5.17.2.4 Mobility between 5GS and GERAN/UTRAN

IP address preservation upon direct mobility between 5GS and GERAN/UTRAN is not supported.

Upon mobility from 5GS to GERAN/UTRAN (e.g. upon leaving NG-RAN coverage) the UE shall perform the A/Gb mode GPRS Attach procedure or Iu mode GPRS Attach procedure (see TS 23.060 [56]).

As a UE option, to support IP address preservation at mobility from EPC to 5GS for PDN connections without 5GS related parameters, a 5GS capable UE may:

• Following mobility from GERAN/UTRAN to EPS, release those PDN connection(s) and re-establish them as specified in clause 4.11.1.5.4.1 of TS 23.502 [3] so that they support interworking to 5GS.

NOTE 1: It is recommended that a UE using this option does not do this behaviour after every change to EPS in PLMNs that do not support 5GS, nor for APNs that do not support mobility to 5GS; and, that such a UE supports storage of the 5GS related parameters while in GERAN/UTRAN. Whether and how the UE is aware of which PLMNs support 5GS and which APNs do not support mobility to 5GS is out of scope of this specification.

To support mobility from EPC to 5GS to EPC to GERAN/UTRAN for PDN connections established in EPC:

NOTE 2: For the use of N7 or N40 interfaces while the UE is in GERAN/UTRAN access, the SMF+PGW-C selected by the MME (using the existing selection procedures described in clause 4.11.0a of TS 23.502 [3] and clause 4.3.8 of TS 23.401 [26]) needs to support functionality (e.g. signalling of GERAN/UTRAN cell identification over N7) specified in Annex L.

• in signalling sent on the N26 interface, the MME should send the TI and BSS Container in the EPS Bearer Context (see Table 7.3.1-3 of TS 29.274 [101]), if there is any, of the EPS bearer to the SMF (V-SMF / I-SMF) via the AMF in the Bearer Context within the PDN Connection IE in the Forward Relocation Request and Context Response messages (TS 29.274 [101]); the SMF (V-SMF / I-SMF) should store the TI and BSS Container and the SMF (V-SMF / I-SMF) should provide the TI and BSS Container to the AMF (as part of a procedure to deliver SM context to AMF) so that the AMF sends the TI and BSS Container of the related EPS bearer in the Bearer Context within the EPS PDN Connection information in any subsequent Forward Relocation Request and Context Response message sent to an MME.

NOTE 3: At mobility from EPC, the SMF+PGW-C / V-SMF / I-SMF receives the TI and BSS Container as part of the UE EPS PDN Connection information from the AMF and stores the TI. At mobility to EPC, the SMF+PGW-C / V-SMF / I-SMF provides the AMF with the TI and BSS Container as part of the UE EPS PDN Connection information. The SMF+PGW-C / V-SMF / I-SMF is not meant to understand the TI/BSS Container nor to use it for any other purpose than providing it back to AMF.

GERAN/UTRAN Mobility Management Bearer Synchronisation procedures will release any dedicated QoS Flows established in 5GS and then the SMF+PGW-C deletes the PCC rule(s) associated with those QoS Flows and informs the PCF about the removed PCC rule(s) as described in clause 4.11 of TS 23.502 [3].

When the UE accesses the network via GERAN/UTRAN over Gn/Gp interface, Secondary PDP Context Activation Procedure is not supported and the SMF+PGW-C rejects the Secondary PDP Context Activation request if UE requested it.

IP address preservation at mobility from EPC to GERAN/UTRAN for PDU sessions established in 5GS is not supported.

With regard to interworking between 5GS and the Circuit Switched domain when the GERAN or UTRAN network is operating in NMO II (i.e. no Gs interface between MSC and SGSN):

• upon mobility from 5GS to GERAN/UTRAN, the UE shall either:
• act as if it is returning after a loss of GERAN/UTRAN coverage (and e.g. only perform a periodic LAU if the periodic LAU timer has expired), or,
• perform a Location Update to the MSC. If the UE is registered for IMS voice and is configured, using Device Management or initial provisioning, to perform additional mobility management procedures when it has moved from a RAT that supports IMS voice over PS sessions to one that does not (see TS 23.060 [56]), it shall follow this option.

Upon mobility from GERAN/UTRAN to 5GS (e.g. upon selecting an NG-RAN cell) the UE shall perform the Registration procedure of "initial registration" type as described in TS 23.502 [3]. The UE shall indicate a 5G-GUTI as UE identity in the Registration procedure if it has a stored valid native 5G-GUTI (e.g. from an earlier registration in the 5G System). Otherwise the UE shall indicate a SUCI.

If a UE in MICO mode moves to GERAN/UTRAN and any of the triggers defined in clause 5.4.1.3 occur, then the UE shall locally disable MICO mode and perform the A/Gb mode GPRS Attach procedure or Iu mode GPRS Attach procedure (see TS 23.060 [56]). The UE can renegotiate MICO when it returns to 5GS during (re-)registration procedure.

In Single Registration mode, expiry of the periodic RAU timer, or, the periodic LAU timer shall not cause the UE to change RAT.

The 5G SRVCC from NG-RAN to UTRAN is specified in the TS 23.216 [88]. After the 5G SRVCC to UTRAN, all the PDU sessions of the UE are released.