09.183GPPGeneral Packet Radio Service (GPRS)Gs interface layer 3 specificationServing GPRS Support Node (SGSN) - Visitors Location Register (VLR)TS
7.1 General description
This procedure is used by the VLR to request from an SGSN an indication when activity (either signalling or data transmission) from an MS is detected. This procedure can be invoked at any time by the VLR. The BSSAP+-ALERT-REQUEST message shall be acknowledged by the SGSN.
7.2 Procedures in the VLR
7.2.1 Alert Initiation
The VLR may start the Non-GPRS alert procedure at any time. When the VLR wants to request to an SGSN that further activity from an MS shall reported by the SGSN, the VLR shall send an BSSAP+-ALERT-REQUEST message to that SGSN. The VLR starts timer T7 when the BSSAP+-ALERT-REQUEST message is sent.
7.2.2 Alert Response
When a BSSAP+-ALERT-ACK message is received, the VLR shall stop the timer T7. The state of the association is not changed.
7.2.3 Alert failure
If a BSSAP+-ALERT-REJECT message is received, the VLR shall stop the timer T7, move the state of the association to Gs-NULL and within this state the association is marked with the contents of the Gs Cause IE.
7.2.4 Alert Indication
The VLR shall not change the state of the association upon reception of an BSSAP+-MS-ACTIVITY-INDICATION message.
7.2.5 Abnormal cases
If no BSSAP+-ALERT-ACK message is received before the timer T7 expires, the VLR shall retransmit the BSSAP+-ALERT-REQUEST message a maximum of N7 times. If no BSSAP+-ALERT-ACK message is received after that, a report shall be made to the O&M system. The state of the association is not changed.
7.3 Procedures in the SGSN
7.3.1 Alert response
The SGSN may receive a BSSAP+-ALERT-REQUEST message at any state of the association. Upon receipt of an BSSAP+-ALERT-REQUEST message from the VLR and if the IMSI is known in the SGSN, the SGSN shall reply with a BSSAP+-ALERT-ACK message and set the NGAF.
7.3.2 Alert failure
If a BSSAP+-ALERT-REQUEST message is received for an IMSI that is unknown at the SGSN, the SGSN shall return a BSSAP+-ALERT-REJECT message to the VLR indicating the Gs Cause IE value ‘IMSI unknown’.
7.3.3 Alert indication
The SGSN shall to report to the VLR upon detection of any activity (either signalling or data) from the MS if the NGAF is set. If the SGSN detects GPRS signalling that leads to a procedure towards the VLR, the SGSN shall follow this procedure and reset the NGAF. If the SGSN detects activity that does not lead to any procedure towards the VLR, the SGSN shall send an BSSAP+-MS-ACTIVITY-INDICATION message towards the VLR and reset the NGAF.