Enabling network calculus-based simulation for TCP congestion control

In this paper, we propose to speed up the simulation performance for TCP-operated networks by incorporating network calculus-based models in a simulation framework. In the simulation framework, we describes the operational properties of TCP congestion control, additive increase and multiplicative decrease (AIMD) and slow start, together with the queue discipline in the domain of network calculus. In order to determine network calculus models for TCP congestion, we first devise a simple TCP throughput model which approximately determines the range of per-flow throughput that one TCP can attain in a given interval, given the number of packet losses in the interval. We then exploit the TCP model to define a collection of network calculus theorems that regulate TCP flows in the range between the upper- and lower-limits on the TCP per-flow throughput in the corresponding interval. Finally, we incorporate the derived rules (theorems) into ns-2 to obtain a network calculus-based (NC-based) simulation, and carry out both the NC-based and the packet-level simulation to evaluate the performance gain and accuracy of the NC-based simulation, where the former is represented in perspective of the execution time (wall time) incurred in conducting the simulations and the later is evaluated in terms of the difference between results obtained in both the packet-level and NC-based simulation. The simulation results indicate that an order of magnitude or more (maximally 30 times) improvement in the execution time is achieved and the performance improvement becomes more salient as the network size increases in perspective of network-link capacities and the number of flows. The discrepancy observed between the NC-based simulation and the packet-level simulation, on the other hand, is minimally 1–2% and maximally 8–12% in a wide spectrum of network topologies and traffic loads. Additionally the results also indicate that the NC-based simulation outperforms a fluid-model-based simulation realized with the use of the time-stepped hybrid simulation technique, and that the performance improvement of the NC-based simulation is still held in IEEE 802.11-based wireless networks and also immune to the type of the simulation platform consisting of ns-2, operating system, and hardware specification.     

基于复杂网络的路网交通拥堵评估仿真模型

为了能够深入分析路网交通拥堵动态演进过程, 为交通拥堵治理提供决策分析工具, 提出了一个基于复杂网络的路网拥堵评估仿真模型。通过将路段阻抗概念引入复杂网络理论, 实现了路网拓扑模型和流量模型的结合; 此外, 还提出了虚拟测试车辆遍历的路网通行能力评估方法, 对不同路网条件下的道路通行能力进行评估。最后在PC系统上实现了完整的路网拥堵评估仿真系统, 并利用该系统对拥堵路段数量、交通流量和路网拓扑结构等因素对路网通行能力的影响进行了仿真分析。仿真实验结果与经典交通理论分析结果一致, 并能反映更多的动态过程信息, 表明该模型能够准确有效地进行交通路网拥堵评估, 可以为相应的交通拥堵管理决策提供依据。     

Reliability consideration of a mobile communication system with network congestion

High quality traffic for networks is urgently needed for mobile communication systems. Mobile stations frequently become unavailable due to communication errors generated by network congestion. Traffic congestion in a network system may occur intermittently and continue for a length of time, sometimes causing communication errors. If congestions happens during communication, communication errors occur and the communication is rejected. This paper considers the problem of reliability in mobile communication systems during congestion by using a recovery scheme. We formulate a stochastic model of a mobile communication system which consists of mobile stations, several base stations and a switching center. When communication errors occur, the system makes a rollback recovery and returns to the recent checkpoint. We derive the mean time to take checkpoint and the expected number of rollback recoveries, handoff, and successful transmissions until communication errors occur. Further, we calculate the expected costs and discuss ways to minimize the costs by analyzing the optimal checkpoint intervals.     

Hub-and-spoke network design with congestion

We consider a hub-and-spoke network design problem with congestion. The model we propose extends current models by taking congestion effects into account. This is achieved through a non-linear cost term in the objective function. We first linearize the model, and then provide a Lagrangean heuristic that finds high-quality solutions within reasonable computational time. The results of the model provide new and realistic insights into the hub-and-spoke network design problem.     

应用MTLAB分析模拟化学振荡的过程

化学振荡反应作为非线性科学研究的真实典型范例,采用传统的计算机语言编程,作理论分析模拟烦琐而困难。本文建议用MATLAB功能强大的solve,jacobian,eig,ode,plot等内部函数,对Lotka-Volterra和Belousov-Zhabotinsky两个典型的化学振荡反应进行理论分析和数值模拟。结果表明MATLAB能够可靠、简便地寻找出振荡动力学区域并对化学动力学方程组数值积分,可以成为化学振荡理论研究的有力工具。     

Bus-based park-and-ride system: a stochastic model on multimodal network with congestion pricing schemes

This paper focuses on modelling the network flow equilibrium problem on a multimodal transport network with bus-based park-and-ride (P&R) system and congestion pricing charges. The multimodal network has three travel modes: auto mode, transit mode and P&R mode. A continuously distributed value-of-time is assumed to convert toll charges and transit fares to time unit, and the users’ route choice behaviour is assumed to follow the probit-based stochastic user equilibrium principle with elastic demand. These two assumptions have caused randomness to the users’ generalised travel times on the multimodal network. A comprehensive network framework is first defined for the flow equilibrium problem with consideration of interactions between auto flows and transit (bus) flows. Then, a fixed-point model with unique solution is proposed for the equilibrium flows, which can be solved by a convergent cost averaging method. Finally, the proposed methodology is tested by a network example.     

