Fast restoration of ATM networks

Asynchronous transfer mode (ATM) is now well recognized as the fundamental switching and multiplexing technique for future broadband ISDN. As these networks will be increasingly relied upon for providing a multitude of integrated voice, data, and video services, network reliability is a key concern. There are several intrinsic features of ATM networks that could potentially be exploited to provide improved restoration techniques, beyond those established for synchronous transfer mode (STM) networks, such as digital cross-connect restoration or self-healing rings. These features include ATM cell level error detection, inherent rate adaptation and nonhierarchical multiplexing. The authors explore the use of these features in developing fast restoration strategies for ATM networks. In particular, they address: (1) ATM error detection capabilities for enhanced failure detection, (2) network rerouting strategies, (3) spare capacity allocation, and (4) network control architecture and related implementation aspects. Their findings suggest that fast network span failure detection and bandwidth-efficient rerouting capabilities can be combined to develop restoration strategies for ATM networks with significantly greater performance-cost ratios when compared to existing STM network restoration strategies     

Le cation méthylènecyclopropyle: Etude théorique. Solvolyse des halogénocyclopropènes et des halogénométhylenecyclopropanes

The solvolysis in acid solution of allylic halides and acetates of methylenecyclopropanes and cyclopropylmethanes does not lead to ring opening when there is an ethoxycarbonyl substituent on the ring methylene. Only the cyclopropenyl derivative is obtained. A study of the electronic structure and the geometry of the possible allylic cation intermediate has been carried out using the MNDO method. In addition, the influence of the substituents on the ring opening of this cation has beeen examined. It appears that the ring opening is disfavoured upon substitution of an alkyl group by alkoxymethyl on the ring methylene group.     

Structure électronique et spectres de resonance paramagnétique électronique des radicaux vinyle et cyclopropyle

Résumé Une méthode de calcul dérivant des théories de champ self-consistant est utilisé pour l'étude des radicaux vinyle et cyclopropyle. L'énergie électronique et les écarts hyperfins dûs aux hydrogènes et au ¹³C ont été évalués en fonction de la géométrie du carbone radicalaire. L'angle C=-H du radical vinyle a été trouvé de l'ordre de 150°. Il est montré que les radicaux vinyle et cyclopropyle ont une structure électronique similaire qui ne peut être totalement expliquée ni par un modèle ni par un modèle classique, chaque radical présentant à la fois les deux caractères.

---

A method of calculation derived from self-consistent field theories is used for a study of vinyl and cyclopropyl radicals. Electronic energy, hydrogens and ¹³C hyperfine splittings are evaluated as a function of the carbon geometry. The C=-H bond angle is found to be about 150°. It is shown that vinyl and cyclopropyl radicals have a similar electronic structure which cannot be completely interpreted neither by a classical model nor by a one, each radical presenting both and character.

---

Zusammenfassung Nach einer auf dem SCF-Verfahren basierenden Methode werden Elektronenenergie, H- und ¹³C-Hyperfeinaufspaltung in Vinyl- und Cyclopropylradikalen in Abhängigkeit vom Bindungswinkel am berechnet, der sich im ersten Fall zu 150° ergibt. Es zeigt sich, daß die Elektronenstruktur der beiden behandelten Radikale ähnlich ist und daß sie weder durch ein reines - noch ein reines -Modell zu beschreiben ist.

     

Perturbative configuration interaction using localized orbitals in the INDO hypothesis. I. Theory and applications to energetical problems

A complete development of the PCILO method is presented for the INDO approximation. The introduction of exchange terms in the theoretical treatment is discussed and a detailed analysis of the successive contributions to the total energy is given. The method is applied to various conformational problems (geometry optimization, rotation, and inversion barriers). Its ability to deal with biological systems is tested on the acetylcholine neurotransmitter. Inversion barriers are highly improved with respect to the corresponding CNDO algorithm. A better account of σ-π exchange phenomena can be expected with the presented PCILO -INDO development.     

