in a telecommunications system such as a global mobile telephone network in which each subscriber unit includes a subscriber identity module , each sim card has fixed memory locations , to which data can be addressed over the air . some of the locations can not be overwritten from the subscriber unit but can be accessed therefrom on the entry of short simple codes , each associated with one of the locations . further fixed memory locations can be read over the air only when the subscriber enters a personal identification number . locking control files are used to control read / write access to the locations respectively .