In the evolution from 4G to 5G, interworking between the Evolved Packet System (EPS) and the 5G System (5GS) is a critical architectural component. 3GPP specifications define precise mechanisms to handle how a User Equipment (UE) manages its registration state when moving between these two domains.
The two primary modes of operation defined for UEs supporting both 5GC and EPC NAS are Single Registration Mode and Dual Registration Mode.
1. Single Registration Mode (N1 Mode <-> S1 Mode)
As defined in 3GPP TS 23.501, Single Registration mode is mandatory for UEs that support both 5GC and EPC NAS. In this configuration, the UE maintains a single coordinated registration state.
Key Architectural Characteristics:
- One Active MM State: The UE holds only one active Mobility Management (MM) state at any given time. It is either in:
- 5GC: 5GMM-REGISTERED state (RM state).
- EPC: EMM-REGISTERED state.
- NAS Mode Exclusivity: The UE operates either in 5GC NAS mode or EPC NAS mode. It cannot be registered to both Core Networks simultaneously.
Mobility and Identity Mapping:
Seamless mobility between the systems relies on the mapping of temporary identities (GUTI):
- Mobility from EPS to 5GC: The UE maps its existing EPS-GUTI to a 5G-GUTI to perform Registration in 5GC.
- Mobility from 5GC to EPC: The UE maps its existing 5G-GUTI to an EPS-GUTI to perform a Tracking Area Update (TAU) or Attach in EPC.
Note on Security Context Retention: When the UE moves from 5GC to EPC, it retains the native 5G-GUTI and the native 5G Security Context. These are stored locally and re-used if the UE returns to 5GC, optimizing the re-registration process.
2. Dual Registration Mode
Dual Registration mode allows for independent operation between the two domains. This mode is specifically intended for interworking between EPS/E-UTRAN and 5GS/NR where simultaneous connectivity or independent registration management is required.
Key Architectural Characteristics:
- Independent Registrations: The UE handles registrations for 5GC and EPC independently.
- Separate RRC Connections: The UE maintains separate Radio Resource Control (RRC) connections for the respective radio access technologies.
- Registration Flexibility: In this mode, the UE may be:
- Registered to 5GC only.
- Registered to EPC only.
- Registered to both 5GC and EPC simultaneously.
Identity Management & Capability Indication:
Unlike Single Registration mode, the UE maintains distinct identities for each domain:
- 5GC: The UE maintains and provides a native 5G-GUTI (if previously allocated) for registrations towards 5GC.
- EPC: The UE maintains and provides a native EPS-GUTI (if previously allocated) for Attach/TAU towards EPC.
UE Network Capability Signaling:
To ensure the network is aware of the UE’s interworking capabilities:
- During E-UTRAN Initial Attach: A UE supporting both NAS stacks must indicate its support of 5G NAS in the UE Network Capability Information Element (IE).
- During Registration to 5GC: A UE supporting both NAS stacks must indicate its support of EPC NAS.
Comparison: Single vs. Dual Registration
| Feature | Single Registration Mode | Dual Registration Mode |
| MM State | Only one active state (5GC or EPC) | Independent states (Can be both) |
| GUTI Handling | Maps EPS-GUTI $\leftrightarrow$ 5G-GUTI | Maintains native GUTIs independently |
| Simultaneous Registration | No | Yes |
| Mandatory Support | Yes (for UEs supporting both NAS) | Optional (Network/UE capability dependent) |
| Context Retention | Retains native 5G Context while in EPC | Maintains separate contexts |