TCP/AWM网络系统的事件触发预设性能拥塞控制EI北大核心CSCD

研究了具有非响应用户数据报协议流干扰的TCP/AWM网络系统的跟踪控制问题.针对网络拥塞控制系统,给出了一种改进的网络模型.将预设性能控制和事件触发机制相结合,提出了一种新的AWM拥塞控制算法.利用改进的边界性能函数限制TCP/AWM网络系统的队列跟踪误差,提高了闭环系统的暂态和稳态性能,保证了所有闭环信号都是有界的.此外,采用基于一位信号传输的事件触发控制协议,当控制信号更新时,仅通过一个二进制信号(0或1)传输信息.与传统的事件触发控制协议相比,不仅考虑了有限的带宽,而且考虑了网络传输的成本以及安全性问题.最后,通过仿真验证和分析,证明了所提方法的可行性和优越性.     

基于Agent的森林火灾系统建模仿真

为研究森林火灾系统的整体发展态势及演化规律,引入面向模式的基于Agent的建模方法,进行了融合自然、社会微观作用机理的计算机仿真研究,以Agent元模型为软件实现基础建立起森林火灾系统模型,有机融合了林火蔓延的元胞自动机模式和协作扑救的网络协作模式.对森林火灾系统模型进行动态仿真实验,实验结果表明了有关模式对系统态势发展的影响,以及基于Agent的建模方法对森林火灾系统研究的有效性.     

立体轨道式码头传送系统建模与仿真

针对集装箱码头立体轨道式传送系统网络的复杂结构,采用面向对象随机Petri网（Object-Oriented Stochastic Petri Net,OOSPN）建模方法,简化结构实现层次化建模,并运用随机Petri网进行性能分析。从而检测出系统各个特征状态的执行效率,有效地发现了系统瓶颈,并提出了改进方案。在此基础上,通过WITNESS仿真软件实现码头作业的模拟。结果表明该方法能有效构建和分析集装箱码头立体轨道式传送系统网络复杂系统,并获得其效率指标和统计数据。     

对等网络的拥塞研究

对等网络的普遍应用带来了网络拥塞。从对等网络的拓扑属性研究网络拥塞，首先分析真实的Gnutella网络的流量，确定节点介数与网络拥塞之间存在关系。接着根据排队论模型从理论上给出了导致网络拥塞的临界负载与网络拓扑属性介数的公式解。最后设计拥塞控制策略，通过增加具有大介数节点的容量和这些节点间连接的方法来减轻网络拥塞。     

Pricing and congestion management in a network with heterogeneous users

This note presents an economic model for a communication network with utility-maximizing elastic users who adapt to congestion by adjusting their flows. Users are heterogeneous with respect to both the utility they attach to different levels of flow and their sensitivity to delay. Following Kelly et al. (1998), we introduce dynamic rate-control algorithms, based on the users' utility functions and delay sensitivities, as well as tolls charged by the system, and examine the behavior of these algorithms. We show that allowing heterogeneity with respect to delay sensitivity introduces a fundamental nonconvexity into the congestion-cost functions. As a result, there are often multiple stationary points of the aggregate net utility function. Hence, marginal-cost pricing-equating users' marginal utilities to their marginal costs-may identify a local maximum or even a saddle point, rather than a global maximum. Moreover, the dynamic rate-control algorithm may converge to a local rather than global maximum, depending on the starting point. We present examples with different user utility functions, including some in which the only interior stationary point is a saddlepoint which is dominated by all the single-user optimal allocations. We also consider variants of the dynamic algorithm and their performance in a network with heterogeneous users. Our results suggest that applying a rate-control algorithm such as TCP (Transmission Control Protocol), even when augmented by some form of implicit or explicit pricing, may have unexpected and perhaps undesirable effects on the allocation of flows among heterogeneous delay-sensitive users.     

Automated interactive simulation modeling system: AISIM

A new general purpose simulation modeling tool, AISIM (Automated Interactive Simulation Modeling System), has been developed by Air Force System Command's Electronic Systems Division under the direction of Air Force Program Element 64740F, Computer Resource Management Technology. The 64740F Program Element develops and transfers into active use the technology, tools, and techniques needed to cope with the explosive growth of computer resources used in mission critical Air Force systems. The program was initiated to:

• 1.1) provide for the transition of computer system developments in laboratories, industry, and academia to system program offices.
• 2.2) develop and apply software acquisition management techniques to reduce life cycle costs.
• 3.3) provide improved software design tools.

AISIM is one of these tools. Among the users of AISIM are Air Force Systems Command Product Divisions, the Defense Mapping Agency, Naval Underwater Systems Command, and the Supreme Headquarters Allied Powers Europe (SHAPE) Technical Center. AISIM is designed to provide quick answers to system performance questions during the conceptual phase of a system's acquisition. AISIM has undergone many tests and is currently in the initial operating capability phase of software development. Initial test results indicate potential for wide application to the study of communication nets, computer system architectures, and data processing functions during the conceptual stages of system development. This paper introduces the reader to AISIM—its model building capabilities and its five user interfaces.     

