总线控制网络中基于优先级CSMA/CD协议的改进

总线控制网络的MAC层协议对网络实时特性具有重要影响。结合优先级CSMA／CD协议的等级原则与Token Bus协议的均衡思想，提出了一种改进的总线优先级轮循CSMA／CD协议。其高级别的总线优先级类似于Token Bus协议中的令牌，在各节点之间轮循传递；使控制网络在遵循整体分级的原则下，相同类型节点的性能局部能够相对均衡。详细阐明了该改进基本思想，概述其具体实现方式，并给出了对比仿真结果。该改进协议对于CAN等基于优先级CSMA／CD协议的总线控制网络具有一定实用价值。     

An extended token bus protocol for embedded networks

The network environment considered in this paper exists in commercial and military products such as Autonomous Aircraft and Vehicles. In these products sensors and processors are utilized for control, diagnosis, repair and communication. We refer to such networks as embedded networks and present a bus protocol suitable for these networks. Two modes of operation have been defined for the proposed protocol (IMAP). Normal mode is defined as that in which token passing is done in a random order and the token remains within a cluster of active stations. The other mode of operation is called the interrupt mode. In this mode, token bus operation is carried out and the token is passed through every station. Performance in terms of channel utilization and delay characteristics of IMAP is compared to the token bus and CSMA/CD performance. The performance models are developed by obtaining time-distance diagrams of token bus and IMAP. It is observed that IMAP has better performance characteristics than the conventional token bus and CSMA/CD.     

Software bottlenecking in client-server systems and rendezvousnetworks

Software bottlenecks are performance constraints caused by slow execution of a software task, in typical client-server systems a client task must wait in a blocked state for the server task to respond to its requests, so a saturated server will slow down all its clients. A rendezvous network generalizes this relationship to multiple layers of servers with send-and-wait interactions (rendezvous), a two-phase model of task behavior, and to a unified model for hardware and software contention. Software bottlenecks have different symptoms, different behavior when the system is altered, and a different cure from the conventional bottlenecks seen in queueing network models of computer systems, caused by hardware limits. The differences are due to the “push-back” effect of the rendezvous, which spreads the saturation of a server to its clients. The paper describes software bottlenecks by examples, gives a definition, shows how they can be located and alleviated, and gives a method for estimating the performance benefit to be obtained. Ultimately, if all the software bottlenecks can be removed, the performance limit will be due to a conventional hardware bottleneck     

Design and performance evaluation considerations of a multimediamedical database

Multimedia information management and communications systems for medical applications have been undergoing design and development. The aim of these activities has been the development of distributed computer systems providing storage, processing, and communication services required by the medical community. One of the main critical issues of such systems is the handling of multimedia information (i.e., text, images, graphics, and voice) in a uniform way and the fast access to images through the network. We present the design architecture and concepts used in our multimedia database system, along with its performance evaluation. The evaluation focuses on the response times of a multimedia medical database server, as seen by remote workstations for various retrieval and image transfer requests. For this purpose, a queueing network model was developed and simulated. Communications between the workstations and the database server are governed by the client-server model using TCP/IP and appropriate application protocols on a 10 Mb/s Ethernet LAN     

CSMA/CD with network partitioning

One of the local area network medium access control standards created by the Institute of Electrical and Electronic Engineers in IEEE Project 802 is the carrier sense multiple access with collision detection (CSMA/CD. These studies show that control. Numerous studies have been reported in the literature on the performance of CSMA/CD. These studies show that CSMA/CD performs well under light network load but not well at heavy load. To improve the performance of CSMA/CD under heavy load, a new concept called network partitioning is presented. Network partitioning allows the network to be partitioned into segments when under heavy load. Partition stations then act as bridges between the segments. The impact of network partitioning on network performance was tested using a simulation model of an Ethernet local area network (an implementation of the CSMA/CD medium access control). The simulation results show that network partitioning can significantly improve the performance of CSMA/CD under heavy load.     

Interactive view-dependent rendering over networks

For a client-server based view-dependent rendering system, the overhead of view-dependent rendering and the network latency are major obstacles in achieving interactivity. In this paper, we first present a multiresolution hierarchy traversal management strategy to control the overhead of view-dependent rendering for low-capacity clients. Then we propose a predictive parallel strategy to overcome the network latency for client-server based view-dependent multiresolution rendering systems. Our solution is to make the client process and the server process run in parallel, using the rendering time to cover the network latency. For networks with long round-trip times, we manage to overlap the network latency for one frame with the rendering time for multiple frames. View-parameters prediction is incorporated to make the parallelism of the client and the server feasible. In order to maintain an acceptable view-dependent rendering quality in the network environment, we develop a synchronization mechanism and a dynamic adjustment mechanism to handle the transient network slowdowns and the changes of the network condition. Our experimental results, in comparison with the sequential method, show that our predictive parallel approach can achieve an interactive frame rate while keeping an acceptable rendering quality for large triangle models over networks with relatively long round-trip times.     

