区块链是一种结合分布式共识、加密、时间戳等方法,在不依赖任何第三方中心化机构的情况下,实现点对点交易、协调以及协作的技术.近几年,区块链技术的不断发展引起了产业界和学术界的极大兴趣.但是,区块链的存储可扩展性问题,提高了区块链设备的门槛,成为了区块链应用落地的瓶颈.介绍了区块链的基本原理和存储模型,分析了当前区块链所面临的存储问题;然后,针对区块链存储可扩展性问题,从链下存储和链上存储这两条技术路线出发,论述了主要的解决方案的原理与思路;最后,总结了提高区块链存储可扩展性的技术研究进展,指出了当前解决方案所面临的问题,为未来的研究工作提供了方向.

随着电子商务的迅速发展,协同过滤技术在推荐领域中,得到了广泛的运用.托攻击问题和数据稀疏性问题,导致推荐结果不理想.研究证明,用户间信任关系可以极大缓解数据稀疏问题,使得推荐更为准确.但是,包含信任关系的推荐算法,大多未能考虑到托攻击对于推荐的影响,使得系统的鲁棒性下降.通过研究包含信任信息的推荐情景中托攻击用户的统计量表现特征,提出一种在信任网络下,检测托攻击用户的TSAD算法.实验证明,该算法能够准确地识别托攻击用户,增强系统的鲁棒性.

基于现有的两阶段虚拟网络映射算法,分析节点综合资源计算标准,改进选取节点映射的评估方法。该虚拟网络映射算法基于虚拟网络中节点的多种属性,即节点CPU计算资源、链路带宽资源、节点相邻节点的数目及拓扑属性。节点映射阶段采用改进之后的资源评估方式进行节点映射,同时采用可重用技术,实现物理网络节点可重复映射,即物理网络中的同一个节点可以被虚拟网络中的多个节点多次映射。该算法不仅有效地减少了映射过程中出现的资源瓶颈问题,而且降低了链路映射的成本,节约了部分映射带宽资源,从而使得网络基础设施提供商可以接受到更多的虚拟网络服务,提高网络运营的网络收益。仿真实验数据表明,该虚拟网络映射算法在虚拟网络请求接受率和网络开销比等参数指标上具有显著提升。

针对传统随机森林算法不能有效、快速地处理海量高维数据,导致分类效果不理想的问题,以提高分类准确率和运行效率为目的,提出了一种基于Hadoop平台的分布式可扩展随机森林算法。该算法采用基于袋外数据集测试准确率变化的特征选择算法,在分布式平台上使用MapReduce构建随机森林,然后通过改变袋外数据的每一列特征获取每一棵决策树对应的特征重要性度量及权重,运用两者的加权求和求得特征重要性排序,其中决策树的权重取决于决策树与集体随机森林预测的一致性。最后,在特征重要性排序的基础上引入了一定的随机性,确保了每棵树的强度,又减少了树与树间的相关性。实验结果表明,相比于传统单机模式下的随机森林特征选择算法,该算法在分类的准确性和运行效率上效果良好。

传统IP网络架构越来越不适应日益增长的网络业务与应用,一种新型的网络架构应运而生,即软件定义网络.它实现数据面和控制面相分离,同时具有集中控制、开放接口、可编程等特性.研究了基于该网络架构的虚拟网络映射问题,即如何将带有约束条件的虚拟网络请求映射到由传统电信商提供的底层物理网络中.首先给出虚拟网络映射问题的定义及目标,然后对现有的多种经典映射算法进行分类,并同时对比与分析,最后在此基础上对未来虚拟网络映射算法提出一些建议与改进思路.

Wireless Sensor Networks(WSNs)consist of many tiny wireless communication and sensing devices. Multi-cast is the fundamental routing service used to deliver messages efficiently from a source to a set of destinations. More-over, multicast protocols try to minimize the consumption of network resources by using the fact that several paths from the source to the destinations can be shared by multiple destinations, and the larger the path shared, the lower overall bandwidth consumption is obtained. In this paper, based on the analysis of the feature of WSNs, the existing multicast routing proto-cols in WSNs are classified and analyzed. The future development trend of multicast protocols in WSNs is discussed finally.

A theoretical calculation model is given for standard deviation of non-degenerated four-wave mixing noise in case that the modulation format is FSK and the launched pulses are chirped Gaussian pulse wave, without ignoring group-velocity walk-off of random bit sequence between channels. Some calculation examples indicate that in discussion of the relationship between deviation frequency and standard deviation of four-wave mixing noise, deviation frequency is found to have the optimal value in considering the influence of multiple channels on the detected channel. The characteristic width value would not affect the selection of optimal value while the small characteristic width value could obtain relatively small four-wave mixing effects. However the value of group velocity dispersion has a major influence on the relation curve between deviation frequency and standard deviation of four-wave mixing noise, and when its absolute value is small, the the relation curve between deviation frequency and the standard deviation of four-wave mixing noise exhibits a growing trend, and when its absolute values increases, the curve fluctuates.

Under the condition that the launched pulses are chirped Gaussian pulses and in considering the randomness of bit pattern and initial phase of bit sequence in each channel,a semi-analytic calculation model applied to evaluate the variance of degenerated four-wave mixing noise in dispersion-management wavelength-division-multiplexing(WDM) system is estabilished.The pulse amplitude expressions for the chirped Gaussian pulse in dispersion-management fiber link are deduced in detail.The dispersion map is composed of two variable fiber segments with same absolute value and opposite sign of dispersion.The simulation indicates that the pulse characteristic widths have obvious impacts on standard variance of FWM noise,while the standard variance of FWM noise increases with the increase of pulse characteristic widths.When the pulse characteristic width is smaller,the chirp parameter value has fairly great influence on the FWM effect.The optimized dispersion management scheme could make the standard variance of FWM noise small.