@book { DerdeutscheMobilfunkmarkt,
    title = "Der deutsche Mobilfunkmarkt",
    author = "Pascal {Gries}",
    year = "2008",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783836619813",
    url = "https://m.diplom.de/document/226176"
}