Modular mobile communication system

The invention relates to a modular system for personal data acquisition and communication, advantageously a cellular telephone system the functions of which can be increased by means of various expansion cards. A host device in the system according to the invention, such as a mobile communication device, need not be equipped with all possible functions in the manufacturing stage but some of the functions can be realized on expansion cards, and the user can add a desired function to the host device according to need. The expansion cards are small and they are installed substantially inside the host device, such as a mobile communication device, so that the entity formed by the host device and the expansion card seems to the user as a single compact device. Advantageously, the expansion cards include, in addition to the hardware needed, also the software required for controlling the operation of the modules.