Explains the concepts and ideas assciated with physical level protocols and provides a summary of the industry's most widely used physical level interface standards. This second edition includes new information that updates the material that has changed since the publication of the first edition. The revised text features a large portion devoted to the ITU-T V series recommendations, digital interfaces, fractional T1 and SONET, and the digital data system. The book's primary focus is on the physical interfaces between a user device, such as a computer or a terminal and a communication line interface device, like a modem, data service unit, or multiplexer. In addition, the book discusses updated information on private-wired systems, serial communications, and new EIA standards. The principal changes from the first edition include information on the V series modems, V.32 bis explanations, and an additional section on next generation modems detailing V.34.