Type-of-service routing in datagram delivery systems

The Internet is expected to support various services, including best-effort services and guaranteed services. For best-effort services, we propose a new approach to achieving type-of-service (TOS) classes with adaptive next-hop routing. We consider two TOS classes, namely, delay-sensitive and throughput-sensitive. As in routing protocols such as OSPF and integrated IS-IS, each node has a different next-hop for each destination and TOS class. Traditionally, a node has a single FCFS queue for each outgoing link, and the next-hops are computed using link measurements. In our approach, we attempt to isolate the two traffic classes by using for each outgoing link a separate FCFS queue for each TOS class; the link is shared cyclicly between its TOS queues. The next-hops for the delay-sensitive traffic adapts to link delays of that traffic. The next-hops for the throughput-sensitive traffic adapts to overall link utilizations. We compare our approach with the traditional approach using discrete-event simulation and Lyapunov analysis (for stability of routes). Our approach offers lower end-to-end delay to the delay-sensitive traffic. A related property is that the routes for the delay-sensitive traffic are more stable, i.e., less oscillations. An unexpected property is that the overall end-to-end delay is lower, because the throughput-sensitive traffic moves away to under-utilized routes     

Asymptotic analysis of single pore gas-solid reactions

The single-pore model for gas-solid reactions proposed by Ramachandran and Smith[1] is used to develop exact, closed-form solutions for the concentrati pore radius and conversion profiles in a long, narrow pore for both cylindrical and slab geometries. For relatively large values of the Thiele modulus, the solutions portray the asymptotic behavior of the reaction scheme in a finite pore. The cases of incomplete conversion associated with pore closure low initial porosities and complete conversion associated with a moving reaction zone at high initial porosities are analyzed. It is shown that the asy results provide very accurate estimates of the overall conversion in applications involving large pore diffusional resistance or fast reactions. The se of the conversion to various physicochemical parameters is also examined. The asymptotic results show good agreement with experimental data for gas-sol     

Interdiffusion and intrinsic diffusion in the Ni AI (δ) phase of the Al-Ni system

Interdiffusion coefficients at 950 to 1150°C and the ratio of intrinsic diffusion coefficients at 1100°C were measured as functions of composition in the NiAl (δ) phase of the Al-Ni system, using a vapor-solid technique. Diffusivity values were also obtained for the Ni₃Al (∈)and Ni (Al) solid solution (ζ) phases from 950 to 1150°C. The interdiffusion coefficient in NiAl (δ) varies several orders of magnitude over the δ phase field with a deep minimum in the diffusivity-composition curve at 48 to 49 at. pct Al. The ratio of intrinsic diffusion coefficients, D_ni/D_aI, in the δ phase also varies with composition from a value of 3 to 3.5 below 50 at. pct Al to 0.1 or less above 50 at. pct Al. Formerly Research Assistant, Department of Mate-rials Science, SUNY at Stony Brook, New York.     

Analysis of the evolution of microcreep during physical aging and mechanical deformation in poly(methyl methacrylate) using a microstructural model

The evolution of poly(methyl methacrylate) (PMMA) during physical aging at 90°C is followed by torsional microcreep tests. On the aged specimen a longitudinal stress is applied which induces a strain of 5 percent after 30 days of creep. The torsional microcreep tests are performed during the longitudinal creep in order to compare the structure evolution of PMMA caused by straining with its evolution measured during aging. The microcreep, for the first 800s, follows a reversible logarithmic law. In this stage the mobile defects achieve their activated form which is perfectly reversible when unloaded. The physical aging reduces this logarithmic part of microcreep. This is due to the decrease of either the number or the volume of the mobile defects. Beyond a critical elongation ? = 1 percent, the longitudinal straining has just the opposite influence, i.e., the logarithmic part of microcreep increases. This critical elongation ? = 1 percent corresponds to the beginning of the steady state longitudinal creep. The transient that precedes this steady state has no detectable influence on the structure of the specimen.     

On the geometry of radiative and thermally conducting gas flows

After formulation of the various dynamical and kinematical relations connecting the flow quantities with the geometrical parameters of the streamline trajectories, the expressions for the tangent, principal normal and binormal vectors and the curvature and torsion of the streamlines have been obtained in terms of the velocity components, pressure, density, thermal conductivity and radiation variables. This is followed by the determination of the pressure gradient along the streamlines, their principal normals and binormals. It has been shown that the radiating character of the gas decreases the pressure gradient whereas thermal conductivity increases it along the streamlines but the pressure remains constant along the binormals and if the streamlines are straight lines, the trajectories of the principal normals lie on the surface of the constant pressure. Finally, the expressions for the vorticity components in terms of the curvature of the streamline and the velocity gradients along the streamline, their principal normals and binormals have been obtained.     

Effect of chemical doping on the electrical conductivity of poly(2,6-dimethyl-1,4-phenylene oxide)

The electrical conductivity of solution-grown poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) films doped with TCNQ, TCNE, TNF, and I₂ has been studied as a function of dopant concentration, temperature (298–353 K), and field (50–2000 V). PPO forms a conductive complex on doping with these strong electron-acceptor organic molecules. As with PPO, two distinct ohmic and nonohmic conduction regions at low and higher fields, respectively, are observed. The conduction properties of doped PPo films result from the formation of an isotropic continuous charge-transfer complex. The existence of such a complex is confirmed by UV-visible absorption spectra measurements. A possible mechanism for the electrical conduction process is discussed. © 1994 John Wiley & Sons, Inc.     

