摘要： There is a growing trend in developing applications for mobile computing systems in which mobile host computers retain their network connections while in transit. This paper proposes an algorithm that enforces a useful property, namely, causal ordering, that delivers messages among mobile hosts. This property ensures that causally related messages directed to the same destination will be delivered in an order consistent with their causality, which is important in applications that involve human interaction such as mobile e-mail and mobile teleconferencing. Such applications are envisioned by the proponents of Personal Communications Services (PCS). Without this property, users may receive and read original messages and the corresponding replies out of order. Our algorithm, when compared with previous proposals, provides an alternative with a low handoff cost, medium message overhead, and low probability of unnecessary inhibition in delivering messages.

关键词： 销量   笔记本电脑   Compaq   笔记本计算机   移动计算   方正  

摘要： 据有关权威机构统计,Compaq笔记本电脑在国内市场的份额已由排名第六位升至第四位。据权威人士分析,Compaq笔记本电脑在国内销量的飞速增长,主要得益于北大方正的加入,关键在于二者能够优势互补:一方面,Compaq公司作为世界级的计算机生产企业,且有优秀的产品;另一

关键词： 硬盘   硬磁盘   磁盘   移动计算  

摘要： 在移动计算应用领域,IBM 提供了 Travelster 系列高效和大容量的2.5英寸硬盘驱动器.它不仅具有同3.5英寸硬盘驱动器同样的功能,而且体积纤巧,只有100克至180克。

关键词： 电池   笔记本电脑   笔记本计算机   移动计算   序幕   情节  

摘要： Lithium Technology 公司9月底宣布,该公司已获得550万美元的私人投资,用于开发下一代高聚合锂电池。这家位于宾夕法尼亚州 PlymouthMeeting 的公司是目前业界唯一能生产这种电池的厂家。该公司许诺将生产一种全系列的电池产品用于极薄的移动计算设备中,包括笔记本电脑、个人数字

关键词： Chemical structure   Oxygen   Phosphates   Noncovalent interactions   Ions  

摘要： Three molecular dynamics simulations of the free, phosphate ion-bound and phosphopeptide-bound C-terminal SH2 domain of phospholipase C-gamma l (PLCC . pY) have been performed to aid in the interpretation of chemical shift data and the elucidation of interatomic interactions at the phosphotyrosine (pTyr) binding region. The simulation of the phosphopeptide complex was carried out with newly developed CHARMM force-field parameters for pTyr, optimized against experimental data and ab initio calculations. The lack of NOEs involving phosphate in the binding pocket had necessitated a chemical shift analysis of the pTyr binding region for a more detailed characterization of the hydrogen bonding interactions involving pTyr. Although most of these interactions are not present in the NMR structure used as the simulation starting point, the system converges early in the simulation to a structure more compatible with the chemical shift data. This is supported by ab initio determination of the H-1(eta) and H-1(epsilon) chemical shifts of the three arginines (Arg 18, 37, and 39) in the pTyr binding pocket based on the PLCC . pY MD structure, which are in accord with the experimental values. The simulation structure of the PLCC . pY complex reveals a more complete picture of interatomic interactions in the pTyr binding pocket than is possible with current chemical shift and NOE approaches alone, thereby permitting the identification of the primary pTyr-recognition residues. This pattern of interactions is strikingly similar to those of crystal structures of related SH2 domains. The simulations also suggest several alternative interpretations of the chemical shift data to those suggested in the experimental investigation (Pascal, S. M., et al. Biochemistry 1995, 34, 11353). This insight is valuable as the observed chemical shifts could result from a number of possible pictures of interactions. The present study demonstrates that the combination of molecular dynamics simulations an

关键词： mobility-shift assay   nonspecific protein-DNA complexes   gel cage  

摘要： Theoretical mobility-shift patterns are computed by solution of conservation equations for electrophoresis coupled with chemical reaction, The chemical reaction term is often formulated in terms of dissociation of the protein-DNA complex in a gel cage. This formulation assumes that once the dissociated protein escapes the cage, it goes down a sink and is totally lost, This implies that the concentration of the escaped protein is too low to affect significantly the rates of protein-DNA association along its migration pathway.

关键词： ab initio computations   chemical shift computations   Ga-71   electric field gradient  

摘要： Ga-71 chemical shifts computed with the GIAO (gange-including atomic orbitals)-SCF method and medium-sized basis sets for optimized geometries are reported for GaMe(3), GaMe(3)-NMe(3), GaCl4-, Ga2Cl6, Ga(OH4)(-), GaH4-, Ga(OH2)(3)(3+), Ga(NCMe)(6)(3+), Ga(C(5)Me(5)), Ga[GaCl4] and Ga(C5H5). Experimental trends are well reproduced, but the slope of the delta(calc) vs. delta(exp) correlation is only 0.93 instead of unity. For selected compounds, the electron-correlated GIAO-MP2, method affords an improved description of the chemical shift range covered (ca. 1400 ppm). Geometry effects on delta(Ga-71) are small in most cases, but can be notable for certain Ga-I species, e.g. for Ga-I[GaCl4], the calculated Ga-I chemical shift of which also depends strongly on aggregation and solvation. delta(Ga-71) values no larger than 153 ppm are computed for LiGaH4 and aggregates thereof, in sharp contrast to the published experimental value of 682 ppm. Based on accurate GIAO-CCSD(T) calculations (at a highly correlated coupled-cluster level), a delta(Ga-71) value of ca. 615 ppm is predicted for GaH3. At the GIAO-MP2 level, delta(Ga-71) = 312 ppm is predicted for GaMe(4)(-), which should be accessible experimentally. The computed sigma(Ga) values in the most strongly shielded Ga-I species approach that of isolated Ga+. Since sigma(Ga) of isolated Ga3+ is similar, the deshielding in molecules containing Ga-III is attributed to large paramagnetic contributions of the bonding MOs in the valence shell. For a number of molecules, trends in experimental delta(Ga-71) linewidths can be rationalized in terms of the relative magnitudes of the electric field gradient at the Ga atom.

关键词： Mobile telecommunication systems  

摘要： We discuss the design and implementation of network and transport layer protocols for mobile networking. The network architecture used is unique in that it separates the mobile network(s) from fixed networks and provides connectivity between the two via special gateways. These gateways split all transport connections and provide QoS guarantees to mobile users for all their open connections. We provide summaries of our protocols and discuss possible improvements.

摘要： NMR chemical shift calculations in combination with geometry optimizations both based on density functional theory were used to identify and to describe dimesitylketone O-oxide (6), which is the first carbonyl oxide that could be generated in solution at - 78 degrees C and investigated by NMR spectroscopy. The conformation of carbonyl oxide 6 is characterized by two orthogonal mesityl rings and a close O,H contact between one of the H atoms of the mesityl methyl groups and the terminal O atom of the carbonyl oxide, The calculated geometry and conformation of 6 are verified by the agreement of calculated and measured NMR chemical shifts, The unusual rearrangement of 6 to an alcohol is explained.

关键词： Computer networks  

摘要： Today a mobile user wants to connect his portable computer: remotely to the central database at home, locally to the printer on the spot and globally to the world-wide-web. To achieve this, different connection lines are available: wireless networks for connecting out in the fields, ISDN or analogue telephone lines when residing in a hotel, Ethernet access at the customer's site. But this connectivity raises a lot of questions, about technical, security or accounting issues. This paper presents the architecture of an environment aiming to support mobile users and dealing with the given problems. Copyright (C) 1996 Elsevier Science Ltd