Cell loss dynamics and output process in a finite-buffer discrete-time queue with correlated arrivals

This paper analyzes the performance of an ATM switching node considering cell arrival correlation. An ATM switching node is modeled as a discrete-time finite-buffer queue. Cell arrivals are assumed to follow a semi-Markovian process, where the number of cell arrivals in a lot depends on the states of the underlying (M-state) Markov chain in the current and previous slots. This paper presents analyses for various characteristics of the cell loss, as well as the distribution function of the cell output process from an ATM switching node. Obtained results include the cell loss probability, the consecutive loss probability, the distribution of loss period lengths, the joint distribution of successive cell interdeparture times, and the distribution of busy and idle periods. Through the numerical results, it is shown that both the correlation and the variation of cell arrivals significantly affect the cell loss and the output process characteristics

Performance Evaluation of a Packetized Voice System Simulation Study

Introduction of the packet switching technique into digitized voice communication may afford great advantages in efficient use of the channel, compared to both circuit-switched and DSI systems. Detailed characteristics, however, have not been obtained because of difficulty in the exact analysis. Hence, simulation models are developed in this paper for the packetized voice transmission system, and various characteristics such as transmission delays and loss probability of voice packets are obtained. We further evaluate three types of voice packet reassembly strategy at t he receiving terminal, and obtain the optimal packet length, which keeps each of overall packet transmission delay and packet loss probability less than a certain permissible values. Comparison among three strategies is also stated

A direct numerical simulation study of vorticity transformation in weakly turbulent premixed flames

Database obtained earlier in 3D Direct Numerical Simulations (DNS) of statistically stationary, 1D, planar turbulent flames characterized by three different density ratios σ is processed in order to investigate vorticity transformation in premixed combustion under conditions of moderately weak turbulence (rms turbulent velocity and laminar flame speed are roughly equal to one another). In cases H and M characterized by σ = 7.53 and 5.0, respectively, anisotropic generation of vorticity within the flame brush is reported. In order to study physical mechanisms that control this phenomenon, various terms in vorticity and enstrophy balance equations are analyzed, with both mean terms and terms conditioned on a particular value c of the combustion progress variable being addressed. Results indicate an important role played by baroclinic torque and dilatation in transformation of average vorticity and enstrophy within both flamelets and flame brush. Besides these widely recognized physical mechanisms, two other effects are documented. First, viscous stresses redistribute enstrophy within flamelets, but play a minor role in the balance of the mean enstrophy Ω ¯ ¯ ¯ within turbulent flame brush. Second, negative correlation u ′ ⋅∇Ω ′ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ between fluctuations in velocity u and enstrophy gradient contributes substantially to an increase in the mean Ω ¯ ¯ ¯ within turbulent flame brush. This negative correlation is mainly controlled by the positive correlation between fluctuations in the enstrophy and dilatation and, therefore, dilatation fluctuations substantially reduce the damping effect of the mean dilatation on the vorticity and enstrophy fields. In case L characterized by σ = 2.5, these effects are weakly pronounced and Ω ¯ ¯ ¯ is reduced mainly due to viscosity. Under conditions of the present DNS, vortex stretching plays a minor role in the balance of vorticity and enstrophy within turbulent flame brush in all three cases

Performance Evaluation of an Integrated Access Scheme in a Satellite Communication Channel

A method for realizing a circuit and packet integrated access scheme in a satellite communication channel is considered. Two kinds of terminals are assumed, namely, bursty terminals for handling bursty traffic and heavily loaded terminals for long-holdingtime message traffic. In this method, the channel frame is divided into two subframes: one is for bursty terminals, and the other is for heavily loaded terminals. The subframe for heavily loaded terminals is further divided into two subchannels, a reservation subchannel (consisting of small slots) and a message subchannel. The bursty terminals transmit their packets in their dedicated subframes on the slotted ALOHA protocol. The heavily loaded terminal having a message transmits, first of all, a reservation packet in a randomly selected small slot of the reservation subchannel to reserve slots in the coming message subchannels. One slot in the same position of each of the succeeding message subchannels is reserved for the terminal until the end-of-use flag, transmitted from the terminal, is received by the satellite. Mean transmission delays for both kinds of traffic in this method are analytically obtained. We show that there exists an optimal frame length which minimizes mean transmission delay for one kind of traffic while keeping mean transmission delay for the other kind under some permissible value

Three reversible states controlled on a gold monoatomic contact by the electrochemical potential

Conductance of an Au mono atomic contact was investigated under the electrochemical potential control. The Au contact showed three different behaviors depending on the potential: 1 $G_{0}$ ($G_{0}$ = $2e^{2}/h$), 0.5 $G_{0}$ and not-well defined values below 1 $G_{0}$ were shown when the potential of the contact was kept at -0.6 V (double layer potential), -1.0 V (hydrogen evolution potential), and 0.8 V (oxide formation potential) versus Ag/AgCl in 0.1 M Na$_{2}$SO$_{4}$ solution, respectively. These three reversible states and their respective conductances could be fully controlled by the electrochemical potential. These changes in the conductance values are discussed based on the proposed structure models of hydrogen adsorbed and oxygen incorporated on an Au mono atomic contact.Comment: 8 pages, 4 figures, to be appeared in Physical Review

