What is TMSI in Telecom?

It stands for a temporary mobile subscriber identity. While the permanent identity is IMSI. To hide the actual identity and secure the communication a mobile network allocates a temporary identity on the initial network attach of a SIM.

There are chances that a hacker tool can intercept radio communication between a mobile device and a wired mobile core network.  If all traffic is based on IMSI, the hacker can connect to the mobile network with the IMSI. It leads to simulating a full mobile device so that hackers can receive calls and SMS.

To hide the actual identity TMSI is used. This has significance only when a mobile device is in roaming under certain areas.  Roaming area change will cause to change in TMSI.

The radio interface keeps mapping to IMSI and TMSI, so that message coming from the core network reaches the correct mobile device. The communication between roaming and the home network is based on om IMSI.

At a time a device may have three TMSIs, one allocated by MSC, one allocated by SGSN, and one allocated by MME.  The length is four bytes. All bits are 1, which represents invalid TMSI.

MSC allocated temporary IMSI is Tmsi, SGSN allocated temporary IMSI is P-Tmsi (packet-TMSI).   Allocated by MME is GUTI.

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