The international mobile subscriber identity (IMSI; /ˈɪmziː/ IM-zee) is a number that uniquely identifies every user of a cellular network. It is stored as a 64-bit field and is sent by the mobile device to the network. It is also used for acquiring other details of the mobile device in the home location register (HLR) or as locally copied in the visitor location register. To prevent eavesdroppers from identifying and tracking the subscriber on the radio interface, the IMSI is sent as rarely as possible and a randomly-generated TMSI is sent instead. Mobile phone identities and data are sometimes scooped up by equipment called an IMSI-catcher or Stingray phone tracker that mimics cellular networks, creating serious privacy and other human rights concerns.
The IMSI is used in any mobile network that interconnects with other networks. For GSM, UMTS and LTE networks, this number was provisioned in the SIM card and for cdmaOne and CDMA2000 networks, in the phone directly or in the R-UIM card (the CDMA equivalent of the SIM card). Both cards have been superseded by the UICC.
