@book { TarifstrukturenderFestnetzanbieter,
    title = "Tarifstrukturen der Festnetzanbieter",
    author = "Sven {Janssen}",
    year = "1999",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832416492",
    url = "https://m.diplom.de/document/217526"
}