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1.
We consider queueing networks which are made from servers exchanging their positions on a graph. When two servers exchange their positions, they take their customers with them. Each customer has a fixed destination. Customers use the network to reach their destinations, which is complicated by movements of the servers. We develop the general theory of such networks and establish the convergence of the symmetrized version of such a network to some nonlinear Markov process. 相似文献
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《Computer Networks》1999,31(21):2287-2292
This paper presents a solution that compatibilizes user mobility and secure access to information servers by means of X.509 certificates with a short validity period. The common approach to compatibilizing user mobility and secure access is based on removable tokens that hold cryptographic information. The use of these techniques restricts user mobility in several ways. Firstly, when specific hardware is required, it must be available in any computer the user may employ to connect from. Secondly, using software that must be added to well-known client programs means that the user must circumscribe to those hosts where the software is installed or install it on his/her own. The solution we present here does not impose any constraints on hardware and, since it is based on the thin client paradigm, software requirements are minimal. The application of X.509 certificates permits the use of (de facto) standard software for accessing the information. Furthermore, since the system uses short term certificates it does not necessitate the user eliminating any traces left behind in the client program after its use. Finally, the token (actually, a diskette) can be used with practically any computer, as it contains all the software and data needed for user authentication, and is based on a thin client written in an architecture-neutral language like Java. The requirements on the computer the user is connecting from are minimal: having a floppy drive and a Java virtual machine. An implementation of the framework described here is in use to provide authorized access to internal servers at CICA. 相似文献
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In the past, several authors have expressed their concerns over the poor congestion control in mobile wireless ad-hoc networks using traditional reference layer model. Many solutions were proposed to handle growing traffic and congestion in the network, using link layer information. Existing solutions have shown difficulties in dealing with congestion with varying packets drop. Moreover, ensuring the superior performance of congestion control schemes with traditional referenced layer model is a challenging issue, due to quick topology changes, dynamic wireless channel characteristics, link-layer contentions, etc. In this paper, we propose an effective cross-layer adaptive transmission method to handle the congestion in mobile wireless ad-hoc networks adequately. Simulation results exemplify the usefulness of the proposed method in handling congestion, and yields better results compared to existing approaches. 相似文献
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借鉴人类社会行为关系的规律和普适计算环境的特点,依据信任的滞后性和服务的前瞻性矛盾的动态演化规律,提出普适计算环境中信任关系的建立、信任度的衡量和更新以及基于信任的级联服务模型;通过直接和间接(第三方推荐)的方法动态地对信任程度进行多角度评估,以级联服务的方式从众多可选择的服务目标中筛选出最符合用户要求的服务,克服了单一设备不能完全满足服务请求属性和服务选择盲目性的缺点,提高服务的质量和效率。 相似文献
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Software development for mobile computers 总被引:1,自引:0,他引:1
The author describes a course that he developed and teaches on software development for mobile computing. The course provides the theory and hands-on experience that students need to develop applications for new mobile computing environments. 相似文献
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《Computers & Mathematics with Applications》2006,51(2):377-386
High quality traffic for networks is urgently needed for mobile communication systems. Mobile stations frequently become unavailable due to communication errors generated by network congestion. Traffic congestion in a network system may occur intermittently and continue for a length of time, sometimes causing communication errors. If congestions happens during communication, communication errors occur and the communication is rejected. This paper considers the problem of reliability in mobile communication systems during congestion by using a recovery scheme. We formulate a stochastic model of a mobile communication system which consists of mobile stations, several base stations and a switching center. When communication errors occur, the system makes a rollback recovery and returns to the recent checkpoint. We derive the mean time to take checkpoint and the expected number of rollback recoveries, handoff, and successful transmissions until communication errors occur. Further, we calculate the expected costs and discuss ways to minimize the costs by analyzing the optimal checkpoint intervals. 相似文献
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Ubiquitous computing systems often involve infrastructure placed either on the body or in the environment. Many issues help determine equipment's optimal placement. We've developed a combined wearable and environmental infrastructure for CareLog, a prototype system for capturing and annotating the behavior of children with autism (CWA). The infrastructure features a wearable personal server, which stores each child's information, and environmental buffers - in this case Internet-isolated cameras that store audio and video streams for short intervals before removing them. If a caregiver wishes to annotate a behavior, the caregiver downloads images from these cameras to the child's personal server; otherwise, the images are irrevocably erased. CareLog demonstrates an interesting compromise between environmental and wearable infrastructures and takes advantages of some of the best attributes of each. 相似文献
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《Computers & Industrial Engineering》2005,48(2):273-288
This paper deals with a multi-server retrial queueing model in which the number of active servers depends on the number of customers in the system. To this end, the servers are switched on and off according to a multi-threshold strategy. For a fixed choice of the threshold levels, the stationary distribution and various performance measures of the system are calculated. In the case of equidistant connection levels, the optimum threshold level is numerically computed. 相似文献
