This signalling flow shows the basic IMSI attach or location update procedure when a UMTS UE registers to a pool-area by mobile identity type IMSI.
UE requests the setup of the RRC connection and sends the Initial_DT including the Intra Domain NAS Node Selector (IDNNS). The IDNNS indicates that the routing parameter is derived from IMSI.
The RNC selects the MSC as described in clause 7.1.3
. The Initial-UE message is generated on RANAP and sent via Iu-CS to the selected MSC/VLR.
The optional security functions can be executed.
Since the UE wasn't registered in the MSC before the Map Update-location procedure to HLR is invoked by the MSC:
The update-location is sent to HLR;
In case the UE was registered in another VLR the HLR sends cancel location procedure to old VLR;
The old VLR acknowledges with Cancel location Ack;
The HLR sends insert-subscriber data to the new VLR;
The new VLR acknowledges with insert-subscriber data Ack;
After finishing the location update procedure the HLR responds with Update-location to the new VLR.
The MSC/VLR assigns a new TMSI to the UE and sends this TMSI value in the location-update accept to the UE. The TMSI contains the NRI of the VLR. The location update accept is carried in the NAS-PDU of the RANAP direct-transfer over Iu interface.
The RNC transparently forwards the NAS-PDU to the UE in the RRC downlink-DT message.
In the UE the new TMSI (including the NRI) and the LA are stored on SIM and the TMSI-reallocation complete is generated. This NAS message is returned to CN in the Uplink-DT via RRC. The mobile shall use this new TMSI next time it performs a mobile originating procedure.
The RNC forwards the NAS-PDU transparently to the MSC/VLR in the NAS-PDU of a Direct-transfer on Iu interface. If the TMSI-reallocation complete is received in the MSC/VLR the TMSI is considered as valid for the UE. From then on the UE will be addressed using this TMSI value.