基于Petri网的道路交叉口建模方法及比较研究

离散Petri网和连续Petri网的各自优势和缺点明显,前者建模准确但复杂度高,后者复杂度较低但模型不够精确。混合Petri网作为两者的结合,能在一定程度上弥补各自的缺点,非常适合用于道路交叉口系统的建模。但是以往的文献中缺乏纯离散模型和混合模型之间的定量比较。提出了一种自上而下的混合Petri网方法,能够快速建立道路交叉口系统模型;以一个4相位固定相序交叉口为例,建立其混合Petri网模型并与纯离散模型进行了定量的比较与分析。通过对不同系统负载下交叉口内交通流排队长度的仿真,并考察混合模型与离散模型之间的误差变化,在一定程度上为模型选择的合理性提供了定量的依据。     

Model of joint servicing of real-time service traffic and data traffic. II

For the model of joint servicing of the real-time service and data traffics, numerical study of the demand servicing indices vs. the model input parameters was carried out. Solution of the problem of estimating the minimal channel resource sufficient to service the demand inflow with the desired performance was considered. Approximate algorithms to calculate the model were constructed, and their accuracy was analyzed numerically.     

Performance analysis of banyan-type multistage interconnection networks under nonuniform traffic pattern

Multistage Interconnection Networks(MINs) have a number of applications in the areas of computer and communications. The most widely researched structure among MIN’s is the (l)banyan type network. It has several variations such as buffered banyan, batcher-banyan, tandem banyan, recirculating banyan and banyan with contention resolution phase. Analytical performance evaluation is crucial for justifying the merit of the design in different operational conditions. While several analytical models have been proposed for the performance evaluation of MINs, they are mainly for uniform traffics. Even the models for nonuniform traffics have several shortcomings such as they only consider output buffered structure or do not consider blocking conditions. In this paper, the more accurate models than any other ones so far have been proposed for the performance evaluation of multibuffered banyan-type MIN’s under nonuniform traffic condition is obtained. The accuracy of proposed models are conformed by comparing with the results from simulation. Firstly, single buffer model is developed. Markov chain is used for the analysis. Multibuffer model is obtained from single buffer model. Simulation is performed using Discrete Evenet Simulaton(DES) method. As a results, proposed model proves to be very accurate.     

Queuing analysis of shared-buffer switches with control scheme under bursty traffic

Shared-buffer switches have many advantages such as relatively low cell loss rate and good buffer utilization, and they are increasingly favoured in recent VLSI switch designs for ATM. However, their performance degrades dramatically under nonuniform traffic due to the monopolization of the buffer by some favoured cells. To overcome this, restricted types of sharing and hot-spot pushout (HSPO) have been proposed, and the latter has been shown by simulation to perform better in all situations. In this paper we develop an analytical model for performance evaluation of a shared-buffer asynchronous transfer mode (ATM) switch with HSPO under bursty traffic. This analytical model is an improved version of the first model ever developed for this purpose. We balance the relative queues to approximate the effects of pushout, while keeping only four state-variables, and our model gives a good agreement with simulation, for calculating throughput and cell loss.     

自相似背景流量模型下的CBR流延迟抖动分析

实时业务是网络中快速增长的业务类型,但网络中需要传送多种业务的混合流量。实时 业务的性能取决于分组延迟抖动。过大的分组延迟抖动将导致语音的中断,画面的停顿和跳跃。延 迟抖动主要是背景流量在边缘路由器的干扰引起的。以往的延迟抖动分析都是假设背景流量为泊松 过程,研究表明这种假设已经不符合当前网络流量的特性。本文将对自相似背景流量下的CBR流的 延迟抖动进行分析,给出其分布函数,并以仿真结果验证其与泊松流量对CBR流的不同影响。     

Managing emergencies and abnormal situations in air traffic control (part II): Teamwork strategies

Team performance has been studied in many safety-critical organizations including aviation, nuclear power plant, offshore oil platforms and health organizations. This study looks into teamwork strategies that air traffic controllers employ to manage emergencies and abnormal situations. Two field studies were carried out in the form of observations of simulator training in emergency and unusual scenarios of novices and experienced controllers. Teamwork strategies covered aspects of team orientation and coordination, information exchange, change management and error handling. Several performance metrics were used to rate the efficiency of teamwork and test the construct validity of a prototype model of teamwork. This is a companion study to an earlier investigation of taskwork strategies in the same field (part I) and contributes to the development of a generic model for Taskwork and Teamwork strategies in Emergencies in Air traffic Management (T²EAM). Suggestions are made on how to use T²EAM to develop training programs, assess team performance and improve mishap investigations.     

