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A hybrid approach to minimize state space explosion problem for the solution of two stage tandem queues

Two stage open queuing networks are used for modeling the subsystem-behaviour in computers and communication networks, mass storage devices, memory servers, and queuing analysis of wireless mobile cellular networks. The queuing analysis of wireless systems is essential in order to quantify the impact of different factors on quality of service (QoS); performance measures so that wireless protocols can be designed and/or tuned in an optimal manner. In that sense two stage open queuing systems are particularly important to model handoff phenomena, especially for the integration of two different systems such as cellular and wireless local area networks (WLANs). Analytical solutions for two-dimensional Markov processes suffer from the state space explosion problem. The numerical difficulties caused by large state spaces, make it difficult to handle multiple servers at the second stage of a tandem queuing system together with server failures and repairs. This study presents a new approach to analytical modeling of open networks offering improvements in alleviating this problem. The proposed solution is a hybrid version, which combines well known spectral expansion, and hierarchical Markov reward rate approaches. Using this approach, two-stage open networks with multiple servers, break-downs, and repairs at the second stage and feedback can be modeled as three-dimensional Markov processes and solved for performability measures. Comparative results show that the new algorithm used for solution, provides a high degree of accuracy, and it is computationally more efficient than the existing approaches. The proposed model is capable of solving other three-dimensional Markov processes.

Modelling and performability analysis of network memory servers

This paper presents an analytical method for the performability evaluation of a proposed network memory server attached to a local area network. The proposed server is expected to improve data storage and retrieval. However, the underlying network will receive an increased amount of traffic which may have a significant effect on the networks performance. The proposed analytical model and its solution can be used to evaluate the performance of such a system. Mean queue lengths and the probability that the NMS queue is full are calculated and presented.

Implementation of Non-Pipelined and Pipelined Data Encryption Standard (DES) Using Xilinx Virtex-6 FPGA Technology

Data encryption process can easily be quite complicated and usually requires significant computation time and power despite significant simplifications. This paper discusses about pipelined and non-pipelined implementation of one of the most commonly used symmetric encryption algorithm, Data Encryption Standard (DES). The platform used for this matter is, Xilinx new high performance silicon foundation, Virtex-6 Field Programmable Gate Array technology. Finite state machine is used only in non-pipelined implementation, and it is not implemented for the pipelined approach. The testing of the implemented design shows that it is possible to generate data in 16 clock cycles when non-pipelined approach is employed. When pipelined approach is employed on the other hand, 17 clock signals are required for the initial phase only, and one clock signal is sufficient afterwards for each data generation cycle. The Very High Speed Integrated Circuit Hardware Description Language (VHDL) is used to program the design.

Path Loss Effect on Energy Consumption in a WSN

Energy consumption of nodes is a crucial factor that constrains the networks life time for Wireless Sensor Networks (WSNs). WSNs are composed of small sensors equipped with low-power devices, and have limited battery power supply. The main concern in existing architectural and optimisation studies is to prolong the network lifetime. The lifetime of the sensor nodes is affected by different components such as the microprocessor, the sensing module and the wireless transmitter/receiver. The existing works mainly consider these components to decide on best deployment, topology, protocols and so on. Recent studies have also considered the monitoring and evaluation of the path loss caused by environmental factors. Path loss is always considered in isolation from the higher layers such as application and network. It is necessary to combine path loss computations used in physical layer, with information from upper layers such as application layer for a more realistic evaluation. In this paper, a simulation-based study is presented that uses path-loss model and application layer information in order to predict the network lifetime. Physical environment is considered as well. We show that when path-loss is introduced, increasing the transmission power is needed to reduce the amount of packets lost. This presents a tradeoff between the residual energy and the successful transmission rate when more realistic settings are employed for simulation. It is a challenging task to optimise the transmission power of WSNs, in presence of path loss, because although increasing the transmission power reduces the residual energy, it also reduces the number of retransmissions required.

