有限负载下802.11 DCF的性能分析及优化

利用三维马尔可夫链和M/G/1/K队列建立了有限负载下DCF机制的性能模型,分析了终端数量、传输负载、二进制指数回退机制及MAC层有限队列对系统性能的影响.基于该模型,推导了有限负载下最大化吞吐量的最优最小竞争窗口的闭式解.仿真结果表明,模型能够有效地预测有限负载下DCF的性能,根据传输负载调整最小竞争窗口大小能够获得最大化吞吐量.     

通过自适应调整最小竞争窗口最大化IEEE 802.11DCF的饱和吞吐量

最大化802.11 DCF的饱和吞吐量对充分利用无线局域网宝贵的带宽资源具有重要意义。该文在分析802.11 DCF的饱和吞吐量与最小竞争窗口、最大回退等级、网络中竞争信道的节点数的关系的基础上,推导了根据网络中竞争信道的节点数,计算最小竞争窗口的最佳值的简单公式。给出了估计竞争信道的节点数并据此动态调整最小竞争窗口的最佳值的自适应算法。同时,该文对估计竞争节点数的算法的准确性和计算最小竞争窗口最佳值的公式的准确性进行了仿真分析,并比较了改进后的802.11 DCF的饱和吞吐量与原802.11 DCF的饱和吞吐量的大小。仿真结果证明了上述公式、算法是准确和有效的。     

基于效用最大化的IEEE 802.11 DCF性能分析及改进

针对802.11 DCF在系统负载较大时不能有效利用带宽资源的缺点,该文提出一种基于效用函数的DCF优化机制(U-DCF)。通过设置站点吞吐量的对数效用函数,将带宽资源的有效利用问题建模为系统效用最大化问题;应用最优化理论将此系统问题等效为可分布式求解的用户问题,即各站点只须独立选择最大化其净效用的竞争参数(CWmin),则系统整体效用也获得最大化。仿真结果表明:与标准DCF相比,U-DCF通过预估系统的当前平均分组长度和竞争站点数来调整竞争参数CWmin,能够显著提高系统的饱和吞吐量,减小分组发送时延和丢帧率。     

IEEE802.11MAC协议中一种改进的退避解决算法

IEEE802.11无线局域网,在共享信道中经历碰撞的网络节点需要随机退避一段时间,这段时间是从竞争窗口中均匀选取,竞争窗口大小由BEB机制动态控制,一些文献研究表明,BEB机制在重负载的情况下,突现出公平性问题和低的吞吐量,本文基于MILD退避机制,提出一种适用于分布式协调功能改进算法。该算法通过修改802.11的MAC层中的DCF(DistributedCoordinationFunction)子协议,改善了IEEE802.11无线局域网在拥塞情况下的性能,提高了网络吞吐量。     

无线传感器网络DCF改进模型建模分析

针对无线网络的IEEE802.11的DCF机制,文章构建一种基于冲突概率p的分析模型,该模型综合考虑了无线网络中的节点数、网络负载及退避算法中的退避阶数及重传次数等影响因素,利用M/G/1/K队列模拟无线网络终端的饱和与非饱和状态,采用二维Markov对无线网络系统建模,由该模型得到网络系统的归一化吞吐量,并分析不同的数据包到达率对网络性能的影响,为预测和优化系统的性能提供理论依据。     

有效提高WLAN吞吐量的数据帧发送策略

设计合理的数据帧发送策略能够提高无线资源利用率.分析了退避机制对系统归一化吞吐量的影响情况,提出一种简单、有效的数据帧发送策略,其中主要包括主动的动态数据帧发送时间调整机制和被动的自适应竞争窗口调整方法,前者能够主动地调节单位时间内注入队列的数据帧数量,后者以被动的方式根据网络当前数据帧碰撞程度快速调整竞争窗口.仿真结果表明,提出的调整机制能够根据网络状态快速地、自适应地优化节点数据分组传输过程中的相关参数,使得网络吞吐量提高接近30%,有效地改善了无线资源使用率.     

