Modeling the IEEE 802.11e HCCA mode

It was shown that the hybrid coordination function control channel access (HCCA) is capable of guaranteeing quality of service in wireless local area networks. However, there is still no comprehensive analytical model for HCCA. Therefore, novel modeling of pure HCCA based on the cyclic-service queueing system is provided in this paper. Our model is general enough to accept a wide range of schedulers and various types of traffic under the finite buffer policy. Via comparisons with simulations, high accuracy of the analytical model is exhibited. Furthermore, some valuable insights and recommendations on how to improve the HCCA performance are revealed by investigating the HCCA mode through the proposed model.     

Fermentation of low-salt miso as affected by supplementation with ethanol.

Steam-cooked soybeans and rice koji were combined (1:1, w/w), mixed with 5% (w/w) NaCl and ground into a fine paste. Samples (30 g) were deposited in nylon/polyethylene plastic bags and supplemented with 10 ml of aqueous ethanol solutions to give concentrations of 0, 2.5, 5, 7.5, 10, 15, 20, and 25% ethanol. Mixtures were homogenized, sealed, and incubated at 28 degrees C for eight weeks. Mold populations were less than 3 log10 CFU/g in all miso products after four weeks of fermentation. Yeast populations increased to 6.1 log10 CFU/g in the control (0% added ethanol) during the first week of fermentation and remained stable throughout the eight-week fermentation period. Yeasts were not detected in products containing 5-25% ethanol. Populations of lactic acid bacteria (LAB) increased to 6 log10 CFU/g after one week of fermentation in products containing 0 and 2.5% ethanol. However, after eight weeks of fermentation, LAB populations in all products were less than 4 log10 CFU/g. Rapid decreases in pH occurred only in products supplemented with 0 or 2.5% ethanol. Percentages of soluble protein in miso products containing various ethanol concentrations during the eight-week fermentation period revealed that protease activity was still active or not greatly inhibited in products supplemented with less than 10% ethanol. In comparison, koji enzymes were comparatively less affected by ethanol than were populations of molds, yeasts, and LAB. Total soluble carbohydrate and glucose contents were higher in products supplemented with 5, 7.5 and 10% ethanol than in other products. Discoloration (browning) during fermentation occurred most rapidly in products supplemented with 5 or 7.5% ethanol. Sensory evaluation of the low-salt (5%) product supplemented with 7.5% ethanol and fermented for eight weeks revealed normal or enhanced flavor ratings compared to ratings for a commercial product.     

Designing a fair scheduling mechanism for IEEE 802.11 wireless LANs

A fair scheduling mechanism called distributed elastic round robin (DERR) is proposed in this letter for IEEE 802.11 wireless LANs operated in a distributed manner. To quantify the fairness, we not only derive its fairness bound, but also observe the fairness through ratios of throughput and weight using a simulation approach. By numerical comparisons among DERR, distributed deficit round robin (DDRR), and IEEE 802.11e, we demonstrate that DERR outperforms the other two mechanisms in performance and fairness.     

Coordinated pursuer control using particle filters for autonomous search-and-capture

In a real-world pursuit-evasion (PE) game, the pursuers often have a limited field-of-view of the evaders and thus are required to search for and detect the evaders before capturing them. This paper presents a unified framework and control algorithm using particle filters (PFs) for the coordination of multiple pursuers to search for and capture multiple evaders given the ability of PF to estimate highly non-Gaussian densities prevalent in search problems. The pursuer control problem is formulated as a stochastic control problem where global objectives function of both searching and capturing are common. To take the evaders’ actions into account, an action measure (AM) is defined over the evaders’ PDs is used to represent the probability that the evader may transit each state in the PD. The global objective functions for search and capture are then decomposed into local objective functions for unification through objective priority weights. Coordination between the pursuers takes place through the multi-sensor update where the observation likelihoods of all pursuers are used in the PF update stage. The control actions of each pursuer are then determined individually, based on the updated PDs given the objective weights, action measures as well as evader importance weights in the case of multiple evaders. The proposed algorithm is tested in three scenarios for its effectiveness. In addition, a parametric study on the average capture time against the initial variances of the target state uncertainty is conducted to test for robustness. Results show that the pursuers are able to capture all the evaders in each case with the capture time for the second and last scenario differing by only 2.9% implying firstly that under the proposed algorithm, the capture time is not proportional to the increase in the number of evaders and also suggested robustness and potential scalability of the proposed algorithm.     

