@book { GSMGlobalSystemforMobileCommunications,
    title = "GSM - Global System for Mobile Communications",
    author = "Elisabeth {Petz}",
    year = "2002",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832456078",
    url = "https://m.diplom.de/document/221090"
}