Performance analysis of priority scheduling mechanisms under heterogeneous network traffic

Priority scheduling principle plays a crucial role in the Differentiated Services (DiffServ) architecture for the provisioning of Quality-of-Service (QoS) of network-based applications. Analytical modelling and performance evaluation of priority queuing systems have received significant attention and research efforts. However, most existing work has primarily focused on the analysis of priority queuing under either Short Range Dependent (SRD) or Long Range Dependent (LRD) traffic only. Recent studies have shown that realistic traffic reveals heterogeneous nature within modern multi-service networks. With the aim of investigating the impact of heterogeneous traffic on the design and performance of network-based systems, this paper proposes a novel analytical model for priority queuing systems subject to heterogeneous LRD self-similar and SRD Poisson traffic. The key contribution of the paper is to extend the application of the generalized Schilder's theorem (originally a large deviation principle for handling Gaussian processes only) to deal with heterogeneous traffic and further develop the analytical upper and lower bounds of the queue length distributions for individual traffic flows. The validity and accuracy of the model demonstrated through extensive comparisons between analytical bounds and simulation results make it a practical and cost-effective evaluation tool for investigating the performance behaviour of priority queuing systems under heterogeneous traffic with various parameter settings.     

基于支持向量回归机的公路货运量预测模型*

为了提高公路货运量预测的能力,应用基于结构风险最小化准则的标准支持向量回归机方法来研究公路货运量预测问题.在选择适当的参数和核函数的基础上,通过对成都公路货运量时间序列进行预测,并与人工神经网络、线性回归分析等方法进行了对比,发现该方法能获得最小的训练相对误差和测试相对误差.     

Prediction of communication delay in torus networks under multiple time-scale correlated traffic

The efficiency of a large-scale multicomputer is critically dependent on the performance of its interconnection network. Current multicomputers have widely employed the torus as their underlying network topology for efficient interprocessor communication. In order to ensure a successful exploitation of the computational power offered by multicomputers it is essential to obtain a clear understanding of the performance capabilities of their interconnection networks under various system configurations. Analytical modelling plays an important role in achieving this goal. This study proposes a concise performance model for computing communication delay in the torus network with circuit switching in the presence of multiple time-scale correlated traffic which is found in many real-world parallel computation environments and has strong impact on network performance. The tractability and reasonable accuracy of the analytical model demonstrated by extensive simulation experiments make it a practical and cost-effective evaluation tool to investigate network performance with various alternative design solutions and under different operating conditions.     

FPGEN: A fast, scalable and programmable traffic generator for the performance evaluation of high-speed computer networks

Testing today’s high-speed network equipment requires the generation of network traffic which is similar to the real Internet traffic at Gbps line rates. There are many software-based traffic generators which can generate packets according to different stochastic distributions. However, they are not suitable for high-speed hardware test platforms. This paper describes FPGEN (Fast Packet GENerator), a programmable random traffic generator which is entirely implemented on FPGA (Field Programmable Gate Array). FPGEN can generate variable packet sizes and traffic with Poisson and Markov-modulated on-off statistics at OC-48 rate per interface. Our work that is presented in this paper includes the theoretical design of FPGEN, the hardware design of the FPGA-based traffic generator board (printed circuit board design and construction) and the implementation of FPGEN on FPGA. Our experimental study demonstrates that FPGEN can achieve both the desired rate and statistical properties for the generated traffic.     

Optimization of robust area traffic control with equilibrium flow under demand uncertainty

For area traffic control road network under realization of uncertain travel demand, a robust signal setting is investigated in this paper. Due to certain hierarchy in a decision-making order, a min–max bilevel program is proposed. A new solution method is presented to determine a Nash–Stackelberg solution where a proposed signal setting is found for area traffic control under demand uncertainty. In order to investigate the robustness of the proposed signal settings, numerical computations are performed for various initial data sets in a medium-sized example road network. Good computational results indicated that the proposed signal settings can successfully reduce a worst-case travel cost substantially while incurring a relatively slight loss of optimality with respect to the optimal deterministic solutions for nominal travel demands. Particularly, our computation results showed that the proposed signal settings become even attractive as demand growth increases under a worst-case realization taken by uncertain travel demands.     

Nonsaturation throughput enhancement of IEEE 802.11b distributed coordination function for heterogeneous traffic under noisy environment

In this paper, we propose a mechanism named modified backoff (MB) mechanism to decrease the channel idle time in IEEE 802.11 distributed coordination function (DCF). In the noisy channel, when signal-to-noise ratio (SNR) is low, applying this mechanism in DCF greatly improves the throughput and lowers the channel idle time. This paper presents an analytical model for the performance study of IEEE 802.11 MB-DCF for nonsaturated heterogeneous traffic in the presence of transmission errors. First, we introduce the MB-DCF and compare its performance to IEEE 802.11 DCF with binary exponential backoff (BEB). The IEEE 802.11 DCF with BEB mechanism suffers from more channel idle time under low SNR. The MB-DCF ensures high throughput and low packet delay by reducing the channel idle time under the low traffic in the network. However, to the best of the authors' knowledge, there are no previous works that enhance the performance of the DCF under imperfect wireless channel. We show through analysis that the proposed mechanism greatly outperforms the original IEEE 802.11 DCF in the imperfect channel condition. The effectiveness of physical and link layer parameters on throughput performance is explored. We also present a throughput investigation of the heterogeneous traffic for different radio conditions.     

