TCP performance over mobile ad hoc networks: a quantitative study

In this paper, we study the performance of the transmission control protocol (TCP) over mobile ad‐hoc networks. We present a comprehensive set of simulation results and identify the key factors that impact TCP's performance over ad‐hoc networks. We use a variety of parameters including link failure detection latency, route computation latency, packet level route unavailability index, and flow level route unavailability index to capture the impact of mobility. We relate the impact of mobility on the different parameters to TCP's performance by studying the throughput, loss‐rate and retransmission timeout values at the TCP layer. We conclude from our results that existing approaches to improve TCP performance over mobile ad‐hoc networks have identified and hence focused only on a subset of the affecting factors. In the process, we identify a comprehensive set of factors influencing TCP performance. Finally, using the insights gained through the performance evaluations, we propose a framework called Atra consisting of three simple and easily implementable mechanisms at the MAC and routing layers to improve TCP's performance over ad‐hoc networks. We demonstrate that Atra improves on the throughput performance of a default protocol stack by 50%–100%. Copyright © 2004 John Wiley & Sons, Ltd.     

Thermal expansion in various phases of Nitinol using TMA

Phase transformation in the near-equiatomic, cold-worked Nitinol, exhibiting shape memory effect, has been investigated through the study of thermal expansion by employing thermomechanical analyser (TMA). The characteristic transformation temperatures determined by this method are compared with those obtained through DSC thermograms. The reliability and sensitivity of the thermal expansion probe have been discussed. The variation of thermal expansion coefficient with temperature in different heat treated samples has been studied in the range 30–120°C. Thermal expansion coefficient is found to be −ve during M↔A transformation and +ve in R↔A transformations. Thermal expansion coefficient in martensitic region in the presence and absence of R-phase has been determined.     

Query Localization Techniques for On-Demand Routing Protocols in Ad Hoc Networks

Mobile ad hoc networks are characterized by multi-hop wireless links, absence of any cellular infrastructure, and frequent host mobility. Design of efficient routing protocols in such networks is a challenging issue. A class of routing protocols called on-demand protocols has recently found attention because of their low routing overhead. We propose a technique that can reduce the routing overhead even further. The on-demand protocols depend on query floods to discover routes whenever a new route is needed. Our technique utilizes prior routing histories to localize the query flood to a limited region of the network. Simulation results demonstrate excellent reduction of routing overheads with this mechanism. This also contributes to a reduced level of network congestion and better end-to-end delay performance of data packets.     

Harnessing the Extracellular Electron Transfer Capability of Geobacter sulfurreducens for Ambient Synthesis of Stable Bifunctional Single-Atom Electrocatalyst for Water Splitting

Single-atom metal (SA-M) catalysts with high dispersion of active metal sites allow maximum atomic utilization. Conventional synthesis of SA-M catalysts involves high-temperature treatments, leading to low yield with a random distribution of atoms. Herein, a nature-based facile method to synthesize SA-M catalysts (M = Fe, Ir, Pt, Ru, Cu, or Pd) in a single step at ambient temperature, using the extracellular electron transfer capability of Geobacter sulfurreducens (GS), is presented. Interestingly, the SA-M is coordinated to three nitrogen atoms adopting an MN₃ on the surface of GS. Dry samples of SA-Ir@GS without further heat treatment show exceptionally high activity for oxygen evolution reaction when compared to benchmark IrO₂ catalyst and comparable hydrogen evolution reaction activity to commercial 10 wt% Pt/C. The SA-Ir@GS exhibits the best water-splitting performance compared to other SA-M@GS, showing a low applied potential of 1.65 V to achieve 10 mA cm⁻² in 1.0 M KOH with cycling over 5 h. The density functional calculations reveal that the large adsorption energy of H₂O and moderate adsorption energies of reactants and reaction intermediates for SA-Ir@GS favorably improve its activity. This synthesis method at room temperature provides a versatile platform for the preparation of SA-M catalysts for various applications by merely altering the metal precursors.     

Enantiospecific total synthesis of (−)-crotanecine

Crotanecine is the necine base component of a number of pyrrolizidine alkaloids. This necine subunit is an amino triol bearing a primary allylic alcohol characterized by an all-cis relationship of its stereocenters. The synthesis of crotanecine has been accomplished using simple yet versatile ring construction approach using ribose as chiral starting point.     

Condition based divergence in synthesis of tetrahydrobenzo[b]pyrans

Research on Chemical Intermediates - An environmentally benign novel strategy is developed for the synthesis of tetrahydrobenzo[b]pyrans by using thiamine mononitrate as a green catalyst, with the...     

Two Distinct Origins of Highly Localized Luminescent Centers within InGaN/GaN Quantum‐Well Light‐Emitting Diodes

The high light‐output efficiencies of In_xGa_1‐xN quantum‐well (QW)‐based light‐emitting diodes (LEDs) even in presence of a large number of nonradiative recombination centers (such as dislocations) has been explained by localization of carriers in radiative potential traps, the origins of which still remain unclear. To provide insights on the highly efficient radiative traps, spectrally resolved photoluminescence (PL) microscopy has been performed on green‐light‐emitting In_0.22Ga_0.78N QW LEDs, by selectively generating carriers in the alloy layers. PL imaging shows the presence of numerous inhomogeneously distributed low‐band‐gap traps with diverse radiative intensities. PL spectroscopy of a statistically relevant number of individual traps reveals a clear bimodal distribution in terms of both band‐gap energies and radiative recombination efficiencies, indicating the presence of two distinct classes of carrier localization centers within the same QW sample. Disparity in their relative surface coverage and photoemission “blinking” characteristics suggests that the deep traps originate from local compositional fluctuations of indium within the alloy, while the shallow traps arise from nanometer‐scale thickness variations of the active layers. This is further supported by Poisson–Schrödinger self‐consistent calculations and implies that radiative traps formed due to both local indium content and interface‐morphology‐related heterogeneities can coexist within the same QW sample.