TMSI Reallocation

In order to ensure continued confidentiality of a user’s IMSI, each time an MS changes LA, a
new TMSI is issued by the VLR.
Exceptional conditions could include a network failure (e.g. a database) where the TMSI
becomes unknown, in which case the network will request IMSI details in order to validate
the subscriber before issuing a new TMSI.

TMSI Re-allocation

• Used to protect a subscriber’s IMSI
• TMSI only unique within a Location Area (LA)
• Outside an LA, TMSI must be combined with LAI to remain
unique
• TMSI re-allocated on LA change (minimum) or as a result of
an exceptional condition.
• Normally takes place in encrypted mode
• Normally tales place in conjunction with another procedure
e.g. Location update, call setup etc

Summary

This Section has covered the following MM Functions:
• Network areas
• MS Mobility States
• Attaching and Detaching from the Network
• Authentication and Security (Section 10)
• Location Updating
• Roaming
• TMSI reallocation