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 Temporary Mobile Subscriber Identity (TMSI)

A TMSI is allocated by the CS domain VLR to protect an MS/mobile user's identity while communicating over the air interface. A TMSI is allocated as part of the successful CS domain location area update mobility management procedure initiated by an MS toward the CN. A TMSI is used by the MSC as the identity of an MS during the air interface Layer 3 mobility management‐related procedures such as sending a paging request message to the MS to notify about an incoming call.

A TMSI has an end‐to‐end significance and has 32 bits in length. TMSI with all 1's is not a valid one. A new TMSI is also allocated to a mobile device on entering a new location area under a different VLR where the MS performs a location area update procedure with the CN. The VLR maintains an association between the IMSI of the mobile device and the allocated TMSI. For multi‐RATs, i.e. GPRS Edge Radio Access Network (GERAN), UTRAN, and E‐UTRAN, capable MS/UE, a TMSI is allocated as a result of the combined mobility management procedures performed by a UE/MS toward the CN.