Showing papers in "Computer Networks and Isdn Systems in 1995"

The Harvest information discovery and access system

Abstract: It is increasingly difficult to make effective use of Internet information, given the rapid growth in data volume, user base, and data diversity. In this paper we introduce Harvest, a system that provides a scalable, customizable architecture for gathering, indexing, caching, replicating, and accessing Internet information.

Survey of ATM switch architectures

Abstract: For reasons of economy and flexibility, BISDN (Broadband Integrated Services Digital Network) is expected to replace existing application-oriented communication networks. ATM (Asynchronous Transfer Mode) is a high-speed packet-switching technique that has emerged as the most promising technology for BISDN. Since early 1980s, a large number of architectures have been proposed for ATM switching. In this paper, we present a descriptive survey of ATM switch architectures, with emphasis on electronic space-division point-to-point switches.

The Fishwrap personalized news system

Abstract: Fishwrap is an experimental electronic newspaper system available at MIT. Fishwrap attempts to address the needs of the freshman integrating into the MIT community. Fishwrap attempts this by balancing an individual's desire for personalization with the need to participate and know about the world at large. This paper describes the scope of the experiment, the underlying technology, and the evaluation by participants. Access to Fishwrap's personalized news system appears as a World Wide Web (WWW) hypertext link. This link calls upon programs to subscribe the individual to the news service. At this time an individual access password is provided. After the initial session students can get Fishwrap from any computer connected to the MIT network with a WWW browser.

Web traffic characterization: an assessment of the impact of caching documents from NCSA's web server

Abstract: We analyze two days of queries to the popular NCSA Mosaic server to assess the geographic distribution of transaction requests. The wide geographic diversity of query sources and popularity of a relatively small portion of the web server file set present a strong case for deployment of geographically distributed caching mechanisms to improve server and network efficiency. The NCSA web server consists of four servers in a cluster. We show time series of bandwidth and transaction demands for the server cluster and break these demands down into components according to geographical source of the query. We analyze the impact of caching the results of queries within the geographic zone from which the request was sourced, in terms of reduction of transactions with and bandwidth volume from the main server. We find that a cache document timeout even as low as 1024 seconds (about 17 minutes) during the two days that we analyzed would have saved between 40% and 70% of the bytes transferred from the central server. We investigate a range of timeouts for flushing documents from the cache, outlining the tradeoff between bandwidth savings and memory/cache management costs. We discuss the implications of this tradeoff in the face of possible future usage-based pricing of backbone services that may connect several cache sites. We also discuss other issues that caching inevitably poses, such as how to redirect queries initially destined for a central server to a preferred cache site. The preference of a cache site may be a function of not only geographic proximity, but also current load on nearby servers or network links. Such refinements in the web architecture will be essential to the stability of the network as the web continues to grow, and operational geographic analysis of queries to archive and library servers will be fundamental to its effective evolution.

PDAs as mobile WWW browsers

Abstract: In this paper we present a WWW frontend for Apple's Personal Digital Assistant (PDA) Newton. At the Telecooperation Office (TecO) we carry out a project researching information retrieval by mobile hand-held devices. In this context we are investigating impacts of PDA architecture (e.g. limited storage capabilities, small display) on the usability of these devices to act as WWW browsers. As a result of these activities, we developed a WWW browser for the Apple Newton PDA. Currently WWW access is restricteted to stationary hosts. It would be a very promising approach to have access to this global information system via mobile devices. PDAs are the pinnacle of modern computer and communication technology and supposed to be in everybody's possesion in a few years. These hand-held systems offer wireless communication and advanced integration: You can carry computer and communication services in your pocket. They can be used as mobile information browsers not only in hospitals or libraries but also in future offices and everyday life where all kind of information access are to be combined in one device. We demonstrate the feasibility of providing access to WWW via mobile hand-held devices. Based on our experiences we will report the special requirements for PDAs as WWW clients. Those requirements are, e.g., pre-processing of graphical data and reflection of small bandwidth wireless communication. We will present the architecture of our PDA WWW browser based on the concept of distributed clients and discuss desirable PDA specific features of the WWW service. Finally we want to present first experiences in using the browser and give an outlook on future developments.

