The MS (Mobile Station) is the final and the main entity in a GSM network for which the connection is being set-up.
An MS is made up hardwares , softwares (e.g. Android OS) and the Subscriber Identity Module (SIM). It is an intelligent plastic card with a microcontroller that constitutes an inherent part of a mobile terminal. The SIM card stores all user-specific data that is relevant to the GSM network.
- Each MS has its own IMEI number to uniquely identify a device. However the SIM is used to personalize his/her own MS as per the desire of user. (e.g. recharging/adding top-up plans)
- The SIM card contains a microcontroller with ROM, RAM and an NVM.
- The ROM (4 to 6 kB) stores the programs implementing the A3 and A8 encryption algorithms.
- The A3 algorithm is used in the user authentication process while the A8 algorithm is used for derivation of the key for encryption of the transmitted data.
- The RAM is relatively small (around 256 bytes). The NVM contains user’s parameters like International Mobile Subscriber Identity (IMSI), Location Area Identifier (LAI), PIN associated to SIM, user’s authentication key Ki etc.
- The SIM card communicates with the phone in the asynchronous serial mode in half-duplex transmission at the data rate equal to 3.2 kbps in each direction.
- The SIM card is portable i.e. it can be moved from phone to phone and still function in the same manner.