具有时间同步和节点定位的工业无线网络协议

针对工业无线网络WIN的特点及传输实时性和空间位置的确定性要求,引入了工业无线网络的通信参考模型和协议架构.然后简单介绍了基于令牌的控制WIN-Z的MAC协议,并对其嵌入的时间同步算法和节点定位方法做了详细介绍.最后利用改进后的GAINS平台和WSN分析与管理平台SNAMP对WIN-Z的MAC协议和CSMA/CA做了数据吞吐量和数据传榆延迟的测试,测试结果表明,该协议保证了信道接入的公平性,减少了数据传输时延,适用于WIN网络.     

应答服务元素(ORSE)及非证实型服务的实现

Mini—MAP中令牌逻辑环的成员站同非成员站之间的通信是通过ORSE实现的。本文给出了这两种站的逻辑模型,ORSE的工作原理及借助ORSE实现非证实型服务请求的过程。最后对一些有关问题进行了讨论。     

基于透明计算模式的I/O Server的设计

设计并实现了一个基于透明计算模式的I/O Server系统，I/O Server和I/O Client是一个在透明计算环境下，支持多操作系统远程启动和运行的网络存储访问服务I/O Manager的2个软件模块，I/O Server工作在服务器端，I/O Client工作在客户端。在透明计算模式中，各客户机硬件与操作系统分离，用户需要的操作系统的应用程序存储在服务器端。在客户机启动时，I/O Server和启动协议将I/O Client下载到端系统上运行，然后I/O Client向I/O Server发出I/O请求，I/O Server对收到的I/O请求加以分析，进行优先级分类，在优先级分时轮转调度I/O请求、操作服务器上的虚拟硬盘文件，并通过预取和缓存策略减少磁盘I/O操作，将处理结果返回给客户端，支持操作系统的远程启动，并为系统运行时的各种请求提供服务。     

自相似通信量对CSMA/CD性能的影响

大量网络通信量测量研究表明,通信量在长时间尺度上的突发性规律可以用分形或自相似模型来描述。为准确评估CSMA/CD协议性能,基于RMD算法产生近似的FBM,并利用近似FMB构造了自相似通信量模型。基于自相似通信量,采用离散事件仿真技术对CSMA/CD协议建立了排队模型,对网络性能进行了分析,通过与通信量服从泊松到达的仿真结果进行对比,说明了自相似特性对网络性能分析的影响。     

Performance analysis of BusNet protocol for backplane bus-based interprocessor communication

Nowadays, backplane bus-based multiprocessor systems often utilize the standard network protocol such as TCP/IP for communication between processors on the backplane bus. In such systems, it is common for the backplane bus to emulate the standard MAC protocols such as CSMA/CD. This paper aims to analyze the delay performance of the MAC emulation-based backplane network by constructing queueing models based on detailed bus operations. For this purpose, we choose BusNet as a target protocol. BusNet is an ANSI standard network protocol and its specification contains basic operations commonly used in most backplane buses. We investigate the throughput-delay characteristics in terms of packet size, block transfer scale, and arbitration scheme. We also compare the packet delay in BusNet with the IEEE 802.3 CSMA/CD network which BusNet is expected to be compatible with. The simulation result shows how an optimal block transfer scale can be determined in respect of the performance trade-off between BusNet and other real-time traffics.     

C/S模式网管系统向B/S模式的迁移与优化

在传统C/S (客户端/服务器)模式的网络管理系统中,存在网络管理便利性差和维护麻烦的缺点,因此提出基于B/S (浏览器/服务器)模式的网络管理系统,将原有C/S模式的网管系统转为B/S模式,并且对系统结构进行了优化,有助于客户方便可靠地监测和管理网络资源.该系统以原网管系统服务端暴露的接口为基础,利用RMI (远程方法调用)技术调用服务端接口与Web服务器实现数据通信,并且整个系统采用负载均衡策略将来自多个浏览器的访问请求均衡地分配至各Web服务器进行处理,同时对单一的负载均衡器进行了双机热备处理,从而大大提升了网络管理系统的整体性能和可靠性.     

SOMR结构网络机器人的核心数据调度

在单操作者多智能体SOMR的网络机器人系统中，网络数据传输中的拥塞是瓶颈问 题．作为操作者的核心计算机同时处理多个网络机器人的上传信息，并进行任务的协调与合 作，起着全局调度的作用，其数据调度方式是多机器人系统工作效率的关键．本文目的即为 解决SOMR的网络机器人系统中的数据调度方式问题，提出一种基于运筹学排队论的核心计算 机数据优化调度方式，数学分析结果表明其数据传输效率高于CSMA/CD和令牌等方式．     

CSMA/CD with reservations in wireless communication for distributed robot system

This paper proposes a medium access protocol, CSMA/CD-R (Carrier Sense Multiple Access/Collision Detection with Reservation), designed for a distributed robot system based on wireless network without any centralized mechanism. It employs stations to reserve a communication channel after communication collision such that it shows better performance than conventional CSMA protocol for the wireless communication of the distributed robot system. The effectiveness and applicability of the proposed protocol are demonstrated by carrying out both computer simulations and real experiments with the developed multi-robot system.     