Refractive Index of Liquid Aluminum Oxide at 0.6328 μm

The real part, n , of the refractive index of pendant drops of laser-heated liquid Al₂O₃ was measured by laser ellipsometry at 0.6328 μm. At temperatures from 2327 to 2600 K, n for the liquid was 1.744 ± 0.016. No significant differences were observed in the results obtained on ruby specimens and in oxygen or argon atmosphres.     

Inside Front Cover: Field‐Driven Biofunctionalization of Polymer Fiber Surfaces during Electrospinning (Adv. Mater. 1/2007)

Surface‐biofunctionalized synthetic polymer fibers composed of a fiber‐forming host polymer and an oligopeptide conjugate can be prepared from electrospinning, report Spontak and co‐workers on p. 87. The conjugate consists of a polypeptide segment and a polymer block that is compatible with the host polymer. Because the more polarizable peptide segment migrates to the surface during electrospinning, peptide surface‐enrichment is achieved in a single step without further treatment.     

Contention-Based Airtime Usage Control in Multirate IEEE 802.11 Wireless LANs

In a multirate wireless LAN, wireless/mobile stations usually adapt their transmission rates to the channel condition. It is difficult to control each station's usage of network resources since the shared channel can be overused by low transmission-rate stations. To solve this problem, we propose a distributed control of stations' airtime usage which 1) always guarantees each station to receive a specified share of airtime, and 2) keeps service for individual stations unaffected by other stations' transmission rates. Such airtime control enables service differentiation or quality of service (QoS) support. Moreover, it can achieve a higher overall system throughput. The proposed airtime usage control exploits the Enhanced Distributed Channel Access (EDCA) of the IEEE 802.11e standard . Two control mechanisms are proposed: one based on controlling the station's arbitration inter-frame space (AIFS) and the other based on the contention window size. We show how the stations' airtime usage is related to the AIFS and contention window size parameters. Using this relation, two analytical models are developed to determine the optimal control parameters. Unlike the other heuristic controls or analytical models, our model provides handles or parameters for quantitative control of stations' airtime usage. Our evaluation results show that a precise airtime usage control can be achieved in a multirate wireless LAN     

Sensing-based opportunistic channel access

Enabled by regulatory initiatives and advanced radio technologies, more flexible opportunistic spectrum access has great potential to alleviate the spectrum scarcity. In this paper, we study the channel selection issue of secondary users in spectrum-agile communication systems. We focus on the sensing-based approach because it is simple and has low infrastructure requirements. We propose a two-step approach for channel selection. The first step is to determine whether or not a channel is idle and thus accessible to secondary users. We propose three algorithms to perform the accessibility check based on measurements of primary signals. Then we address the question whether an accessible channel is a good opportunity for a secondary user. Xin Liu received her Ph.D. degree in electrical engineering from Purdue University in 2002. She is currently an assistant professor in the Computer Science Department at the University of California, Davis. Before joining UC Davis, she was a postdoctoral research associate in the Coordinated Science Laboratory at UIUC. Her research is on wireless communication networks, with a focus on resource allocation and dynamic spectrum management. She received the best paper of year award of the Computer Networks Journal in 2003 for her work on opportunistic scheduling. She received NSF CAREER award in 2005 for her research on “Smart-Radio-Technology-Enabled Opportunistic Spectrum Utilization.” Sai Shankar N received his PhD degree from the department of Electrical Communication Engineering from Indian Institute of Science, Bangalore, India for his work in the area of performance modeling of ATM networks. In 1998, He was awarded the German Fellowship, DAAD, in the department of mathematics, University of Kaiserslautern, Gernany to work on queueing approaches in manufacturing. In 1999, he joined Philips Research, Eindhoven, the Netherlands, where he served as Research Scientist in the department of New Media Systems and Applications working on various problems involving Hybrid, Fiber, Co-axial Cable (IEEE 802.14) Networks and IP protocols and provided efficient algorithms to improve protocol efficiency. In the year 2001 he joined Philips Research USA, Briarcliff Manor, NY and worked in the area of Wireless LANs/Ultra Wide Band (UWB), Cognitive Radios and Cooperative Communications. He was a key member in shaping the QoS related issues in IEEE 802.11e standardization and was one of the prime authors and inventors of the WiMEDIA UWB MAC protocol for which he was nominated as one of the five finalists in the Innovator of the year category by EE Times in year 2004. Besides these he has authored nearly 10 papers in the area of cognitive radios and holds fundamental patents on the design of MAC for cooperative communications. He has chaired numerical conferences and technical sessions and has delivered more than ten tutorials in leading international conferences of which two will appear in IEEE COMSOC webpage. He is also the Senior Member of the IEEE and has authored more than 50 conference and journal papers and holds more than 35 patents. Currently Sai Shankar is with Standards Engineering department of Qualcomm Inc. working on issues related to UWB and cognitive radios.