WebMap: a graphical hypertext navigation tool

Abstract: WebMap is intended to help avoiding the well-known “lost in hyperspace” situation. It does this by creating and updating a two-dimensional graphical map of a user's “journey through hyperspace” by dynamically analyzing the navigation actions. WebMap communicates with the Mosaic WWW browser, so user interactions with Mosaic are immediately reflected in WebMap's graphics window. WebMap also may be used to control Mosaic, e.g., to cause it to load a new document into one of its windows or store a document in PostScript format.

Ethical Web agents

Abstract: As the Web continues to evolve, the sophistication of the programs that are employed in interacting with it will also increase in sophistication. Web agents, programs acting autonomously on some task, are already present in the form of spiders. Agents offer substantial benefits and hazards, and because of this, their development must involve not only attention to technical details, but also the ethical concerns relating to their resulting impact. These ethical concerns will differ for agents employed in the creation of a service and agents acting on behalf of a specific individual. An ethic is proposed that addresses both of these perspectives. The proposal is predicated on the assumption that agents are a reality on the Web, and that there are no reasonable means of preventing their proliferation.

The ISO reference model for open distributed processing: an introduction

Abstract: The ISO Reference Model of Open Distributed Processing (RM-ODP) consists of four parts: an Overview of the reference model, the Descriptive Model, the Prescriptive Model, and the Architectural Semantics. The four parts provide the concepts and rules of distributed processing to ensure openness between interacting distributed application components. Openness is a combination of characteristics: i.e. scalability, accessibility, heterogeneity, autonomy and distribution. The RM-ODP introduces the concept of viewpoint to describe a system from a particular set of concerns, and hence to deal with the complexity of distributed systems. While all the viewpoints are relevant to the description and design of distributed systems, the computational and engineering models are the ones that bear most directly on the design and implementation of distributed systems. From a distributed software engineering point of view, the computational and engineering viewpoints are again the most important; they reflect the software structure of the distributed application most closely. In this introductory paper, we concentrate on the computational and engineering viewpoints.

Beyond the Web: manipulating the real world

Abstract: This paper describes a WWW site that allows users to “leave the Web” and interact with the real world. An interdisciplinary team of anthropologists, computer scientists and electrical engineers collaborated on the project, desigining a system which consists of a robot arm fitted with a CCD camera and a pneumatic system. By clicking on an ISMAP control panel image, the operator of the robot directs the camera to move vertically or horizontally in order to obtain a desired position and image. The robot is located over a dry-earth surface allowing users to direct short bursts of compressed air onto the surface using the pneumatic system. Thus robot operators can “excavate” regions within the environment by positioning the arm, delivering a burst of air, and viewing the image of the newly cleared region. This paper describes the system in detail, addressing critical issues such as robot interface, security measures, user authentication, and interface design. We see this project as a feasibility study for a broad range of WWW applications.

A flexible shared-buffer switch for ATM at Gb/s rates

Abstract: This paper presents the architecture of a very high-speed VLSI packet switch and its performance. The switch, called PRIZMA, is suited for broadband telecommunications, based on ATM, the Asynchronous Transfer Mode. However, the concept is not restricted to ATM-oriented architectural environments. There may be applications within private networks, independent of whether they are ATM-based. There may also be other potential applications such as multiprocessor interconnection. The architecture of the PRIZMA switch follows the architecture of its lower-speed earlier version (H. Ahmadi et al., Int. J. Digital Analog Cabled Syst. 2 (4) (1989) 277–287) to a large degree: It is based on a single-chip switch element that exploits the performance advantage of output queuing and from which larger, self-routing single-stage or multistage switch fabrics can be constructed in a modular way. However, compared to the precursor, higher performance is achieved by output queues that now are configured as a dynamically shared memory. This shared memory can also be expanded by linking multiple switch elements. Owing to novel parallel structures inside the switch element, VLSI implementation is possible for transmission rates on the order of a gigabit per second per port. In the last section of this paper, performance results are presented for a switch in a single-stage configuration as well as for the case of a three-stage switch fabric.

