11 research outputs found
On the study of the optimistic unchoking algorithms and incentive mechanisms of BitTorrent
Optimistic unchoking plays an important role in BitTorrent Peer-to-Peer (P2P)[46, 45, 48, 4] file sharing networks. Peers use optimistic unchoking to find upload bandwidth information about their neighbors. However, free-riders can also take advantage of optimistic unchoking and download from the network without uploading anything. In this thesis, a novel optimistic unchoking algorithm for BitTorrent is proposed. The main purposes of our algorithm are to prevent free-riding and to improve the efficiency of optimistic unchoking. A stochastic model is then proposed to analyze the performance of my algorithm. We also verify the results by simulations. BitTorrent also have a built-in incentive mechanism called "Tit-for-Tat" [4] to prevent free-riding. Basically, a peer will upload to other peers (default is four) that give it the highest download rate. In this thesis, We will show that by adjusting the upload rate and the number of uploads, a selfish peer can take advantage of the "Tit-for-Tat" [4] to improve its download rate. However, this strategy of the selfish peer is harmful to the whole network. If many peers take the same strategy, the performance of the whole network will be significantly decreased. It is then theoretically proved that the "Tit-for-Tat" [4] is not an optimal incentive mechanism. To solve this problem, We propose a new incentive mechanism for BitTorrent. With this new mechanism, even if all peers are selfish, the performance of the whole network can still be maintained at a very high leve
Noisy weak-lensing convergence peak statistics near clusters of galaxies and beyond
Taking into account noise from intrinsic ellipticities of source galaxies, in
this paper, we study the peak statistics in weak-lensing convergence maps
around clusters of galaxies and beyond. We emphasize how the noise peak
statistics is affected by the density distribution of nearby clusters, and also
how cluster-peak signals are changed by the existence of noise. These are the
important aspects to be understood thoroughly in weak-lensing analyses for
individual clusters as well as in cosmological applications of weak-lensing
cluster statistics. We adopt Gaussian smoothing with the smoothing scale
in our analyses. It is found that the noise peak
distribution near a cluster of galaxies depends sensitively on the density
profile of the cluster. For a cored isothermal cluster with the core radius
, the inner region with appears noisy containing on average
peaks with for and the true peak
height of the cluster , where denotes the convergence signal to
noise ratio. For a NFW cluster of the same mass and the same central , the
average number of peaks with within is . Thus a
high peak corresponding to the main cluster can be identified more cleanly in
the NFW case. In the outer region with , the number of high
noise peaks is considerably enhanced in comparison with that of the pure noise
case without the nearby cluster. (abridged)Comment: 10 figures, ApJ in pres
Statistical Properties of Supercluster-Like Filaments from Cosmological Simulations
We study large-scale structures from numerical simulations, paying particular
attention to supercluster-like structures. A grid-density-contour based
algorithm is adopted to locate connected groups. With the increase of the
linking density threshold, the foam- like cosmic web is subsequently broken
into individual supercluster-like groups and further halos which are in
accordance to groups with the linking density threshold {\rho}/{\rho}= 1 +
{\delta} = 80. By analyzing sets of cosmological simulations with varying
cosmological parameters, we find that an universal mass function exists not
only for halos but also for low-density supercluster-like groups until the
linking density threshold decreases to a density where the global percolation
of large-scale structures occurs. We further show that the mass functions of
different groups can be well described by the Jenkins form with the parameters
being dependent on the linking density threshold. On the other hand, these low-
density supercluster-like groups cannot be directly associated with the
predictions from the excursion set theory with effective barriers obtained from
dynamical collapse models, and the peak exclusion effect must be taken into
account. Including such an effect, the mass function of groups with the linking
density threshold 1 + {\delta} = 16 is in good agreements with that from the
excursion set theory with a nearly flat effective barrier. A simplified
analysis of the ellipsoidal collapse model indicates that the barrier for
collapses along two axes to form filaments is approximately flat in scales,
thus we define groups identified with 1 + {\delta} = 16 as filaments. We then
further study the halo-filament conditional mass function and the filament-halo
conditional mass function, and compare them with the predictions from the
two-barrier excursion set theory. The shape statistics for filaments are also
presented.Comment: Accepted by Ap
Fast methods for low-frequency and static EM problems
Electromagnetic effects play an important role in many engineering problems. The fast and accurate numerical methods for electromagnetic analysis are highly desired in both the low-frequency analysis and the static analysis.
In the first part of this thesis, a low-frequency stable domain decomposition method, the augmented equivalence principle algorithm (A-EPA) with augmented electric field integral equation (A-EFIE), is introduced for analyzing the electromagnetic problems at low frequencies. The A-EFIE is first employed as a inner current solver for the EPA algorithm so that it improves the low-frequency inaccuracy issue. This method, however, cannot completely remove the low-frequency breakdown. To overcome it, the A-EPA with A-EFIE is studied and developed so that it has the capability to solve low-frequency problems accurately.
In the second part, novel Helmholtz decomposition based fast Poisson solvers for both 2-D and 3-D problems are introduced. These new methods are implemented through the quasi-Helmholtz decomposition technique, i.e. the loop-tree decomposition. In 2-D cases, the proposed method can achieve O(N) complexity in terms of both computational cost and memory consumption for moderate accuracy requirements. Although computational costs become higher when more accurate results are needed, a multilevel method by using the hierarchical loop basis functions can obtain the desired efficiency. The same idea can be extend to 3-D case for exploiting a new generation of fast method for electrostatic problems.published_or_final_versionElectrical and Electronic EngineeringDoctoralDoctor of Philosoph
Effect of Electrospinning Network Instead of Polymer Network on the Properties of PDLCs
In this study, polymer-dispersed liquid crystal (PDLC) membranes were prepared by combining prepolymer, liquid crystal, and nanofiber mesh membranes under UV irradiation. EM, POM, and electro-optic curves were then used to examine the modified polymer network structure and the electro-optical properties of these samples. As a result, the PDLCs with a specific amount of reticular nanofiber films had considerably improved electro-optical characteristics and antiaging capabilities. The advancement of PDLC incorporated with reticulated nanofiber films, which exhibited a faster response time and superior electro-optical properties, would greatly enhance the technological application prospects of PDLC-based smart windows, displays, power storage, and flexible gadgets