Review of road traffic control strategies

Traffic congestion in urban road and freeway networks leads to a strong degradation of the network infrastructure and accordingly reduced throughput, which can be countered via suitable control measures and strategies. After illustrating the main reasons for infrastructure deterioration due to traffic congestion, a comprehensive overview of proposed and implemented control strategies is provided for three areas: urban road networks, freeway networks, and route guidance. Selected application results, obtained from either simulation studies or field implementations, are briefly outlined to illustrate the impact of various control actions and strategies. The paper concludes with a brief discussion of future needs in this important technical area.     

SCADA系统安全通信面临的挑战及未来发展趋势

近年来,监控与数据采集（SCADA）系统的安全性问题引起了极大关注,为此,一些研究机构提出了很多安全机制,国际标准化组织也发布了多个SCADA系统安全标准。主要阐述SCADA安全通信所面临的技术挑战及应对措施,主要从密钥管理、入侵检测、智能设备的研发以及系统替换等方面进行概述;重点关注SCADA网络安全倡议中所提出的安全协议;以及未来能量栅格包括SCADA网络中嵌入式系统设备的安全功能和通信功能的实现问题。     

Deadline‐aware TCP congestion control for HTTP adaptive streaming services

Over the last decades, video streaming services have gained a lot of popularity, representing over 70% of the total consumer Internet traffic worldwide. While originally dedicated streaming protocols have been used to deliver these services, for several years, the video streaming industry is steadily shifting to HTTP adaptive streaming (HAS)–based delivery over TCP. With each video segment, implicit delivery deadlines are associated in order to avoid playout interruptions. However, TCP is known to be far from minimizing the number of deadline‐missing streams. In this paper, 2 deadline‐aware congestion control mechanisms are proposed, based on a parametrization of the traditional TCP New Reno congestion control strategy. By introducing deadline awareness at the transport layer, the modulation of the congestion window size is dynamically adapted to steer the aggressiveness of the considered stream to optimize the delivery of video streaming services. The proposed approaches are thoroughly evaluated in an HAS video‐on‐demand scenario over tree‐based delivery networks. It was shown that in a realistic scenario, the proposed approaches can outperform traditional congestion control strategies by up to 12% in terms of estimated mean opinion score, mainly by reducing the average video freezing time by more than 85%.     

Flow Control Based on Buffer Classes

The problem of combining flow control and deadlock prevention is considered from a technical point of view. A possible solution is provided by congestion control based on buffer classes, which have been implemented within the GMD-network. This approach facilitates the realization of many variants of flow control mechanisms by adaptation of buffer class limits to the traffic conditions. Some of these algorithms have been studied by simulation methods and are presented in this contribution.     

Videoconferencing over packet-based networks

The Internet explosion is driving the need for new collaboration tools which will enable two or more users to share data, audio, and video. The real-time packet-based solutions which are emerging differ considerably from the circuit-switch solutions which have existed for some time now. In this paper, we present one such packet-based approach, the Multimedia Multiparty Teleconferencing (MMT) system, which was fully implemented as a research prototype. Using MMT as an example, we address some of the fundamental issues related to videoconferencing systems in a packet-based environment, and discuss the differences with the traditional circuit-switch approaches, namely, the ITU H.320 standard. In particular, MMT is a distributed solution, while H.320 is centralized. The use of multicast and a novel video-mixing technique to facilitate the distributed solution are presented. Furthermore, MMT audio and video streams are susceptible to congestion and packet loss in the shared media packet-based environment, while H.320 streams use dedicated connections. As such, synchronization, error resilience, and dynamic rate control schemes for the packet-based system are presented     

