114 research outputs found
Validation of simulated real world TCP stacks
The TCP models in ns-2 have been validated and are widely used in network research. They are however not aimed at producing results consistent with a TCP implementation, they are rather designed to be a general model for TCP congestion control. The Network Simulation Cradle makes real world TCP implementations available to ns-2: Linux, FreeBSD and OpenBSD can all be simulated as easily as using the original simplified models. These simulated TCP implementations can be validated by directly comparing packet traces from simulations to traces measured from a real network. We describe the Network Simulation Cradle, present packet trace comparison results showing the high degree of accuracy possible when simulating with real TCP implementations and briefly show how this is reflected in a simulation study of TCP throughput
Network Simulation Cradle
This thesis proposes the use of real world network stacks instead of protocol
abstractions in a network simulator, bringing the actual code used in
computer systems inside the simulator and allowing for greater simulation
accuracy. Specifically, a framework called the Network Simulation
Cradle is created that supports the kernel source code from FreeBSD, OpenBSD
and Linux to make the network stacks from these systems available to the
popular network simulator ns-2.
Simulating with these real world network stacks reveals situations where the
result differs significantly from ns-2's TCP models. The simulated
network stacks are able to be directly compared to the same operating system
running on an actual machine, making validation simple. When measuring the
packet traces produced on a test network and in simulation the results are
nearly identical, a level of accuracy previously unavailable using traditional
TCP simulation models. The results of simulations run comparing ns-2 TCP
models and our framework are presented in this dissertation along with
validation studies of our framework showing how closely simulation resembles
real world computers.
Using real world stacks to simulate TCP is a complementary approach to using
the existing TCP models and provides an extra level of validation. This way of
simulating TCP and other protocols provides the network researcher or engineer
new possibilities. One example is using the framework as a protocol
development environment, which allows user-level development of protocols with
a standard set of reproducible tests, the ability to test scenarios which are
costly or impossible to build physically, and being able to trace and debug
the protocol code without affecting results
Lecture capture using large interactive display systems
There are various software technologies that allow capture and redelivery of lectures. Most of these technologies however rely on the use of proprietary software, often requiring extra efforts from the lecturer in terms of the initial preparation of the lecture material, or in editing and annotating after the lecture to make the material suitable for the students. To review the material students then require access to the proprietary software. This paper describes a system for the lightweight capture of lecture presentations, based on the use of a low-cost large interactive display surface, together with standard Microsoft PowerPoint™ presentation software. The captured version of the presentation includes the original lecture slides, graphical annotations made by the lecturer during the lecture, and the audio recording of the lecture; all saved as a PowerPoint file. In addition, the system adds some annotations and index slides to allow quick and easy access to different segments of the presentation. Presentations can be replayed in part or in full as required, preserving all of the content of the live lecture
Weak top-down constraints for unsupervised acoustic model training
Typical supervised acoustic model training relies on strong top-down constraints provided by dynamic programming alignment of the in-put observations to phonetic sequences derived from orthographic word transcripts and pronunciation dictionaries. This paper investi-gates a much weaker form of top-down supervision for use in place of transcripts and dictionaries in the zero resource setting. Our pro-posed constraints, which can be produced using recent spoken term discovery systems, come in the form of pairs of isolated word exam-ples that share the same unknown type. For each pair, we perform a dynamic programming alignment of the acoustic observations of the two constituent examples, generating an inventory of cross-speaker frame pairs that each provide evidence that the same subword unit model should account for them. We find these weak top-down con-straints are capable of improving model speaker independence by up to 57 % relative over bottom-up training alone. Index Terms — speaker independent acoustic models, unsuper-vised training, spectral clustering, top-down constraints 1
Application of polychaetes in (de)coupled integrated aquaculture: an approach for fish waste bioremediation
ABSTRACT: Development of benthic components within integrated multi-trophic aquaculture (IMTA) systems warrants more attention, and the development of polychaetes as an extractive component in IMTA systems is ongoing. This study estimates the bioremediation potential of Capitella sp. and Ophryotrocha craigsmithi for coupled and decoupled salmon-driven IMTA. In coupled IMTA, polychaetes receive fresh faeces, while in decoupled IMTA, preservation of faeces is applied. Respiration and ammonia excretion rates were measured for polychaetes fed fresh, oven-dried or acidified salmon faeces, and combined with nutrients incorporated into tissue growth, to estimate nutrient requirements. Nutrient requirements were subsequently used to evaluate bioremediation potential. Metabolic rates were highest for O. craigsmithi and contributed notably to their overall nutrient requirement (20-30%). For the 2 polychaete species, nutrient requirements ranged from 5 to 26 mg C and from 2 to 6 mg N g-1 AFDW d-1. These requirements were comparable with or higher than other polychaete species, highlighting the potential for fish waste bioremediation by Capitella sp. and O. craigsmithi. Preserved diets reduced bioremediation potential 1.5 and 3-5 times for, respectively, Capitella sp. and O. craigsmithi. Assuming that polychaetes are efficient fish-faeces convertors, the bioremediation potential indicates that benthic cultivation units containing 65000-95000 ind. m-2 of Capitella sp. or 36000-194000 ind. m-2 of O. craigsmithi can convert the daily organic waste flux deposited below an average salmon farm. These densities were within ranges reported for wild populations, indicating that, based on the bioremediation potential, development of benthic IMTA with these 2 polychaete species seems realistic and efficient for waste conversion.publishedVersio