Some FAQs about usage pricing

Abstract: This is a list of Frequently Asked Questions about usage-based pricing of the Internet. We argue that usage-based pricing is likely to come sooner or later and that some serious thought should be devoted to devising a sensible system of usage-based pricing.

Auto-FAQ: an experiment in cyberspace leveraging

Abstract: Intelligent behavior requires, in one form or another, access to an enormous reservoir of knowledge and information. Unfortunately, intelligent systems are currently limited by a severe knowledge acquisition bottleneck. In most cases, the cost of getting knowledge is simply too high. Hand-coding is often too arduous or too time-consuming, while learning algorithms are often too slow or handcuffed by insufficient training data. This paper explores the idea of harnessing computer networks to overcome the knowledge acquisition bottleneck. We introduce the idea of a CYLINA (CYberspace Leveraged INtelligent Agent)—an intelligent system that gains knowledge/information through interactions with a large population of network users. Instead of depending on the big efforts of a few knowledge engineers, CYLINAs rely on small, incremental contributions from a large population of experts. Our thesis is that the shear volume of interaction will allow significant knowledge to be acquired in a short amount of time. We consider potential applications for CYLINAs, then focus on Auto-FAQ, an experimental system currently under development at GTE Laboratories. Auto-FAQ is a question-answering system. Its intent is to make information typically found in USENET News FAQs much more accessible (It has many other uses as well.) Users ask questions in natural language forms. These questions index directly into the systems infobase. Infobase entries are question-answer pairs. Answers can be raw text, URLs, or Links into existing entries in the system's infobase. By using the system recursively, users can explore entire subjects with a series of questions. Facilities exist to tag gaps in the systems knowledge base. When a gap is found, it is posted to a public list. Individuals in the cyberspace community can search the list, volunteer expertise, and fill in gaps as appropriate. A version of Auto-FAQ is currently operating on a private network at GTE Laboratories. The system is currently able to answer basic questions about itself, WWW, and Mosaic. Future plans are to make Auto-FAQ and its associated software available on the global Internet.

Synchronizable test sequences of finite state machines

Abstract: The finite state machine (FSM) model is commonly used for specifying communicati on protocols and other types of distributed systems. With the use of multiple te sters for an FSM, the synchronization between inputs from different testers beco mes a problem. A synchronizable test sequence of an FSM is a test sequence for w hich the synchronization problem either does not exist or can be solved by commu nication between testers. In this paper, we consider two testing strategies for an FSM: port-based testing, which does not allow testers for the FSM to communic ate with each other, and group-based testing, which divides the ports of the FSM into groups and allows the testers for ports in the same group to communicate w ith each other. For each type of testing, we define a necessary and sufficient c ondition under which a test sequence of an FSM is synchronizable and show how to generate a set of testers according to a given test sequence. Also, we discuss the issues of test sequence generation and fault detection, and present the results of some empirical studies.

Supporting the real-time requirements of continuous media in open distributed processing

Abstract: The field of distributed systems is now focusing on standards for Open Distributed Processing (ODP) and entering a stage of some maturity. However, it is important that ongoing standardisation remains responsive to emerging technological demands such as the emergence of distributed multimedia computing. This new field introduces continuous media data types such as digital audio and video which have new, real-time requirements not fully supported in current ODP proposals. This paper describes the results of collaborative research at Lancaster University and CNET, France into the handling of multimedia and continuous media in an ODP environment. The aim of the paper is to present a comprehensive statement of requirements for continuous media in ODP and to aid the evolution of standards by making specific proposals on how the evolving ODP standards could be enhanced.

A formal semantics for the ODP computational model

Abstract: The ODP computational model is an essential part of the ODP Reference Model that provides an abstract, language-independent programming framework for open distributed systems. This paper presents a formal operational semantics for an essential subset of the ODP computational model. The semantics takes the form of a rewriting theory that provides a generic and abstract true-concurrency semantics for the ODP computational model.

