09.183GPPGeneral Packet Radio Service (GPRS)Gs interface layer 3 specificationServing GPRS Support Node (SGSN) - Visitors Location Register (VLR)TS
8.1 General description
This procedure is used by the SGSN to indicate to the VLR that the MS has been IMSI detached from GPRS service and therefore the association between the SGSN and the VLR has to be deactivated. This procedure only applies to MSs that are not in the Gs-NULL state at the SGSN. The procedures specified in this section apply to GPRS detach indication initiated by the MS or by the network as specified in 3GPP TS 03.60.
The procedure is also used by the SGSN to indicate to the VLR when a Location Update procedure has been rejected by the SGSN.
The Explicit IMSI detach from GPRS services procedure aborts any other ongoing procedure related to this MS on the Gs interface in the SGSN and in the VLR.
The VLR and the MS should be synchronised as to whether the PBCCH or the BCCH is used, for any of the subsequent paging. In order to achieve this, the SGSN shall attempt to inform the VLR about the detach event by using a retry scheme if the initial delivery of the BSSAP+-GPRS-DETACH-INDICATION message fails.
8.2 Procedures in the SGSN
8.2.1 Explicit GPRS detach initiation
The SGSN shall send a BSSAP+-GPRS-DETACH-INDICATION message to a VLR if:
– The SGSN receives a GPRS only detach from the MS ;
– The SGSN performs network-initiated GPRS detach procedure ; or
– The combined Routing and Location Area Update procedure is rejected at the SGSN.
If the SGSN receives a Detach Request from an MS and the state of the association to a VLR for that MS is not Gs-NULL, the SGSN shall check the detach type indicated in the message. If the MS is indicating GPRS detach the SGSN shall send a BSSAP+-GPRS-DETACH-INDICATION message to the VLR indicating ‘MS initiated IMSI detach from GPRS service’.
If the SGSN decides to perform a network-initiated GPRS detach and the state of the association to a VLR for that MS is not Gs-NULL, the SGSN shall send a BSSAP+-GPRS-DETACH-INDICATION message to the VLR indicating ‘SGSN initiated IMSI detach from GPRS service’.
If the combined Routing and Location Area Update procedure is rejected at the SGSN for a MS with an association state different from Gs-NULL, the SGSN shall send a BSSAP+-GPRS-DETACH-INDICATION to the VLR indicating ’GPRS services not allowed’. The SGSN then sends, for example, an Attach Reject message as specified in 3GPP TS 04.08.
After the sending of the BSSAP+-GPRS-DETACH-INDICATION message, the SGSN shall move the state of the association to Gs-NULL. The SGSN shall start timer T8 upon transmission of the BSSAP+-GPRS-DETACH-INDICATION message and if timer T6-1 is running, timer T6-1 shall be stopped and reset.
8.2.2 Explicit GPRS detach Response
The SGSN shall not wait for the reception of the BSSAP+-GPRS-DETACH-ACK message before sending (if needed) the confirmation of the detach to the MS.
8.2.3 Abnormal cases
If no BSSAP+-GPRS-DETACH-ACK message is received by the SGSN to a previous BSSAP+-GPRS-DETACH-INDICATION message before timer T8 expires, the SGSN shall repeat the BSSAP+-GPRS-DETACH-INDICATION message a maximum of N8 times. If no BSSAP+-GPRS-DETACH-ACK message is received after that, a report shall be made to the O&M system. The state of the association during the acknowledgement procedure remains Gs-NULL.
8.3 Procedures in the VLR
When a VLR receives a BSSAP+-GPRS-DETACH-INDICATION message, the VLR shall send a BSSAP+-GPRS-DETACH-ACK message to the sending SGSN. The state of the association for the MS shall be moved from any state to Gs-NULL. The VLR marks the association as ‘IMSI detached for GPRS services’ with the reason indicated in the IMSI detach from GPRS service type IE.
If the VLR’s implicit detach timer is not running then, the VLR shall set and restart the implicit detach timer upon reception of a BSSAP+-GPRS-DETACH-INDICATION message. If the VLR’s implicit detach timer is running (ie the state of the association was already Gs-NULL) then, the reception of a BSSAP+-GPRS-DETACH-INDICATION message shall not affect the VLR’s implicit detach timer.