How it Works
A UE that supports only EPS based Dual Connectivity with secondary RAT NR:
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Always performs initial access through E-UTRA (LTE-Uu) but never through NR.
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Performs EPS NAS procedures over E-UTRA (that is, Mobility Management, Session Management and so on) as defined in 3GPP TS 24.301.
A UE that supports camping on 5G Systems with 5GC NAS:
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Performs initial access either through E-UTRAN that connects to 5GC or through NR towards 5GC.
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Performs initial access through E-UTRAN towards EPS, if supported and needed.
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Performs EPS NAS or 5GC NAS procedures over E-UTRAN or NR respectively (that is, Mobility Management, Session Management, and so on) depending on whether the UE requests 5GC access or EPS access, if the UE also supports EPS NAS.
For interworking with EPS, the UE that supports both 5GC and EPS NAS can operate in one of the following modes:
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Single-registration mode: UE has only one active MM state (either RM state in 5GC or EMM state in EPS) and it is either in 5GC NAS mode or in EPS NAS mode (when connected to 5GC or EPS, respectively).
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Dual-registration mode: UE handles independent registrations for 5GC and EPS using separate RRC connections. In this mode, the UE may be registered to 5GC only, EPS only, or to both 5GC and EPS.
Networks that support interworking with EPS, may support interworking procedures that use the N26 interface or interworking procedures that do not use the N26 interface.
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Interworking procedures with N26 support provide IP address continuity on inter-system mobility to UEs that support 5GC NAS and EPS NAS and that operate in single registration mode. Interworking procedures using the N26 interface, enables the exchange of MM and SM states between the source and target network.
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Networks that support interworking procedures without N26 support procedures to provide IP address continuity on inter-system mobility to UEs operating in both single-registration mode and dual-registration mode. For interworking without the N26 interface, IP address preservation is provided to the UEs on inter-system mobility by storing and fetching PGW-C+SMF and corresponding APN or DNN information via the HSS+UDM.
Important | Interworking of SMF and EPS currently works only with the N26 interface. |