the invention relates to modems designed to allow transmission of data between a terminal and a network . to improve the operating versatility of a modem or its utilization by a smartcard , the modem is equipped according to the invention with a receptacle for receiving a smartcard , with a connector for this card and a card interface circuit , and the microcontroller of the modem also controls the card interface circuit . this microcontroller thus constitutes the kernel of the modem and at the same time the kernel of a card reader . the communication with the card is effected by commands known as ‘ at ’ and the systematic prefix sequence ‘ at ’ is followed by a second systematic prefix sequence if a communication operation between the terminal and the card is to be effected . the second prefix sequence is ‘+ g ’.