A summary of the 2012 JHU CLSP Workshop on Zero Resource Speech Technologies and Models of Early Language Acquisition
We summarize the accomplishments of a multi-disciplinary workshop exploring the computational and scientific issues surrounding zero resource (unsupervised) speech technologies and related models of early language acquisition. Centered around the tasks of phonetic and lexical discovery, we consider unified evaluation metrics, present two new approaches for improving speaker independence in the absence of supervision, and evaluate the application of Bayesian word segmentation algorithms to automatic subword unit tokenizations. Finally, we present two strategies for integrating zero resource techniques into supervised settings, demonstrating the potential of unsupervised methods to improve mainstream technologies.5 page(s
Probing correlations in the exciton landscape of a moir\'e heterostructure
Excitons are two-particle correlated bound states that are formed due to
Coulomb interaction between single-particle holes and electrons. In the
solid-state, cooperative interactions with surrounding quasiparticles can
strongly tailor the exciton properties and potentially even create new
correlated states of matter. It is thus highly desirable to access such
cooperative and correlated exciton behavior on a fundamental level. Here, we
find that the ultrafast transfer of an exciton's hole across a type-II
band-aligned moir\'e heterostructure leads to a surprising sub-200-fs upshift
of the single-particle energy of the electron being photoemitted from the
two-particle exciton state. While energy relaxation usually leads to an
energetic downshift of the spectroscopic signature, we show that this unusual
upshift is a clear fingerprint of the correlated interactions of the electron
and hole parts of the exciton quasiparticle. In this way, time-resolved
photoelectron spectroscopy is straightforwardly established as a powerful
method to access exciton correlations and cooperative behavior in
two-dimensional quantum materials. Our work highlights this new capability and
motivates the future study of optically inaccessible correlated excitonic and
electronic states in moir\'e heterostructures.Comment: 32 pages, 4 main figures, 5 supplemental figure
Formation of moir\ue9 interlayer excitons in space and time
Moir\ue9 superlattices in atomically thin van der Waals heterostructures hold great promise for extended control of electronic and valleytronic lifetimes1-7, the confinement of excitons in artificial moir\ue9 lattices8-13 and the formation of exotic quantum phases14-18. Such moir\ue9-induced emergent phenomena are particularly strong for interlayer excitons, where the hole and the electron are localized in different layers of the heterostructure19,20. To exploit the full potential of correlated moir\ue9 and exciton physics, a thorough understanding of the ultrafast interlayer exciton formation process and the real-space wavefunction confinement is indispensable. Here we show that femtosecond photoemission momentum microscopy provides quantitative access to these key properties of the moir\ue9 interlayer excitons. First, we elucidate that interlayer excitons are dominantly formed through femtosecond exciton-phonon scattering and subsequent charge transfer\ua0at the interlayer-hybridized Σ valleys. Second, we show that interlayer excitons exhibit a momentum fingerprint that is a direct hallmark of the superlattice moir\ue9 modification. Third, we reconstruct the wavefunction distribution of the electronic part of the exciton and compare the size with the real-space moir\ue9 superlattice. Our work provides direct access to interlayer exciton formation dynamics in space and time and reveals opportunities to study correlated moir\ue9 and exciton physics for the future realization of exotic quantum phases of matter
Ultrafast dynamics of bright and dark excitons in monolayer WSe and heterobilayer WSe/MoS
The energy landscape of optical excitations in mono- and few-layer transition
metal dichalcogenides (TMDs) is dominated by optically bright and dark
excitons. These excitons can be fully localized within a single TMD layer, or
the electron- and the hole-component of the exciton can be charge-separated
over multiple TMD layers. Such intra- or interlayer excitons have been
characterized in detail using all-optical spectroscopies, and, more recently,
photoemission spectroscopy. In addition, there are so-called hybrid excitons
whose electron- and/or hole-component are delocalized over two or more TMD
layers, and therefore provide a promising pathway to mediate charge-transfer
processes across the TMD interface. Hence, an in-situ characterization of their
energy landscape and dynamics is of vital interest. In this work, using
femtosecond momentum microscopy combined with many-particle modeling, we
quantitatively compare the dynamics of momentum-indirect intralayer excitons in
monolayer WSe with the dynamics of momentum-indirect hybrid excitons in
heterobilayer WSe/MoS, and draw three key conclusions: First, we find
that the energy of hybrid excitons is reduced when compared to excitons with
pure intralayer character. Second, we show that the momentum-indirect
intralayer and hybrid excitons are formed via exciton-phonon scattering from
optically excited bright excitons. And third, we demonstrate that the
efficiency for phonon absorption and emission processes in the mono- and the
heterobilayer is strongly dependent on the energy alignment of the intralayer
and hybrid excitons with respect to the optically excited bright exciton.
Overall, our work provides microscopic insights into exciton dynamics in TMD
mono- and bilayers.Comment: 27 pages, 5 figure
Ultrafast nano-imaging of dark excitons
The role and impact of spatial heterogeneity in two-dimensional quantum
materials represents one of the major research quests regarding the future
application of these materials in optoelectronics and quantum information
science. In the case of transition-metal dichalcogenide heterostructures, in
particular, direct access to heterogeneities in the dark-exciton landscape with
nanometer spatial and ultrafast time resolution is highly desired, but remains
largely elusive. Here, we introduce ultrafast dark field momentum microscopy to
spatio-temporally resolve dark exciton formation dynamics in a twisted
WSe/MoS heterostructure with 55 femtosecond time- and 500~nm spatial
resolution. This allows us to directly map spatial heterogeneity in the
electronic and excitonic structure, and to correlate these with the dark
exciton formation and relaxation dynamics. The benefits of simultaneous
ultrafast nanoscale dark-field momentum microscopy and spectroscopy is
groundbreaking for the present study, and opens the door to new types of
experiments with unprecedented spectroscopic and spatiotemporal capabilities.Comment: 39 pages, 4 main figures, 8 supplemental figure
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