TMSIs

Temporary Mobile Subscriber Identity (TMSI) is a temporary identifier assigned by a mobile network to a subscriber to protect IMSI on the radio interface. It is used in GSM and its successors to identify the subscriber during signaling such as attach, routing area updates, and paging, without transmitting the permanent IMSI.

In GSM/UMTS networks, the TMSI is allocated by the Visiting Location Register (VLR) or equivalent network element when a subscriber registers in a new location area or routing area. The TMSI is valid for that area and is used together with the Location Area Identity (LAI) or Routing Area Identification to locate the subscriber. The network stores a mapping between the TMSI and the IMSI. When paging is required, the network can address the subscriber by TMSI rather than IMSI, reducing exposure of IMSI.

TMSIs may have additional variants in specialized domains: P-TMSI (Packet TMSI) is used in GPRS/GSM data services to identify the subscriber in packet-switched contexts. With newer networks such as LTE, an SAE-TMSI (sometimes called S-TMSI) is used to identify the user within the radio interface without revealing the IMSI.

Security and privacy concerns include the fact that while TMSI reduces IMSI exposure, it does not provide full anonymity and may be reused or tracked across handovers if not refreshed. Frequent reallocation and encryption help mitigate risks.

See also: IMSI, LAI, VLR, MME.