Kinetic effect of boron on the thermal stability of Si–(B–)C–N polymer-derived ceramics

The isothermal mass loss of two polymer-derived ceramics with compositions SiC_1.4N_0.9 and SiC_1.5N_1.0B_0.05 were measured as a function of time using thermal gravimetric analysis at various temperatures ranging between 1580 and 1720 °C. The process of mass loss, attributed to the reaction Si₃N₄ + 3C → 3SiC + 2N₂^↑, takes substantially more time for the boron-containing ceramic compared with the boron-free one. The continuous formation of SiC crystallites as the product of the reaction between Si₃N₄ and C was revealed through X-ray diffraction (XRD) measurements during the course of the reaction. The kinetics of this reaction was studied using a generalized model for the analysis of chemical reaction kinetics. Consequently, the effective activation energies for the Si₃N₄ degradation were estimated to be 11.6 ± 0.5 eV and 17.1 ± 0.7 eV for the Si–C–N and Si–B–C–N ceramics, respectively. Moreover, the results obtained indicate that the dominant mechanisms of the Si₃N₄ degradation are strongly influenced by the presence of boron. For the Si–C–N ceramic, the chemical reaction at interfaces of the reactants and the crystallization of SiC as the reaction product are proposed to be the main probable stages controlling the progress of the investigated reaction. However, the local diffusion of C out of BNC_x turbostratic layers surrounding the Si₃N₄ nanocrystals and the gas (N₂) release from the reaction zone are suggested to be the most plausible processes limiting the progress of Si₃N₄ degradation for the Si–B–C–N ceramic.     

Efficient support for client/server applications over heterogeneousATM networks

We present a new network design problem that is applicable for designing virtual paths (VPs) in an asynchronous transfer mode (ATM) network to efficiently support client/server applications. We present several alternatives for the solution, compare their properties, and focus on a novel “greedy” solution, which we prove to optimize certain important criteria (namely, the network overhead for a request/response and the utilization of bandwidth and routing table resources). We also present simulation results that demonstrate the performance and scalability of our solution. In addition, we propose a new efficient bandwidth allocation scheme which is tailored for client/server applications over ATM networks     

Distributed optical control plane for dynamic lightpath establishment in translucent optical networks based on reachability graph

In transparent optical networks, physical layer impairments (PLIs) incurred by non-ideal optical transmission media accumulate along an optical path, and the overall effect determines the optical feasibility of the lightpaths. In addition, transparent optical networks suffer from inefficient wavelength utilization, as a connection request may be rejected because of non-availability of a common wavelength on all the links along the chosen route. To increase optical reach, resource utilization, and average call acceptance ratio (and hence revenues), network operators are resort to translucent optical networks. In these networks a limited number of regenerators are placed at a selected set of nodes. In this scenario development of an optical control plane which is aware of PLIs, location and number of regenerators, is of paramount importance for on-demand lightpath provisioning. In this paper, we propose a novel approach of constructing a reachability graph of the physical network considering PLIs and regenerators. If there is no transparent path in the physical network, we route the connections with multiple transparent segments on the reachability graph. We propose efficient mechanisms and corresponding GMPLS protocol extensions for impairment and regenerator aware routing and wavelength assignment (IRA-RWA) in translucent optical networks. The simulation results suggest that our proposed approach together with LSP stitching signaling mechanism is feasible to implement and close to deployment.     

On the future of wavelength routing networks

This article discusses the possible applications for optical networks based on wavelength division multiplexing and how they compete and complement current high-speed networks (SONET, ATM). We first outline the best-case scenario for this technology and describe the spectrum of proposed optical networks (WDM links, passive optical access networks, broadcast-and-select networks, and wavelength routing networks). Then we focus on wavelength routing networks and describe their advantages and disadvantages relative to other competing alternatives for very-high-speed networks. Finally, we analyze the different markets for such networks in the telco and data communications arena     

Optical layer survivability-an implementation perspective

This paper looks at several aspects of optical layer protection techniques from an implementation perspective. We discuss the factors that affect the complexity of optical protection schemes, such as supporting mesh instead of ring protection, handling low-priority traffic, and dealing with multiple types of failures. The paper also looks at how the client layer interacts with the optical layer with respect to protection, in terms of how client connections are mapped into the optical layer, and how protection schemes in both layers can work together in efficient ways. Finally, we describe several interesting optical protection implementations, focusing on the ones that are different from conventional SONET-like implementations.     

