Subscriber Identity Module (SIM)

SIM is a microchip
Except for emergency calls, a GSM mobile phone cannot be used without the SIM.
In GSM terminology, the term MS refers to the combination of a SIM and an ME
The SIM stores three types of subscriber related information:
Fixed data stored before the subscription is sold: e.g. IMSI, authentication key and security algorithms
Temporary network data: e.g. the location area of the subscriber and forbidden PLMNs
Service data: e.g. language preference etc..,
A SIM contains information for GSM network operations.
This information can be related to the mobile subscriber, GSM services or PLMN.
The data storage requirements of a SIM are divided into two categories: mandatory and optional.
The major task of a SIM is to store data.
The SIM has an area of non-volatile memory which is used to store information specific to a particular subscriber and this includes the subscriber’s unique international mobile subscriber identity (IMSI) number.
This number is used to identify each individual subscriber within the GSM network
The SIM will also contain the subscriber’s secret authentication key, Ki, the authentication algorithm, A3, and the cipher key generation algorithm, A8.
The language preference indicator is also located in the SIM and this is used to indicate the language to be used on the MS screen.
The items described above are mandatory and must be present in any SIM that conforms to the GSM specifications.
The SIM may also contain a number of optional items which will include the subscriber’s abbreviated dialling numbers.
The SIM may also contain a list of the last number(s) that the subscriber has dialled and an area of storage for the subscriber’s short messages.
Inserting an SIM card into an ME effectively personalises the equipment to the particular subscriber.