@book { VoIPVoiceoverInternetProtocol,
    title = "VoIP - Voice over Internet Protocol",
    author = "Christian {Öhler}",
    year = "2004",
    publisher = "GRIN Verlag",
    address = "München, Germany",
    doi = "10.3239/9783638254755",
    url = "https://www.hausarbeiten.de/document/22018"
}