When a UE moves from EPC to 5GC, the UE always performs Registration procedure.
When a UE moves from 5GC to EPC, the UE performs either Tracking Area Update or Initial Attach.
The UE performs Tracking Area Update procedure if
Both the UE and the EPC support "attach without PDN connectivity", or
The UE has at least one PDU Session for which Session Continuity is supported during interworking, i.e. the UE has EPS Bearer ID and mapped EPS QoS parameters received as described in clause 126.96.36.199.
The UE performs an initial attach procedure if
The UE is registered without PDU Session in 5GC or the UE is registered only with PDU Session for which Session Continuity is not supported during interworking to EPC; and
Either the UE or the EPC does not support attach without PDN connectivity.
In the case of network sharing the UE selects the target PLMN ID according to clause 5.18.3 of TS 23.501.
Clause 188.8.131.52.2 covers the case of idle mode mobility from 5GC to EPC. UE performs Tracking Area Update procedure in E-UTRA/EPS when it moves from NG-RAN/5GS to E-UTRA/EPS coverage area.
The procedure involves a Tracking Area Update to EPC and setup of default EPS bearer and dedicated bearers in EPC in steps 1-11 and re-activation, if required.
The AMF verifies the integrity of the TAU request message:
The AMF determines for a PDU Session whether to retrieve context including mapped UE EPS connection from V-SMF (in the case of HR roaming) or from the SMF+PGW-C (in the case of non roaming or LBO roaming) as follows:
If the AMF determines that one or more of the EBI(s) can be transferred, the AMF sends Nsmf_PDUSession_ContextRequest to the V-SMF or SMF+PGW-C and includes in the message EBI value(s) if any that cannot be transferred.
The EBI values(s) that cannot be transferred is determined by the AMF if the target MME does not support 15 EPS bearers, i.e. the AMF determines the EBI values in range 1-4 as not to be transferred to EPS and if there are still more than 8 EBI values associated with PDU Sessions, the AMF then determines EBI value(s) not to be transferred to EPS based on S-NSSAI and ARP as specified in clause 184.108.40.206.1 of TS 23.501.
The AMF does not retrieve the context for a PDU Session that cannot be transferred to EPS due to no EBI allocated, or allocated EBIs not transferrable, or combination of the two.
In non-roaming or LBO roaming, the AMF retrieves context that includes the mapped EPS Bearer Contexts.
The AMF provides in Nsmf_PDUSession_ContextRequest the target MME capability to the PGW C+SMF in the request to allow the the SMF+PGW-C to determine whether to include EPS Bearer context for Ethernet PDN type or non-IP PDN Type or not.
If the AMF includes in Nsmf_PDUSession_ContextRequest EBI list not to be transferred and if the EBI value of the QoS Flow associated with the default QoS Rule is included in that list, the SMF+PGW-C shall not return the PDN Connection context (which implies the whole PDU Session is not transferred to EPS), otherwise if the EBI value of the QoS Flow associated with the default QoS Rule is not included in the EBI list not to be transferred, the V-SMF or SMF+PGW-C shall not provide the EPS bearer context(s) mapped from QoS Flow(s) associated with that list.
When the AMF sends Nsmf_PDUSession_ContextRequest to the V-SMF or the SMF+PGW-C, the AMF indicates whether the target MME supports User Plane Integrity Protection with EPS.
The above steps are performed with all the SMF+PGW-Cs corresponding to PDU Sessions of the UE which are associated with 3GPP access and have EBI(s) allocated to them.
In Home Routed roaming, the AMF requests the V-SMF to provide SMF Context by using Nsmf_PDUSession_ContextRequest.
For Non-roaming or roaming with local breakout scenario, if the CN Tunnel Info for EPS bearer(s) have not been allocated before, the SMF sends N4 Session Modification Request to PGW-U+UPF to establish the tunnel for each EPS bearers and PGW-U+UPF provides the PGW-U Tunnel Info for each EPS bearers to SMF+PGW-C.
For PDU Sessions that are anchored a UPF, in non-roaming or roaming with local breakout, the SMF+PGW-C returns mapped EPS bearer contexts, which includes PGW-C control plane tunnel information of the PDN connection corresponding to the PDU session, EBI for each EPS bearer, PGW-U tunnel information for each EPS bearer and EPS QoS parameters for each EPS bearer. For PDU Sessions with PDU Session Type Ethernet, if the UE and target MME supports Ethernet PDN type, the SMF+PGW-C provides SM Context for Ethernet PDN Type, otherwise if the UE or target MME does not support Ethernet Type but support non-IP Type, the SMF+PGW-C provides SM Context for non-IP PDN Type. For PDU Sessions with PDU Session Type Unstructured, the SMF provides SM Context for non-IP PDN Type. In home routed roaming, V-SMF provides the SM Context.
If the UP integrity protection policy for the EPS bearer context is set to "Required", the V-SMF or the PGW C+SMF shall not provide the EPS bearer context unless the MME supports User Plane Integrity Protection with EPS and the UE supports User Plane Integrity Protection with EPS.
For PDU Sessions that are anchored at an NEF, the SMF returns an SCEF+NEF ID and an EBI for each PDN connection corresponding to a PDU Session.
If the SMF+PGW-C has marked that the status of one or more QoS Flows are deleted in the 5GC but not synchronized with the UE yet according to clause 220.127.116.11, the SMF+PGW-C does not return to the AMF the EPS context(s) if all its associated QoS Flows are marked as deleted, that is, the SMF+PGW-C returns to the AMF the EPS bearer contexts mapped from QoS Flows where at least one of the QoS Flow for the EPS bearer is not marked as deleted.
