TMSI (Temporary Mobile Subscriber Identity)

The Temporary Mobile Subscriber Identity is a short identifier assigned by the VLR or MME to a subscriber so that the IMSI does not have to be sent over the air on every transaction. The TMSI is reallocated periodically to limit linkability, but in 2G and 3G the procedure is unauthenticated, allowing an IMSI catcher to force the device to reveal its IMSI. 5G strengthens this with 5G-GUTI and SUCI-based concealment of the long-term identity.

Categories: Radio Access NetworkCore NetworkIdentity and Subscriber

TMSI in context

The radio access network is the layer where mobile devices attach to the operator's infrastructure. Attacks in this layer include IMSI catching, rogue base stations, downgrade attacks and jamming, and defenses hinge on mutual authentication, integrity-protected signaling and Open RAN supply-chain hygiene.

The mobile core is where subscriber sessions, mobility and policy live. In 4G it is the EPC (MME, HSS, S/PGW); in 5G it is the Service-Based Architecture with AMF, SMF, UPF, AUSF, UDM and the NRF. Core exposure is the difference between a single-subscriber leak and a fleet-wide breach.

To place TMSI in the wider telecom-security picture, review 5G-GUTI, MNO, Encryption, IMSI Catcher (Cell Site Simulator), SUCI and USIM.

Related terms

Related topic hubs

  • 5G Security 5G SBA, SEPP, SUCI, 5G-AKA, N32 and service-based interface security.
  • NAS Security Non-Access Stratum security, integrity and ciphering algorithms.
  • IMSI Catcher Rogue base stations, IMSI capture and detection.
  • Diameter Security 4G/LTE Diameter threats across IPX/roaming and DEA defenses.

More from the TelcoSec Glossary

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