@book { VoiceoverIPimMobilfunk,
    title = "Voice over IP im Mobilfunk",
    author = "Mario {Schulz}",
    year = "2008",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783836622882",
    url = "https://m.diplom.de/document/226377"
}