Voice packetization and compression in broadband ATM networks

Some methods of supporting voice in broadband ISDN, (B-ISDN) asynchronous transfer mode (ATM), including voice compression, are examined. Techniques for voice compression with variable-length packet format at DS1 transmission rate, e.g., wideband packet technology (WPT), have been successfully implemented utilizing embedded adaptive differential pulse code modulation (ADPCM) coding, digital speech interpolation (DSI), and block-dropping schemes. For supporting voice in B-ISDN, voice compression techniques are considered that are similar to those used in WPT but with different packetization and congestion control methods designed for the fixed-length ATM protocol at high speeds. Possible approaches for packetization and implementation of variable-bit-rate voice coding schemes are described. ADPCM and DSI for voice coding and compression and cell discarding (CD) for congestion control are considered. The advantages of voice compression and CD in broadband ATM networks are demonstrated in terms of transmission bandwidth savings and resiliency of the network during congestion     

A survey of cross-layer design for VANETs

Recently, vehicular communication systems have attracted much attention, fueled largely by the growing interest in Intelligent Transportation Systems (ITS). These systems are aimed at addressing critical issues like passenger safety and traffic congestion, by integrating information and communication technologies into transportation infrastructure and vehicles. They are built on top of self organizing networks, known as a Vehicular Ad hoc Networks (VANET), composed of mobile vehicles connected by wireless links. While the solutions based on the traditional layered communication system architectures such as OSI model are readily applicable, they often fail to address the fundamental problems in ad hoc networks, such as dynamic changes in the network topology. Furthermore, many ITS applications impose stringent QoS requirements, which are not met by existing ad hoc networking solutions. The paradigm of cross-layer design has been introduced as an alternative to pure layered design to develop communication protocols. Cross-layer design allows information to be exchanged and shared across layer boundaries in order to enable efficient and robust protocols. There has been several research efforts that validated the importance of cross-layer design in vehicular networks. In this article, a survey of recent work on cross-layer communication solutions for VANETs is presented. Major approaches to cross-layer protocol design is introduced, followed by an overview of corresponding cross-layer protocols. Finally, open research problems in developing efficient cross-layer protocols for next generation transportation systems are discussed.     

Congestion and flow control in Signaling System No. 7-impacts ofintelligent networks and new services

Focuses on the transient performance analysis of the congestion and flow control mechanisms in CCITT Signaling System No. 7 (SS7). Special attention is directed to the impacts of the introduction of intelligent services and new applications, e.g., Freephone, credit card services, user-to-user signaling, etc. In particular, the authors show that signaling traffic characteristics like signaling scenarios or signaling message length as well as end-to-end signaling capabilities have a significant influence on the congestion and flow control and, therefore, on the real-time signaling performance. One important result of the performance studies is that if, e.g., intelligent services are introduced, the SS7 congestion and flow control does not work correctly. To solve this problem, some reinvestigations into these mechanisms would be necessary. Therefore, some approaches, e.g., modification of the signaling connection control part (SCCP) congestion control, usage of the SCCP relay function, or a redesign of the message transfer part (MTP) flow control procedures are discussed in order to guarantee the efficacy of the congestion and flow control mechanisms also in the future     

Internet路由器中拥塞控制机制研究的现状与展望

目前,Internet路由器中拥塞控制机制研究的热点在于:在不破坏网络可扩展性的前提下,如何在路由器上提供拥塞控制机制。该文从信息与激励的角度出发,以部分流状态、核心路由器状态无关、拥塞计费等为3个基本思路,对该领域中已提出的主要研究方案进行了分类阐述和比较分析,总结了其最新研究进展,为下一步的研究提出了新的课题和设想。     

蜂窝网络中用户流量的地域分布预测分析与研究

Early detection and rapid resolution network congestion can considerably improve network capacity. Consequently, much research has been carried out on predicting traff ic patterns in 3G networks. This paper introduces an access point centric approach that is implemented by two prediction models, the traffic abstraction model and the order-k Markov model. Traffi c predictions are carried out to support the congestion control in the semi-smart antenna systems. The simulation result shows that the cumulative error rate is below 25% even carrying out multi-step-ahead predictions.     

TCP/IP协议的拥塞控制策略

网络拥塞问题会导致网络性能下降,对网络正常运行有很大影响,严重时会导致死锁的产生.文中分析了网络拥塞的产生的原因,并着重讨论了传统端到端的TCP拥塞控制的策略和方法,传统TCP拥塞控制策略提出了改进策略和网络层的IP拥塞控制策略.最后对基于TCP和IP拥塞控制策略进行了对比探讨.     

