存储区域网中光纤通道和iSCSI协议的性能分析

在存储环境中,SCSI协议是被广泛采用的块存储管理协议。不同的传输机制,如,并行SCSI、光纤通道和iSCSI(internetSCSI)都可以被用于传输SCSI协议数据。对于并行SCSI来说,传输距离短、连接的设备数目少是它的主要缺点。FC和iSCSI是下一代SCSI传输机制的主要竞争对手。通过对FC和iSCSI两个协议之间不同方面的分析,评价FC和iSCSI是否满足吉比特速率、可扩展互连设备和满足存储环境和存储协议的需要。     

Throughput-delay performance of packet-switching multiple-access channel with power capture

Power capture is the ability of a receiver to receive correctly the strongest of several messages that arrive during overlapping intervals. The presence of power capture in multiple-access packet switching channels introduces inequality in the access conditions to the channel when packets arrive at the receiver at different power levels. Two models for such channels under slotted ALOHA protocol with power capture are considered. In the first, a packet is transmitted at one of K possible power levels, and it is received correctly if no other packet is transmitted at the same or higher level. The second model is of a ground radio network where a received packet's power is inversely proportional to the distance it propagates. In this model, a packet transmitted from a distance r is received correctly if no other packet is transmitted from a distance smaller than a · r (a ? 1). A technique developed by Lam (1974) is generalized to obtain the throughput-delay characteristics of the channel for both models. The effect of high power traffic on the lower power traffic is discussed, and it is shown that the ‘Sisyphus distance’ phenomenon predicted by Abramson (1977) for the case a = 1 in the second model does not exist in the more realistic case of a > 1.     

开放接入点的安全可信接入

随着无线网络公众服务的推进,免费的开放接入点爆发式增长。针对开放接入点日益突出的安全问题,尤其是伪造接入点带来的个人隐私信息泄露,钓鱼攻击,漏洞攻击等,提出了一种基于接入点行为分析的安全可信接入方法。该方法参考了垃圾邮件判定的相关技术,采用D-S证据理论,建立一个推理规则库（包含SSID判别,Beacon密度,AP的物理层特性,数据转发行为特性等）,通过接入点行为证据的分析和合成,完成对接入点可信性的判定,从而避免无线客户端接入非法的接入点。通过典型实例验证了该方法。     

支持多类型业务的受限负载EDCA性能分析

无线局域网增强的分布式信道接入(EDCA)具有支持多类型业务的功能。为准确有效地分析EDCA机制的性能,提出一个考虑多类型业务的受限负载二维马尔可夫性能分析模型。该模型通过引入空闲、后回退等状态来精确刻画饱和与非饱和负载下系统的性能,并考虑EDCA重传次数、后碰撞过程等特征。仿真结果验证了模型的有效性,系统的最大吞吐量出现在非饱和负载情况下。     

On the analysis of a symmetric polling system with single-message buffers

A performance analysis is provided for a polling system consisting of statistically identical stations with single-message buffers and Poisson arrival streams. Switchover and message service times are assumed to be generally distributed. Some errors in the past analysis are pointed out. We express such performance measures as the mean polling cycle time, the mean message response time, and the mean number of messages at an arbitrary time in terms of the mean number of massages served in a polling cycle. Our mean message response time reduces to that for an FCFS M/G/1//N queue (machine interference model) in the limit of zero switchover time.     

负载波动对无线局域网的影响机理与对策分析

负载波动是引起无线局域网络吞吐量和稳定性低的直接原因之一,其主要诱因是站点接入密度的大幅度变化和数据冲突程度的剧烈振荡。分析了无线局域网络的数据冲突避免机制,在探究负载波动的原因及其对无线局域网络影响机理的过程中提出相应对策,通过构造数学模型和建立实验环境,仿真验证了所设计策略的效果,对仿真结果进行了分析。     

A cross-efficiency fuzzy Data Envelopment Analysis technique for performance evaluation of Decision Making Units under uncertainty

The paper presents a novel cross-efficiency fuzzy Data Envelopment Analysis (DEA) technique for evaluating different elements (Decision Making Units or DMUs) under uncertainty. In order to evaluate the performance of several DMUs while dealing with uncertain input and output data, the presented technique employs triangular fuzzy numbers. A fuzzy triangular efficiency is associated to each DMU through a cross evaluation obtained by a compromise between suitably chosen objectives. Results are then defuzzified to provide a ranking of the DMUs. The proposed method is applied to the performance evaluation of healthcare systems in a region of Southern Italy. The DMU data uncertainty derives from ongoing reforms and the reported assessment is conducted firstly in order to evaluate and rank the efficiency of the considered healthcare systems, and subsequently to assess the evolution of the performance of one of the most affected among these DMUs by the reform plans. The case study demonstrates the model ease of application, its discriminative power among DMUs when compared to a more classical fuzzy DEA approach, and the usefulness in planning and validating targeted reforms in the case of healthcare systems.     

无线局域网的速率自适应算法设计与仿真分析

目前的IEEE802.11在物理层提供了不同的传输速率,因此根据不同的信道条件可以选择一个合适的传输速率,以使系统吞吐量达到最大。在WLANs中,当有多个用户向同一个接入点传输数据时,碰撞就会产生。Collision-Aware Rate Adaptation（CARA）算法能够有选择地启用Request-To-Send/Clear-To-Send（RTS/CTS）识别出这种碰撞,很大程度上避免了由于碰撞引起的吞吐量下降。但随着碰撞的增加,CARA吞吐量还是有明显下降。为了提高吞吐量,提出了一种新的速率自适应算法Adaptive Avoid Collision Rate Adaptation（Adaptive-ACRA）,该算法根据信道条件不同改变门限值,有效地提高了大量碰撞存在情况下的吞吐量。最后,通过大量NS2仿真证明了新算法提高系统吞吐量的有效性。