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ZEEB ROAD, ANN ARBOR, Ml 48106 18 BEDFORD ROW, LONDON WC1R 4EJ, ENGLAND 8006353 Shoch , John F. DESIGN AND PERFORMANCE OF LOCAL COMPUTER NETWORKS Stanford University Ph.D. 1979 University Microfilms International 300 N. Zeeb Road, Ann Arbor, MI 48106 18 Bedford Row, London WC1R 4EJ, England Copyright 1979 by Shoch, John F. All Rights Reserved PLEASE NOTE: In all cases this material has been filmed 1n the best possible way from the available copy. Problems encountered with this document have been Identified here with a check mark ^ . 1. Glossy photographs 2. Colored Illustrations 3. Photographs with dark background \/ 4. Illustrations are poor copy • 5. Print shows through as there 1s text on both sides of page 6. Indistinct, broken or small print on several pages _________ throughout 7. Tightly bound copy with print lost in spine 8. Computer printout pages with indistinct print 9. Page(s) lacking when material received, and not available from school or author _______ 10. 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(Electrical Engineering) I certify that I have read this thesis and that in my opinion it is fully adequate, in scope and quality, as a dissertation for the degree o f Doctor o f Philosophy. Approved for the University Committee on Graduate Studies: 0 / _ Dean o f Graduate Studies Abstract As computing systems continue to increase in performance and decrease in price, and as we witness the emergence of yet more powerful personal computers, it becomes even more important to provide high performance communications systems to interconnect these resources. The work reported here represents an investigation into the emerging technology of local computer networks -- in general, these are communications systems designed to tie together some number of computers and terminals within a distance of roughly one kilometer, with a data rate on the order of 106 bits per second. Local networks are typically used to support a wide range of applications, including terminal access, file transfer, and more general forms of multi-machine processing. This investigation has been broken down into two major components: a comparison of alternative architectures for local networks, and a systematic effort to measure the performance characteristics of one such network. The discussion of local networks begins with the identification of four major dimensions to the design space: the nature of the underlying physical media, physical connectivity, logical connectivity, and control disciplines used for accessing shared resources. Using that taxonomy, and a set of criteria for evaluating specific local networks, we examine several dozen alternative designs. Within the whole range of the design space, however, most networks fall into one of five major families: —Partially connected systems using store-and-forward techniques. —Star networks or hierarchical configurations. —Rings and loops. -Radio-based approaches using packet broadcasting. —Multiaccess bus structures. From that evaluation and analysis, one of the most attractive designs is the use of a shared passive bus, run with a distributed control procedure such as carrier sense multiple access with collision detection (CSMA/CD). This approach first emerged in the design of the Ethernet communications network ("Ethernet"); the second half of the investigation consists of a systematic effort to assess the performance of an Ethernet local network. The subject of these experiments was one of the largest local nets in use, supporting over 120 computers, or hosts. This system was first measured under normal load, examining various performance characteristics: error rates, iv reliability, traffic volume, utilization, packet lengths, traffic patterns, inter-packet arrival times, overhead, and more. Performance was then measured under conditions of high load and overload, using a special set of tests programs to generate artificially high levels of traffic, while controlling as many as 90 hosts at one time. From these experiments we have been able to conclude that this approach to local networking performs quite well. Under normal load there are very few errors, low delay, and the network easily supports the offered traffic. Under extremely high load we find that the network utilization remains high (over 97%) and the system does not become unstable. Based upon these results, the final portion of the work examines some future opportunities for using Ethemet-style local networks: carrying packet voice traffic, and providing access to Ethernet systems from standardized interfaces (such as the CCITT recommendation X.25). v Acknowledgements The completion of this particular piece of work provides a welcome opportunity to acknowledge some long-standing personal and intellectual debts. It was Bob Metcalfe who first introduced me to the field of computer networks, and in the years since then we have worked together on many aspects of these problems. This work reflects, to a great extent, the result of his insight, criticism, and encouragement In addition, Gio Wiederhold and Forest Baskett have made many valuable contributions to the discussion which follows, and helped tremendously in keeping things moving forward. Jon Hupp was of immeasurable help in designing and carrying out many of the measurements which are reported in the later chapters. Yet a systems effort of this sort involves the help of many people -- we prospered because of continual interaction with and support from the whole "networking crew" at Xerox in Palo Alto, including David Boggs, Ed Taft, Larry Stewart, Yogen Dalai, and Hal Murray. The Xerox Palo Alto Research Center (Parc) provided probably the only environment of its kind where one could carry out this kind of investigation. It was Alan Kay, however, who first kindled my interest in Computer Science by demonstrating that it was not only fun to write programs, but that this was also a remarkably V rich area for sustained intellectual inquiry. Since that time, many different people have, helped to feed that flame -- in encounters they may have long forgotten. I only hope that these many uncounted friends in the Learning Research Group, at Parc, and at Stanford' University will recognize the ways in which their individual contributions are reflected in what follows. And there is a very special set of friends who, through the years, have done so much to help establish a convivial environment in which to work, or who have provided some of the needed moral support: Bert Sutherland and Bob Taylor at Parc; and also David E. Shoch, Gertrude W. Shoch, and Lori A. Goetz, who have been through it all. To everyone, my thanks. John F. Shoch Stanford, California August, 1979 Table of Contents Abstract iv Acknowledgements vi Table of Contents vii List of Figures xi Part I: Design of local networks . 1 1. Introduction 2 2. Alternative architectures for local networks 6 2.1. Introduction 6 2.2. Dimensions of the design space 6 2.2.1. Point-to-point vs. broadcast channels 7 2.2.2. Physical connectivity, or topology 7 2.2.3.

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