User Identity Confidentiality

Subscriber identity confidentiality provides for the privacy of GSM PLMN subscribers
identities by ensuring that the IMSI is not made available or disclosed without authorisation.
This capability provides protection against location tracing mobile subscribers by listening to
the signalling exchanges on the radio path.
Protection of a subscriber’s IMSI is achieved through the issuing of a temporary replacement
ID. This is known as the subscriber’s Temporary Mobile Subscriber Identity (TMSI –
pronounced ‘timsie’).
This identity is allocated by the VLR when a subscriber first affiliates to the network and may
be subject to change each time the subscriber re-connects to the network or when it moves
between MSCs (and hence VLRs).

Protecting User ID Confidentiality

• Benefits of user ID confidentiality include:
• Ensures IMSI is not disclosed without authorisation
• Prevents location tracking using air interface data
• Prevents user ID from being extracted from signalling
information
• Implemented using replacement Temporary Mobile
Subscriber Identity (TMSI)