09.183GPPGeneral Packet Radio Service (GPRS)Gs interface layer 3 specificationServing GPRS Support Node (SGSN) - Visitors Location Register (VLR)TS
The Gs interface connects the databases in the MSC/VLR and the SGSN. The procedures described in this technical specification are used to co-ordinate the location information of MSs that are IMSI attached to both GPRS and non-GPRS services. The Gs interface is also used to convey some circuit switched related procedures via the SGSN.
The basis for the interworking between a VLR and an SGSN is the existence of an association between those entities per MS. An association consists of the SGSN storing the number of the VLR serving the MS for circuit switched services and the VLR storing the number of the SGSN serving the MS for packet switched services. The association is only applicable to MSs in class-A mode of operation and MSs in class-B mode of operation.
All the messages described in this TS use the SCCP class 0 connectionless service.
When the return option in SCCP is used and the sender receives an N_NOTICE indication from SCCP, the sending entity shall report to the Operation and Maintenance system (see ITU-T Q.714).
The behaviour of the VLR and the SGSN entities related to the Gs interface are defined by the state of the association for an MS. Individual states per association, i.e. per MS in class-A mode of operation and MS in class-B mode of operation, are held at both the VLR and the SGSN.
4.1 Association at the VLR
The states associated to the Gs interface in the VLR are specified in this subclause. The state diagram at the VLR is shown in figure 4.1. The state diagram does not include the message error handling specified in clause 16.
4.1.1 States at the VLR
There is no association with an SGSN for the MS and therefore the VLR considers that the MS is IMSI detached for GPRS services. In this state no BSSAP+-MS-INFORMATION-REQUEST or BSSAP+-MM-INFORMATION-REQUEST messages are sent to the SGSN. The VLR may initiate paging on the Gs interface if the ‘Confirmed by Radio Contact’ restoration indicator in the VLR is set to ‘false’ (see 3GPP TS 03.07). Any message from the SGSN is ignored apart from the BSSAP+-LOCATION-UPDATE-REQUEST message.
The VLR has received a BSSAP+-LOCATION-UPDATE-REQUEST message from the SGSN. In this state the VLR may be waiting for the outcome of the Update Location procedure from the HLR. The VLR shall send BSSAP+-PAGING-REQUEST messages to MSs in class-A and MSs in class-B mode of operation via only the Gs interface.
The VLR considers that the MS is attached to both GPRS and non-GPRS services. In this state the VLR sends BSSAP+-PAGING-REQUEST messages to MSs in class-A mode of operation and and MSs in class-B mode of operation via only the Gs interface. The VLR can perform the MS Identification procedure and the MM information procedure.
Figure 4.1/3GPP TS 09.18: State diagram at the VLR
4.2 Association at the SGSN
The states and MM context variables associated to the Gs interface in the SGSN are specified in this subsection. The state diagram at the SGSN is shown in figure 4.2. The state diagram does not include the message error handling specified in section 16.
4.2.1 MM context variables at the SGSN
Set to ‘false’ when the SGSN has received a reset indication from the VLR. The SGSN shall request to the MS, upon reception of the next routeing area update (either routeing area update only or combined routeing and location area update) procedure, to re-attach to non-GPRS services if the MS is still IMSI attached to non-GPRS services.
Set to ‘true’ when the SGSN restarts after a failure. The ‘SGSN-Reset’ variable is unique within an SGSN and it applies to all the MM context stored in the SGSN.
4.2.2 States at the SGSN
There is no association with a VLR for the MS and therefore the SGSN considers that the MS is IMSI detached of non-GPRS services. In this state the SGSN accepts BSSAP+-PAGING-REQUEST messages to MSs only if the ‘SGSN-Reset’ restoration indicator in the SGSN is set to ‘true’.
The SGSN has sent a BSSAP+-LOCATION-UPDATE-REQUEST message to the VLR. In this state the SGSN waits for the outcome of the Location Update for non-GPRS procedure at the VLR before sending the response to the MS. In this state the SGSN accepts BSSAP+-PAGING-REQUEST messages.
The SGSN stores an association for that MS. In this state the SGSN performs the Location Update for non-GPRS services procedure towards the VLR for MSs in class-A and MSs in class-B mode of operation when the MS moves to a new LA.
Figure 4.2/3GPP TS 09.18: State diagram at the SGSN