09.183GPPGeneral Packet Radio Service (GPRS)Gs interface layer 3 specificationServing GPRS Support Node (SGSN) - Visitors Location Register (VLR)TS
12.1 General description
This procedure is used by the SGSN to inform to the associated VLRs about the recovery from an internal failure that has affected the association with the VLRs.
The SGSN recovery procedure shall be handled in such a way that the signalling load on the VLR and SGSN does not create any overload problem.
12.2 Procedures in the SGSN
12.2.1 SGSN Reset Initiation
In the event of a failure at the SGSN which has resulted in the loss of VLR association information on some MSs, the SGSN shall move from any state to the Gs-NULL state for all the associations with VLRs per MS. The SGSN shall also set the ‘SGSN-Reset’ MM context variable to ‘true’ and start the timer T12-1. When the timer T12-1 expires the ‘SGSN-Reset’ MM context variable is set to ‘false’. The value of the timer T12-1 shall be longer that the periodic routing area update timer at the SGSN.
A BSSAP+-RESET-INDICATION message shall be sent to all the VLRs connected to the SGSN by Gs interfaces. The BSSAP+-RESET-INDICATION message indicates to the VLR that all the associations with that particular SGSN for all the MSs registered in the VLR are no longer reliable. The normal procedures for updating the association are still applicable (sections ‘Location Update for non-GPRS services procedure’, ‘Explicit IMSI detach from GPRS services procedure’, ‘Explicit IMSI detach from non-GPRS services procedure’ and ‘Implicit IMSI detach from non-GPRS services procedure’). The SGSN shall also start timer T12-2.
12.2.2 SGSN Reset Response
Upon receipt of a BSSAP+-RESET-ACK message, the SGSN shall stop the timer T12-2.
12.2.3 Abnormal cases
If the SGSN does not receive a BSSAP+-RESET-ACK message from that VLR before the T12-2 timer expires, the SGSN shall retransmit the BSSAP+-RESET-INDICATION message. The retransmission is repeated a maximum of N12 times. If no BSSAP+-RESET-ACK is received after a report shall be to made the O&M system.
12.3 Procedures in the VLR
Upon receipt of a BSSAP+-RESET-INDICATION message from the SGSN, the VLR is informed that all the associations with that SGSN for all the MSs registered in the SGSN are no longer reliable because the SGSN may have lost information about the state of the MSs for that VLR and during the failure the SGSN may have missed signalling messages. The VLR shall set the ‘Confirmed by Radio Contact’ restoration indicator to ‘false’ in all the associations containing the restarted SGSN. If the ‘Confirmed by Radio Contact’ restoration indicator is ‘false’ the VLR may send paging messages on both the Gs and the A interface.
The VLR sends a BSSAP+-RESET-ACK message to the SGSN. This indicates to the SGSN that all the associations for the MSs which have an association with that SGSN will be moved to the Gs-NULL state.