DCF传输MPEG-4视频流的研究

IEEE 802.11无线局域网的基本接入方式是分布式协调功能(DCF)。当无线局域网中的负载变大时,采用DCF的站点可能在成功发送之前需要延迟几秒以上。这将严重影响MPEG-4视频流等实时应用的性能。该文针对实时应用提出了一种DCF的改进机制L-DCF,并介绍了在L-DCF下如何传输MPEG-4视频流。仿真结果表明,与DCF相比,采用L-DCF的MPEG-4视频流将经历较少的时延和抖动,同时又具有与采用DCF时相同的吞吐量与公平性,因此能够获得更好的播放质量。     

IEEE 802.11b DCF三维Markov非饱和链路模型

为了准确评估实际网络中IEEE 802.11b分布式协调功能(Distributed Coordination Function,DCF)的系统性能,提出了一种三维Markov链路模型,该模型综合考虑了DCF协议退避计数器冻结状态和有限重传次数,并结合M/M/1/K排队模型给出了在负载有限的情况下DCF系统性能的理论模型。仿真结果表明,该模型能准确地预测IEEE 802.11b DCF协议的系统性能。     

多类别终端无线局域网实时业务性能研究

针对具有不同MAC参数和应用类型的终端在IEEE802.11DCF无线局域网内共存的情况,使用离散Markov模型分析信道在饱和状态时的竞争特性,研究实时业务吞吐量、竞争延迟和丢失率等性能指标与协议参数和数据帧长度的关系。计算和仿真实验结果的对比验证了分析模型的正确性,表明部分终端协议参数的改变会引起所有业务性能变化,而合理调整最小竞争窗口和最大回退次数可以改善实时业务的性能。     

新的改进IEEE 802.11 DCF性能的退避机制

分布式协调功能DCF是IEEE802.11标准最基本的媒体接入方法,它的核心是载波检测多址接入/冲突避免（CSMA/CA）机制,通过退避算法,减少碰撞的概率。提出了一种新的退避机制改进IEEE802.11DCF饱和吞吐量性能,建立了三维马尔可夫链网络模型详细研究分析,同时利用NS2对所提出的机制进行仿真,比较了改进后的802.11DCF饱和吞吐量与原802.11DCF的饱和吞吐量的大小,仿真结果证明了算法的准确有效。     

A Lightweight Scheme of Differentiated Services for Home Networking

Lightweight mechanisms are included in the differentiated services architecture (diffserv) in order to provide differentiated services in the Internet. A Premium service lightweight scheme based on the delay ratio criterion with key-benefit a periodically developed conservative control mechanism is provided. Our model consists of a M/M/1/K queue for high priority (HP) packets and a finite queue birth–death (b–d) model for low priority streams. Simulation experiments under certain load and service conditions guarantee a constant delay ratio. The use of the algorithm is extended to different Home Networking Technologies (ADSL, Wireless Local Network, etc.) providing solutions to many of them. This revised version was published online in September 2006 with corrections to the Cover Date.     

Buffer Management in a Packet Switch

Consider a single packet switch with a finite number of packet buffers shared between several output queues. An arriving packet is lost if no free buffer is available, as in the CIGALE network. It has been observed by simulation that if load increases too much, congestion may occur, i.e., throughput declines; it appears that the busiest link's queue tends to hog the buffers. Therefore, we will limit the queue length and when the queue is full the packet will be dropped. We expect that this restricted buffer sharing policy will avoid congestion under conditions of heavy load. A queueing model of a packet switch is defined and solved by local balance. Loss probability is evaluated, and values of queue limit to minimize loss are found; they depend on load. A Square-Root rule is introduced to make the choice of queue limit independent of load. For a sample switch, with three output links, a comparison is made between performance under different buffer sharing policies; it is shown that restricted sharing prevents congestion by making throughput an increasing function of load.     

Markov-modulated flow model for the output queues of a packetswitch

The output queues of an M×N packet switch are studied using a Markov-modulated flow model. The switching element is a central server which sequentially routes packets from the inputs to the outputs. The focus is on systems in which the server speed is such that the bulk of the queuing takes place in the output queues. The conventional point process approach neglects the impact of switching and transmission time. An attempt is made to account for these finite system speeds by using a Markov-modulated continuous flow to approximate the arrival process to an output queue. This model captures the dependency between arrivals at different outputs and reflects the fact that packet arrivals and departures are not instantaneous. The output queue content distribution is obtained, for both infinite and finite buffer systems, from the spectral expansion of the solution of a system of differential equations. Numerical examples and comparisons with the results of an M/M/1 approximation are presented     

