可靠多媒体多播传输协议

对于大规模多媒体多播应用来说,一个有效发现和修正传输错误的可靠多媒体多播协议是必要的。该文研讨了多媒体多播应用中的允许延迟和分组丢失率问题,提出了一种基于协议转接概念的可靠多播传输协议(RMTP)。RMTP协议聚焦在允许延迟上提供多媒体服务质量保证,转接节点放置在多播树上,数据恢复在两个转接节点之间进行。RMTP协议转接能立即满足重传需求和减少分组的复制数量。最后,给出了RMTP协议的性能分析及与SRM协议的比较。研究表明RMTP协议为多媒体多播传输提供了一种新的有效途径。     

基于动态范围的移动组播协议

提出在基于范围的移动组播协议（RBMoM）的基础上根据用户的移动特性、业务量及网络特性为每个主机动态地确定最优服务范围的算法,形成了基于动态范围的移动组播协议（DRBMoM）．该协议通过设计一个代价函数并求解使代价函数最小化的服务范围来实现减小组播分组传输时延和组播树重构频率的折衷．分析表明,代价函数值随主机的业务量和移动速度的增加而增加;最优服务范围随主机业务量的增加而减小,随主机移动速度的增加而增加．在性能对比中发现,DRBMoM的代价函数值小于RBMoM的代价函数值,表明DRBMoM的确增强了RBMoM的性能;在与同类方案的比较中,发现DRBMoM的平均切换信令代价稍高于同类方案,但其平均组播分组传输代价及最终的代价函数值均小于同类方案．     

一种基于超立方体的应用层组播协议

提出了一种基于超立方体结构的应用层组播协议-comcast．ComCast协议通过反馈机制实现了可靠组播。并通过建立逻辑连接的方法解决了容错性问题；采用NS2对ComCast协议进行了仿真，并与另外一种基于超立方体的协议HyperCast进行了比较，比较指标包括路径伸展率、链路压力、数据传送率．结果表明，虽然ComCast协议的链路压力较高。但是其路径伸展率较低。同时在节点正常和出错两种情况下都具有较好的数据传送率．     

加权选择管理节点的分层可靠组播协议

参与协同的各个用户分别加入各局部组播组,通过计算节点的处理器能力、内存大小、带宽、接收队列状态及吞吐性能等性能参数,得出各节点的性能指标(即权重),从中选取最优者作为管理员,次优者作为管理员候选者,候选者在管理员失效的状况下成为管理员.管理员负责处理局部组播组的ACK报文及报文重传.并引入流量控制,使得大量的交互信息不会引起网络状况的严重恶化.并利用NS2对协议进行了仿真实现.     

考虑主机容量的应用层组播协议研究

目前,对于组播协议的研究主要是基于应用层组播与IP组播的,但这两种组播方式的目标主要在于提高组播吞吐量、改善延迟等方面,而对于最大化利用主机容量却甚少考虑。鉴于此,提出一种通过考虑主机容量因素,使得主机容量达到最大化利用的协议(BCCALM),从而避免浪费主机资源,并解决了某一中继主机连接超过其可承载数目的主机数目时而致使中继主机负担较重、延迟增加的问题。该协议采用分层分群结构,基本思想是对中继主机的出度进行限制,同时使其出度达到其可以承载的最大数目。     

一种优化时延的机会协同组播调度策略

无线组播面临的最大问题是各个用户信道状态的不均匀性和波动性,无法同时满足所有用户的服务质量需求。无线蜂窝通信系统中,为实现可靠组播,数据包不可避免地需要被重传多次,组播的时延也因此增加。本文我们首先分析了协同组播调度CMS策略的时延,并且推导了基于信道信息全知条件下的最小时延。另外,我们在部分信道信息未知条件下提出了一种机会协同组播策略来优化组播时延。仿真数据显示,我们提出的机会协同组播调度OCMS策略在时延性能上几乎达到了我们所分析的CMS策略的最小值而且明显优于其他调度策略。     

三层交换网下的组播服务网设计和实现

该文对组播技术进行深入分析,研究组播的实现技术。最后应用组播技术在锐捷可提供组播性能的交换设备上成功地实现了组播网络的部署。然后,在深入研究组播服务器技术和实现方式的基础上,我们成功地配置了组播服务器。这样,就建立了一个完整的组播网络,为学生访问视频教学资源、视频/电话会议等提供了方便。同时,组播网络节省了单位主干网有限的带宽,减轻了链路设备和服务器设备的负担,同时减小了链路设备存储转发时延。     