Network-Adaptive Rate Control and Unequal Loss Protection with TCP-Friendly Protocol for Scalable Video Over Internet

Video communication with Quality of Service (QoS) is an important and challenging task. To have QoS provision at application level in the current best-effort Internet, rate control, congestion control and error control are several effective approaches. In this paper, we propose a new network-adaptive rate control and Unequal Loss Protection (ULP) scheme in conjunction with TCP-friendly congestion control for scalable video streaming. Our proposed approach is capable of simultaneously controlling congestion and packet loss occurred across the Internet. More specifically, we first dynamically estimate the available network bandwidth on the fly. Then, TCP-friendly congestion control is performed to smoothly adjust the sending rate for transmission of continuous media. Considering the characteristic of scalable video, unequal loss protection at packet level is adopted for different video layers while performing congestion control. Consequently, a fixed-length and priority-based packetization scheme is introduced to enhance the capability of loss protection and improve the efficiency of network-bandwidth utilization. Moreover, Rate-Distortion (R-D) based bit allocation is proposed to minimize the expected end-to-end distortion. Simulation results demonstrate the effectiveness of our proposed scheme.     

Improving Network Performance by Ameliorating TCP Congestion Control Mechanism

With the rapid growth of rate-based services and wireless applications,improving Transmission Control Protocol (TCP) congestion control has been becoming more important in the network research field. This paper first briefly introduces the Additive-Increase Multiplicative-Decrease (AIMD) (a,b) algorithm,and then presents an improved TCP congestion control mechanism (D-AIMD) whose principles and simulation results are discussed in detail. This mechanism can be easily implemented with lower additional overheads and can efficiently improve network performance.     

Subspace identification for closed-loop 2-D separable-in-denominator systems

Identification for closed-loop two-dimensional (2-D) causal, recursive, and separable-in-denominator (CRSD) systems in the Roesser form is discussed in this study. For closed-loop 2-D CRSD systems, under feedback control condition, there exists some correlation between the unknown disturbances and future inputs which offers the fundamental limitation for utilizing standard open-loop 2-D CRSD systems subspace identification methods. In other words, the existing open-loop subspace approaches will result in biased estimates of plant parameters from closed-loop data. In this study, based on orthogonal projection and principal component analysis, novel 2-D CRSD subspace identification methods are developed, which are applicable to both open-loop and closed-loop data. Additionally, the whiteness external excitation case is discussed and subsequently modified instrument variables are adopted to improve the proposed subspace algorithm. An illustrative example of the injection molding process and several numerical examples are used to validate consistency and efficiency of the proposed subspace approaches for 2-D CRSD systems.     

Performing Flexible Control on Low-Cost Microcontrollers Using a Minimal Real-Time Kernel

In recent years, approaches to control performance and resource optimization for embedded control systems have been receiving increased attention. Most of them focus on theory, whereas practical aspects are omitted. Theoretical advances demand flexible real-time kernel support for multitasking and preemption, thus requiring more sophisticated and expensive software/hardware solutions. On the other hand, embedded control systems often have cost constraints related with mass production and strong industrial competition, thus demanding low-cost solutions. In this paper, it is shown that these conflicting demands can be softened and that a compromise solution can be reached. We advocate that recent research results on optimal resource management for control tasks can be implemented on simple multitasking preemptive real-time kernels targeting low-cost microprocessors, which can be easily built in-house and tailored to actual application needs. The experimental evaluation shows that significant control performance improvement can be achieved without increasing hardware costs.     