Predicting local distributions of erosion–corrosion wear sites for the piping in the nuclear power plant using CFD models

The erosion–corrosion (E/C) wear is an essential degradation mechanism for the piping in the nuclear power plant, which results in the oxide mass loss from the inside of piping, the wall thinning, and even the pipe break. The pipe break induced by the E/C wear may cause costly plant repairs and personal injures. The measurement of pipe wall thickness is a useful tool for the power plant to prevent this incident. In this paper, CFD models are proposed to predict the local distributions of E/C wear sites, which include both the two-phase hydrodynamic model and the E/C models. The impacts of centrifugal and gravitational forces on the liquid droplet behaviors within the piping can be reasonably captured by the two-phase model. Coupled with these calculated flow characteristics, the E/C models can predicted the wear site distributions that show satisfactory agreement with the plant measurements. Therefore, the models proposed herein can assist in the pipe wall monitoring program for the nuclear power plant by way of concentrating the measuring point on the possible sites of severe E/C wear for the piping and reducing the measurement labor works.     

Investigating the burning characteristics of electric cables used in the nuclear power plant by way of 3-D transient FDS code

Burning characteristics of electrical cables are one of the key parameters for the fire hazard assessment of nuclear power plants (NPPs) since the cables are the essential sources of fire in the plants. A three-dimensional (3-D) transient computational fluid dynamics (CFD) code_FDS is adopted in this paper to simulate these characteristics related to the cable burning. Being one of the NRC licensing fire codes, the FDS includes the thermal-hydraulic equations, the turbulence model and the chemical combustion model, etc. In order to assess the CFD fire models used in this code, a burning test using the control cable with the outer jacket of polyvinylchloride (PVC) and the inner insulation of cross-linked polyethylene (XLPE) is conducted. The measured parameters associated with the burning characteristics include the heat release rate (HRR), O₂ depletion, and CO and CO₂ production, etc. Except the amount of O₂ consumption, the predicted transient behaviors of other parameters can reproduce the measured data. Based on the chemical combustion model in the FDS code, this discrepancy may be essentially resulted from the default value of hydrogen fraction (H_frac) contained in the soot since the soot yield for the burning of PVC material is high enough that the uncertainty in the H_frac value has a prominent effect on the amount of O₂ consumption. This explanation can be confirmed by a benchmark calculation for simulating a burning test with the polymethylmethacrylate (PMMA) fuel of low-soot yield. The present simulation works can provide the useful information for the plant staff or the researcher as they would perform the fire hazard analysis in the NPPs using the FDS code.     

Fair round robin binary countdown to achieve QoS guarantee and fairness in WLANs

How to guarantee both quality of service (QoS) and fairness in wireless local area networks (WLANs) is a challenging issue. To touch this issue, a fair medium access control (MAC) scheme called fair round robin binary countdown (FRRBC) adopting the eminent concepts of allowance and binary countdown is proposed in this paper. FRRBC can guarantee QoS for both audio and video with the aid of adaptive adjustment on system parameters and some extra rules designed according to delay bounds. Using multiple mapping functions from allowances to fixed-bit binary numbers to indicate different priorities, FRRBC not only provides guaranteed system performance but also achieves good fairness. Finally, we demonstrate that FRRBC can significantly outperform the enhanced distributed channel access (EDCA) (IEEE 802.11 WG in IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. Amendment 8: Medium Access Control (MAC) Quality of Service Enhancements, 2005) and the synchronized medium access control (SYN-MAC) (Wu HY et al. in Mobile Netw Appl 11:627–637, 2005) because of its superiority to offer guaranteed QoS for both audio and video, low delay jitter, low blocking ratio, and good fairness. Of course, FRRBC can be illustrated to be a better choice than the enhanced distributed elastic round robin (EDERR) (Ferng HW, Liau HY in IEEE Trans Mobile Comput 8(7):880–894, 2009).     

