The subject of computer communications is changing very rapidly. Improvements in terminal access, aligned with the development of timesharing, has brought hands-on experience to a large number of non specialist users. Computer networks have made available vast computing resources and data banks to these users. This book is for anyone familiar with using computers who wishes to understand the techniques used in computer communications. It is also an introduction to the architecture of present day computer communication systems. I would like to thank Roland lbbett, Steve Treadwell, Peter Kirstein and Del Thomas for their invaluable advice and encouragement. My thanks also to Malcolm Stewart and the staff at Macmillan. The late Gareth Pugh encouraged my interest in computer communications and provided the opportunity to develop the material for this book. The text was formatted on a UNIX computer system: I am grateful to Professor Kirstein for permission to use this system. I am indebted to NEC Telecommunications Europe for the use of a spinwriter printer on which the master copy was produced. Finally, no amount of words can express my debt to Jo this project and Rosemary for patiently bearing with over the last three years.