Mosaic access to real-time data from the TOGA-TAO array of moored buoys

Abstract: The TAO Array is a major component of the global climate monitoring system, consisting of approximately 70 moored buoys, spanning the Equatorial Pacific Ocean from 95W in the eastern Pacific to 137E in the west, and telemetering atmospheric and oceanographic data in real time to shore-based computers via the NOAA Argos satellite system. The TAO array allows, for the first time, observationally coherent basin-wide fields of wind and upper ocean thermal and current structure to be constructed in real time for assimilation into numerical models and for diagnostic studies. The TAO array is supported by an international consortium, involving cooperation between the United States, France, Japan, Korea and Taiwan. Real-time data from the TOGA-TAO Array of moored buoys is being made available through a Mosaic interface designed by the TAO Project office at NOAA's Pacific Marine Environment Laboratory, in Seattle, Washington. At present, displays include latitude-time plots, longitude-time plots and latitude-longitude plots of surface variables, and are being expanded to include additional views of the four-dimensional data set from this array of buoys, including animations of the buoy data and comparisions of the buoy data with operational analyses model data from the National Meteorological Center.

A broadband ring network: multichannel optical slotted ring

Abstract: This paper presents a multichannel optical ring network for broadband integrated services. The network uses a multichannel architecture with wavelength division multiplexing (WDM) and photonic packet switching devices for its access mechanism. The electronic speed bottleneck is removed out of the ring, which allows utilization of the full bandwidth for the optical fiber transmission medium. For compatibility with B-ISDN, the network uses a similar cell structure to that of an ATM based cell with a slotted ring concept. Fiber optic delay line matched filters are used as a means of address detection at the destination nodes. The network architecture is described and a design of the optical switching node is presented. A novel technique is devised in order to measure the average transfer delay of a cell in the multichannel slotted ring network. The analytical results show an excellent agreement with simulation results over a broad range of parameters. The results show that the major part of the cell transfer delay is coming from the propagation delay from a source to a destination. It is also observed that the throughput characteristic of the network depends upon the aggregated transmission capacity of the network.

HTML to the max: a manifesto for adding SGML intelligence to the World-Wide Web

Abstract: HTML demonstrates that SGML markup is useful for networked information. How can it be made even more useful? One way is to extend the tag set from HTML to HTML2, etc. We argue here for a more radical approach: full SGML awareness in WWW. We believe the difficulties are small, the cost affordable, and the advantages overwhelming. SGML is a metalanguage for defining markup languages; HTML is just one instance of this infinite family. At present, documents in other SGML document types must be translated into HTML for display by a Mosaic client—sometimes this imposes unacceptable information loss. WWW browsers could handle other SGML document types without translation by launching a general-purpose SGML browser to view them, as they now launch graphics viewers; a better solution overall would be to build SGML display into the WWW browsers themselves. Either way, display of an SGML document would be controlled by a style sheet using a small number of display primitives (“bold”, “line break”, etc.) to specify the rendition of each element type. For “well-known” document type definitions (DTDs) like HTML, style sheets could be distributed with the browser, or built in. For other DTDs, the browser would fetch a style sheet from the server. Using style sheets, browser software can also make it easy to customize document display. DTDs and style sheets can be designed to accommodate extensions, ensuring that authors can make small extensions to the tag set with no change whatsoever in the target browsers and virtually no performance penalty.

Performance of inter-media synchronization in distributed and heterogeneous multimedia systems

Abstract: Future multimedia environments are expected to be distributed and heterogeneous. In such environments, media sites ranging from sophisticated workstations to simple media capture and display subsystems such as ISDN videophones and audiophones (together referred to as Mediaphones ) that lack the capability to run clock synchronization algorithms will be connected directly to the network. Towards such environments, we develop mechanisms and protocols for synchronous access to multimedia services over integrated networks. In the inter-media synchronization technique we present, for facilitating synchronous retrieval, a multimedia server computes at the time of recording of a multimedia object a range of relative temporal positions or stamps (called an RTS interval ) for each media unit that it receives from mediaphones. During playback, the multimedia server detects RTS mismatches between media with the help of feedback messages transmitted back by the mediaphones, and steers the mediaphones back to synchrony. We propose predictive policies for resynchronization and compare their performance for video/audio playback with other resynchronization policies, such as conservative, aggressive, and probabilistic. These performance studies reveal that predictive policies perform uniformly well at all levels of asynchrony, unlike their conservative and aggressive counterparts. Moreover, in most cases, predictive policies match probabilistic policies in effectiveness, without imposing the same computational demands as the probabilistic policies.

