5.17.1.1 General

Clause 5.17.1 describes the UE and network behaviour for the migration from EPC to 5GC.

Deployments based on different 3GPP architecture options (i.e. EPC based or 5GC based) and UEs with different capabilities (EPC NAS and 5GC NAS) may coexist at the same time within one PLMN.

It is assumed that a UE that is capable of supporting 5GC NAS procedures may also be capable of supporting EPC NAS (i.e. the NAS procedures defined in TS 24.301 [13]) to operate in legacy networks e.g. in the case of roaming.

The UE will use EPC NAS or 5GC NAS procedures depending on the core network by which it is served.

In order to support smooth migration, it is assumed that the EPC and the 5GC have access to a common subscriber database, that is HSS in the case of EPC and the UDM in the case of 5GC, acting as the master data base for a given user as defined in TS 23.002 [21]. The PCF has access to the UDR that acts as a common subscriber database for a given user identified by a SUPI using the Nudr services defined in TS 23.502 [3].

A UE that supports only EPC based Dual Connectivity with secondary RAT NR:

• always performs initial access through E-UTRA (LTE-Uu) but never through NR;
• performs EPC NAS procedures over E-UTRA (i.e. Mobility Management, Session Management etc) as defined in TS 24.301 [13].

A UE that supports camping on 5G Systems with 5GC NAS:

• performs initial access either through E-UTRAN that connects to 5GC or NR towards 5GC;
• performs initial access through E-UTRAN towards EPC, if supported and needed;
• performs EPC NAS or 5GC NAS procedures over E-UTRAN or NR respectively (i.e. Mobility Management, Session Management etc) depending on whether the UE requests 5GC access or EPC access, if the UE also supports EPC NAS.

When camping on an E-UTRA cell connected to both EPC and 5GC, a UE supporting EPC NAS and 5GC NAS shall select a core network type (EPC or 5GC) and initiate the corresponding NAS procedure as specified in TS 23.122 [17].

In order to support different UEs with different capabilities in the same network, i.e. both UEs that are capable of only EPC NAS (possibly including EPC based Dual Connectivity with NR as secondary RAT) and UEs that support 5GC NAS procedures in the same network:

• eNB that supports access to 5GC shall broadcast that it can connect to 5GC. Based on that, the UE AS layer indicates "E-UTRA connected to 5GC" capability to the UE NAS layer. In addition the eNB broadcasts the supported CIoT 5GS Optimisations that the UE uses for selecting a core network type.
• It is also expected that the UE AS layer is made aware by the UE NAS layer whether a NAS signalling connection is to be initiated to the 5GC. Based on that, UE AS layer indicates to the RAN whether it is requesting 5GC access (i.e. "5GC requested" indication). The RAN uses this indication to determine whether a UE is requesting 5GC access or an EPC access. RAN routes NAS signalling to the applicable AMF or MME accordingly.

NOTE: The UE that supports EPC based Dual Connectivity with secondary RAT only does not provide this "5GC requested" indication at Access Stratum when it performs initial access and therefore eNB uses the "default" CN selection mechanism to direct this UE to an MME

The 5GC network may steer the UE from 5GC based on:

• Core Network type restriction (e.g. due to lack of roaming agreements) described in clause 5.3.4.1.1;
• Availability of EPC connectivity;
• UE indication of EPC Preferred Network Behaviour; and
• Supported Network Behaviour.

The UE that wants to use one or more of these functionalities not supported by 5G System, when in CM-IDLE may disable all the related radio capabilities that allow the UE to access 5G System. The triggers to disable and re-enable the 5GS capabilities to access 5G System in this case are left up to UE implementation.