The Cambridge ring and Unix

In March 1980, a VAX 11/780

1 VAX is a trademark of Digital Equipment Corp.

was installed at the Computing Laboratory of the University of Kent. The VAX runs UNIX as a user service. This paper discusses what has been learnt from the integration of an existing operating system (UNIX) into a local area network based on the Cambridge ring. The approach which has been evolved attempts to make the network connections invisible to the operating system so that the utilities running on the system can access the network as if they were talking to standard peripherals. The paper gives an overview of the ring and the protocols in use at Kent; and points to alternative approaches to the implementation which could be used in a different environment.     

Monitoring tool for the Cambridge Ring LAN

A Cambridge Ring monitor station can detect breaks in a LAN, as well as pinpoint nodes which are causing data corruption. However, no facility exists for inspecting the data actually being transmitted by a corrupting node. A design for a Cambridge Ring probe is described which enables such data capture. Moreover, this probe can be used in conjunction with an oscilloscope for low-level fault finding, or together with a logic state analyser for high-level fault finding and/or performance measurement. Preliminary results are presented which demonstrate the usefulness of the probe for the former. A discussion of how the probe can be used for the latter is also included.     

Low-level programming in modula-2: An experience with local area network access

Access to a computer network, which is currently a topic of some importance to the high-level language programmer, can be provided in a number of ways. This paper describes one such example, where users of the language Modula-2 were given access to a local area network. The author discusses the type of problem that needs to be overcome, when a language such as Modula-2, which was designed as part of an integrated systems architecture, is used for low-level programming in a different architectural environment.     

Message oriented programming—A resource based methodology

The development of new technology and hardware systems provides many new opportunities for their exploitation. These opportunities also present us with certain responsibilities—namely, to develop appropriate tools for the orderly and well founded management of these systems. In this report we present a methodology for the development (and analysis) of programs based on the structuring principle of processes which synchronize their activities by message passing. The basis of the methodology is the resource and this concept generalizes that of abstract data type to the message-passing environment. The methodology is a natural outgrowth of methodologies for sequential programs and parallel programs developed for shared address spaces. We illustrate our ideas via solutions to some well known problems such as the bounded buffer problem, the readers and writers problem, the dining philosophers problem and the unreliable medium problem.     

浅议局域网的构建

随着信息技术的飞速发展,网络的应用已经深入到人们日常生活的每一个角落,局域网的身影更是随处可见,大到一个跨国公司、政府机关,小到一个家庭、学生宿舍局域网的组建。企业在组建网络架构的时候,要充分考虑到内部网络组建和外部网络组建这两部分,这两部分组件内容都有自己的需求,要根据具体的需求,设计实际的内部网络和外部网络。文中充分考虑到企业的实际情况,以现代网络技术为基础,对企业的网络进行构建。     

A monitor tool for a network based on the cambridge ring

An evolving network is subject to technological advances and increasing user demands through out its lifetime. Continuous monitoring is therefore required to understand its behaviour in the operational environment and to provide an aid to planning. A monitor tool has been developed to meet such requirements in a heavily used service network. A decentralized but integrated approach has been adopted. An active collection allows accumulated measurements to be retrieved. The measurements required are determined and the constraints imposed by existing mechanisms and the methods chosen are discussed. Flexible user facilities are available to allow selected measurements to be collected and displayed, and these are described. Daily sessions of network-wide monitoring have been undertaken. The overhead incurred is quantified. The sessions revealed communications software errors and anomalous behaviour of a network node. Further analysis shows the symptoms of the latter, the result of a hardware fault. Enhancements which would increase the flexibility of the tool are also suggested.     

Coupling experimental design—digital simulation of junctions for the development of complex tolerance chains

This paper presents a new functional tolerancing method developed for analyzing tri-dimensional variations of mechanical assemblies. The behavior of a junction is simulated with Microsoft EXCEL to quantify the influence of geometrical variations of a given part on a functional requirement. The solver determines the critical situation of the mechanism with respect to the contact constraints and the location and orientation tolerances. The result of this stage is introduced into an experimental design so as to define a relationship between the variation of the functional requirement and the geometric tolerances specified for the surfaces of the parts. The complete inequality ensuring tolerance synthesis is obtained by adding the defects of all junctions forming the vector loop.     

SimMan—A simulation model for workforce capacity planning

Motivated by the difficulty in predicting the actual performance of workforce capacity planning solutions obtained from mathematical models, we develop a simulator, SimMan, to serve as a test bed for evaluating the effectiveness and robustness of different planning options and assignment rules. To facilitate the ease of use, the simulator is developed in C++ language with a modular structure so that different users or planners can customize the modules according to their specifications. We further develop a mathematical model for the workforce capacity planning problem, where the uncertainty of the demand is handled by the safety stock concepts. We also suggest a number of practical planning alternatives and assignment rules based on information from IBM. These rules are implemented as modules in SimMan and tested numerically to provide managerial insights.     

