to be able to derive formulas in order to apply them just as, to quote Professor Perry, "He should not have to be able to design a watch in order to tell time ... The material for this book has been drawn from my own experience in the field. Inevitably, however, I have used CCITT and Bell System publications for references and in some cases quotation, and I gratefully acknowledge permission for their use. I am also grateful to Stromberg Carlson Corporation for their earlier encour­ agement and support without which this book would not have been possible. Thanks are also due to Fred Hadfield for his advice and assistance in the preparation of the many figures and to my wife Ada for her support and patience as I pursued the demanding but interesting task of producing the text.