Off-Line and On-Line Preconcentration Techniques for the Determination of Phenylureas in Freshwaters

Abstract

Phenylurea herbicides are analysed by reversed-phase liquid chromatography using UV detection at 244 nm after a concentration step in order to determine ppb or sub-ppb levels in drinking and river waters. With an average UV detection limit of 5 ng, a 500 ml sample volume is necessary to reach the 10 ppt level for spiked LC grade water samples and enables easy determination of concentrations below the ppb level for river water samples. Off-line and on-line methods are compared for the concentration step. Off-line concentration consists in a liquid sorption on n-octadecyl silica (C18) and elution by a suitable organic solvent. Polar phenylureas have low retention volumes on C18 silica and consequently the length of the concentration column has to be 10 cm to concentrate them at the ppb level from 100 ml of water and longer for lower levels of detection. Nevertheless, we show that increasing the size of the concentration column does not improve the limits of detection because of the numerous interferences also concentrated when percolating high volumes of water. On-line technology can be used only with short precolumns and requires a sorbent with a great retention for phenylureas. The copolymer-based PRP-1 is found to be an excellent sorbent and it is then possible to apply on-line precolumn technology with preconcentration through two precolumns (10 × 21 mm ID) in series, the first one being packed with C18 silica and the other with the PRP-1 polymer. Interfering compounds are then trapped onto the first precolumn acting as a filter and common phenylurea-breakthrough volumes on the PRP-1 precolumn are higher than 500 ml. Knowing the amounts preconcentrated on both precolumns and using UV and electrochemical detection help the identification of phenylureas in river water.     

End-to-end performance models for variable bit rate voice overtandem links in packet networks

Analytical models are presented for computing the end-to-end voice call performance in a packet network that drops the less significant bits in voice packets during periods of congestion. These models provide information about the end-to-end quality likely to be experienced in future packet-switched integrated services networks. An existing single-node bit-dropping model is modified to include the situation resulting when the overall arrival process at an internal node consists of a mix of packets of different sizes due to bit dropping at previous nodes. A detailed model to capture bit-dropping effects in a tandem connection of nodes is presented. The model includes the effect of load fluctuations at each node, and also takes into account the dependencies in bit dropping experienced by a voice packet at successive nodes in a tandem connection. The model also incorporates the internodal dependence when reductions in packet service times occur at intermediate nodes due to bit dropping at previous nodes. Two approximation procedures are discussed that serve as upper and lower bounds. In particular, the upper bound is shown to be very tight for a practical range of loads, and hence serves as a good approximation with significant computational simplicity     

Determination of polar aniline derivatives in aqueous environmental samples using on-line liquid chromatographic preconcentration techniques

Summary On-line precolumn sample handling is used to enrich polar aniline derivatives in order to preconcentrate them prior to their separation. Liquid-solid extraction is possible with a cation-exchanger precolumn after acidification of water samples at pH 3 and a clean-up in order to remove the high amounts of inorganic cations present in natural samples. Since inorganic removal cannot be total, overloading of the ion exchanger occurs rapidly. The volume which can be directly percolated through the cation-exchanger precolumn cannot exceed 30 ml and the amount preconcentrated is not sufficient for a determination at the 100 ppt level. A two-step preconcentration procedure is carried out in order to increase the sample volume: the direct percolation of samples through the cation-exchanger precolumn is avoided and the clean-up step is no longer necessary. Aniline derivatives are preconcentrated in their neutral form at pH 7 by a 9-cm long column packed with the copolymer-based PRP-1 sorbent; then, a small volume of water-methanol at pH 2 allows the cationic compounds alone to be desorbed from the PRP-1 column in their protonated form and to be transferred to a 1-cm long cation-exchanger precolumn. This precolumn is then coupled to an analytical C18 column and its content on-line analysed by an acetonitrile gradient. The PRP-1 column acts as a powerful filter to many neutral interferents and aniline derivatives can be thus determined from 150-ml drinking water samples with 10–50 ppt UV detection limits.Dedicated to Roland W. Frei     

Direct Synthesis of Peptide-Containing Silicones: A New Way to Bioactive Materials

A simple and efficient way to synthesize peptide-containing silicone materials is described. Silicone oils containing a chosen ratio of bioactive peptide sequences were prepared by acid-catalyzed copolymerization of dichlorodimethylsilane, hybrid dichloromethyl peptidosilane, and Si(vinyl)- or SiH-functionalized monomers. Functionalized silicone oils were first obtained and then, after hydrosilylation cross-linking, bioactive polydimethylsiloxane (PDMS)-based materials were straightforwardly obtained. The introduction of an antibacterial peptide yielded PDMS materials showing activity against Staphylococcus aureus. PDMS containing RGD ligands showed improved cell-adhesion properties. This generic method was fully compatible with the stability of peptides and thus opened the way to the synthesis of a wide range of biologically active silicones.