A Call-Admission Control (CAC) Algorithm for Providing Guaranteed QoS in Cellular Networks

Future broadband wireless access systems are expected to integrate various classes of mobile terminals (MTs), each class with a different type of quality of service (QoS) requirement. When the load on a wireless network is high, the guarantee of QoS for each class of MTs is a challenging task. This study considers two classes of MTs—profiled MTs and nonprofiled or regular MTs. It is assumed that profiled users require a guaranteed QoS. The measure of QoS is the probability of forced termination of a call that was allowed to access the network. Two previous handoff prioritization schemes—(i) prerequest scheme and (ii) guard channel scheme—decrease handoff failure (and hence forced termination). In this work, we compare and contrast both the schemes through extensive simulation and we find that neither guard channel nor channel prerequest scheme can guarantee a desired level of QoS for the profiled MTs. We then propose a novel call-admission control (CAC) algorithm that can maintain any desired level of QoS, while the successful call completion rate is very high. In the proposed algorithm, the new call arrival rate is estimated continuously, and when the estimated arrival rate is higher than a predetermined level, some new calls are blocked irrespective of the availability of channels. The objective of this new call preblocking is to maintain a cell's observed new call arrival rate at no more than the predetermined rate. We show that the proposed method can guarantee any desired level of QoS for profiled users.     

Protecting multicast sessions in WDM optical mesh networks

Recent advances in wavelength-division-multiplexing (WDM) technology are expected to facilitate bandwidth-intensive multicast applications. However, a single fiber (bundle) cut on such a network can disrupt the transmission of information to several destination nodes on a "light tree"-based multicast session. Thus, it is imperative to protect multicast sessions e.g., by reserving resources along backup trees. We show that, if a backup tree is directed-link-disjoint to its primary counterpart, then data loss can be prevented in the event of any single link failure. We provide mathematical formulations for efficient routing and wavelength assignment (RWA) of several multicast sessions (including their backup trees for dedicated protection) at a globally optimum cost. We present these formulations for networks equipped with two kinds of multicast-capable switch architectures: one using the opaque (O-E-O) approach and the other using transparent (all-optical) approach. We expand our formulations to accommodate sparse splitting constraints in a network, in which an optical splitter has limited splitting fanout and each node has a limited number of such splitters. We develop a profit-maximizing model that would enable a network operator to be judicious in selecting sessions and simultaneously routing the chosen ones optimally. We illustrate the solutions obtained from solving these optimization problem formulations for a representative-size network.     

Mechanisms of cavity growth in a fine-grained 7475 Al superplastic alloy

Under creep conditions, cavity growth may be controlled by vacancy diffusion or power-law creep. The two growth mechanisms are examined with reference to a superplastic 7475 Al alloy as a function of test temperature, superimposed strain rate and starting grain size. It is found that power-law cavity growth by plastic deformation of the matrix surrounding the cavities dominates at all test conditions. In addition, growth and interlinkage of cavities is an important parameter in controlling the ease and type of fracture and is enhanced by the ease of superplastic deformation.     

WDM-based local lightwave networks. II. Multihop systems

For pt.I see ibid., vol.6, no.3, p.12-27, 1992. A survey of wavelength-division-multiplexing (WDM)-based local lightwave networks is presented. The general characteristics of multihop systems are discussed, and various multihop approaches are reviewed. The construction of optimal structures based on minimizing the maximum link flow and optimizations based on minimization of the mean network packet delay are also reviewed. Regular topologies that have been studied as candidates for multihop lightwave networks, including the perfect shuffle, the de Bruijn graph, the toroid, and the hypercube, are discussed. Near-optimal node placement algorithms and shared-channel multihop systems are also discussed     

The creep behavior of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/SiC nanocomposites

In an investigation of the creep properties of silicon nitride/silicon carbide nanocomposites, the micro-nano type composites with nano-SiC at intragranular and inter-granular regions show behavior not much different from that of silicon-nitride monolith. The improvement of creep resistance is modest, up to about one order of magnitude decrease in steady-state creep rate. The creep rate parameters such as activation energy and stress exponent measured for this type of nanocomposite are within the range of those of silicon nitride. This evidence suggests that a special strengthening mechanism may not be necessary for this type of material. Nano-nanocomposites show remarkably lower creep rates, possibly pointing to a new creep mechanism such as solid-state diffusion. For more information, contact A.K. Mukherjee, University of California-Davis, Department of Chemical Engineering and Materials Science, Davis, CA 95616; e-mail akmukherjee@ucdavis.edu.     

