Imperial College of Science and Technology, University of London, Department of Computing. HIGH EFFICIENCY, CHARACTER-ORIENTED, LOCAL AREA NETWORKS by Martin Cripps This thesis is submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy and the Diploma of Imperial College of Science and Technology, January 1988. For Clare Attempt the end His reasons are as two grains of wheat but never stand to doubt hid in two bushels of chaff. nothing's so hard You shall seek all day ere you find them but search and when you have found them will find it out they are not worth the search. Robert Herrick (1591-1674) William Shakespeare (1564-1616) 1 ABSTRACT This thesis explores the problem of interconnecting character-oriented devices over local area networks by investigating significant aspects of hardware, software, protocol and operational factors. It proposes effective and efficient solutions which were tested during a full-scale experiment The results of that experiment demonstrate convenient, cost-effective and reliable operation. The novelty of this investigation arises from its character-oriented approach. Much work has been carried out by others on local area networks which transfer blocks of data efficiently, however, a large majority of installed devices operate on a character-by-character basis and will continue so to do for some considerable time. This study is approached through analysis of the low efficiency of international standard networks for this class of device which defines the scope of this work. An original analysis of the potential mechanisms which can be used to give high efficiency and low delay for this class of transfer is then derived. This leads to the proposal of an entirely new structure for interconnection with its associated protocols. A unidirectional, baseband bus is developed and shown to have a unique combination of highly desirable properties which are not available together in any ring, mesh or simple-bus structure. These properties are: simple cabling, unbroken network media, densely packed data on the media, low delay, fault location in normal operation and reconfiguration in the event of partial failure. The protocols which are proposed are specifically aimed at small-data transfer and it is shown how, by using contention-free access, they approach optimal efficiency while maintaining deterministic behaviour. The logical and electrical characteristics of the new bus structure (Synchronet) and its access mechanisms are presented and their performance demonstrated. The Synchronet structure also has more general capabilities, such as for large block transfers, and could be the subject of further work. A full-scale experiment (Sonet), set up to demonstrate the overall viability of the proposals, is discussed with its hardware and software aspects. Specific modifications for a Synchronet to operate at extremes of speed and efficiency are also shown. The experimental results are described including the novel fault detection and fault tolerant aspects of the system. The value of Sonet was illustrated during the four years (1983-87) that it formed the major network in DoC at ICST. The general principles remain valid, however, as with all scalable technologies, improvements in technology are directly mirrored in improvements in the overall network. The use of the new technology and protocols proposed here provides for efficient small to medium sized, character- oriented networks. It is shown how they can be used to provide distributed multiplexer feeders to existing standard networks with simple, single-path wiring giving a cost-effective and convenient way of connecting dispersed, character-oriented devices. 2 ACKNOWLEDGEMENTS As I have been working in this area for, to use the Registry's phrase, some considerable time, there are many more people I wish to thank for their advice, assistance and kindness than I can possibly list here, but they all have my continuing gratitude. I particularly wish to thank my longtime friend and former supervisor Peter Throsby for all his guidance and help. I also wish to thank my colleagues at Imperial College during this time: John Benbow and David Willis of the Centre for Computing and Automation; Doug Barnes, Ed Davies, Don Monro, Mike Reeve, Chris Sowden and Ian Stinson of the Department of Computing; and Barry Belcher of the Department of Aeronautics for their specific contributions to this work and to my sanity. I would like to record my gratitude to the Wolfson Foundation for the generous grant which established the Wolfson Microprocessor Research Unit at Imperial College. Some of the work described in this thesis was greatly facilitated by the equipment available in the Unit, and I doubt that it would have been undertaken at all if