Modern computer networks now circle the world, but the transmission of information between them depends on the many different protocols that define the behaviour of the sender and receiver. It is clear therefore, that the accurate description of these protocols is important if harmonious communication is to be maintained. In this book the authors use the formal specification language PSF to provide an unambiguous description of several communication protocols of varying levels of complexity, ranging from the alternating bit protocol to the token ring protocol. Beginners, as well as professionals in the field of communication protocols, will benefit from both the methods of specification described and the protocols discussed in this book.