A structured P2P network based on the small world phenomenon

In this paper, we propose a new structured P2P overlay network, named SW-Uinta(small-world). In order to reduce the routing latency, we firstly construct the Uinta network in which both physical characteristics of network and data semantic are considered. Furthermore, based on Uinta, a nondeterministic caching strategy is employed to allow for poly-logarithmic search time while having only a constant cache size. Compared with the deterministic caching strategy proposed by previous P2P systems, the nondeterministic caching strategy can reduce communication overhead for maintaining the routing cache table. Cache entries in the cache table of peer nodes can be updated by subsequent queries rather than only by running stabilization periodically. In the following, a novel cache replacement scheme, named the SW cache replacement scheme, is used to improve lookup performance, which has proved to satisfy the small-world principle. So we call this network SW-Uinta(small-world). After that, according to the theoretical analysis, it can be proved that SW-Uinta(small-world) can get O((log ² N)/k) search time with O(k) cache size. Lastly, the performance of SW-Uinta(small-world) is compared with those of other structured P2P networks such as Chord and Uinta. It shows that SW-Uinta(small-world) can achieve improved object lookup performance and reduce maintenance cost.

一种数据块关系指导的缓存替换方法

提出了一种数据块关系指导的缓存替换方法BDP(block correlations directed replacement policy).数据块关系所表示的空间局部性可以用来指导存储系统的缓存替换策略.BDP利用历史访问信息和实时访问信息,预测数据块未来的空间局部性特征,并设计数据块驻留时间的控制策略,减少现有替换算法对空间局部性预测失败带来的失效惩罚.同时给出了BDP的有效缓存模型.模拟结果显示,对大多数实际系统工作负载,BDP的失效率相对LRU算法下降11%～38%,优于现有的考虑空间局部性的替换算法.     

一种基于不放回抽样的图像混沌加密算法

针对现有基于混沌序列的图像加密算法的缺陷,提出一种基于不放回抽样的图像混沌加密算法。该算法采用可变模运算和间接地址映射技术,将混沌序列一对一、高度随机地转换为图像像素地址,实现图像的位置置乱和灰度扰乱加密。对混沌序列的分布无严格要求,使混沌序列的参数和初值可以作为密钥被使用。实验结果表明,与现有同类算法相比,该算法性能优越、运算速度快且易于实现。     

Optimal replacement policy for a two-dissimilar-component cold standby system with different repair actions

In this paper, a cold standby repairable system consisting of two dissimilar components and one repairman is studied. When failures occur, the repair of both component 1 and component 2 are not ‘as good as new’. The consecutive operating times of component 1 after repair constitute a decreasing geometric process, while the repair times of component 1 are independent and identically distributed. For component 2, its failure is rectified by minimal repair, and the repair time is negligible. Component 1 has priority in use when both components are good. The replacement policy N is based on the failure number of component 1. Under policy N, we derive the explicit expression of the long-run average cost rate C(N) as well as the average number of repairs of component 2 before the system replaced. The optimal replacement policy N*, which minimises the long-run average cost rate C(N), is obtained theoretically. If the failure rate r(t) of component 2 is increasing, the existence and uniqueness of the optimal policy N* is also proved. Finally, a numerical example is given to validate the developed theoretical model. Some sensitivity analyses are provided to show the influence of some parameters, such as the costs for replacement and repair, and the parameters of the lifetime and repair time distributions of both components, to the optimal replacement policy N* and corresponding average cost rate C(N*).     

Optimal number of minimal repairs with cumulative repair cost limit for a two-unit system with failure rate interactions

A discrete replacement model is presented that includes a cumulative repair cost limit for a two-unit system with failure rate interactions between the units. We assume a failure in unit 1 causes the failure rate in unit 2 to increase, whereas a failure in unit 2 causes a failure in unit 1, resulting in a total system failure. If unit 1 fails and the cumulative repair cost till to this failure is less than a limit L, then unit 1 is repaired. If there is a failure in unit 1 and the cumulative repair cost exceeds L or the number of failures equals n, the entire system is preventively replaced. The system is also replaced at a total failure, and such replacement cost is higher than the preventive replacement cost. The long-term expected cost per unit time is derived using the expected costs as the optimality criterion. The minimum-cost policy is derived, and existence and uniqueness are proved.     

无证书签名方案的分析及改进

对一个无证书签名方案进行安全性分析,指出该方案不能抵抗公钥替换攻击和恶意的KGC攻击,即攻击者可以通过替换签名者的公钥来伪造任意消息的签名,恶意的KGC(Key Generation Center)可以获取用户的私钥。分析结果显示该方案不能满足无证书签名方案的安全性要求,同时为了应对这两种攻击,提出了改进的方案。     

WWW业务访问特性分布研究

WWW业务表现为一系列的访问序列。而Web Server和Proxy Server的日志很好地记录了这种访问序列的过程及特性。WWW业务的特性研究是Web Server、Web中间件研究和人工合成Web负载的基础。分析了一个Web Server和两个Proxy Server的日志,重点研究了Web页面请求的概率分布、Web静态文档大小的概率分布（含传输文档）、Web静态文档的访问距离的概率分布,并将分析结果同相关文献的结果进行了对比,同时通过试验证实了在使用Size作为Web缓存替换依据时,还应该考虑Web文档的访问频率。     

新铌酸盐化合物K4CuNb8O23及其Cu^2＋与Nb^5＋的类质同象代换

1986年Wanklyn等曾报道存在化合物K_5Cu_2Nb_(11)O_(31),并给出其X射线粉晶谱线,但未见进一步指标化和确定其所属晶系及晶胞参数的报道。本研究表明,存在一种新铌酸盐晶体K_4CuNb_8O_(23),其成分及X射线谱异于K_5Cu_2Nb_(11)O_(31)。 用分析纯K_2CO_3、Nb_2O_5及CuSO_4·5H_O按一定比例配制并于1100—1300℃间烧成,     

环翼式桥墩环翼式挡板最佳延伸长度试验

为研究环翼式桥墩环翼式挡板的最佳延伸长度,进行了不同流速和不同环翼式挡板延伸长度下环翼式桥墩的局部防冲刷试验。试验结果表明:环翼式挡板的延伸长度不同,桥墩的冲刷效果有较大的差别,当环翼式挡板的延伸长度与桥墩的半径相同时,与无挡板的桥墩相比最大冲坑深度可减小57.6%,近底垂向流速最大可减小70.4%,近底紊动强度也明显减小,防冲刷效果显著。     

锚索索力损失对预应力闸墩动态特性的影响

某水电站泄洪闸采用大吨位预应力闸墩承受弧形闸门支臂传递的巨大水推力,通过近2 年的闸墩锚索 拉力监测发现,锚索张拉力损失部分超过15%,并尚未收敛,且闸墩在宣泄洪水过程中曾出现过强烈振动现象. 为进一步明确锚索拉力损失是否对闸墩动态特性(包括模态特性、动位移与动应力特性)产生影响,建立了闸墩 -基础-锚索整体有限元模型,通过制作闸墩水力学模型并采用力传感器测量作用于闸墩上的整体面荷载,获取 不同运行工况下的闸墩整体水动力荷载时程线,以此为有限元模型输入荷载进行闸墩动态响应计算,分析了泄 洪闸预应力闸墩主、次锚索不同索力损失对闸墩的动态特性的影响,研究成果可为工程安全运行提供重要 参考.