the Wolfson Unit had not existed. I would like to thank Professor Bruce Sayers for his supervision, support and for giving me the incentive to submit this thesis. Finally I would like to thank the External Examiners for their valuable suggestions for improvements which have been incorporated into this thesis. CONTENTS Abstract 1 Acknowledgements 2 Contents 3 1.0 INTRODUCTION 8 1.1 Locality Of A Network 9 1.2 Shared Access To A Network 10 1.3 Geographical Coverage Of A Network 10 1.4 Network Topology 12 1.5 Network Data Rates 13 1.6 Allocation Of Network Bandwidth 14 1.7 Operation Of Network Algorithms 15 1.8 Multiplexing as a Network Mechanism 19 1.9 Network Servers and Functions 20 1.10 Operating System Support, History, Software & Protocols 21 1.11 ISO Protocols 23 1.12 Interconnection of Networks 25 1.13 Reliability and Fault-Tolerant Aspects of Networks 26 1.14 Security Aspects of Networks 27 1.15 Cabling for Networks 28 2.0 EFFICIENCY OF NETWORKS 30 2.1 Efficiency of CSMA/CD (Ethernet) 32 2.2 Efficiency of Token-Passing Ring 35 2.3 Efficiency of Cambridge Ring 37 2.4 Efficiency of Mesh Networks 38 2.5 Efficiency of Multiplexer-based Solutions 39 2.6 Summary of Network Efficiencies 40 3.0 IMPROVING EFFICIENCY AND MINIMISING DELAY 41 3.1 Unit of Transfer 43 3.2 Synchronisation of Transfer 44 3.3 Minimising the Overhead of Routing 45 3.4 Minimising the Overhead of Control 45 3.5 Minimising Redundancy for Error Control 46 3.6 Summary of Methods for High Efficiency and Low Delay 47 4.0 THE PROPOSED SOLUTION: SYNCHRONET 48 4.1 Synchronet Topology and Data Flows 48 4.2 Synchronet Dynamic Bus Timing 51 4.3 Synchronet Connection Unit 51 4 4.4 Synchronet Frame Formats 52 4.5 Link Management 53 4.6 Lazy Scanning Mechanism 53 4.7 Connection Forcing Mechanism 55 4.8 A Uniform Synchronet 55 4.9 Implementing a Synchronet 56 4.10 Additional Features of Synchronet 57 4.11 Summary of the Proposed System 58 5.0 THE SONET EXPERIMENT AND HISTORY 59 5.1 Sonet Topology and Data Flows 60 5.2 Sonet Servers and Central Facilities 63 5.3 Interfaces to the Network Control Units 64 5.4 Flow Control and Break Conditions 65 5.5 Establishment and Control of Virtual Channels 67 6.0 SYNCHRONET AND SONET HARDWARE DESIGN 70 6.1 Media and Driver Design 70 6.2 The Sonet Line Driver 72 6.3 The Synchronet and Sonet Receiver Designs 73 6.4 Sonet Timing and Data-transfer Circuits 73 6.5 Sonet Network Server Unit's Circuits 76 7.0 THE SONET SYSTEM SOFTWARE 79 7.1 Network Server Unit Software 79 7.2 Network Connection Unit Software 82 7.3 Communication Between NSU and NCUs 85 7.4 Self Testing Facilities in Sonet 87 7.5 Network Protocol Generation 89 8.0 EVALUATION OF SYNCHRONET AND SONET 90 8.1 Efficiency of Synchronet 90 8.2 Efficiency of the Experimental Sonet 91 8.3 Cabling for Sonet 93 8.4 Interconnection with Synchronet and Sonet 94 8.5 Security Aspects of the Experimental Sonet 94 8.6 Fault Monitoring and Finding in Synchronet 95 8.7 Fault Monitoring and Finding in Sonet 96 9.0 APPLICATIONS TO EXISTING NETWORKS 98 10.0 CONCLUSIONS 100 5 11.0 CITED REFERENCES AND BIBLIOGRAPHY 102 APPENDIX A Symbols and Abbreviations 107 B Protocol Generation ROM Code 108 C Original NCU Primary Self Test 109 D NCU Store Arrangement 111 E NSU Store Arrangement 114 F Sample Test Procedure for an NTU 115 LIST OF FIGURES Figure 1.1 Categorisation of Networks by Area Served. Figure 1.2 Categorisation of Networks by Topology. Figure 1.3 Total Network Data Rates Figure 1.4 User Interface Data Rates Figure 1.5 Hubnet topology Figure 1.6 Extending a Star Network using Multiplexers Figure 1.7 Comparison of KERMTT and NET File Transfer Regimes Figure 1.8 Layers in the ISO Standard Model Figure 1.9 Typical OSI Packet Showing Peer Headers for MAP Figure 1.10 Repair of Ring Using Duplicate Path Figure 2.1 Ethernet Frame Format Figure 2.2 Ethernet Efficiency (for 2, 5 and 64 competing stations) Figure 2.3 Token-passing Ring Protocol Efficiency Figure 2.4 Token-passing Ring Frame Format Figure 2.5 Token-passing Ring Efficiency (for 2,5 and 64 competing stations) Figure 2.6 Cambridge Ring Frame Format Figure 2.7 Typical Mesh Frame Format (eg X-25) Figure 2.8 Summary of Typical Efficiencies Showing Area of Interest Figure 3.1 Addressing Mechanisms for Networks Figure 3.2 Network Characteristics by Unit of Transfer Figure 3.3 Path Access Methods Figure 4.1 Synchronet Unidirectional Bus Figure 4.2 Synchronet Data Flows Figure 4.3 Synchronet Data Transmission in Slots Figure 4.4 Synchronet Frame Format Figure 4.5 Partial Scan - Action Table Figure 4.6 Sample Commands for Uniform Synchronet Figure 4.7 Synchronet Station Hardware Plate 5.1 Sonet NCU and NTU and Main Cable Plate 5.2 Sonet Network Connection Unit Construction Figure 5.1 Overall Design of Sonet Figure 5.2