Smart pumpless loop for micro-channel electronic cooling using flat and enhanced surfaces

Two-phase cooling of a square simulated electronic device surface of 21.3 mm side was successfully carried out without the need for a pump. This smart, passive cooling system incorporates a self-enhancing and self-sustaining mechanism, wherein the system inherently enhances its cooling capacity by increasing the velocity of the two-phase mixture along the boiling surface when an increase in heat flux is sensed. Other practical attributes of this pumpless loop are small liquid inventory requirements and absence of the incipient boiling temperature drop. It is shown small surface tension and contact angle render dielectric coolants such as FC-72 ideally suited for flow in narrow gaps. These unique properties are responsible for very small bubble size, precluding any appreciable blockage of the replenishment liquid flow even in narrow gaps. Critical heat flux (CHF) was found to generally increase with decreasing boiler gap. CHF for flat, micro-channel (with 0.2 mm rectangular fins) and mini-channel (with 1.98 mm rectangular fins) surfaces was 4.5, 5.9, and 5.7 times greater than for pool boiling from a flat surface for corresponding gaps. A pressure drop model was formulated to predict coolant mass flow rate, boiling surface inlet and exit velocities, and pressure drop components throughout the loop. The model predictions illustrate the pumpless loop's self-sustaining and self-enhancing attributes, and relate CHF trends to those of the two-phase mixture acceleration along the boiling surface.     

Influence of surface texturization on the light trapping andspectral response of silicon solar cells

A quantitative model that explains the spectral response, internal quantum efficiency, total short-circuit current, open-circuit voltage, and efficiency of high-efficiency solar cells with textured front surface and Lambertian back-surface reflectors is presented. A comparison of the textured cell characteristics is made with those of planar cells, and the separate roles of the front surface reflection coefficient and internal quantum efficiency in enhancing the short-circuit current have been investigated. It is shown that, in the case of large diffusion lengths, almost all the contribution to the increase of spectral response on texturization is due to the reduced reflection coefficient whereas, for small diffusion lengths, there is a significant increase in internal quantum efficiency on texturization, especially in the region of higher wavelengths. However, there is a small decrease in open-circuit voltage for large diffusion lengths, whereas no significant change is observed for small diffusion lengths on texturization. Nevertheless, there is a net gain in power conversion efficiency which is larger for smaller diffusion lengths     

Wavelength conversion in WDM networking

Wavelength conversion has been proposed for use in wavelength-division multiplexed networks to improve efficiency. This study highlights systems challenges and performance issues which need to be addressed in order to incorporate wavelength conversion effectively. A review/survey of the enabling technologies, design methods, and analytical models used in wavelength-convertible networks is provided     

In vivo cytogenetic studies on mice exposed to acesulfame-K--a non-nutritive sweetener

Acesulfame-K, a sweetening agent, was evaluated in vivo for its genotoxic and clastogenic potentials. Swiss albino male mice were exposed to the compound by gavage. Bone marrow cells isolated from femora were analysed for chromosome aberrations. Doses of 15, 30, 60, 450, 1500 and 2250 mg of acesulfame-K/kg body weight induced a positive dose-dependent significant clastogenicity (trend test alpha < 0.05). These doses were within the no-toxic-effect levels (1.5-3 g/kg body weight in rats) reported by the Joint Expert Committee for Food Additives of the World Health Organization and the Food and Agriculture Organization of the United Nations. In view of the present significant in vivo mammalian genotoxicity data, acesulfame-K should be used with caution.     

Measuring software dependability by robustness benchmarking

Inability to identify weaknesses or to quantify advancements in software system robustness frequently hinders the development of robust software systems. Efforts have been made to develop benchmarks of software robustness to address this problem, but they all suffer from significant shortcomings. The paper presents the various features that are desirable in a benchmark of system robustness, and evaluates some existing benchmarks according to these features. A new hierarchically structured approach to building robustness benchmarks, which overcomes many deficiencies of past efforts, is also presented. This approach has been applied to building a hierarchically structured benchmark that tests part of the Unix file and virtual memory systems. The resultant benchmark has successfully been used to identify new response class structures that were not detected in a similar situation by other less organized techniques