Sharif Digital Repository

The IEEE 802.11 contention-based distributed coordination function has been used in ad hoc networks, and so far, a comprehensive mathematical analysis of such multihop systems has not been reported in the literature due to complicating factors such as hidden terminals and the unreachability problem. In this paper, we propose a new analytical model based on the parallel space-time Markov chain for IEEE 802.11 medium access control (MAC) in multihop ad hoc networks. The proposed scheme is able to model the hidden-terminal and unreachability phenomena. Extensive evaluation is conducted to demonstrate the accuracy of the proposed framework. © 2009 IEEE  

The increasing demand for Web services encourages commercial cloud service providers to publish their own services with various functional and nonfunctional capabilities in different cloud platforms. The aggregation of atomic services from multiple service repositories is the main idea of the service composition concept in multiclouds. The cloud Web service composition is a suitable way for satisfying users' complex requests by integrating services from different clouds in order to create a new value-added composite service. The time required to serve a composite service by a multicloud environment is an important parameter, which depends on different factors, ranging from the service... 

The increasing demand for Web services encourages commercial cloud service providers to publish their own services with various functional and nonfunctional capabilities in different cloud platforms. The aggregation of atomic services from multiple service repositories is the main idea of the service composition concept in multiclouds. The cloud Web service composition is a suitable way for satisfying users' complex requests by integrating services from different clouds in order to create a new value-added composite service. The time required to serve a composite service by a multicloud environment is an important parameter, which depends on different factors, ranging from the service... 

Infrastructure-as-a-Service (IaaS) clouds not only have to meet business requirements but also need to consider other important metrics that influence the quality of service, such as performance, availability, and power consumption. In this paper, we present a Stochastic Activity Network (SAN) based analytical approach that simultaneously computes these metrics under a given load for IaaS cloud data centers. The model's high-level abstraction of IaaS clouds allows us to evaluate different resource management strategies and features, including cloud federation and dispatching policies. In a first step, we model a single rack including its autonomous local manager responsible for scheduling... 

The reliability of wireless sensor networks (WSNs) depends on both sensing coverage and reliable transmission of collected data provided by the target cluster of sensors in the proximity of the phenomenon to the observer. In this paper, we establish a probabilistic fundamental quantitative notion for performance-critical applications on reliable information transfer and propose a new analytical model to calculate the coverage-oriented reliability of cluster-based WSNs. Here, the packet loss probability is modeled as a function of two main parameters. The first is the probability of link failure when the node's buffer at the end point of the link is full (discarding the packet) or when the... 

Wireless Sensor Network (WSN) should be capable of fulfilling its mission, in a timely manner and without loss of important information. In this paper, we propose a new analytical model for calculating RRT (Reliable Real-Time) degree in multihop WSNs, where RRT degree describes the percentage of real-time data that the network can reliably deliver on time from any source to its destination. Also, packet loss probability is modeled as a function of the probability of link failure when the buffer is full and the probability of node failure when node's energy is depleted. Most of network properties are considered as random variables and a queuing-theory based model is derived. In this model,... 

Micro actuators are an irreplaceable part of motion control in miniaturized systems and are intended to have a high range of deformation, high accuracy, large force, and quick response. In this article, an analytical model for a hybrid thermopiezoelectric micro actuator is developed in which a double lead-zirconnate-titanate piezoceramic (PZT) beam structure consisting of two arms with different lengths are used. Governing differential equation of motion and electrical field are derived and solved. Out of parametric studies it was observed that, under application of temperature and voltage gradients, the deflection of the actuator shows different trends depending on the geometry of the micro... 

Resolver, as an electromagnetic sensor, is widely used in different industrial applications such as servo motor feedback applications because of its reliable performance in the presence of undesired external factors in a harsh environment. Among different types of resolvers, variable reluctance (VR) resolvers draw attention due to their robust structure with a winding-less solid rotor and low-cost manufacturing. In this article, analytical modeling based on the magnetic equivalent circuit (MEC) is employed to investigate the output signals of the proposed toroidal winding arrangements for VR resolver with a 5-X rotor. Time-stepping finite element analysis (TSFEA) is used for all the... 

Purpose: This research attempts to formulate an analytical model for the studying and assessment of necessary factors that have strong effects on the level of Iranian Technology Park competitiveness. The result of this model leads to increasing the effectiveness and efficiency of Technology Parks planning stage. Methodology/Approach: The main framework of this research is based on studying the projects and research, which in their methods use the Porter's diamond model or SWOT analytical method. On the other hand, this framework is customized for the future needs of Technology Parks and clarifies the kind of information that must be gathered. Finding: Identification of the core competencies... 

An analytical model is proposed to study the effect of particle size on melting enthalpy and entropy of metallic nanoparticles (NPs). The Mott's and Regel's equations for melting entropy in the combination of core average coordination number (CAC) and surface average coordination number (SAC) of freestanding NPs are considered. Clusters of icosahedral (IC), body centered cubic (BCC), and body centered tetragonal (BCT) structure without any vacancies and defects are modeled. Using the variable coordination number made this model to be in good agreement with experimental and molecular dynamic (MD) results of different crystal structures. The model predicts melting entropy and enthalpy of... 

