(57) Abstract ISDN modem is programmed has in order to operate either of plural ISDN protocol due to one, beforehand, in addition the setup program (200). Plural protocol (201,204,207), one where the connection for the aforementioned switch succeeds is found until, or until everything fails, it is tested automatically one each concerning the modem which is connected to the ISDN switch. When connection succeeds, when succeeding in connection, the protocol which is used is selected to the use after the connecting. When all protocol which is programmed beforehand fails, input device (211) can be offered by the user, can input into one for trial substitution protocol selection because of this.
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