Analytical modelling and simulation of small scale, typical and highly available Beowulf clusters with breakdowns and repairs

Beowulf clusters are very popular because of the high computational power they can provide at reasonably low costs. However, the most pressing issues of today’s cluster solutions are the need for high availability and performance. Cluster systems are clearly prone to failures. Even if cover is provided with some probability c, there would be reconfiguration and/or rebooting delays to resume the operation following a failure. In this paper, the performability modelling of both typical and highly available Beowulf multiprocessor systems is presented. The models developed provide a large degree of flexibility to evaluate the performability of typical and highly available Beowulf cluster systems.

Joint-State Modeling for Open Queuing Networks with Breakdowns, Repairs and Finite Buffers

Networks of queues are extensively used in modelling transaction processing systems and nodes in communication networks. Today, many practical queuing systems with finite queuing capacity are widely used in communication and computing. In such systems, servers are prone to failures. In this paper, the performance modelling of a multi-node system, with finite capacity heterogeneous nodes, serving internal and external arrivals of jobs is considered. An analytical model has been developed, and solved for performability measures. Joint-state modelling approach is used for this purpose. In an attempt to improve accuracy in performability evaluation, assumptions made have been minimised and a series of computations have been performed for open queuing networks with breakdowns, repairs, and finite queuing capacity. Results obtained from the analytical model are presented comparatively with simulation results and IPP modelling.

Modelling Wireless Sensor Networks for Performability Evaluation

The higher demand for use of Wireless Sensor Technology, in the presence of complexities of various deployment environments, and application areas such as wireless multimedia sensor networks, call for the need to improve performance and availability of WSNs. This paper seeks to justify the need to model both performance and availability of WSNs together, parting from the current independent approaches and provides a systematic modelling approach for performability of WSNs. This has further been necessitated by positive research findings facilitating repair and replacement of faulty and dead sensor nodes and communication links. Two different analytical solution approaches are employed for performability modelling of a WSN cluster, and simulation results presented are in agreement with the analytical approximations.

Modelling and analysis of vertical handover in highly mobile environments

Various types of wireless networks have been developed and deployed including 3G, WLAN, WiMAX, LTE and LTE Advanced. User connectivity and network performance can be improved using vertical handover techniques which involve switching between available networks in heterogeneous environments. In this respect, recently there has been an increased interest in the integration of cellular and WLAN systems. In this study, the integrated heterogeneous wireless systems are modelled using two-stage open queuing systems. The proposed analytical model builds a framework for acceptable levels of QoS in heterogeneous environments. This paper gives the detailed analysis of the integrated cellular/WLAN systems based on the deployment of guard channels and buffering at base station for highly mobile users in the cellular systems. Numerical results are obtained using the exact spectral expansion solution approach. These results are then analysed in terms of operational spaces and are shown to be useful for vertical handover decision management.

Fault-Tolerant Two-Stage Open Queuing Systems With Server Failures at Both Stages

Two-stage open queuing systems are used to model and evaluate interaction between two systems in computer and communication networks. Application areas include two-stage internetworking mechanisms, memory servers, and interaction of wireless communication systems. Realistic features, such as finite capacities, feedback from one stage to another, and failures of servers usually complicate analytic solutions. This study presents a new analytical model and solution approach for two-stage open queuing systems with feedback, blocking, and multiple servers, as well as failures at both stages. Unlike the existing studies, systems considered in both stages can be fault tolerant. Numerical results presented comparatively with simulation show that the new approach performs well, in terms of accuracy and computation time.

A mathematical model for performability of Beowulf clusters

Beowulf clusters have become very popular as an alternative to supercomputers world-wide. However, the most pressing issues of todays cluster solutions is the need for high availability and performance. Such systems, clearly, are prone to break-downs. Even if cover is provided with some probability c, there will be reconfiguration and/or rebooting delays to resume the operation of a cluster. In this paper, the performance modelling for Beowulf multiprocessor systems is presented. For these systems, one head processor and several identical processors serving a common stream of arriving jobs is considered. To account for delays due to reconfiguration and rebooting, such systems are modelled and solved for exact performability measures, for both bounded and unbounded queuing capacities, using the spectral expansion method.