Stability of Networked Control Systems Under a Multiple-Packet Transmission Policy

This paper is concerned with stability analysis of discrete-time networked control systems subject to packet loss under a multiple-packet transmission policy with the packet dropping probability of the communication channel bounded from above. Necessary and sufficient conditions for stability are obtained. In addition, the packet dropping margin as a measure of stability robustness of a system against packet loss is defined and its formula is derived. A design method is proposed for enhancing stability robustness subject to the constraint of a set of prescribed nominal closed-loop poles.      

基于状态响应的分组预约多址接入协议研究

为了克服经典PRMA-HS协议中的双稳态问题和分组发送允许概率的静态性缺点,提出了一种基于状态响应的改进型分组预约多址接入协议.该协议能够利用卫星基站反馈信息,根据系统状态实时调整分组发送允许概率,使系统始终处在最佳状态.利用计算机对理想信道和随机误差信道条件下的协议性能进行了仿真,实验结果表明,采用状态响应机制能够克服经典PRMA-HS协议的双稳态问题,提高了协议的稳定性,增加了系统容量,降低了分组丢弃率.     

Joint call and packet QoS in cellular packet networks

This paper proposes a novel call admission control scheme capable of providing a combination of call and packet level quality of service requirements in cellular packet networks. Specifically, we propose a distributed call admission control scheme called PFG, which maximizes the wireless channel utilization subject to a predetermined bound on the call dropping and packet loss probabilities for variable-bit-rate traffic in a packet-switched wireless cellular network. We show that in wireless packet networks, the undesired event of dropping an ongoing call can be completely eliminated without sacrificing the bandwidth utilization. Extensive simulation results confirm that our scheme satisfies the hard constraint on call dropping and packet loss probabilities while maintaining a high bandwidth utilization.     

Calculation of highly structured stability margins

The problem of assessing the robust stability of a closed-loop system subject to plant uncertainty is addressed. Earlier techniques allow only a limited amount of structure information uncertainty and thus can often lead to conservative results. A technique for increasing the bound over the stability margin is presented which may be applied together with the cross-condition number to compute a stability margin for highly structured uncertainty regions. A sample application is given to show the improvements that can be obtained by using the proposed technique in practical situations.     

Nonlinear RED: A simple yet efficient active queue management scheme

Among various active queue management schemes (AQM), random early detection (RED) is probably the most extensively studied. Unlike the existing RED enhancement schemes, we replace the linear packet dropping function in RED by a judicially designed nonlinear quadratic function. The rest of the original RED remains unchanged. We call this new scheme Nonlinear RED, or NLRED. The underlying idea is that, with the proposed nonlinear packet dropping function, packet dropping becomes gentler than RED at light traffic load but more aggressive at heavy load. As a result, at light traffic load, NLRED encourages the router to operate in a range of average queue sizes rather than a fixed one. When the load is heavy and the average queue size approaches the pre-determined maximum threshold (i.e. the queue size may soon get out of control), NLRED allows more aggressive packet dropping to back off from it. Simulations demonstrate that NLRED achieves a higher and more stable throughput than RED and REM, another efficient variant of RED. Since NLRED is fully compatible with RED, we can easily upgrade/replace the existing RED implementations by NLRED.     

Design and analysis of a self-tuning feedback controller for the Internet

Active queue management (AQM) is an effective method used in Internet routers for congestion avoidance, and to achieve a tradeoff between link utilization and delay. The de facto standard, the random early detection (RED) AQM scheme, and most of its variants use average queue length as a congestion indicator to trigger packet dropping. This paper proposes a novel packet dropping scheme, called self-tuning proportional and integral RED (SPI-RED), as an extension of RED. SPI-RED is based on a self-tuning proportional and Integral feedback controller, which considers not only the average queue length at the current time point, but also the past queue lengths during a round-trip time to smooth the impact caused by short-lived traffic dynamics. Furthermore, we give theoretical analysis of the system stability and give guidelines for selection of feedback gains for the TCP/RED system to stabilize the average queue length at a desirable level. The proposed method can also be applied to the other variants of RED. Extensive simulations have been conducted with ns2. The simulation results have demonstrated that the proposed SPI-RED algorithm outperforms the existing AQM schemes in terms of drop probability and stability.     

