26 Mobile terminated CS service delivery via an alternative MME in MME pool

23.0073GPPRelease 18Restoration proceduresTS

This procedure is an optional feature for VLR and MME. It enables the network to continue delivering mobile terminated CS services to UEs via an alternative MME in the MME pool where the UE is located when the MME to which the UE was registered fails without restart or fails for a long duration.

NOTE 1: UEs in idle mode are not aware of an MME failure until they need to send some uplink data or signalling (e.g. a periodic Tracking Area Update) or until they are forced to re-attach e.g. via the network trigerred service restoration procedure. Without support of the procedure defined in this clause, UEs that remain under LTE may not be able to receive mobile terminated CS services for a long duration after an MME failure without restart or a long MME failure.

The following requirements shall apply if the VLR and MME support this feature.

When the VLR has to page the UE for a mobile terminated CS service (e.g. upon receipt of an incoming CS call), if the VLR detects that the MME serving the UE is no longer in service, the VLR should send an SGs paging request with a CS restoration indicator to one alternative MME in the same MME pool. The VLR should load-balance the paging requests among the available MMEs in the pool during the restoration procedure.

The VLR may know the set of MMEs pertaining to the same MME pool by local configuration or by checking the MME Group ID within the MME name that MMEs signal to the VLR in the SGsAP-LOCATION-UDATE-REQUEST, SGsAP-RESET-INDICATION or SGsAP-RESET-ACK messages. The MME should send an SGsAP-RESET-INDICATION message to the VLR after restart.

The VLR may detect that an MME is no longer in service if there are no more SCTP associations in service with that MME.

NOTE 2: Semi-permanent SCTP associations are established between the MME and VLR, i.e. the SCTP associations remain up under normal circumstances.

The VLR should adjust its paging retransmission delay to avoid requesting again the UE to re-attach before the restoration procedure completes.

The MME shall behave as specified in clause 14.1.3 upon receipt of an SGs paging request not including the CS restoration indicator.

The MME shall accept the SGs paging request and proceed as follows upon receipt of an SGs paging request including the CS restoration indicator:

– if the IMSI is unknown by the MME, or if the IMSI is known and the UE is marked as EMM-DEREGISTERED, the MME shall send the paging request with the location information provided by the VLR, regardless of the value of the "MME-Reset" indicator. If no such location information is provided, the MME may either page the UE in all the tracking areas corresponding to that MME or in the tracking areas served by the MME and by the VLR, or reject the paging request per operator policy. The paging request shall include the IMSI and the CN domain indicator set to "PS" to request the UE to re-attach;

– if the IMSI is known by the MME and the UE is considered to be attached to both EPS and non-EPS services or for SMS only (for an SGs paging request with an ‘SMS indicator’), the MME shall page the UE based on the location information stored in the MME.

The MME may support and apply this procedure to a UE using extended idle mode DRX for MT-SMS service, and if so, the MME shall page the UE with the S-TMSI.

NOTE 3: How the alternative MME knows the UE’s DRX parameter or the extended DRX parameters to page the UE is implementation dependent.

Upon receipt of a paging request including the IMSI and the CN domain indicator set to "PS", or upon receipt of a Service Reject with Cause #10 – Implicitly detached (when using S-TMSI paging), the UE re-attaches to one MME of the pool (that may not be necessarily the MME that initiated the paging procedure towards the UE) and a new SGs association is established with the VLR. This may be a different VLR than the VLR that initiated the SGs paging procedure, e.g. if Intra Domain Connection of RAN Nodes to Multiple CN Nodes is deployed for GERAN or UTRAN (see 3GPP TS 23.236 [24]).

NOTE 4: The UE can receive a Service Reject with Cause #10 – "Implicitly detached" upon being paged by an alternative MME with the S-TMSI assigned by the MME that has failed. The UE then re-attaches to the network as specified in clause 5.6.2.2.1 of 3GPP TS 24.301 [19].

If the new SGs association is established towards the same VLR, the VLR should repeat the SGs paging request after the UE has re-attached to non-EPS services. The MT CS service or SMS is then delivered according to normal procedures.

If the new SGs association is established towards a different VLR, the MT CS service may be delivered via the new VLR using Mobile Terminating Roaming Retry or Mobile Terminating Roaming Forwarding (see 3GPP TS 23.018 [23]); the on-going MT SMS is retransmitted by the SMS-SC using the existing SMS procedures (SMS alert).

Subsequent MT CS services are delivered as per normal procedures.

NOTE 5: A UE with ISR active before the MME failure and using GERAN or UTRAN radio access will not receive the paging request sent by the alternative MME. The VLR can deliver the MT CS service by paging the UE on the A/Iu interface as per existing principles of 3GPP TS 29.118 [14] when the UE does not respond to a first paging on the SGs interface. Paging the UE on the A interface fails if the network operates in NMO I and the UE is in PS connected mode in GERAN.

See 3GPP TS 29.118 [14] for a comprehensive description.