Performance of Segnet — a simulation study

This paper describes the performance evaluation of Segnet, a CSMA/CD-based protocol with dynamic network segmentation. Segnet is shown to be suitable for network environments where the traffic pattern is highly localized into groups of stations clustered together. Some further improvements in performance are also shown to accompany the incorporation of store-and-forward buffers in Segnet stations. This is because degradations in performance caused by the segmenting of the network are minimized by temporary storage of packets which would otherwise have to be abandoned.     

仿真排队系统的统计计算研究

通过C 语言来仿真排队系统，该系统的数学模型为M／M／m/模型。然后用统计计算的方法来分析和研究仿真得到的数据，从而得出该系统的统计分布规律。     

A network screen editor implementation

A screen-based editor has been implemented by the authors. Starting from the view that the screen image should at all times reflect the intended contents of the file, a scheme has been developed which requires the minimum compromise from this philosophy but at the same time provides a sophisticated tool which can be accessed from unmodified low-cost ASCII terminals connected via a local area network. Good performance is achieved by the adoption of a special buffer forwarding technique in the terminal concentrator machines. A further design goal was that the user's view of the editor should be independent of either the terminal type or of the host machine operating system.     

基于局域网的某型防空导弹实时飞行动态监控系统

该文介绍了一个结合仿真和计算机网络技术，构造的“基于局域网（LAN）的某型防空导弹实时飞行动态监控系统”，详细说明了它的功能结构，硬件组成和主要的技术特点，为构造其他仿真系统提供了一些参考经验。     

Using simulation for facility design: A case study

A discrete event simulation model was developed and used to estimate the storage area required for a proposed overseas textile manufacturing facility. It was found that the simulation was able to achieve this because of its ability to both store attribute values and to show queuing levels at an individual product level. It was also found that the process of undertaking the simulation project initiated useful discussions regarding the operation of the facility. Discrete event simulation is shown to be much more than an exercise in quantitative analysis of results and an important task of the simulation project manager is to initiate a debate among decision makers regarding the assumptions of how the system operates.     

A knowledge-based approach to the local area network design problem

Computer networks are an essential tool for people in business, industries, government, and schools. With the rapid rate of change in network technology and products, and the emergence of highly sophisticated network users, network design has become an increasingly complex task. Although the computer society aims at agreeing to a series of international standards for describing network architectures, the design of a computer network remains an ill-structured problem that lends itself perfectly to expert systems solutions. We propose an expert system that is able to design local area networks meeting the requirements specified by the user. Rules and guidelines pertaining to local area network design are formulated and incorporated into the knowledge base. The system is built on an object-oriented paradigm. The object-oriented approach and the hierarchical rule structure paradigm are discussed. We also employ the blackboard technique through which rules can access dynamic objects conveniently.     

A simulation model to study the impact of resource changes on motor vehicle division field offices

This paper presents a network-based simulation model for the microcomputer to solve the resource balancing problem. The specific application discussed is systems planning for Motor Vehicles operations; however, the approach is adaptable to many organizations. The developed software is an interactive concept program, and can be used by people with no experience in computer programming.     

A simulation study of the performances of HDLC controlled links

Among the exchanging link protocols, HDLC is becoming more and more popular. But the standards which define it contain options and parameters to be selected according to the actual environment.A model of the information exchange phase of a balanced HDLC link is presented. The choice of two parameters (the maximum window width and a timer constant) is discussed with the help of simulation results. Information frames are of random length and two types of Supervisory frames are used (RR and REJ). The link has been simulated in saturated and nonsaturated situations with different transmission error rates. P/F checkpointing is not used, line propagation times are neglected.The illustrated performances include the link efficiency and various delays associated with the transmission of Information frames.     

A simulation based decision model for a fish processing facility

The single, most important production method in the fish processing industry is quick-freezing. The quick-freezing process in a typical fish processing plant is described. The network simulation model is then developed using the imulation anguage for lternative odeling (SLAM). The results from the model include utilization of machines and workers, in-process inventory levels, and throughput times. Sensitivity analysis is performed on some of the major factors that can affect the efficiency of the system. A front-end interface to the simulation model is developed which facilitates the user in entering input parameters to the model without the need to learn the simulation language. The model provides a methodology for evaluating alternative production strategies for a fish processing facility.     

Ceres-net: A low-cost computer network

This paper describes a local-area network, its hardware and software structure, and its protocol. The principal objective was to minimize the complexity of hardware and software, yet to offer an adequate functionality and performance for a system of up to 30 workstations with local file stores. The primary benefit of the achieved simple structure and low-overhead protocol is the system's reliability.