n / 38 3.4.3 Failure and Recovery / 38 3.4.4 Error Control / 39 3.4.5 Sequencing / 39 Bisync Protocol / 39 HDLC Protocol / 41 SDLC Overview / 41 System Network Architecture (SNA) / 42 Polling Explained / 43 Transmission Codes / 43 Code and Speed Converters / 44 Treatment of Errors / 45 3.12.1 Redundancy Checks / 47 3.12.2 Error-Correcting Codes / 47 3.12.3 Loop Check / 48 3.12.4 Effect of Errors / 48 Front-End Processors / 49 3.13.1 Selection and Evaluation / 49 Terminal Technology / 51 CRT Display Terminals / 51 4.1.1 Control Unit / 52 4.1.2 Keyboard Structure / 53 4.1.3 Display Schemes / 54 4.1.4 Terminal Considerations / 54 Printing Technologies / 56 Graphic Terminals / 62 Facsimile Technology / 63 4.4.1 Stored Switching System / 66 4.4.2 Image Resolution / 66 4.5 Integrated Workstations / 67 CONTENTS xi Network Management / 68 5.1 Network Control Design Considerations / 68 Elementary Monitoring / 69 Redundancy and Compatability / 70 Network Testing / 71 Centralized Troubleshooting / 71 Network Control Center / 73 5.6.1 Alarms / 73 5.6.2 Automatic Trunk Testing / 74 5.6.3 Maintenance and Management Reports / 74 Specialized Test Equipment / 75 5.7.1 Line Analyzers / 76 5.7.2 Breakout Boxes / 77 5.7.3 Error Rate Testers / 78 5.7.4 Data Monitors / 79 5.8 Network Security / 79 Network Structures / 82 Topology Tradeoffs / 82 6.1.1 Front-End and Back-End Networks / 84 6.2 Local Area Networks / 86 6.2.1 The Ethernet Way / 87 6.2.2 LAN Cable Medium Choices / 89 Distributed Communications / 91 Message Switching Networks / 93 Integrated Voice and Data / 95 Shared Resources / 96 Packet Switching Networks / 98 6.7.1 The History of X.25 / 99 6.7.2 How They Work / 100 6.7.3 Packet Switching in Operation / 102 Network Design Considerations / 103 Satellite and Carrier Services / 107 Satellite Overview / 107 7.1.1 Overcoming Attenuation / 109 7.1.2 Satellite Communications Structure / 110 CONTENTS 7.2 Selecting a Transmission Method / 111 7.2.1 Private Alternatives / 113 7.2.2 Measured Use Services / 114 7.3 Facsimile Services