An access protocol to support multimedia traffic over hybrid fiber/coax systems

Abstract: Hybrid fiber/coax plant is being widely deployed to deliver broadband interactive services to the home. Many of these services require a two-way high-speed channel, for example, extending the office LAN into the home. This is a challenging problem over HFC because most of the plant has only a small fraction of the bandwidth allocated to the upstream path, it is a noisy path and it is not possible to sense the medium prior to transmitting. For these reasons a protocol tuned to the medium is necessary. The centralized priority reservation (CPR) protocol is specifically designed for HFC plant with a primary goal of keeping the protocol within the station as simple as possible, consistent with efficient use of the channel. The basic CPR protocol has been described previously. In this paper we describe the application of CPR to the transmission of multimedia type traffic and the performance that is obtained. It supports multiple levels of priority in the transmission of asynchronous data with fine control over the level of priority. It also supports synchronous streams of information. Channel capacity not required by the streams is available to the asynchronous data.

A design model for open distributed processing systems

Abstract: This paper proposes design concepts that allow the conception, understanding and development of complex technical structures for open distributed systems. The proposed concepts are related to, and partially motivated by, the present work on Open Distributed Processing (ODP). As opposed to the current ODP approach, the concepts are aimed at supporting a design trajectory with several, related abstraction levels. Simple examples are used to illustrate the proposed concepts.

WAXweb : a MOO-based collaborative hypermedia system for WWW

Abstract: This paper describes the development of a networked collaborative hypermedia system intended to support groups of writers and scholars in writing and publishing hypertext fiction and criticism. The current system supports the importation of individually-developed Storyspace hypertext documents into a MUD-based collaborative workspace for integration and expansion, and allows for the immediate publication of these dynamically-generated multimedia documents onto the World-Wide Web. In addition, we provide a forms-based writing and linking interface to the text, so that writers can write using either the MUD-based or the forms-based authoring tools. Since the HTML is generated dynamically from our underlying database, we have added the capabilities to: allow for user negotiation of content and bandwidth (e.g., only small versions of the pictures, with the audio and text in German, with movies), provide the bandwidth-intensive media from distributed mirror sites. This system is being used by dozens of students, writers, and theorists around the world to support such projects as: hypertext writing and theory classes at Brown and Vassar, a hypermedia version of David Blair's feature-length film, WAX or the Discovery of Television among the Bees, a collaborative women's hypertext fiction writing group, and the creation of an electronic journal to discuss the impact of technology on writing practice.

Sun Web: user interface design for Sun Microsystem's internal web

Abstract: This paper presents the methods used to design the user interface and overall structure of the internal Web pages for Sun Microsystems. Sun has an extensive set of information available on the WWW with our home page as the access point, but we also wanted to provide employees access to internal information that could not be made available to the Internet at large. Much of the user interface work was done in a few weeks with four usability studies completed in a single week: card sorting to discover the users' view of the information space, an icon intuitiveness study, and two tests of the actual interface. The conclusions from this project are that a uniform user interface structure can make a Web significantly easier to use and that “discount usability engineering” can be employed to base the design on user studies even when project schedules are very tight.

Specification of multiparty audio and video interaction based on the reference model of Open Distributed Processing

Abstract: The Reference Model of Open Distributed Processing (RM-ODP) is an emerging ISO/ITU-T standard. It provides a framework of abstractions based on viewpoints, and it defines five viewpoint languages to model open distributed systems. This paper uses the viewpoint languages to specify multiparty audio/video exchange in distributed systems. To the designers of distributed systems, it shows how the concepts and rules of RM-ODP can be applied. The ODP ?binding object? is an important concept to model continuous data flows in distributed systems. We take this concept as a basis for multiparty audio and video flow exchanges, and we provide five ODP viewpoint specifications, each emphasising a particular concern. To ensure overall correctness, special attention is paid to the mapping between the ODP viewpoint specifications.

