IMSI & TMSI

International Mobile Subscriber Identity ( IMSI):

Each registered user is uniquely identified by its international mobile subscriber identity (IMSI). It is stored in the subscriber identity module (SIM). A mobile station can only be operated if a SIM with a valid IMSI is inserted into equipment with a valid IMEI.

There are following parts of an IMSI:

·         Mobile Country Code (MCC): 3 decimal places, internationally standardized.

·         Mobile Network Code (MNC): 2 decimal places, for unique identification of mobile network within the country.

·         Mobile Subscriber Identification Number (MSIN): Maximum 10 decimal places, identification number of the subscriber in the home mobile network.

Temporary Mobile Subscriber Identity (TMSI):

The VLR, which is responsible for the current location of a subscriber, can assign a temporary mobile subscriber identity (TMSI) which has only local significance in the area handled by the VLR. It is stored on the network side only in the VLR and is not passed to the HLR.

Together with the current location area, TMSI allows a subscriber to be identified uniquely and it can consist of up to 4x8 bits.