Dependable Traffic Control Strategies for Urban and Freeway Networks

Traffic congestion is a growing problem in many countries around the world. It has been recognized that instead of constructing more roads and freeways to counter this problem it is prudent to improve the utilization of existing road network through a judicious combination of advances in control engineering, communication and information technology. The traffic control architecture proposed in this paper is a combination of communicating Urban Traffic Control Architecture (UTCA) and Freeway Traffic Control Architecture (FTCA). The UTCA combines context-awareness, Cyber-Physical Systems (CPS) principles, and Autonomic Computing System (ACS) principles to optimize traffic congestion and enforce safety in urban traffic network. The UTCA includes a network of adaptive intersection traffic controllers and their immediate supervisory systems, who are also networked. The central piece of each traffic controller is an arbiter, which is a mini CPS. It is aware of the traffic dynamics at the intersection managed by it, by virtue of continuous input from monitoring sensors. Due to this context-awareness ability and its communication ability to exchange traffic information with its neighbors, it can execute policy-based reactions in order to enable safe and efficient traffic throughput at its intersection. Each urban traffic supervisory system is designed with ACS principles in order to minimize downtime caused by environmental emergencies and maximize security of the subsystem under it. A supervisory subsystem will also collect global traffic flow information and contextual constraints from its neighbors. Based on this input it will modify policies and communicate them to its traffic controller for timely adaptation. The urban traffic flowing into freeway traffic will be mediated by Intelligent Ramp Meters (IRM). An IRM interacts with the urban traffic control system and its nearest Intelligent Roadside Unit (IRSU) to regulate the flow of traffic from urban to freeway network. The FTCA consists of a network of mutually interacting IRSUs which monitor traffic flow, communicate with IRMs for providing traffic guidance for freeway drivers. An IRSU will communicate with the vehicles in the zone managed by it in order to provide information on rerouting when road and weather conditions warrant it. It also facilitates exchange of information between vehicles, guide them in lane changes and maintaining safe distance in order to avoid collision.     

一种基于ECN的无线TCP拥塞控制机制

针对无线异构链路环境中传统TCP拥塞控制机制效率较低的问题,本文提出一种基于ECN的多级反馈算法。该算法在ECN的基础上可以根据RTT动态地给网络划分等级并进行概率反馈,改变了ECN的二元特性,有效提高了无线数据传输效率。仿真结果表明该算法可降低丢包率,减少拥塞次数,提高吞吐量。     

延迟容忍网络中拥塞控制算法分析

先引入延迟容忍网络中拥塞控制的概念与基础知识,其中介绍了几种常用的拥塞检测方式和拥塞研究方向,然后重点介绍了3种拥塞控制算法的实现并在仿真平台下对这3种算法进行仿真分析,最后对延迟容忍网络中拥塞控制算法的发展前景进行了展望.     

Methods, Tools, and Observations on Flow Control in Packet-Switched Data Networks

Flow control covers several concepts. In an end-to-end connection, it intends to adapt sender's output to receiver's capabilities. In a shared resources communication network, it attempts to prevent congestion by restricting traffic flows. Flow control mechanisms are built with a small number of basic throttling tools. However, they are always tied into a resource management problem, which depends on a large variety of environmental parameters. Techniques used in end-to-end connections are summarized. Congestion control in store-and-forward packetswitching networks is covered extensively, and techniques used or proposed are discussed. Memoryless networks, such as broadcast local networks are shown to be simpler systems in the area of flow control. In conclusion, arguments are made for a service oriented approach, which would optimize resource usage through differentiated service characteristics.     

Broad-band active vibration suppression using PPF focused on industrial application

Due to the demand for higher accuracy and lower acoustic noise levels, there is a growing interest for active vibration suppression in industrial application. Many control concepts have been proposed in this field that are suited to introduce damping in systems with a large number of modes. However, tuning is often focussed on reduction of one dominant mode. Such control approaches are less transparent when several dominant modes are involved. A combined loop-shaping and root-locus tuning approach is proposed to reduce several dominant modes with low-order controllers. The tuning approach is experimentally evaluated on a vibrating plate using piezo's for both sensing and actuating. Based on the given tuning rules and tradeoffs, a single-input-single-output (SISO) feed-back controller is designed and implemented. This low authority control approach shows that besides local performance, the global damping of the plate is also improved, which consequently results in reduction of vibration levels over the whole plate for a broad range of frequencies. As a results, acoustic noise radiation is reduced significantly.