4 Description of the SGs association between a VLR and an MME

29.1183GPPMobility Management Entity (MME) - Visitor Location Register (VLR) SGs interface specificationRelease 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

4.1 General

CS fallback function and SMS delivery via the CS core network is realized by reusing Gs interface mechanisms as defined in 3GPP TS 29.018 [16] on the interface between the MME in the EPS and the VLR. This interface is called SGs interface.

NOTE: Within this specification, the term VLR refers to MSC/VLR or MSC Server/VLR.

The SGs interface connects the databases in the VLR and the MME. The procedures described in the present document are used to co-ordinate the location information of UEs that are IMSI attached to both EPS and non-EPS services. The SGs interface is also used to convey some circuit switched related procedures via the MME.

The basis for the interworking between a VLR and an MME is the existence of a SGs association between those entities per UE. The SGs association is applicable to UEs which are configured to use CS fallback and SMS over SGs, or SMS over SGs only. The SGs association is not applicable if the subscriber data indicates that the subscription is for packet only. The SGs association is also not applicable if the MME is registered for SMS for the UE as specified in 3GPP TS 23.272 [7].

In NB-S1 mode, the SGs association is also applicable to UEs supports NB-S1 mode only to use SMS over SGs only.

The behaviour of the VLR and the MME entities related to the SGs interface are defined by the state of the SGs association for a UE. Individual SGs association states are maintained at both the VLR and the MME for each UE.

4.2 SGs association at the VLR

4.2.1 General

The states associated to the SGs interface in the VLR are specified in subclause 4.2.2 and the state diagram at the VLR is shown in figure 4.2.2.1. The state diagram does not include the message error handling specified in clause 7.

4.2.2 States at the VLR

SGs-NULL

There is no SGs association with an MME for the UE and therefore the VLR considers that the UE is IMSI detached for EPS services. In this state no SGsAP-MM-INFORMATION-REQUEST messages are sent to the MME. The VLR may initiate paging on the SGs interface if the "Confirmed by Radio Contact" restoration indicator in the VLR is set to "false" (see 3GPP TS 23.007 [4]). Any message from the MME is ignored except SGsAP-LOCATION-UPDATE-REQUEST, SGsAP-IMSI-DETACH-INDICATION and SGsAP-EPS-DETACH-INDICATION.

LA-UPDATE-PRESENT

The VLR has received an SGsAP-LOCATION-UPDATE-REQUEST message from the MME. In this state, the VLR may be waiting for the outcome of the Update Location procedure from the HSS, if the IMSI is not known in the VLR. For UEs which are configured to use CS fallback and SMS over SGs, or SMS over SGs only, the VLR sends SGsAP-PAGING-REQUEST messages via the SGs interface.

SGs-ASSOCIATED

The VLR considers that the UE is attached to both EPS and non-EPS services. For UEs which are configured to use CS fallback and SMS over SGs, or SMS over SGs only, the VLR sends SGsAP-PAGING-REQUEST messages via the SGs interface. The VLR can perform the MM information procedure.

NOTE: Receipt of an SGsAP-RESET-INDICATION message from the MME may change or not the state of the SGs interface of all the associations associated to the restarted MME, see subclause 5.8.3.

Figure 4.2.2.1: State diagram at the VLR

4.3 SGs association at the MME

4.3.1 General

The MM context variables associated to the SGs interface in the MME are specified in subclause 4.3.2 and states associated to the SGs interface in the MME are specified in subclause 4.3.3. The state diagram at the MME is shown in figure 4.3.3.1. The state diagram does not include the message error handling specified in clause 7.

4.3.2 MM context variables at the MME

VLR-Reliable:

Boolean set to "false" when the MME has received a reset indication from the VLR. The MME may:

– upon reception of a combined tracking area update request from a UE that is still attached for non-EPS services, perform immediately the location update for non-EPS services procedure; or

– upon reception of a periodic tracking area update request from a UE that is still attached for non-EPS services, dependent on network configuration and operator policy,

– perform a network initiated detach with detach type "IMSI detach" immediately after the completion of the periodic tracking area update procedure; or

– perform immediately the location update for non-EPS services procedure.

MME-Reset:

Boolean set to "true" when the MME restarts after a failure. The "MME-Reset" restoration indicator is unique within an MME and it applies to all the MM contexts stored in the MME.

4.3.3 States at the MME

SGs-NULL

There is no SGs association with a VLR for the UE and therefore the MME considers that the UE is IMSI detached for non-EPS services. In this state the MME accepts SGsAP-PAGING-REQUEST messages to UEs only if:

– the "MME-Reset" restoration indicator in the MME is set to "true"; or

– the MME supports the CS restoration indicator set in the Additional paging indicators information element in the SGs-PAGING-REQUEST message.

LA-UPDATE-REQUESTED

The MME has sent an SGsAP-LOCATION-UPDATE-REQUEST message to the VLR. In this state the MME waits for the outcome of the Update Location for non-EPS services procedure at the VLR before sending the response to the UE. In this state the MME accepts SGsAP-PAGING-REQUEST messages.

SGs-ASSOCIATED

The MME stores an SGs association for the UE. In this state the MME performs the location update for non-EPS services procedure towards the VLR, e.g. when the location area is changed. All conditions describing when to trigger the location update for non-EPS services procedure are listed in subclause 5.2.2.2.

Figure 4.3.3.1: State diagram at the MME

4.3.4 State transitions in the MME without SGsAP signalling

During the tracking area update procedure or routing area update procedure (see 3GPP TS 23.401 [7B]) and if the SGs association is not SGs-NULL, the MME may change the state of the SGs association to SGs-NULL upon receipt of a SGSN Context Acknowledge (see 3GPP TS 29.060 [16A]) or Context Acknowledge message without ISR activation (see 3GPP TS 29.274 [17A]) from the target network.

During the inter-system handover from S1 mode to A/Gb mode or Iu mode, or handover from S1 mode to S1 mode (see 3GPP TS 23.401 [7B]) and if the SGs association is not SGs-NULL, the MME may change the state of the SGs association to SGs-NULL upon receipt of a Forward Relocation Complete (see 3GPP TS 29.060 [16A]) or Forward Relocation Complete Notification message without ISR activation (see 3GPP TS 29.274 [17A]) from the target network.

If the MME receives a normal attach request, attach request for emergency bearer services or a normal tracking area update request from the UE in state SGs-ASSOCIATED, the MME shall change the state of the SGs association to SGs-NULL.

If it is required by network configuration or operator policy, the MME shall change the state of the SGs association to SGs-NULL if the following conditions are fulfilled:

– the MME receives an EXTENDED SERVICE REQUEST message with service type set to "mobile originating CS fallback emergency call or 1xCS fallback emergency call";

– the state of the SGs association is not SGs-NULL; and

– the MME determines that the UE is not available for PS services at the target RAT as described in 3GPP TS 23.272 [7].

4.3.5 State transitions in the VLR without SGsAP signalling

Based on configuration, during PS to CS SRVCC from E-UTRAN (see 3GPP TS 23.216 [26]) and if the SGs association is not SGs-NULL, the VLR may change the state of the SGs association to SGs-NULL upon receipt of a Relocation Complete/Handover Complete message from target RNS/BSS.