6.5 Provision of the IMSI to the BSC

3GPP43.055Dual Transfer Mode (DTM)Release 16Stage 2TS

6.5.1 General

To enable the described implementation of the GPRS class A mode of operation, the BSS and the PCU are required to perform the co-ordination of the allocation of radio resources for both domains. That co-ordination is performed with the IMSI as it is described in the following clauses.

The IMSI shall be provided to the BSC during:

1. call establishment;

2. session establishment; and

3. external handover.

6.5.2 Call establishment

The BSC triggers the establishment of the SCCP connection with the MSC. The MSC shall provide the IMSI to the BSC in a new message: Common ID message. This message can be sent either on the SCCP Connection Confirm message or immediately after, once the connection is already established.

6.5.3 Session establishment Downlink session establishment

Both in the READY and the STANDBY states:

– the IMSI is sent from the SGSN in the PS PAGING BSSGP PDUs;

– the IMSI and the TLLI are sent from the SGSN in the DL-UNITDATA. Uplink session establishment

At the establishment of an uplink TBF, the BSC can identify whether it knows the IMSI of a mobile station for which it has its TLLI. If the IMSI is not known, the BSC can initiate the RA-Capability-Update procedure and request the IMSI (and the RA capabilities) of the mobile to the SGSN.

– If there is a change of cell and the mobile station was in packet transfer mode, it performs a cell update procedure, which can be done with user data and therefore hold for a long time, possibly with no downlink message. During this time the mobile station may not be able to monitor the broadcast channels and miss paging messages. This error situation is prevented by the BSS initiating the RA-Capability-Update procedure.

– The RA-Capability-Update procedure may fail if the SGSN does not have the IMSI (or the RA Capabilities) of the mobile station. This may happen when there is a change of SGSN (hence also of RA). In this case, the mobile station sends the RAU Request message on the uplink TBF and then goes back to (packet) idle mode, where it can receive possible paging messages. When the SGSN answers with the RAU Accept message (after retrieving the PDP and MM contexts from the old SGSN), the DL-UNITDATA message contains the IMSI and the TLLI.

6.5.4 External handover

During CS handover procedure, the IMSI is included in the (BSSMAP) HANDOVER REQUEST message from the MSC to the target BSC. The identifiers included in the (BSSGP) PS HANDOVER REQUEST message from the SGSN to the target BSC during the PS Handover procedure are defined in 3GPP TS 43.129 [13].