支持主机移动的组播协议的性能评价

使用移动IP协议可以使主机移动到Internet的任何区域都能够保持与其它节点之间的通信。IP组播能够高效传输大量数据到一组接收者,可有效利用网络带宽。随着大量移动设备接入Internet,支持主机移动的组播协议已成为人们研究的问题之一。文章首先给出评价支持主机移动的组播协议性能的标准,然后详细介绍了目前已经提出的主要移动组播协议,并进行了性能分析和对比,最后探讨了移动组播的研究方向和发展趋势。     

SCSRM:分割连接的卫星可靠组播方案

针对端到端方案在卫星可靠组播中存在的缺点,提出了一种分割连接的卫星可靠组播方案SCSRM,并从差错控制机制、拥塞控制机制、反馈抑制策略等方面对运行于卫星网络部分的传输协议Sat-MTCP进行了详细的设计.仿真结果表明,即使在较差的信道条件下,SCSRM方案也可以达到较好的Goodput性能;由于采用了有效的反馈抑制策略和差错恢复策略,当接收者数量增大时,Goodput性能基本保持平稳,表现出良好的可扩展性;通过可用带宽的合理分配,保证了带宽公平性.     

多组共享式应用层组播算法

利用单播传输路径的重叠特性所构建的叠加组播树可以部分模拟IP层的有源组播,而单组会话中成员主机在网络中分布的不足可以通过多组会话中的主机来弥补。该文根据这一特点提出了一种基于多组会话成员共享的应用层组播算法,该方法采用了源主机和接收主机之间的单播传输路径和多组协作机制,为每个组播源建立单独的组播树。通过模型分析,该文算法所构建的组播树可以比单组会话计算方法获得较大优势的链路利用率。     

Distributed route repair for increasing reliability and reducing control overhead for multicasting in wireless MANET

The Multicast Ad hoc On-Demand Distance Vector (MAODV) routing protocol is proposed for achieving multicast in a Mobile Ad hoc Network (MANET) while reducing bandwidth waste and energy power consumption. In MANET, packets transmission through a multicast tree may always have unreliable links caused by node mobility or lack of energy, and thus significantly degrades the performance. MAODV uses a broadcast-type local repair mechanism to find an alternative route to the multicast tree when some breaks happen on the tree. Although the local repair mechanism provides a specified time-to-live (TTL) to limit the repair range and the hop-count to the group leader, a large number of broadcast-type Route Request (RREQ) messages extensively yields control overhead and requires a large amount of power consumption to send control messages. Thus, this paper proposes a unicast-type multihop local repair protocol for multicast MANETs to recover lost links efficiently while achieving several advantages: increasing network reliability, increasing packet delivery rate, minimizing the number of control messages and reducing repair delay. Moreover, the optimal number of hops used in the multihop neighbor table is analyzed mathematically. Numerical results indicate that the proposed approach outperforms other repair approaches in terms of successful repair rate, control message overhead and packet delivery rate.     

云存储多异构文件联合延迟尾概率凸优化分析

随着擦除码技术的流行,分布式存储中高数据可靠性和高空间效率存储性能逐渐实现,但是降低尾部延迟仍然是一个有待解决的问题。为此,提出一种量化和优化擦除编码存储系统尾延迟的算法框架。对于任意服务时间分布和异构文件,推导给出尾部延迟上界。提出了一个优化模型,使得所有文件在服务器上放置的加权延迟尾概率和访问请求文件的服务器选择共同最小化,并证明了其非凸问题特性,以便采用一种高效的交替优化算法求解。此外,通过描述延迟分布尾部的渐近行为,以闭合形式对任意擦除编码存储的服务延迟的尾部指数进行数学量化,证明了基于概率调度的算法是（渐近）最优的。实验结果表明,在实际工作负载下擦除编码存储系统的尾部延迟显著降低。     

缓冲交叉开关交换结构性能分析

分析了一种缓冲交叉开关交换结构在突发流量到达下的性能.通过建立分析模型,给出了每个输入端口拥有单个或多个输入队列的缓冲交叉开关结构的饱和吞吐.结果显示,对于单输入队列结构而言,随着突发平均长度的增加,饱和吞吐迅速从1下降,并收敛于0.5.随着每个输入端口输入队列数目的增加,饱和吞吐率逐渐接近1.仿真实验验证了分析模型的准确性.该结果可以用于指导基于缓冲交叉开关的路由交换设备的优化设计.     