The Bit Complexity of Distributed Sorting

We study the bit complexity of the sorting problem for asynchronous distributed systems. We show that for every network with a tree topology T, every sorting algorithm must send at least bits in the worst case, where is the set of possible initial values, and Δ _T is the sum of distances from all the vertices to a median of the tree. In addition, we present an algorithm that sends at most bits for such trees. These bounds are tight if either L=Ω(N ^1+ε ) or Δ _T =Ω(N ² ). We also present results regarding average distributions. These results suggest that sorting is an inherently nondistributive problem, since it requires an amount of information transfer that is equal to the concentration of all the data in a single processor, which then distributes the final results to the whole network. The importance of bit complexity—as opposed to message complexity—stems also from the fact that, in the lower bound discussion, no assumptions are made as to the nature of the algorithm. Received May 2, 1994; revised December 22, 1995.     

Merits of low-density WDM line systems for long-haul networks

Conventional wisdom claims that the higher the number of wavelengths supported by a dense wavelength-division multiplexing transmission system's line system the better, since the high costs of amplifiers are split among more revenue-generating entities. For example, a 20/spl lambda/ link must cost less than 25% of the cost of an 80/spl lambda/ link to compete at full (80/spl lambda/) fill, assuming four 20/spl lambda/ links need to be deployed in parallel. We show that this is not necessarily the case if network considerations and technology evolution are taken into account. In particular, we show that a 20/spl lambda/ link can cost as high as 64-79% of the cost of an 80/spl lambda/ link and still be competitive at full fill, while providing much lower initial cost. This is because of the cost reduction of technology over time, economic considerations (net present value), nonuniform capacity needs in real networks, and how network maintenance can be achieved. We also reason why such lower end systems can indeed achieve the required cost reduction.     

Predeployment of resources in agile photonic networks

This paper shows that the main challenge in remotely configurable optical networks lies in how networks should be designed and sized for dynamic traffic in the face of uncertain forecasts, so as to minimize capital expenses, predominantly due to the need to predeploy (or overprovision) resources in preparation for the next connection request. In addition, this paper demonstrates how the operational savings promised by such capabilities measure up against the added capital expenses. Consequently, new ways to provision resources in the network to minimize these expenses are proposed. The results are based on real-world long-haul networks and costs.     

Attenuated Listeria monocytogenes carrier strains can deliver an HIV-1 gp120 T helper epitope to MHC class II-restricted human CD4+ T cells

Listeria monocytogenes is a facultative intracellular pathogen which, following uptake by macrophages, escapes from the phagosome and replicates in the cytoplasm. This property has been exploited using recombinant L. monocytogenes as a carrier for the intracytoplasmic expression of antigens when MHC class I-restricted cytotoxic T lymphocyte responses are required. Much less is known of the ability of these bacteria to trigger MHC class II-restricted responses. Here, we demonstrate that after ingestion of L. monocytogenes expressing a T helper epitope from the gp120 envelope glycoprotein of HIV, human adherent macrophages and dendritic cells can process and present the epitope to a specific CD4+ T cell line in the context of MHC class II molecules. No significant differences were observed when the attenuated strains were trapped in the phagolysosome or impaired in the capacity to spread intracellularly or from cell to cell. Similar results were obtained using carrier proteins that were either secreted, associated with the bacterial surface, or restricted to the bacterial cytoplasm. A dominant expression of the TCR Vbeta 22 gene subfamily was observed in specific T cell lines generated after stimulation with the recombinant strains or with soluble gp120. Our data show that in this in vitro system L. monocytogenes can efficiently deliver antigens to the MHC class II pathway, in addition to the well-established MHC class I pathway. The eukaryotic cell compartment in which the antigen is synthesized, and the mode of display seem to play a minor role in the overall efficiency of epitope processing and presentation.     

Protection interoperability for WDM optical networks

The failure of a single optical link or node in a wavelength division multiplexing (WDM) network may cause the simultaneous failure of several optical channels. In some cases, this simultaneity may make it impossible for the higher level (SONET or IP) to restore service. This occurs when the higher level is not aware of the internal details of network design at the WDM level. We call this phenomenon “failure propagation.” We analyze three types of failure propagation, called “bottleneck,” “connectivity,” and “multiple groups.” Then we present a solution based on the definition of appropriate requirements at network design and a WDM channel placement algorithm, protection interoperability for WDM (PIW). Our method does not require the higher level to be aware of WDM internals, but still avoids the three types of failure propagation mentioned above. We finally show the result on various network examples