An adaptive‐aware energy and queue improvement of AOMDV

Because the node energy and network resources in the wireless sensor network (WSN) are very finite, it is necessary to distribute data traffic reasonably and achieve network load balancing. Ad hoc on‐demand multipath distance vector (AOMDV) is a widely used routing protocol in WSN, but it has some deficiencies: establishes the route by only using hop counts as the routing criterion without considering other factors such as energy consumption and network load; forwards route request in fixed delay resulting in building the nonoptimal path; and cannot update the path status after built paths. For the deficiency of AOMDV, this paper proposes a multipath routing protocol adaptive energy and queue AOMDV (AEQAOMDV) based on adaptively sensing node residual energy and buffer queue length. When sending a routing request, the forwarding delay of the routing request is adaptively adjusted by both the residual energy and the queue length of the intermediate node; when establishing routes, a fitness is defined as a routing criterion according to the link energy and the queue load, predicting the available duration of the node based on the energy consumption rate and adjusting the weight of the routing criterion by the available duration of the node; after the routes are established, the path information status are updated via periodically broadcasting Hello that carries the path information with the minimum fitness, making the source node update the path information periodically. By using NS‐2, simulations demonstrate that compared with AOMDV, AEQAOMDV has obvious improvements in increasing packet delivery ratio, reducing network routing overhead, reducing route discovery frequency, and decreasing the network delay. And AEQAOMDV is more suitable for WSN.     

分级复合服务策略的多站循环服务系统

对于多站(多队列)循环服务系统,本文提出一种可动态调整服务量的分级复合型新策略,以适应站点负荷非均衡系统。文章讨论的主要内容有:(1)新策略简介;(2)新策略系统排队服务模型特点;(3)新策略系统在队列容量有限时的队列状态转移规律;(4)队列状态转移概率算法;(5)队列状态概率的非常规算法。     

Performance analysis of cellular mobile communication systems fordata transmission

Buffers are used to overcome the data losses due to the interruption of data transmission when a mobile station is handed over in a cellular network. Data traffic from a wireline network to its mobile data terminal, such as access to large commercial databases or data transmission from a main computer to its remote mobile terminal, is studied. Compound Poisson (CP) is used to approximate the arrival process of the data traffic, and a modulated D (MD) distribution is used to approximate the service and handover process. The performance of the queueing model CP/MD/1/N is analyzed. Probability generating functions and characteristic functions are used to evaluate the mean queue length and the mean burst delay for an infinite buffer system and the overflow probabilities versus the buffer size for a finite buffer system. A method is presented to find the probability transition matrix entries by recursively taking derivatives of the probability generating function of the number of the characters arriving during the service-time. The queue length distributions for a finite buffer, both at departure instants and at arbitrary time instants are derived. Comparison with simulation indicates that the model is accurate. The numerical results of the model confirm the effectiveness of the scheme     

Minimization of the total loss rate for two finite queues in series

Two finite-capacity loss queues in series, with Poisson arrival at the first queue and with Erlang service time distribution at both queues are considered. It is assumed that the service rate at the second queue is held fixed and that a maximum buffer space is globally available. For this model, the problem of optimally choosing the service rate at the first queue and the buffer space distribution over the two queues is considered in order to minimize the steady-state total network loss rate. It is shown that a decrease of the service rate at the first queue can reduce the total network loss rate only if this lower service rate decreases the variability of the process feeding the second queue. Numerical results show that, when optimal values are assigned to the parameters of the model, a significant reduction of the network loss rate can be achieved, especially when the network load is moderate and when the service time distributions have a small variance     

Models for Analysis of Packet Voice Communications Systems

In a packet voice communication system, packets are fed to a common queue by a number of independent voice sources and are removed from this queue on a first-come-first-serve basis for transmission over a communication link of finite capacity. Each voice source alternates between active periods, during which packets are generated at regular intervals, and inactive periods, during which no packets are generated. In this paper, we discuss three models, a semi-Markov process model, a continuous-time Markov chain model, and a uniform arrival and service model, to assess the queueing behavior of such systems. Numerical results obtained from each of the three models are compared to each other, to results obtained from a discrete event simulation program, and to results obtained from anM/D/1analysis. Parameters of the model are the average duration of active and inactive periods, the packet generation rate, the communication link capacity, and the total number of voice sources. Conclusions are drawn regarding which models appear to be most appropriate in the parameter ranges investigated.     

基于赋时层次有色Petri网的Multi-Agent调度系统建模

介绍了基于Multi-Agent的分布式环境扫描系统的结构模型.依据赋时层次有色Petri网(HTCP-net)的理论,应用建模、仿真工具CPN Tools建立了基于优先级的任务调度算法和最短等待队列动态负载均衡调度算法的系统调度模型.仿真结果表明,该调度模型有效满足了系统周期性重复访问网站的任务需求.