Section: 5.19.3 MME/SGSN or HSS initiated Dedicated Core Network Reselection
Content: If DCNs are deployed, this procedure is used by the HSS to update (i.e. add, modify or delete) the UE Usage Type subscription parameter in the serving node. This procedure may result in change of serving node of the UE. This procedure may also be used for MME/SGSN initiated serving node change for UEs, e.g. when configuration about the UE Usage Types served by MME/SGSN is changed. This procedure may also be used after a handover procedure by the target MME/SGSN to redirect a UE to a serving node of another DCN. If UE assisted DCN selection feature is supported by the Core Network, the UE is provided with the new DCN-ID. The subscription change may be applied to a large number of UEs and similar considerations as in the case of MME/SGSN rebalancing specified in clause 4.3.7.3 should be applied to avoid sudden redirection of UEs that could overload the core network nodes (and possibly the RAN if paging is needed). Figure 5.19.3-1: MME/SGSN or HSS Initiated Dedicated Core Network Reselection Steps 1 and 2 apply for HSS initiated Dedicated Core Network Reselection procedure only. 1. The HSS sends an Insert Subscriber Data Request (IMSI, Subscription Data) message to the MME/SGSN. The Subscription Data includes UE Usage Type information or UE Usage Type withdrawal information. NOTE 1: If the UE Usage Type subscription change or withdrawal needs to be applied for a large number of subscribers, the HSS should stagger the insertion of subscription changes to serving nodes, e.g. based on OAM. 2. The MME/SGSN updates the stored Subscription Data and acknowledges the Insert Subscriber Data Request message by returning an Insert Subscriber Data Answer (IMSI) message to the HSS. The procedure ends if the MME/SGSN can continue to serve the UE. 3. If the MME/SGSN decides to transfer the UE immediately to another CN or if the MME/SGSN determines that DCN-ID immediately needs to be updated and the UE is in idle-mode, the MME/SGSN pages the UE. Alternatively the MME waits until the UE becomes active. If MME/SGSN decides to transfer the UE to another CN either Steps 4 through 7 or Step 8 occur. Steps 4 through 7 occur if the UE is already in connected mode or UE enters connected mode by initiating data transfer. Step 8 occurs if the UE is in idle mode and performs a TAU/RAU procedure. If the MME/SGSN determines that only DCN-ID shall be updated in the UE (i.e. serving CN node is kept) only step 4 and 5 occurs. 4. Either triggered by the paging or by uplink data the UE initiates NAS connection establishment. Alternatively the UE initiates NAS connection establishment by sending a TAU/RAU Request. 5. When a NAS connection already exists or when a NAS connection is established for initiating data transfer, the MME/SGSN triggers the GUTI Reallocation/P-TMSI Reallocation procedure. If DCN-ID is available and the MME/SGSN determines that the UE shall be updated with a new DCN-ID, the new DCN-ID shall be included in the GUTI Reallocation Command/P-TMSI Reallocation Command. If the MME/SGSN determines that a CN node re-selection needs to be performed, a non-broadcast TAI/RAI shall be included. 6. The MME/SGSN releases RAN resources and UE is moved to idle mode. NOTE 2: If a large number of UEs need to be offloaded the MME/SGSN should not release RAN resources for all UEs immediately to avoid sudden redirection of UEs that could overload the core network nodes (and possibly the RAN if paging is needed). The MME/SGSN should wait until the release is performed due to inactivity. 7. The non-broadcast TAI/RAI triggers the UE to immediately start the TAU/RAU procedure. If available, the new DCN-ID shall be sent from the UE to the RAN. The MME/SGSN receives the TAU/RAU Request message. 8. The UE performs a TAU/RAU request. The MME/SGSN receives the TAU/RAU Request message. 9. If the UE Usage Type for the UE has been added or modified and if it is not served by the MME/SGSN, or if the UE Usage Type has been withdrawn from the HSS subscription data and subscriptions without UE Usage Type are not served by the MME/SGSN, the MME/SGSN triggers the NAS Message redirection procedure of clause 5.19.1 to redirect the UE. This is followed by step 7 of clause 5.19.2 where the TAU/RAU procedure completes at the MME of the selected DCN.