Throughput and delay analysis of free access tree algorithm with mini-slots

This paper analyzes throughput and delay performance of two kinds of free access tree algorithms with mini-slots, and the other is that ternary feedback information is available. In these algorithms Q number of mini-slots are provided within a slot to offer feedback information on the state of the channel. The maximum throughputs of binary and ternary feedback algorithms are analytically obtained. It is also shown that the highest maximum throughput of 0.56714 is achieved when Q approaches infinity and mini-slot overhead goes to zero. The lower bound of the average transmission delays in these algorithms is analytically derived. The obtained lower bound is also a lower bound of the average delay of the whole class of free access algorithms

Saccharification of lignocellulosic biomass under mild condition using ionic liquid

Biomass is expected to be an alternative resource to fossil resources. In this study, the development of a biomass conversion method into the valuable chemical, reducing sugar, was examined. For the conversion, thermochemical technology was focused on for its advantage of short reaction time, and ionic liquid was focused on as a reagent to overcome the unpreferable disadvantage of thermochemical technology, which is the low selectivity. Cedar and crystalline cellulose were pretreated with ionic liquid for reforming into desirable precursors of reducing sugar. Especially when they were pretreated by 1-ethyl-3-methylimidazolium methylphosphonate at 150°C for 1 h, the pretreatment worked effectively by decreasing the crystallinity of samples. Pretreated cedar and crystalline cellulose were converted into reducing sugar under hydrothermal conditions, respectively, by 39 C-% and 90 C-%. Recovery of ionic liquid was also examined. When cedar was used as a material, lignin was dissolved into ionic liquid through pretreatment, which was undesirable because of difficult separation thereof. When crystalline cellulose was used as a material, 98.3% of the ionic liquid was recovered after the conversion with the highest yield of reducing sugar (90 C-%)

Application of conditioned structure functions to exploring influence of premixed combustion on two-point turbulence statistics

International audienceIn order to investigate the influence of combustion-induced thermal expansion on turbulent flow within a premixed flame brush, a new method is introduced. The method consists in analyzing structure functions of the velocity field, which characterize velocity difference in two points A and B, with the structure functions being conditioned to various events; (i) unburned reactants in both points, (ii) combustion products in both points, (iii) intermediate states of the mixture in both points, (iv) the reactants in one point and the products in another point, (v) the reactants in one point and an intermediate state in another point, and (vi) the products in one point and an intermediate state in another point. Such structure functions and relevant probabilities are defined in the paper. Subsequently, the structure functions and the probabilities are extracted from Direct Numerical Simulation (DNS) data obtained from two statistically 1D, planar, fully-developed, weakly turbulent, premixed flames characterized by two significantly different (7.53 and 2.50) density ratios, with all other things being approximately equal. Obtained results indicate that (i) the conditioned structure functions differ significantly from the mean structure functions and (ii) the newly introduced approach could convey information important for understanding fundamentals of flame-turbulence interaction and finding issues that require further research. In particular, application of the approach to the aforementioned DNS data shows that the combustion-induced thermal expansion substantially affects small-scale two-point velocity statistics in the incoming constant-density turbulent flow of unburned reactants within a premixed flame brush

A DNS study of the physical mechanisms associated with density ratio influence on turbulent burning velocity in premixed flames

International audienceData obtained in 3D direct numerical simulations of statistically planar, 1D weakly turbulent flames characterized by different density ratios σ are analyzed in order to study the influence of thermal expansion on flame surface area and turbulent burning rate. Obtained results show that, on the one hand, pressure gradient induced within the flame brush due to heat release in flamelets significantly accelerates unburned gas that deeply intrudes into combustion products in a form of an unburned mixture finger, thus, causing large-scale oscillations of the turbulent burning rate and flame brush thickness. Under conditions of the present simulations, contribution of this mechanism to creation of flame surface area is substantial and is increased by the density ratio, thus, implying an increase in the burning rate by σ. On the other hand, the total flame surface areas simulated at σ = 7.53 and 2.5 are approximately equal to one another. Apparent inconsistency between these results implies existence of another thermal expansion effect that reduces the influence of the density ratio on the flame surface area and burning rate. Investigation of the issue shows that the axial flow acceleration by the combustion-induced pressure gradient not only straightforwardly creates flame surface area by pushing a finger tip into products, but also mitigates wrinkling of the flame surface (the side surface of the finger) by turbulent eddies. The latter effect is attributed to a high-speed (at σ = 7.53) axial 1 flow (a jet) of unburned gas, which is induced by the axial pressure gradient within the flame brush (and the finger). This axial flow acceleration reduces a residence time of a turbulent eddy in an unburned zone of the flame brush (e.g. within the finger). Therefore, the capability of the eddy for wrinkling the flamelet surface (e.g. the side finger surface) is weakened due to a shorter residence time