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Mobile software applications have to cope with a particular environment that involves small size, limited resources, high autonomy requirements, competitive business models and many other challenges. To provide development guidelines that respond to these needs, several practices have been introduced; however, it is not clear how these guidelines may contribute to solve the issues present in the mobile domain. Furthermore, the rapid evolution of the mobile ecosystem challenges many of the premises upon which the proposed practices were designed. In this paper, we present a survey of the literature on software assurance practices for mobile applications, with the objective of describing them and assessing their contribution and success. We identified, organized and reviewed a body of research that spans in three levels: software development processes, software product assurance practices, and software implementation practices. By carrying out this literature survey, we reviewed the different approaches that researchers on Software Engineering have provided to address the needs that raise in the mobile software development arena. Moreover, we review the evolution of these practices, identifying how the constant changes and modernization of the mobile execution environment has impacted the methods proposed in the literature. Finally, we introduced discussion on the application of these practices in a real productive setting, opening an area for further research that may determine if practitioners have followed the proposed assurance paradigms. 相似文献
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《Journal of Network and Computer Applications》2007,30(1):384-413
Design of servers to meet the quality of service (QoS) requirements of interactive video-on-demand (VOD) systems is challenging. Recognizing the increasing use of these systems in a wide range of applications, as well as the stringent service demands expected from them, several design alternatives have been proposed to improve server throughput. A buffer management technique, called interval caching, is one such solution which exploits the temporal locality of requests to the same movie and tries to serve requests from the cache, thereby enhancing system throughput.In this paper, we present a comprehensive mathematical model for analyzing the performance of interactive video servers that use interval caching. The model takes into account the representative workload parameters of interactive servers employing interval caching and calculates the expected number of cached streams as an indication of the improvement in server capacity due to interval caching. Especially, user interactions, which sensitively affect the performance of interval caching, are realistically reflected in our model for an accurate analysis. A statistical admission control technique has also been developed based on this model. Using this model as a design tool, we apply the model to measure the impact of different VCR operations on client requests and rejection probability, as well as the effect of cache size. 相似文献
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Vipul Mathur Author Vitae Varsha Apte Author Vitae 《Journal of Systems and Software》2009,82(1):39-55
A Web server, when overloaded, shows a severe degradation of goodput initially, with the eventual settling of goodput as load increases further. Traditional performance models have failed to capture this behavior. In this paper, we propose an analytical model, which is a two-stage and layered queuing model of the Web server, which is able to reproduce this behavior. We do this by explicitly modelling the overhead processing, the user abandonment and retry behavior, and the contention for resources, for the FIFO and LIFO queuing disciplines. We show that LIFO provides better goodput in most overload situations. We compare our model predictions with experimental results from a test bed and find that our results match well with measurements. 相似文献
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本文针对电网的输电阻塞问题,首先对输电阻塞问题进行了分析和假设,运用线性回归模型对数据进行了拟合,得出了各输电线路的有功潮流与各发电机组出力之间的近似关系;然后,对其进行结果检验和误差分析。最后,得出其最大相对误差为0.0247,合理的设计出了下一时段各机组出力的分配预案,并得到了最优的机组出力分配方案。 相似文献
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针对在IPv6无线网络切换过程中, 短暂的链路连接断开会导致反复丢包; 同时在切换过程中, 由于带宽的改变, 在新的接入点就有可能发生丢包或资源浪费以及拥塞。提出一种以TCP协议为基础的路径损耗确认(TCP-PLACK)机制来代替TCP选择确认机制(TCP-SACK)。每当一个TCP接收方在断开或切换后而连接到一个新的接入点时, 上述的TCP-PLACK机制就会发送一个特殊的确认, 其中包含有在新接入点丢包的详细信息和可用带宽。收到这个确认, 发送方会重新发送丢失的包, 并在新接入点根据带宽的可用性调整发送速率。实验结果表明, 该机制有利于改善IPv6网络切换过程中的丢失恢复和速率控制问题。 相似文献
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P. De Wilde 《Neural computing & applications》1998,7(1):26-36
This paper models information flow in a communication network. The network consists of nodes that communicate with each other, and information servers that have a predominantly one-way communication to their customers. A neural network is used as a model for the communication network. The existence of multiple equilibria in the communication network is established. The network operator observes only one equilibrium, but if he knows the other equilibria, he can influence the free parameters, for example by providing extra bandwidth, so that the network settles in another equilibrium that is more profitable for the operator. The influence of several network parameters on the dynamics is studied both by simulation and by theoretical methods.The author was with the Intelligent Systems Unit, BT Laboratories, Martlesham Heath, Ipswich IP5 7RE, UK. 相似文献
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针对车道识别与偏离预警算法在准确性、可靠性和计算效率方面存在的问题,提出一种基于双曲线模型的车道识别与偏离预警算法。首先,在图像预处理基础上通过特征点搜索筛选道路边缘点,采用双曲线构建道路模型,利用最小二乘原理拟合道路参数,再根据拟合车道线及邻近点信息构建车道置信度函数,将置信度大于设定阈值的车道线作为最终检测结果;然后,根据相邻帧车道线连续变化的特点,在前帧拟合道路线附近使用粒子滤波算法进行道路边缘点筛选、拟合以及置信度计算,实现对车道线的跟踪;最后,在图像坐标系中建立时空联合预警模型,对车道偏离行为进行预警。在PC平台上进行的算法实现与道路实验结果表明:所提方法在一般路况下,具有92%的车道识别和偏离识别正确率和40ms/帧的平均处理速度,满足车道偏离预警应用要求。 相似文献
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针对一类非线性函数未知的非线性离散系统,提出一种新的基于广义模糊双曲正切模型的参考模型自适应控制器设计方法,并利用Lyapunov稳定性理论证明了该控制器是全局渐近稳定的.仿真例子证明了该方法的有效性. 相似文献