The AMF responds with a Context Response message as in step 5 in clause 18.104.22.168 of TS 23.401 carrying mapped MM context (including mapped security context), Return preferred and SM EPS UE Context (default and dedicated GBR bearers) to the MME. If the verification of the integrity protection fails, the AMF returns an appropriate error cause. Return preferred is an optional indication by the AMF of a preferred return of the UE to the 5GS PLMN at a later access change to a 5GS shared network. The AMF may start an implementation specific (guard) timer for the UE context.
From the received context and the Tracking Area indicated by the RAN, the MME can determine whether the UE is performing Inter-RAT mobility to or from NB-IoT.
Steps 6-12 from clause 22.214.171.124 (Tracking Area Update procedure with Serving GW change) in TS 23.401 are performed with following addition and modification:
In the step 10, if the PDU Session (PDN connection) has QoS Flows that do not have EPS bearer ID(s) assigned, the SMF+PGW-C deletes the PCC rule(s) associated with those QoS Flows and informs the PCF about the removed PCC rule(s). If there are QoS Flow(s) with PCC rule(s) that do not have allocated TFT packet filters (due to number exceeding limit), the SMF+PGW-C deletes those PCC rule(s) and informs the PCF about the removed PCC rule(s).
In the step 10, if the MME does not indicate support of User Plane integrity protection, or the new eNB does not support User Plane integrity protection, or the UE does not support User Plane Integrity Protection with EPS and the UP integrity protection policy is set to "Required" then the SMF+PGW-C releases the bearers associated with the PDN CONNECTION.
In the step 11, the SMF+PGW-C requests the PGW-U+UPF to establish the tunnel for each EPS bearer by providing SGW-U Tunnel Info.
In step 10, the SMF+PGW-C may need to report some subscribed event to the PCF by performing an SMF initiated SM Policy Association Modification procedure as defined in clause 4.16.5. If the mapped EPS bearers are not included in Modify Bearer Request, the SMF+PGW-C deletes the PCC rule(s) associated with the QoS Flows corresponding to those mapped EPS bearers.
Step 9a from clause 126.96.36.199 (Tracking Area Update procedure with Serving GW change) in TS 23.401 with the modification captured in clause 188.8.131.52.3
If the SCEF connection is to be established, the steps 9-13 are replaced with the steps 2-3 from clause 184.108.40.206 of TS 23.682. The SCEF+NEF ID and the EBI received from the AMF are included in the Create SCEF Connection Request.
The HSS+UDM invokes Nudm_UECM_DeregistrationNotification to notify the AMF associated with 3GPP access with reason as 5GS to EPS Mobility. If the timer started in step 6 is not running, the old AMF removes the UE context. Otherwise, the AMF may remove UE context when the timer expires.
The AMF requests the release of the PDU Session(s) which is associated with 3GPP access and not expected to be transferred to EPC, i.e. AMF requests the release of:
PDU Session(s) whose corresponding SMF+PGW-C(s) are not contacted by AMF for SM context because the AMF determines that none of EBI(s) for the PDU Session can be transferred to EPS at step 5a; and
PDU Session(s) for which the SM context retrieval failed at step 5c.
The AMF requests the release of the SM context in the V-SMF only and the V-SMF releases resource in the V-UPF, for Home Routed PDU Session with EBIs allocated. The 5GC may also keep UE context to allow the use of native security parameters when UE moves back from EPS to 5GS later.
If PCC is enabled, the AMF initiates AM Policy Association Termination procedure as defined in clause 220.127.116.11 and UE Policy Association Termination procedure as defined in clause 18.104.22.168.
Registration associated with the non-3GPP access in the AMF is not removed (i.e. an AMF that was serving the UE over both 3GPP and non-3GPP accesses does not consider the UE as deregistered over non 3GPP access and will remain registered and subscribed to subscription data updates in UDM).
When the UE decides to deregister over non-3GPP access or the old AMF decides not to maintain a UE registration for non-3GPP access anymore, the old AMF then deregisters from UDM by sending a Nudm_UECM_Deregistration service operation, unsubscribes from Subscription Data updates by sending an Nudm_SDM_Unsubscribe service operation to UDM and releases all the AMF and AN resources related to the UE.
Steps 17-21 from clause 22.214.171.124 (Tracking Area Update procedure with Serving GW change) in TS 23.401 with the following modification:
The MME may provide the eNodeB with a PLMN list in the Handover Restriction List taking into account the last used 5GS PLMN ID and the Return preferred indication. The Handover Restriction List contains a list of PLMN IDs as specified by clause 5.2a of TS 23.251 for eNodeB functions.
The MME may not release the signalling connection with the UE based on the indication received in the step 1 that the UE is moving from 5GC.
[conditional] Step 19 from clause 126.96.36.199.1 applies.
If some of the QoS Flow(s) for an EPS bearer were marked as deleted, the SMF+PGW-C may initiate bearer modification as specified in clause 5.4.3 of TS 23.401 to remove the TFT filter(s) corresponding to the Packet Filter Set(s) in the QoS rules.
If data forwarding tunnel information (i.e. Forwarding F-TEID) is received from MME in step 14 of Figure 188.8.131.52A-1 of TS 23.401 in step 4 above, the AMF provides the data forwarding tunnel info to (V-)SMF in Nsmf_PDUSession_UpdateSMContext Request. The (V-)SMF provides the data forwarding tunnel info to (V-)UPF if data is buffered in (V-)UPF.
The (V-)SMF and (V-)UPF forwards data using the provided SGW forwarding tunnel above. The (V-)SMF also initiates a timer to release the SM context or data forwarding tunnel of the PDU session.
The data received by the Serving GW on the forwarding tunnel is forwarded on the (newly) established tunnel to eNB.