冲突减弱型LEO多址协议性能仿真分析

针对PRMA-HS协议中系统阻塞概率较大的问题,引入微时隙概念,提出了具有冲突减弱功能的MPRMA-HS协议,通过对可用时隙的再划分,增加系统竞争时隙数量,以降低系统拥塞。建立了协议模型,利用平衡点分析法对协议的接入阻塞概率、系统丢包率和吞吐率进行了理论推导。通过计算机仿真给出了协议的性能曲线,并与PRMA-HS进行了对比。结果表明,MPRMA-HS协议所能支持的最大用户终端数比PRMA-HS协议提高11.9%,而接入阻塞概率和丢包率均小于PRMA-HS协议。     

Performance of PRMA in a mobile radio channel

The performance of the packet reservation multiple access (PRMA) protocol in a real mobile radio channel that suffers from propagation impairments including multipath fading, shadowing and distant-dependent path loss, is studied by simulation. Significant performance degradations, measured by the number of simultaneous conversations supportable in a cell subject to a packet dropping probability of 1%, are observed. Additionally, receiver capture is also found to be ineffective in increasing this supportable number of simultaneous conversations. A modified PRMA protocol is then introduced, and shown to be capable of improving performance through minimizing premature loss of reservations and avoiding additional collisions that arise due to the channel errors.     

存在数据包丢失NCS系统的补偿估计器的建模与分析

对同时存在网络诱导时延及数据包丢失的一类网络控制系统进行了研究和分析。针对时延网络系统存在的数据包丢失,设计出包丢失估计补偿器,用补偿器的信息来更新控制器并建立系统模型,从而有效降低了时延和丢包对系统的影响,最终提高系统性能。与此同时,通过李亚普诺夫稳定性理论来对闭环系统进行了稳定性分析,并给出控制器的设计方法。最后,通过了实例仿真,证实了带补偿器和估计器的网络控制系统设计的有效性。     

一种选择性丢包机制的分析与设计

随机早期捡测算法(RED)在产生丢包时简单地丢弃到达的数据包，由于Web流占据了网络上75％左右的带宽，而Web流的特点是数据包比较少，这就使得早期丢包的效果难以体现。本文提出一种选择性丢包机制，引进一个类似公平队列的虚队列，检测发遥速率过大的流，筛选出丢包的候选链接，拥塞时丢弃候选链接在队列申的第一个连接的数据包。模拟实验结果表明，与队头丢包相比，采用选择性丢包的RED算法可进一步降低网关的丢包率，提高吞吐率，缩短Http的响应时间，提高队列的稳定性。     

基于区分服务的分层多播拥塞控制算法

多媒体多播应用在Internet上的广泛部署对拥塞控制提出了要求,分层多播是适应网络异构性较为有效的方案.为了克服现有分层多播存在的拥塞响应延时大、吞吐率抖动剧烈和不满足TCP友好的问题,给出了一个基于区分服务的分层多播模型,提出了一种基于区分服务的分层多播拥塞控制算法DSLMCC(DiffServ-based layered multicast packet dropping),在边缘路由器上引入了基于概率的区分优先级的分组标记算法,在核心路由器上采用区分优先级的分组丢弃算法.仿真结果表明,该算法能够有效地改进区分服务网络上的分层多播拥塞控制的性能,具有较快的拥塞响应速度、较好的稳定性和公平性,并且较好地适应了网络的异构性.     

PacketScore: a statistics-based packet filtering scheme against distributed denial-of-service attacks

Distributed denial-of-service (DDoS) attacks are a critical threat to the Internet. This paper introduces a DDoS defense scheme that supports automated online attack characterizations and accurate attack packet discarding based on statistical processing. The key idea is to prioritize a packet based on a score which estimates its legitimacy given the attribute values it carries. Once the score of a packet is computed, this scheme performs score-based selective packet discarding where the dropping threshold is dynamically adjusted based on the score distribution of recent incoming packets and the current level of system overload. This paper describes the design and evaluation of automated attack characterizations, selective packet discarding, and an overload control process. Special considerations are made to ensure that the scheme is amenable to high-speed hardware implementation through scorebook generation and pipeline processing. A simulation study indicates that packetscore is very effective in blocking several different attack types under many different conditions.     

基于启发式失效率函数的主动队列管理

研究主动队列管理技术的REI)和REM的丢包率和队长稳定性关系;基于启发式丢包处理方法,提出一种 采用启发式失效率函数计算丢弃概率函数以减小丢包率的拥塞控制算法(HFA)。在轻量级负载情况下,该算法性能 和自适应随机早期检测与随机指数标记相当;而在重量级负载情况下,不仅丢包率的均值和方差小,而且瞬时队长在 设定的目标范围,可以明显地减小队列长度的波动性。在N}2上进行的大量实验结果表明,所提算法具有瞬时队长 控制在预期范围、抖动性小、丢包率及丢包率方差小等优点。     

