The usage of data communications and computer networks are ever in creasing. It is one of the few technological areas which brings benefits to most of the countries and the peoples of the world. Without it many industries could not exist. It is the objective of this book to discuss data communications in a readable form that students and professionals all over the world can understand. As much as possible the text uses dia grams to illustrate key points. Most currently available data communications books take their view point from either a computer scientists top-down approach or from an electronic engineers bottom-up approach. This book takes a practical ap proach and supports it with a theoretical background to create a textbook which can be used by electronic engineers, computer engineers, computer scientists and industry professionals. It discusses most of the current and future key data communications technologies, including: Data Communications Standards and Models; Local Area Networks (Ethernet, Token Ring and FDDI); Transmission Control ProtocollInternet Protocol (TCPIIP); High-level Data Link Control (HDLC); X.25 Packet-switching; Asynchronous Communications (RS-232) and Modems; Pulse Coded Modulation (PCM); Integrated Digital Services Network (ISDN); Asynchronous Transfer Mode (ATM); Error Control; X-Windows. The chapters are ordered in a possible structure for the presentation of the material and have not been sectioned into data communications areas.