesian network structures to learn the structure with very minimal errors. SGA has been proven to discover Bayesian networks with greater accuracy than existing classical genetic algorithms. We present empirical results to prove the accuracy of SGA in predicting the Bayesian network structures. IGI PUBLISHING This paper appears in the publication, Bayesian Network Technologies: Applications and Graphical Models edited by A. Mittal and A. Kassim