Input to state stability of min-max MPC controllers for nonlinear systems with bounded uncertainties

Min-max model predictive control (MPC) is one of the control techniques capable of robustly stabilize uncertain nonlinear systems subject to constraints. In this paper we extend existing results on robust stability of min-max MPC to the case of systems with uncertainties which depend on the state and the input and not necessarily decaying, i.e. state and input dependent bounded uncertainties. This allows us to consider both plant uncertainties and external disturbances in a less conservative way.It is shown that the input-to-state practical stability (ISpS) notion is suitable to analyze the stability of worst-case based controllers. Thus, we provide Lyapunov-like sufficient conditions for ISpS. Based on this, it is proved that if the terminal cost is an ISpS-Lyapunov function then the optimal cost is also an ISpS-Lyapunov function for the system controlled by the min-max MPC and hence, the controlled system is ISpS. Moreover, we show that if the system controlled by the terminal control law locally admits certain stability margin, then the system controlled by the min-max MPC retains the stability margin in the feasibility region.     

一种新的被动队列管理算法

针对因网络拥塞而产生的丢包问题,基于弃头方式提出了一种新的被动队列管理算法(Drop Front n based on Wavelet and Shuffled frog leaping,DFWS)。该算法首先利用小波变换对实际流量的长相关特性进行刻画,并利用混合蛙跳方法获取网络队长,同时结合弃头方式提出丢包策略和丢包概率。最后,以实际数据进行仿真实验,深入研究了网络队长、丢包数量和服务率之间的关系。结果发现,与其他算法相比,DFWS具有较好的适应性。     

一种基于优先级的分组丢弃算法PRED

针对实时业务如语音，分析了现有的Internet的缓冲管理算法——Drop Tail算法的缺陷，提出了一种基于优先级的分组丢弃算法——PRED算法。PRED算法体现了分组的相对优先级，并能自适应地对分组的服务级别进行平滑地升级或降级，因而很适于作为DiffServ网络中的分组丢弃算法。仿真结果证明了理论分析的正确性。     

基于HJB方程的无线传感器网络系统Minimax控制器设计

针对工业环境下无线传感器网络系统在受到外部较大干扰时的系统稳定性问题,提出Hamilton-JacobiBellman (HJB)方程与Minimax控制相结合的方法.首先,针对无线传感器网络在复杂工况环境下出现的网络时延和连续丢包有界的情况,给出具有时延和丢包的无线传感器网络系统模型;然后,在Minimax性能指标函数下,利用HJB方程设计系统的Minimax最优控制器,进一步通过检验函数得出有关最大干扰的表达形式,从而推导出系统稳定的充分条件;最后,通过数值算例和仿真验证系统在突发较大干扰时采用所提方法的可行性和有效性.     

显示拥塞指示标记的主动队列管理研究

主动队列管理对于解决网路拥塞具有重要意义。针对PID主动队列管理算法在调节队列长度时有较大的丢包率这一缺点,提出一种显示拥塞指示标记即ECN标记的PID主动队列管理算法。该算法用显示拥塞指示标记取代丢包机制,用于通知源端网络即将发生拥塞,采用PID控制器实现反馈控制,保证系统的稳定性。仿真结果表明,显示拥塞指示标记的主动队列管理算法适用于多变的网络环境,比PID算法具有低丢包率、低延时和高吞吐量的特点。     

Stability analysis of model-based networked distributed control systems

Networked distributed control systems (NDCSs) face serious challenges such as delays and packet dropouts induced by the communication network employed to connect local controllers of interacting subsystems. These two network-induced shortcomings may degrade the performance or even destabilize NDCSs. This paper is concerned with the problem of stability analysis and stabilization of the NDCSs, featuring both random delay and random packet loss in their communication networks. A model-based networked distributed control framework is proposed to stabilize the NDCS consisting of discrete-time subsystems interconnected through their states. In this control framework, to compensate for the adverse effects of these two network-induced shortcomings, an interaction estimator is provided in each local controller; in addition to a main control unit. This estimator uses the explicit model of the subsystems to estimate the evolution of the states of interacting subsystems, when information about their actual values is not available. A model for the NDCS subject to both random packet loss and random delay is developed. By providing a 3-step interaction estimating algorithm, the closed-loop model-based networked distributed control system (MB-NDCS) is formulated as a time-dependent impulsive system. Then, a quadratic Lyapunov function is constructed to derive a linear matrix inequality (LMI) based sufficient condition for stability analysis of the overall impulsive system. Finally, an illustrative example of a network of interconnected chemical reactors with recycle is presented to show the effectiveness of the proposed approach.