| 1. |
- Brogren, M, et al.
(författare)
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Optical properties, durability, and system aspects of a new aluminium-polymer-laminated steel reflector for solar concentrators
- 2004
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 82:3, s. 387-412
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Tidskriftsartikel (refereegranskat)abstract
- A newly developed aluminium-polymer-laminated steel reflector for use in solar concentrators was evaluated with respect to its optical properties, durability, and reflector performance in solar thermal and photovoltaic systems. The optical properties of the reflector material were investigated using spectrophotometer and scatterometry. The durability of the reflector was tested in a climatic test chamber as well as outdoors in Alvkarleby (60.5degreesN, 17.4degreesE), Sweden. Before ageing, the solar weighted total and specular reflectance values were 82% and 77%, respectively, and the reflector scattered light isotropically. After I year's outdoor exposure, the total and specular solar reflectance had decreased by less than 11%. However, after 2000 h in damp heat and 1000 W/m(2) simulated solar radiation, the optical properties had changed significantly: The light scattering was anisotropic and the total and specular solar reflectance values had decreased to 75% and 42%, respectively. The decrease was found to be due to degradation of the protective polyethylene terephthalate (PET) layer, caused by UV radiation and high temperature. The conclusions are that the degradation is climate dependent and that PET is not suitable as a protective coating under extreme conditions, such as those in the climatic test chamber. However, the results from outdoor testing indicate that the material withstands exposure in a normal Swedish climate.
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| 2. |
- Carlsson, Patrik, et al.
(författare)
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Delay Performance in IP Routers
- 2004
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Konferensbidrag (refereegranskat)abstract
- The main goals of the paper are towards an understanding of the delay process in best-effort Internet for both non-congested and congested networks. A dedicated measurement system is re-ported for delay measurements in IP routers, which follows specifications of the IETF RFC 2679. The system is using both passive measurements and active probing. Dedicated application-layer software is used to generate UDP traffic with TCP-like characteristics. Pareto traffic models are used to generate self-similar traffic in the link. The reported results are in the form of several impor-tant statistics regarding processing delay of a router, router delay for a single data flow, router delay for more data flows as well as end-to-end delay for a chain of routers. We confirm results reported earlier about the fact that the delay in IP routers is generally influenced by traffic characteristics, link conditions and, at some extent, details in hardware implementation and different IOS releases. The delay in IP routers usually shows heavy-tailed characteristics. It may also occasionally show extreme values, which are due to improper functioning of the routers.
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| 3. |
- Carlsson, Patrik, et al.
(författare)
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Matching Multi-Fractal Process Parameters Against Real Data Traffic
- 2002
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Konferensbidrag (refereegranskat)abstract
- Recent analyses of real data/internet traffic indicate that data traffic exhibits long-range dependence as well as self-similar or multi-fractal properties. By using mathematical models of Internet traffic that share these properties we can perform analytical studies of network traffic. This gives us an opportunity to analyse potential bottlenecks and estimate delays in the networks. Processes with multi-fractal properties can be modeled by multiplying the output of Markov Modulated Rate Processes (MMRP) [1] each defined by four parameters. The MMRP are easily used in stochastic fluid flow modeling. This model is also suited for analysis of other traffic types e.g. VoIP and thus, it allows for integration of different traffic types, i.e. time-sensitive voice traffic with best-effort data traffic. Using this model we can calculate performance parameters for each individual stream that enters the system/model. In this paper we show how to construct a multi-fractal process that is matched to measured data from MMRP sub processes.
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| 4. |
- Carlsson, Patrik, et al.
(författare)
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Modelling of Ethernet Traffic on Multiple Timescales
- 2004
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Konferensbidrag (refereegranskat)abstract
- Ethernet is one of the most common link layer technologies, used in local area networks, wireless networks and wide area networks. There is however a lack of traffic models for Ethernet that is usable in performance analysis. In this paper we use such a model. The model operates on matching multiple moments of the bit rate at several timescales. In order to match the model parameters to measured traffic, five methods have been developed. We use this to model three different links; the BCpOct89 Bellcore trace, an Internet access link and an ADSL link. Our results show that, as the number of sources present on an Ethernet link grows, the model becomes better and less complex.
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| 5. |
- Carlsson, Patrik, et al.
(författare)
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Obtaining Reliable Bit Rate Measurements in SNMP-Managed Networks
- 2002
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Konferensbidrag (refereegranskat)abstract
- The Simple Network Management Protocol, SNMP, is the most widespread standard for Internet management. As SNMP stacks are available on most equipment, this protocol has to be considered when it comes to performance management, traffic engineering and network control. However, especially when using the predominant version 1, SNMPv1, special care has to be taken to avoid erroneous results when calculating bit rates. In this work, we evalute six off-the-shelf network components. We demonstrate that bit rate measurements can be completely misleading if the sample intervals that are used are either too large or too small. We present solutions and work-arounds for these problems. The devices are evaluated with regards to their updating and response behavior.