A three-dimensional full-cell CFD model used to investigate the effects of different flow channel designs on PEMFC performance

A three-dimensional “full-cell” computational fluid dynamics (CFD) model is proposed in this paper to investigate the effects of different flow channel designs on the performance of proton exchange membrane fuel cells (PEMFC). The flow channel designs selected in this work include the parallel and serpentine flow channels, single-path and multi-path flow channels, and uniform depth and step-wise depth flow channels. This model is validated by the experiments conducted in the fuel cell center of Yuan Ze University, showing that the present model can investigate the characteristics of flow channel for the PEMFC and assist in the optima designs of flow channels. The effects of different flow channel designs on the PEMFC performance obtained by the model predictions agree well with those obtained by experiments. Based on the simulation results, which are also confirmed by the experimental data, the parallel flow channel with the step-wise depth design significantly promotes the PEMFC performance. However, the performance of PEMFC with the serpentine flow channel is insensitive to these different depth designs. In addition, the distribution characteristics of fuel gases and current density for the PEMFC with different flow channels can be also reasonably captured by the present model.     

Modeling and Cost Analysis of an Improved Movement-Based Location Update Scheme in Wireless Communication Networks

A movement based location update (MBLU) scheme is an LU scheme, under which a user equipment (UE) performs an LU when the number of cells crossed by the UE reaches a movement threshold. The MBLU scheme suffers from ping-pong LU effect. The ping-pong LU effect arises when the UE that moves repetitively between two adjacent cells performs LUs in the same way as in the case of straight movement. To tackle the ping-pong LU effect encountered by the original MBLU (OMBLU) scheme, an improved MBLU (IMBLU) scheme was proposed in the literature. Under the IMBLU scheme, the UE performs an LU when the number of different cells, rather than the number of cells, visited by the UE reaches the movement threshold. In this paper we develop an embedded Markov chain model to calculate the signaling cost of the IMBLU scheme. We derive analytical formulas for the signaling cost, whose accuracy is tested through simulation. It is observed from a numerical study based on these formulas that 1) the signaling cost is a downward convex function of the movement threshold, implying the existence of an optimal movement threshold that can minimize the signaling cost, and that 2) the reduction in the signaling cost achieved by the IMBLU scheme relative to the OMBLU scheme is more prominent in the case of stronger repetitiveness in the UE movement. The model developed and the formulas derived in this paper can guide the implementation of the IMBLU scheme in wireless communication networks.     

Key management protocol with end-to-end data security and key revocation for a multi-BS wireless sensor network

Focusing on a large-scale wireless sensor network with multiple base stations (BS), a key management protocol is designed in this paper. For securely relaying data between a node and a base station or two nodes, an end-to-end data security method is adopted by this protocol. Further employing a distributed key revocation scheme to efficiently remove compromised nodes then forms our key management protocol celled multi-BS key management protocol (MKMP). Through performance evaluation, we show that MKMP outperforms LEDS Ren et al. (IEEE Trans Mobile Comput 7(5):585–598, 2008) in terms of efficiency of resilience against the node capture attack. With the analysis of key storage overheads, we demonstrate that MKMP performs better than mKeying Wang et al. (A key management protocol for wireless sensor networks with multiple base stations. In: Procceedings of ICC'08, pp 1625–1629, 2008) in terms of this overhead.