Large network operators are increasingly transferring their own service areas to specialized electrical companies. The book on telecommunications has therefore been structured so that electrical companies can install or support customer connections of current telecommunications technology in a model manner at any time. The technical book covers the entire classic telecommunications technology that is installed, maintained, and repaired by the specialized trades at customer sites. Special attention is given to the connection technology of analog and digital telecommunications. The functionality of telephone systems is discussed, along with their planning, parameterization, and documentation. In addition to analog connection techniques, the digital ISDN connection is described in detail and explained based on an electrical engineering education. Modern customer connections such as DSL are also included.