An adaptive directional MAC protocol for ad hoc networks using directional antennas

A directional antenna can bring benefits in terms of power consumption,spatial reuse,etc.To exploit the advantage of directional antennas and improve the transmission throughput highest,this paper proposes an adaptive directional MAC protocol(ADMAC).By varying the transmission strategy according to the usage of the channel,nodes can send RTS and CTS packets omni-directionally or directionally.Also,this paper proposes a calculation method of virtual carrier sensing with collision avoidance.By the method,ADMAC protocol makes more pairs of nodes transmit and receive data simultaneously without interferences than other MAC protocols for directional antennas.The paper compares the simultaneous delivering nodes and network throughput of ADMAC with DMAC,DVCS and SDMAC under different experiment parameters.The simulation results show that the throughput of ADMAC is higher than the throughput of DMAC,DVCS and SDMAC protocols.     

Reward only is not enough: Evaluating and improving the fairness policy of the P2P file sharing network eMule/eDonkey

Limiting the threat of free-riding behavior is an important design issue for peer-to-peer (P2P) file sharing networks. However, the fairness policy that rewards contributors with credit in one of the most popular P2P file sharing networks, eMule/eDonkey, hasn’t been thoroughly studied. In this paper, motivated by our experiments with the eMule/eDonkey network, we firstly theoretically analyze the content exchange process with credit in eMule/eDonkey and then verify the mathematical model by an agent-based simulation. Both the numerical and simulation-based results confirm our discovery in the experiments that eMule/eDonkey’s local credit strategy can not provide enough fairness as it doesn’t explicitly punish free-riders. To overcome this drawback, we propose a new free-riding control scheme, which can simply maintain the current credit local structure and take advantage of the credit policy. Extensive numerical evaluation and simulation indicate that this scheme significantly improves system fairness.     

A quantitative comparison of the effects of modeling approaches on system verification using a controlled challenge problem

Requirements Engineering - To reduce program risks, engineering methods capitalizing on modeling and machine assistance have been extensively investigated within systems engineering (and more...     

Discrete logarithms inGF(p)

Several related algorithms are presented for computing logarithms in fieldsGF(p),p a prime. Heuristic arguments predict a running time of exp((1+o(1)) ) for the initial precomputation phase that is needed for eachp, and much shorter running times for computing individual logarithms once the precomputation is done. The running time of the precomputation is roughly the same as that of the fastest known algorithms for factoring integers of size aboutp. The algorithms use the well known basic scheme of obtaining linear equations for logarithms of small primes and then solving them to obtain a database to be used for the computation of individual logarithms. The novel ingredients are new ways of obtaining linear equations and new methods of solving these linear equations by adaptations of sparse matrix methods from numerical analysis to the case of finite rings. While some of the new logarithm algorithms are adaptations of known integer factorization algorithms, others are new and can be adapted to yield integer factorization algorithms.     

聚丙烯固相法接枝改性

聚丙烯固相法接枝不饱和单体是一种简便易行的本体法技术。与其它接枝方法相比，具有成本低、接枝率高、时间短、无溶剂回收、不妨害接枝物的进一步应用等优点。本文介绍了聚丙烯固相法接枝不饱和单体的进展，并较详细地讨论了影响接枝反应的各种因素。     

Removal of mineral material from solvent-refined coal by solvent extractions

Solvent refined coal (SRC), a coal extract which is highly polyaromatic in character and relatively high in N, O and S content, is derived from the direct liquefaction of coal. SRC contains mineral matter in the form of soluble organometallic compounds which can not be removed by filtration. These soluble species can be detrimental to the hydrocracking catalyst used in the upgrading of the SRC to high-value liquid fuels. These species, however, can be separated from the SRC by extraction with benzene or tetralin as solvents so that the mineral matter is mainly concentrated in the solvent-insoluble fraction, the other fraction are then readily hydrocracked, as demonstrated in this work.     