Optimal NT policies for M/G/1 system with a startup and unreliable server

This paper studies the control policies of an M/G/1 queueing system with a startup and unreliable server, in which the length of the vacation period is controlled either by the number of arrivals during the idle period, or by a timer. After all the customers are served in the queue exhaustively, the server immediately takes a vacation and operates two different policies: (i) the server reactivates as soon as the number of arrivals in the queue reaches to a predetermined threshold N or the waiting time of the leading customer reaches T units; and (ii) the server reactivates as soon as the number of arrivals in the queue reaches to a predetermined threshold N or T time units have elapsed since the end of the completion period. If the timer expires or the number of arrivals exceeds the threshold N, then the server reactivates and requires a startup time before providing the service until the system is empty. Furthermore, it is assumed that the server breaks down according to a Poisson process and his repair time has a general distribution. We analyze the system characteristics for each scheme. The total expected cost function per unit time is developed to determine the optimal thresholds of N and T at a minimum cost.     

基于时延的Flash Crowd 控制模型

提出了一种session级别的flash crowd控制策略SGAC(session-granularity admission control),将session控制粒度和request控制粒度相结合,采用请求平均返回时延作为检测和控制的依据.对session采取一旦接受就完成的策略,在实现对服务器过载控制的同时,保护用户session的完整性,并能自动调节新session的准入速率,以提高服务器利用率.采用真实HTTP Log进行模拟,结果表明,SGAC方法能够有效控制服务器过载,保护session的完整性,提高服务器利用率,降低接入端路由器计算开销,保护有价值的交易session.     

Multi-server threshold queues with hysteresis

A queueing system with K servers operates in the following manner. When the system is empty, arriving customers are attended by only one server. A set of forward threshold values is defined so that at epochs at which the number of customers in the system exceeds a forward threshold value an extra server is added. Similarly, a set of reverse threshold values is defined so that at epochs at which the number of customers in the system becomes less than a reverse threshold value one server is removed. The forward and reverse threshold values are assumed to be different. Closed-form occupancy distribution and mean delay of customers in the system are obtained by the Green's function method.     

短波数据通信可靠多播协议研究

针对短波数据通信的特点,提出了一种可靠多播中反馈信息的信道争用方式,使第二代短波通信实现可靠多播传输。同时将改进的可靠多播协议RMTP应用到短波可靠多播传输中,首次提出“一层发送,逐层补充,分层分步反馈”的思想。通过仿真表明,改进后的RMTP协议满足可靠多播传输要求,与未使用该协议的可靠多播传输相比多播传输延时明显缩短。     

Understanding and improvement of the selection of replica servers in key–value stores

In current large-scale distributed key–value stores, the tail latency of the hundreds of key–value access operations generated by an end-user request determines the response time of this request. Therefore, this tail latency has great impacts on the user experience and revenue. Replica selection algorithms, which select the best replica server for the service of each key–value access operation as much as possible, is the key to cut the tail latency of these key–value access operations. This paper summarizes current replica selection algorithms, including both the algorithms employed by current key–value stores and the classic algorithms of other similar systems. These algorithms are classified into three categories: information-agnostic, client-independence and feedback, according to their demanded information. As a step further, simulation-based performance analysis of these algorithms is conducted. The result brings us the insights that the response time (RPT) is useful to measure the service rate, but will lead to the herd behaviors. Moreover, the number of outstanding key–value access operations (OSKs) is helpful to both the selection of the fastest replica server and the avoidance of herd behaviors. Based on these insights, we design the L2 algorithm by assembling the basic ideas of the Least OSK algorithm and the Least RPT algorithm. The L2 algorithm is much simpler than the recently proposed C3 algorithm and has a similar best performance with C3 as confirmed by the simulation and experimental results.     

移动边缘计算技术在高铁通信网络中的应用

随着高铁的日益普及,在高铁列车与地面之间建立移动数据通道,满足车地数据传输以及旅客上网的需求,成为越来越迫切的问题.现有GSM-R和LTE-R的解决方案,还存在带宽小、时延大、传输不稳定等问题.为此,本文提出采用移动边缘计算（Mobile Edge Computing,简称为MEC）技术来优化高铁通信网络.主要思想是在车厢和基站部署MEC服务器,经过两级MEC服务器的协同配合,达到复用空口链路、提升无线传输稳定性和降低时延的目的,并最终提升用户体验.通过实际网络试验结果显示,该方案可显著提升传输速率、减小传输时延.