The development of low-cost digital integrated circuits has brought digital switching from a concept to an economic reality. Digital switching systems have now found worldwide acceptance and there are very few new switch­ ing systems being considered either for design or application which are not digital. Digital technology has created new opportunities for innovation including the integration of digital transmission and switching, the combi­ nation of voice and data services in one switching entity, and the design of switching systems which are economical over a broad range of sizes. In the strict sense, the term "digital switching" refers to a system which establishes a message channel between two terminations where infor­ mation is represented in digital form. In more common usage, a digital switch usually contains a time-divided network composed of logic gates and digital memory to accomplish the switching function. The intent of this book is to provide an introductory level explanation of the principles of digital switching. These principles apply to both public and PABX switching. The book is aimed at those who apply, design, main­ tain, or simply wish to understand digital switching techniques. An electri­ cal engineering degree is definitely not required for comprehension. We have concentrated on explaining digital switching techniques without the use of detailed mathematics. However, each chapter contains a comprehen­ sive list of references which will lead the reader to sources for a more in-depth study of the many subjects covered.