Platoon dispersion prediction under the condition of adjacent cycle traffic flow overlapping based on Support Vector Regression

Coordinated signal control can improve the continuity of vehicular traffic flow movement and reduce delay. Cycle Flow Profile is the base for calculating coordinated signal control parameters. Platoon dispersion characteristic determines the Cycle Flow Profile. So, improving platoon dispersion prediction accuracy can obtain significant benefit for signal coordination. When the velocities of the vehicles vary greatly, faster vehicles of next cycle can catch up the slower vehicles of the current cycle. Traffic flow overlapping of adjacent cycle is an important characteristic. Support Vector Regression is adopted to predict platoon dispersion and compares prediction accuracy with Robertson formula. The results are encouraging; Support Vector Regression has higher prediction accuracy.     

Transient process relaxation time research under the condition of self-similar traffic input in wireless networks

In the article, the results of measuring the traffic in wireless networks are presented. The results provide evidence of the traffic being notably self-similar. The two following models were developed in order to estimate the parameters of a wireless network operating along with the input traffic being self-similar: a model for systems with no restrictions on the quantity of the orders in the system lines and a model for systems with limited buffer volumes. The two types of incoming traffic that each system was simulated for were the Poisson and self-similar traffic.The main objective of the article is to figure out the relaxation time and dependences of the relaxation time and loss probability on different parameters of the simulated systems.     

Applications of sketches in network traffic measurement: A survey

Accurate and timely network traffic measurement is essential for network status monitoring, network fault analysis, network intrusion detection, and network security management. With the rapid development of the network, massive network traffic brings severe challenges to network traffic measurement. However, existing measurement methods suffer from many limitations for effectively recording and accurately analyzing big-volume traffic. Recently, sketches, a family of probabilistic data structures that employ hashing technology for summarizing traffic data, have been widely used to solve these problems. However, current literature still lacks a thorough review on sketch-based traffic measurement methods to offer a comprehensive insight on how to apply sketches for fulfilling various traffic measurement tasks. In this paper, we provide a detailed and comprehensive review on the applications of sketches in network traffic measurement. To this end, we classify the network traffic measurement tasks into four categories based on the target of traffic measurement, namely cardinality estimation, flow size estimation, change anomaly detection, and persistent spreader identification. First, we briefly introduce these four types of traffic measurement tasks and discuss the advantages of applying sketches. Then, we propose a series of requirements with regard to the applications of sketches in network traffic measurement. After that, we perform a fine-grained classification for each sketch-based measurement category according to the technologies applied on sketches. During the review, we evaluate the performance, advantages and disadvantages of current sketch-based traffic measurement methods based on the proposed requirements. Through the thorough review, we gain a number of valuable implications that can guide us to choose and design proper traffic measurement methods based on sketches. We also review a number of general sketches that are highly expected in modern network systems to simultaneously perform multiple traffic measurement tasks and discuss their performance based on the proposed requirements. Finally, through our serious review, we summarize a number of open issues and identify several promising research directions.     

Strategies for designing distributed systems: case studies in the design of an air traffic management system

The air traffic management system in the USA is an example of a distributed problem-solving system. It has elements of both cooperative and competitive problem-solving. It includes complex organizations such as Flight Operations Centers, the FAA Air Traffic Control Systems Command Center (ATCSCC), and traffic management units at en route centers that focus on daily strategic planning, as well as individuals concerned more with immediate tactical decisions (such as air traffic controllers and pilots). The design of this system has evolved over time to rely heavily on the distribution of tasks and control authority in order to keep cognitive complexity manageable for any one individual operator, and to provide redundancy (both human and technological) to serve as a safety net to catch the slips or mistakes that any one person or entity might make. Within this distributed architecture, a number of different conceptual approaches have been applied to deal with cognitive complexity and to provide redundancy. These approaches can be characterized in terms of the strategy for distributing: (1) control or responsibility, (2) knowledge or expertise, (3) access to data, (4) processing capacity, and (5) goals and priorities. This paper will provide an abstract characterization of these alternative strategies for distributing work in terms of these 5 dimensions, and will illustrate and evaluate their effectiveness in terms of concrete realizations found within the National Airspace System.     

From trees to forest: relational complexity network and workload of air traffic controllers

In this paper, we propose a relational complexity (RC) network framework based on RC metric and network theory to model controllers' workload in conflict detection and resolution. We suggest that, at the sector level, air traffic showing a centralised network pattern can provide cognitive benefits in visual search and resolution decision which will in turn result in lower workload. We found that the network centralisation index can account for more variance in predicting perceived workload and task completion time in both a static conflict detection task (Study 1) and a dynamic one (Study 2) in addition to other aircraft-level and pair-level factors. This finding suggests that linear combination of aircraft-level or dyad-level information may not be adequate and the global-pattern-based index is necessary. Theoretical and practical implications of using this framework to improve future workload modelling and management are discussed.     

A Model of Inventory Control. II

A model of inventory control is considered. It is described by a semi-Markovian random walk with a negative drift at an angle of (0° < < 90°), with positive random jumps, delays, and absorbing screens at zero and at a #gt; 0. The Laplace transformation is found for the distribution of the first moment of the stoking up of a warehouse, and its first and second moments are explicitly obtained.