The motivation for this book stems from an early exposure to the book Ap plied Mechanics by John Perry. Professor Perry strove to encourage his readers to understand the applications and use of mathematics in engineering with out insisting that they become immersed in pure mathematics. The following text uses this approach to the application of telecommunications switching. Readers wishing to study the derivation and proof of formulas will be able to do so using relevant references. The existence of low-cost programmable calculators frees practicing engineers from much laborious calculation, allowing more time for creative design and application of the art. The reader should not need 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.