Electrical post insulators are important components of electrical substations since any type of failure in such insulators leads to the breakdown of the local network. Although the electrical substations are often in service condition, any horizontal excitation due to the earthquake, or any extreme event, may cause lateral deformation and damage to the post insulators. Hollow composite post insulators, a new and evolving technology, have a very complex mechanical behavior due to their materials and connections. To date, the design of such post insulators has been based on the limited test results available in the literature. Most of experiments have been conducted on small-scale specimens... 

The stretched hypercube has recently been introduced as an attractive alternative to the well-known hypercube. Previous research on this network topology has mainly focused on topological properties, VLSI and algorithmic aspects of this network. Several analytical models have been proposed in the literature for different interconnection networks, as the most cost-effective tools to evaluate the performance merits of such systems. This paper proposes an analytical performance model to predict message latency in wormhole-switched stretched hypercube interconnection networks with fully adaptive routing. The analysis focuses on a fully adaptive routing algorithm which has been shown to be the... 

This paper presents a novel, sliding, A-shaped microrobot with nanometric resolution for precision positioning applications. The microrobot is actuated near its natural frequency using a piezoelectric stack actuator to produce translational motion. The dynamic modeling of the mechanism is based on the assumptions of the linear piezoelectric behavior and the Coulomb friction model. Using this model the required condition for generating net motion is found. The suitability of three simple, friction-based locomotion modes for implementation on the proposed device is addressed. Influences of some important configuration parameters on the behavior of the microrobot, based on defined criteria, are... 

A depleted naturally fractured gas condensate reservoir with extremely tight matrix permeability in the center of Iran is used for gas storage. Due to the tightness of the matrix, simulation of this process may present a unique challenge in terms of the importance of diffusion process in flow behavior of the injected gas in matrix blocks. This article presents a multimechanistic (Darcian-type flow and Fickian-type flow) mathematical model to investigate the importance of diffusion process as a function of important rock and fluid parameters. The authors' approach consists of the following steps: (a) development of a 1-D governing equations for a single matrix block consists of a gas... 

In the present study, a novel theoretical model is developed, based on classical laminate theory, to predict the equivalent mechanical properties of the re-entrant lattice structures, which composed of continuous fiber reinforced composite struts. Three main mechanism of stretching, flexing and hinging are considered and a general closed-form formulation is derived to estimate the auxetic honeycomb's elastic and shear modulus as well as Poisson's ratios. In spite of previous studies in which the response of honeycomb structures is modeled using beam theory, here, each strut of unit cell is expressed as a composite laminate with orthotropic mechanical properties and classical laminate theory... 

Interaction of Carbon Nanotubes (CNTs) and nanovoids embedded in a metal matrix is studied comparatively. For this purpose, two nearby CNTs/nanovoids are modeled as two similar cylindrical inclusions/holes in an infinite matrix. The nonlocal stresses around the CNTs/nanovoids are obtained by applying the integral constitutive equation of the nonlocal elasticity to the stresses from the complex stress potential method. Also, in order to bring different nonlocality effects of dissimilar media into account, the influence function of the nonlocal elasticity is modified. Effects of the CNTs/nanovoids size and distance as well as far-field loading ratio on the stress state in the matrix are... 

Modeling of the laser-assisted machining (LAM) process requires an accurate estimate of the surface absorption coefficient. In addition to the laser type and material properties, this parameter is dependent on the state of the process in terms of surface quality, texture, oxidation state, and temperature change during the machining process. In this article, two methods are presented for calculating this coefficient based on the surface temperature obtained from measurement by a pyrometer and an analytical model of the process. In the first method, the pyrometer is fixed on a lathe bed while in the second method, a co-moving pyrometer is used. Temperature measurements in both methods are... 

Most slab-on-girder steel bridges have been designed and constructed without seismic resistance consideration; as a result, their members, connections, and more commonly their substructure are not sized and detailed to provide the required ductile response needed during major earthquakes. A ductile seismic retrofit solution proposed in this investigation consist of converting of existing end diaphragms into ductile end diaphragms and replacing the end cross-frames and the lower lateral-braced panels adjacent to the supports by special ductile diaphragms. Consequently, this creates ductile fuses to protect the rest of the super- and substructure. This paper illustrates the typical seismic... 

Use of infrared light for indoor wireless communications has received considerable attention recently. In this paper, we present results obtained from a large set of measurements performed at the University of Ottawa. We investigate impacts of receiver rotation and shadowing on the properties of indoor infrared channels. This paper contains a description of a measurement system developed and used throughout the experiments. Measurement results are used to find and present methods to estimate variations of channel path loss for small changes in the receiver direction, using statistical techniques. This would be useful for generating samples of channel path loss for system performance... 

The fault-tolerant capability is a prerequisite for electrical safety-critical applications. In this regard, multi-phase induction motors are well-known actuators used to provide this feature. Compared to conventional three-phase machines with distributed windings, induction machines having concentrated windings could be more fault-tolerant because of increasing the number of phases. In addition, concentrated windings have shorter end-winding and less required copper weight which makes it a lower cost solution compared to distributed windings. However, concentrated windings adversely increase the distortion of the air gap flux density which affects motor performances. In this paper, a...