Outlines the basics of signal transmission techniques and temporal combining of digital streams in digital transmission systems. The principles of construction of equipment of terminal stations and the line path are considered. The main characteristics and diagrams of the construction of IKM-15 transmission systems, "Zone-15," IKM-30 IKM-120, IKM-480 are given. The basics of their design and operation are outlined. This textbook is the first specialized publication on the subject "Digital Transmission Systems."