Structural and transport effects of doping perfluorosulfonic acid polymers with the heteropoly acids, H3PW12O40 or H4SiW12O40

A perfluorosulfonic acid (PFSA) polymer with pendant side chain -O(CF₂)₄SO₃H was doped with the heteropoly acids (HPAs), H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀. Infrared spectroscopy revealed a strong interaction between the HPA and the PFSA ionomer. Modes associated with the peripheral bonds of the HPA were shifted to lower wave numbers when doped into PFSA membranes. Small-angle X-ray scattering (SAXS) measurements showed the presence of large crystallites of HPA in the membrane with d spacings of ca. 10 Å, close to the lattice spacing observed in bulk HPA crystals. Under wet conditions the HPA was more dispersed and constrained the size of the sulfonic acid clusters to 20 Å at a 5 wt% HPA doping level, the same as in the vacuum treated ionomer samples. Under conditions of minimum hydration the HPA decreased the E_a for the self-diffusion of water from 27 to 15 kJ mol⁻¹. The reverse trend was seen under 100% RH conditions. Proton conductivity measurements showed improved proton conductivity of the HPA doped PFSAs at a constant dew point of 80 °C for all temperatures up to 120 °C and at all relative hummidities up to 80%. The activation energy for proton conduction generally was lower than for the undoped materials at RH ≤80%. Significantly the E_a was 1/2 that of the undoped material at RHs of 40 and 60%. A practical proton conductivity of 113 mS cm⁻¹ was observed at 100 °C and 80% RH.     

The heterogeneous nucleation of microcellular foams assisted by the survival of microvoids in polymers containing low glass transition particles. Part I: Mathematical modeling and numerical simulation

The existing models based on classical nucleation theory are not able to explain satisfactorily the nucleation phenomenon of microcellular foams in thermoplastics. Here, we extend the analysis of Kweeder (24), who developed a new model that considers the presence of microvoids, resulting from the thermal processing history of the polymer, as potential nucleation sites. The nucleation model “concentrates” on the stresses and thus void formations in the rubber particles. Since these are pre-existing microvoids, bubble nucleation depends on the survival of these voids to grow rather than the formation of a new phase as modeled by classical nucleation theory. The population of viable microvoids with a sufficiently large radius to survive and overcome surface and elastic forces has been modeled to yield the cell density. A log-normal distribution, which relates to the rubber particle size, has been used to model the distribution of microvoids in the polymer composite material. The model depends on various process parameters such as saturation pressure, foaming temperature, concentration of nucleating agents, solubility of the blowing agent in the polymer, and the modulus. High impact polystyrene (HIPS) was added to polystyrene to obtain polymers with different concentrations of rubber gel particles, the nucleating agent, and used here for this study.     

Graft polymerization of vinyl acetate onto granular starch: Comparison on the potassium persulfate and ceric ammonium nitrate initiated system

This work was undertaken to discuss in depth the vital differences in the morphological development during synthesis, and properties of starch‐g‐poly‐(vinyl acetate) copolymers using two different initiators, potassium persulfate (KPS) and ceric ammonium nitrate (CAN). KPS‐initiated system gave relatively low values of grafting ratio and grafting efficiency, indicating a great tendency for the formation of poly(vinyl acetate) homopolymer (PVAc). Yet, higher values were seen for the CAN‐initiated system. Transmission electron microscope observations indicated a relatively broad distribution of latex particles for the KPS‐initiated system. The surface potential of latex particles was about ?3.5 mV, which turned out to be insufficient to maintain stability of latex particles. On the other hand, a uniform particle size distribution was found for the CAN‐initiated system, as the surface potential of latex particles was 21.5 mV. Moreover, radicals on starch molecules were generated directly through a redox reaction with positively charged ceric ion. The hydrophobic PVAc chains were thus grafted on starch, resulting in an amphiphilic graft copolymer, which provides a sufficient stabilization degree as a role of surfactant to render a relatively uniform distribution of latex particles. The synthesized starch‐g‐poly(vinyl acetate) copolymers were further converted to starch‐g‐poly(vinyl alcohol) through saponification, which were subjected to evaluations regarding the biodegradation and cell culture capability. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3017–3027, 2006     

Thermal alteration of a cyclic fatty acid produced by a flaxseed extract

Methyl 8-[2-(cis-pent-2′-enyl)-3-oxo-cis-cyclopent-4-enyl] octanoate (I) is the methyl ester of a cyclic fatty acid synthesized enzymically from an incubation of linolenic acid with an extract of flaxseed (Linum usitatissimum L.). A proposed trivial name for I is methyl 12-oxo-cis-10, 15-phytodienoate (12-oxo-PDA). The evidence presented indicated that compound I has thecis configuration of the carbon chains with respect to the cyclopentenone ring. Treatment with acid, base, or heat isomerized I to a second product (II) that has thetrans configuration of the carbon chains. Prolonged heat treatment of II yielded a third cyclic product, methyl 12-oxo-9(13),cis-15-phytodienoate (III).