Some features of two fieldbuses of the IEC 61158 standard

The IEC 61158 is on the way to become an international fieldbus standard after a long period of contrasts and misunderstandings. The final standard document results from the joining of eight among the most diffused fieldbus protocols.This paper takes into consideration one of the most popular fieldbus of the final standard document, Profibus, and examines the differences with the original IEC 61158 Technical Specification (which is also included in the standard). The analysis concerns the data link layer of the two fieldbuses: Profibus is based on a token passing technique, while the IEC fieldbus realises a mixed access combining token passing with a centralised scheduling procedure managed by a special node named Link Active Scheduler (LAS). Two parameters of the data link layers have been in particular considered: the cycle time and the medium access efficiency. In particular, the evaluation of the cycle times has been carried out using analytical models which are valid under the hypothesis that the token holding times of the stations are sufficiently long to satisfy all the service requests coming from the users, that is a typical situation of a network not overloaded. Considerations have also been made to investigate the behaviours of the cycle times when the network traffic increases.     

CSMA with collision avoidance

Advances in local area networking have allowed users to run many different applications on one system. Users are now asking for greater access, more functions, more power and greater reliability. This requires that the system tasks should be distributed, and means that the interconnection system used should be highly reliable; ‘passive’, so that no failed component can bring the entire system down; support high through-put; and operate on low-cost cable. This paper describes a broadband transmission system based on a single, passive coaxial cable which detects possible collisions before the data is sent. Examples of two remote stations and two adjacent stations competing for transmission are given, along with performance comparisons of CSMA/CD and CSMA/CA.     

基于IEEE802．15．4a的无线测控网络协议

为满足工业现场测控任务对网络通信实时性的要求,提出一种基于IEEE80215．4a的无线测控网络协议,其中的令牌控制子层采用令牌机制管理各站点对无线信道的访问,克服了CSMA／CA机制无序竞争的缺陷,保证数据传输的时间确定性,介绍协议栈模型、逻辑令牌环动态建立过程以及令牌故障的自恢复机制,并在硬件平台NanoNET上加以实现。实验结果表明,该协议能够在强电磁干扰环境中稳定运行,具备实时特性。     

Conventional and early token release scheduling models for the IEEE 802.5 token ring

We develop analyticalscheduling models for both the original IEEE 802.5 token ring protocol and a recent extension to the original protocol that allows early token release (ETR). A scheduling model is an abstraction that supports reasoning about the timing correctness of a given set of real-time messages scheduled on the network. Scheduling analysis of the original IEEE 802.5 token ring protocol has previously been discussed in Strosnider and Marchok (1989) and Pleinevaux (1992) in the context of improving responsiveness of soft deadline aperiodic messages. In contrast, this paper develops schedulability conditions for arbitrary periodic message sets. The main contributions of this work are: Scheduling models for both the original protocol and ETR protocol; comparison for maximum achievable utilizations for the two protocols; comparison between the original protocol and ETR from a schedulability viewpoint. We also demonstrate the utility of our scheduling models to select network operating parameters such as maximum packet size, and to quantify effects of parameters such as the number of stations, and network size on schedulability.     

对等式测控网的设计与实现

为了提高单片机测控网络系统的灵活性,进一步降低测控网络的成本,解决传统通迅网络中主机负担过重的问题,提出了对等式单片机测控网络新结构。通过对系统结构的特殊设计,取消主机并采用令牌环总线的通信方式,使节点间实现对等通信成为可能,并保证了通信的可靠性。经过实验室构建实际系统,验证了方案的可行性。该方案突破了传统的主从式通讯模式,克服了现在一般数据采集站中从站间通信困难的缺点;具有数据吞吐量大、节点与节点间通信渠道宽、主机的功能被彻底地取代等优点,对于构建低成本分布式测控网络具有实用价值。     

An approximation for mean waiting times in cyclic server systems with nonexhaustive service

The cyclic server system has been the subject of considerable research over the last few years. Interest in analyzing such systems has gained momentum due to their application in the performance analysis of token ring networks. In this paper we consider cyclic server systems with nonexhaustive service discipline. The performance measures of interest here are the mean waiting times at the nodes in the system. Exact analysis of such systems for these performance measures is very difficult in general, and a number of approximation schemes have been proposed in the past to evaluate these quantities. This paper presents a new approximation technique that gives accurate estimates of these mean waiting times, based on extensive validation with simulations.