TCP 拥塞控制的混杂建模与分析

互联网中的拥塞控制是一个离散事件与连续变量相互作用的混杂动态过程．假定拥塞事件为一个更新过程序列，采用含有一个状态的随机混杂自动机模型，描述了TCP拥塞控制中的AIMD过程．通过分析发送端拥塞窗口的演化行为，发现拥塞窗口的演化过程与TCP连接的吞吐量具有稳定的统计特征，与初始条件无关．解释了当路由器采用去尾策略时，引起的流同步现象与分析结果的一致性．最后对分析结果进行了仿真验证．     

Epidemiological modeling with a population density map-based cellular automata simulation system

We present a new numerical, two-dimensional cellular automata framework for simulation of the spread of an infectious disease in a region with non-homogenous spatial population distribution. For the simulation the real map of the population density in Poland is used, where the sources of the infection are located. Presented model is a combination of SEIR and IBM models complemented with additional factors like variable population density, death, birth and some stochastic parameters to reflect the more realistic population dynamics. In proposed model the states of individuals are tracked through time like in IBM model and the evolution of the whole system is described by SEIR transition function which determines how cells interact with their neighbours, influencing global behavior of the system. Presented model requires less complicated input than IBM models and is less expensive computationally. We explore influenza as the contagious disease in our map-based simulation. The results of the simulation show the spreading-rate of the disease and can be used to describe possible actions for preventing pandemic.     

中观交通流建模与系统仿真研究综述*

在简要描述了宏微观交通流仿真的建模思路和仿真软件开发情况的基础上,重点对近四十年来中观交通流仿真建模的研究成果进行了总结和归类,并对不同中观仿真模型的优缺点进行了比较;最后指出了中观交通流建模与仿真的研究方向。     

基于Petri网的情境感知系统建模与仿真*

情境感知系统对情境信息的利用程度在不断提高,对服务的多样性提出了更高要求,但是传统的情境感知系统的建模方法难以满足对系统动态描述和模拟的需求。通过对传统的Petri网进行扩展,提出了情境感知系统的赋时着色Petri网模型;并结合仿真工具CPN Tools对模型进行模拟和验证,进而发现系统设计中存在的潜在不足,对其优化改进。以痴呆老人智能辅助系统为例,证明了该方法的有效性。     

新型伺服控制系统的建模仿真与分析

分析了伺服控制系统的工作原理,提出一种新型伺服控制系统建模方法。在Matlab/Simulink环境下采用模块化的方法构建了系统的仿真模型,系统采用双闭环控制,并对该模型进行了仿真。仿真结果表明,该系统具有良好的动态和静态特性,验证了该方法的有效性,为实际伺服控制系统的设计和调速提供了新的方法。     

主被动关节柔性树形机器人系统动力学建模与仿真

进一步完善空间算子代数理论体系,研究了带有主被动关节的树形柔性机器人系统符号动力学高效率建模的方法.利用拓扑结构和低序体阵列的方法描述了树型机器人系统.采用空间算子研究了欠驱动树型多体系统的描述方法.根据系统中铰的驱动情况分别对铰链定义为主动铰和被动铰,建立树形机器人系统的运动学,然后通过铰链的类型分别按照两次从系统的顶端到基座的顺序、一次从基座到顶端的顺序进行了系统铰接体惯量的递推、系统冗余力的递推、广义加速度和广义主动力的递推,最后将分叉处的广义力和广义加速度矢量相加得到树形机器人系统的广义动力学模型.通过上述递推过程建立动力学模型,实现了高效率O(N)(N为系统自由度)次的计算效率.最后以加拿大树形空间机械臂为例做仿真验证了本研究内容的正确性和高效性.     

Development and application of intelligent system modeling and simulation platform

As a common support of M&S (Modeling and Simulation) applications, M&S language and platform can effectively improve M&S capability for complex objects and systems. This paper proposes an Intelligent system Modeling and Simulation Language (IMSL) which integrates artificial intelligence theory and M&S technology to describe intelligent model structures, behaviors, facts, knowledge, and rules. Meanwhile, an M&S platform is implemented to provide a basic support for M&S of intelligent systems, featured as its convenience, efficiency and problem solving ability. Our research work has been focused on M&S methods and language specification of intelligence systems, intelligent simulation resource libraries and integrated development environment. As an application instance, M&S of a C3I (Command, Control, Communication and Information) system is conducted to evaluate the performance of IMSL and the IMSL platform. And the simulation result proves that IMSL can work effectively in solving practical problems.     

大规模交通网络实时模拟系统

以元胞自动机模型为微观模型，以CS模型为指导构建二维的大规模交通网络，利用多线程机制实现了整个模拟系统的高速运行。具体的模拟实验显示：该模拟系统完全可以满足实时模拟的需要，对于包含400个十字路口的交通网络，模拟系统每更新一次所需要的时间小于1s。