A VLR in which a mobile subscriber is roaming may also assign a Temporary Mobile Subscriber Identity or TMSI ( also known in some places as TIMSI) to address and identify the Mobile subscriber, It is used in place of the IMSI and makes it much more difficult for anyone else to determine the identity of the subscriber as it is no longer required to periodically transfer IMSI over the air interface, TMSI is only valid for the current VLR and is not passed to the HLR  The TMSI is stored in both the VLR database and on the mobile subscriber’s SIM-card.

A SGSN may also assign a P-TMSI or Packet-Temporary Mobile Subscriber Identity to a mobile subscriber attached with it to avoid the need of the transmission of the IMSI