International Mobile Subscriber Identity or with abbreviated version IMSI is used to globally identify a mobile subscriber. When a user purchases a SIM or a USIM and subscribes to a mobile network, a built-in unique number is registered in the network database and becomes effective.
The IMSI is permanently stored in the USIM and subscription information in network user databases such as HLR or HSS. As well as in LTE network the IMSI can be temporary stored in MME, SGW and PGW during initial attach procedure, after termination the connection the IMSI will be removed on the mentioned network elements.
The IMSI can have a maximum length of 15-digits.
IMSI is a combination of PLMN and Mobile Subscriber Identification Number (MSIN). The MSIN identifies a mobile subscriber within a PLMN.
In the case of analog cellular network, the MSIN was used to route the call. In next generation cellular systems, temporary numbers are assigned to the handset instead of IMSI, to avoid any security issues. This temporary number is TMSI and it is frequently changed.