According to LTE and Beyond, an IMSI is a unique International Mobile Subscriber Identity that is given to each user in a GSM, EPS or UMTS system, while the TMSI is the Temporary Mobile Subscriber Identity given to visiting mobile subscribers. A TMSI is typically used instead of an IMSI to protect a user's identity and to make it harder to eavesdrop through radio interfacing.
According to Princeton University, an IMSI, is associated with all GSM and UMTS network mobile phone users. This ID is left in the phone inside the SIM card and is sent to the network by the SIM. The Home Location Register stores information about the phone, and the IMSI can be used to access that information's details. An IMSI is rarely sent because this protects users from eavesdropping and tracking on the radio interface.
Instead of using an IMSI, a TMSI may be used to protect a person's identity. A TMSI consists of 4 octets, according to LTE and Beyond. A TMSI is stored in a SIM, and it is normally used in paging situations. The TMSI is randomly generated and can be sent in place of the IMSI, protecting the user. Additionally, the IMSI can be used in any mobile network that can be connected with other networks. It is typically a 15-digit ID, but it may be shorter in some cases.