On communicating sequential processes

Abstract: A protocol for communicating sequential processes under different service conditions has been suggested. A timing mechanism has been incorporated to support synchronous and real time applications. Network performance has been evaluated in such a case by computer simulation. Tradeoff between throughput and average packet delay at a node has been found out at different packet generation rates. Commitment control has been incorporated in the protocol using an immediate acknowledgement scheme and the system performance has been observed.

Towards a basic reference model of open distributed processing

Abstract: The Basic Reference Model of ODP introduces a number of basic concepts in order to provide a common basis for the development of a coherent set of standards. To achieve this objective, a clear understanding of the basic concepts is one prerequisite. This paper makes an effort at clarifying some of the basic concepts independently of standardized or non-standardized formal description techniques. Among the basic concepts considered here are: agent, action, interaction, interaction point, architecture, behaviour, system, composition, refinement, and abstraction. In a case study, it is then shown how these basic concepts can be represented in a formal specification written in temporal logic.

Creating a web analysis and visualization environment

Abstract: Due to the rapid growth of the World-Wide Web, resource discovery has become an increasing problem. As an answer to the demand for information management, a third generation of World-Wide Web tools will evolve: Information gathering and processing agents. This paper describes WAVE (Web Analysis and Visualization Environment), a 3D interface for World-Wide Web information visualization and browsing. It uses the mathematical theory of concept analysis to conceptually cluster objects. So-called “conceptual scales” for attributes, such as location, title, keywords, topic, size, or modification time, provide a formal mechanism that automatically classifies and categorizes documents, creating a conceptual information space. A visualization shell serves as an ergonomically sound user interface for exploring this information space.

A flexible traffic shaper for high speed networks: design and comparative study with Leaky Bucket

Abstract: Maximizing bandwidth utilization and providing performance guarantees, in the context of multimedia networking, are two incompatible goals. Heterogeneity of the multimedia sources calls for effective traffic control schemes to satisfy their diverse Quality of Service (QoS) requirements. These include admission control at connection set up, traffic control at the source ends and efficient scheduling schemes at the switches. The emphasis in this paper is on traffic control at the source end. Most multimedia sources are bursty in nature. Traffic shapers have been mainly studied hitherto from the point of view of their effectiveness in smoothing the burstiness. The Leaky Bucket (LB) scheme, to cite an example, is a mean rate policer smoothing at the token generation rate. Studies on bursty sources show that burstiness promotes statistical multiplexing at the cost of possible congestion. Smoothing, on the other hand, helps in providing guarantees at the cost of utilization. Thus need for a flexible scheme which can provide a reasonable compromise between utilization and performance is imminent. Recent studies have also questioned the suitability of LB for policing real-time traffic due to the excessive delays. We argue for a policy which is less stringent on short term burstiness than the LB. We propose a new traffic shaper which can adjust the burstiness of the input traffic to obtain reasonable bandwidth utilization while maintaining statistical service guarantees. The performance study is conducted in two parts. In the first part, we study the effect of varying the shaper parameters on the input characteristics. In the second part, we dimension our scheme and a LB equivalently and compare the mean and peak rate policing behavior with delay and loss as the performance parameters. Adopting a less stringent attitude towards short term burstiness is shown to result in considerable advantage while policing real-time traffic. Future research possibilities in this topic are explored.

Optimizing a generalized polling protocol for resource finding over a multiple access channel

Abstract: We consider a generalized polling protocol to locate (or find) resources known only by name in a distributed system interconnected by a bus-based multiple access channel. The set of nodes in the system are partitioned into a set of disjoint groups. To locate a resource, the searching node sequentially polls each one of the groups until the resource is found. This scheme is a generalization of both pure polling and broadcast. Our basic aim is to show how to obtain an optimal division of the nodes into groups. To that end, a typical generalized polling protocol is analyzed and an efficient algorithm is given that provides a group division minimizing the expected cost per resource location operation.