UE identifier in idle mode

UE in idle mode can be identified by IMSI(International Mobile Subscriber Identity), TMSI(Temporary Mobile Subscriber Identity) or P-TMSI(Packet Temporary Mobile Subscriber Identity).

CS core network allocates TMSI and is unique within a location area. MCC, MNC, LAC (Location area code) are specified along with TMSI to ensure that UE identity is unique globally.

PS core network allocates P-TMSI and is unique within a routing area.MCC, MNC, LAC and RAC (Routing area code) are specified along with P-TMSI to ensure that UE identity is unique globally.

IMSI is fixed for a specific USIM and is not allocated by the network during registration.

TMSI and P-TMSI are used in preference to the IMSI to help protect the confidentiality of the UE identity as they are less vulnerable to attacks since they are not fixed. It is usual to reallocate the TMSI as part of the location area update procedure and to reallocate the P-TMSI as part of the routing area update procedure.

3GPP TS 25.331 specifies that when a UE transmits RRC Connection Request message to enter RRC Connected mode then it should include an initial UE identity. The order of preference for which is specified as TMSI, P-TMSI, IMSI and International Mobile Equipment Identity(IMEI). The IMEI is only used if none of the other three identities are available. The IMEI identifies UE rather than USIM. It would be necessary to include the IMEI when a UE without a USIM is used to make an emergency call. The RNC includes the same initial UE identity within the RRC Connection Setup or RRC Connection Reject messages. Other UE identities, e.g. U-RNTI, H-RNTI, E-RNTI, C-RNTI are assigned once the UE is in RRC Connection mode.