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| 6. |
- Constantinescu, Doru, et al.
(författare)
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Measurement of One-Way Internet Packet Delay
- 2004
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Konferensbidrag (refereegranskat)abstract
- The paper reports on a dedicated measurement system for delay measurements in IP routers, which follows specifications of the IETF RFC 2679. The system uses both passive and active measurements. Dedicated application-layer software is used to generate traffic. Pareto traffic models are used to generate self-similar traffic in the link. Both packet inter-arrival times and packet sizes are matching real traffic models. A passive measurement system is used for data collection that is based on using several so-called Measurement Points, each of them equipped with DAG monitoring cards. Hashing is used for the identification and matching of packets. The combination of passive measurements and active probing, together with using the DAG monitoring system, gives us an unique possibility to perform precise traffic measurements as well as the flexibility needed to compensate for the lack of analytic solutions.
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| 7. |
- Erman, David, et al.
(författare)
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Measurement and Analysis of BitTorrent Signaling Traffic
- 2004
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Konferensbidrag (refereegranskat)abstract
- BitTorrent is a second generation Peer-to-Peer application that has been recently developed as an alternative to the classical client-server model to reduce the load burden on content servers and networks. The protocol relies on the use of swarming techniques for distributing content. No search functionality is built into the protocol, and the signaling is geared only towards an efficient dissemination of data. The paper reports on measurement and analysis of BitTorrent traffic collected at the Blekinge Institute of Technology (BIT), Karlskrona, Sweden. We measure and analyze data from local BitTorrent client sessions at BIT. The characteristics of the signaling traffic exchanged among the participating peers in a BitTorrent distribution swarm are investigated. A dedicated approach based on combining instrumentation at the application layer with flow identification and extraction at the transport layer is used for traffic measurements.
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| 8. |
- Fiedler, Markus, et al.
(författare)
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Identification of performance degradation in IP networks using throughput statistics
- 2003
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Konferensbidrag (refereegranskat)abstract
- To be able to satisfy their users, interactive applications like video conferences require a certain Quality-of-Service from heterogeneous networks. This paper proposes the use of throughput histograms as Quality-of-Service indicator. These histograms are built from local, unsynchronized, passive measurements of packet streams from the viewpoint of an application. They can easily be exchanged between sender and receiver, and their comparison provides information about severity and type of a potential bottleneck. We demonstrate the usefulness of these indicators for evaluating the transport quality perceived by a video conferencing application and its users in the presence of a bottleneck.
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| 9. |
- Fiedler, Markus, et al.
(författare)
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Voice and Multi-fractal Data in the Internet
- 2001
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Konferensbidrag (refereegranskat)abstract
- In the Internet era there is a need to understand how traditional Public Switched Telephone Network (PSTN) services and the new Internet services can coexist. The best-effort based IP network will have to maintain the level of service that customers from the PSTN world expect. This study contributes to the traffic engineering of such a system by presenting an analytical model for integrating Voice over IP (VoIP) traffic with multi-fractal data traffic. This model is based on the stochastic fluid flow model. Effects on delay and loss performance of adding VoIP traffic to a data link as well as capacity requirements to maintain a certain quality of service are discussed
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| 10. |
- Hinz, A, et al.
(författare)
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Simulation of transients in heterogeneous catalysis: a comparison of the step- and pulse-transient techniques for the study of hydrocarbon oxidation on metal oxide catalysts
- 2000
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Ingår i: Chemical Engineering Science. - 0009-2509. ; 55:20, s. 4385-4397
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Tidskriftsartikel (refereegranskat)abstract
- The responses to step and pulse transients with reactants have been simulated for conventional step-transient conditions at atmospheric pressure and pulsing under vacuum using a temporal analysis of products (TAP) reactor, respectively. Propene oxidation over an oxide catalyst with the participation of lattice oxygen was chosen as a model reaction. The mechanism comprises adsorption of the reactants, a series of surface reaction steps and desorption of the products. For this type of reaction one of the reactants, namely oxygen, is a constituent of the catalyst and the oxygen coverage is assumed to be one at the start of the transient, Simulations were performed for all variants with one of the reaction steps becoming rate limiting at steady-state conditions. Both types of transients are compared for the same reaction mechanism and rate constants, neglecting catalyst restructuring. The results show that the visual features of step responses are often quite conclusive about the rate limiting step of the reaction mechanism, while pulse responses in this regard are less conclusive unless they are modelled or a desorption step is rate limiting. The two transient methods are compared regarding their usability for the study of reaction mechanisms. (C) 2000 Elsevier Science Ltd. All rights reserved.
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