Performance analysis of wireless LANs: an integrated packet/flow level approach

In this paper we present an integrated packet/flow level modelling approach for analysing flow throughputs and transfer times in IEEE 802.11 WLANs. The packet level model captures the statistical characteristics of the transmission of individual packets at the MAC layer, while the flow level model takes into account the system dynamics due to the initiation and completion of data flow transfers. The latter model is a processor sharing type of queueing model reflecting the IEEE 802.11 MAC design principle of distributing the transmission capacity fairly among the active flows. The resulting integrated packet/flow level model is analytically tractable and yields a simple approximation for the throughput and flow transfer time. Extensive simulations show that the approximation is very accurate for a wide range of parameter settings. In addition, the simulation study confirms the attractive property following from our approximation that the expected flow transfer delay is insensitive to the flow size distribution (apart from its mean)

TDMA-Medienzugriffsverfahren im Rueckkanal passiver optischer ATM-Zugangsnetze

Our modern information society is built on the availability and exchange of information as an integral part of both economic and private life. For example, the amount of traffic carried per year by the world-wide Internet is growing both due to the increasing popularity of private and business usage of the Internet and also due to the increasing number and size of documents available in the World Wide Web (WWW). The information and services offered via the WWW -from scientific research reports to culture and travel information, telephone directory services, dictionaries, train time tables and even online shopping -make this medium very attractive also for private use. The availability of powerful computers, the integration of 'classic' data services for file and document exchange (ftp and e-mail) into easy-to-use WWW browsers also facilitate the growth of world-wide date networks. Today, residential broadband network access is mostly realized via copper pairs (telephone network with analog or digital subscriber lines), coaxial cables (TV cable networks), radio technology (terrestrial or satellite) or even the power supply infrastructure (powerline communication, PLC). Several studies [20 41, 47, 50, 69, 70, 87, 136, 154] claim that in the future, passive optical networks (PON) will gain importance in broadband access. One example for a project building an ATM PON demonstrator wars the BAF (Broadband Access Facilities) project funded by the European Union [26]. In such ATM PON networks special attention needs to be paid to the upstream media access control if the network is required to offer certain quality of service guarantees such as low delay variation on the one hand and statistical multiplexing of the traffic from different subscribers on the other hand. In this thesis, the mechanisms for upstream media access control to be used in passive ATM access networks are classified, studied and compared with respect to their performance, particularly concentrating on delay variation issues. (orig.)Available from TIB Hannover: RA 2233(74) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Sample-path large deviations for tandem queues with Gaussian inputs

We focus on a (two-node) tandem queue, fed by a large number of Gaussian inputs. With service rates and buffer sizes at both modes scaled appropriately, Schilder's sample-path large deviations theorem can be applied to calculate the asymptotics of the overflow probability of the second queue. More specifically, we derive a lower bound on the exponential decay rate of this overflow probability and present an explicit condition for the lower bound to match the exact decay rate. Examples show that this condition holds for a broad range of frequently used Gaussian inputs

4D XCT OF SYNTHETIC ARTERIAL GRAFTS: TOWARDS AN IMPROVED DESIGN

A common treatment of atherosclerosis, one of the most common cardiovascular diseases, involves the placement of a bypass made of synthetic grafts, such as Dacron or Gore-Tex. However, an important mismatch in the mechanical properties between the native tissue and the graft can result in complications, especially for small-diameter vessels (<6mm). Therefore, detailed analysis of the global and local mechanical properties of synthetic graft materials, in comparison to native tissue, is essential to further improve vascular graft properties and reducing the risk of graft failure. In this study, we applied 4D X-ray microfocus computed tomography (4D XCT) to a Dacron graft. Local strain maps were calculated based on conventional Digital Volume Correlation (DVC)and three different methods of incremental DVC were compared

Improving the resilience in IP networks

Quality of service issues of IP networks are mostly related to guaranteeing bandwidth for flows. However, many interactive real-time applications also require this bandwidth in an uninterrupted fashion. The paper describes how multipath routing and local failure reaction can be employed to provide uninterrupted QoS to applications. We show how multipath route sets can be found in reasonably meshed networks and how multipath routing can be used to save on the spare capacity required in case of link failures

Performance analysis of WLANs: an integrated packet/flow level approach

We present an integrated packet/flow level model for WLAN performance analysis. It captures the statistical characteristics of the transmission of individual packets at the MAC layer, and includes the system dynamics due to the initiation and completion of data flow transfers. The processor sharing-based model is analytically tractable and yields a simple approximation for the expected flow transfer time. Extensive simulations show that the approximation is very accurate for a wide range of parameter settings