There can be single or dual registration mode UE supports to register with 5GC and EPC
Single Registration Mode:
- In single registration mode there is only one active mobility state at any given time.
- UE can be either in 5GC NAS mode or EPC NAS mode.
- UE maps EPC-GUTI to 5G-GUTI during mobility between EPC and 5GC.
- UE keeps 5G context for re-use when moving from 5GC to EPC
Dual Registration mode:
- In Dual registration mode UE keeps independent registration for 5GC and EPC.
- In this mode UE maintains 5G-GUTI and EPC-GUTI independently.
- UE can perform 5GC or EPC re-registration/TAU using corresponding GUTIs.
Inter-working architecture without N26 interface
5G to EPS mobility without N26 interface flow
Message flow steps
- Initially UE is registered to 5GC and establishes the PDU session.
- In order to move to EPS system, UE triggers Attach request(to preserver IP address) to EPC.
- Alternatively UE sends a Tracking area Update which fails and then UE initiates an Attach procedure. In this case there is no IP address preservation.
- Single Registered mode UEs provide a EPS-GUTI derived from 5GC-GUTI if available else uses IMSI.
- Dual Registered mode UEs provide 4G GUTI(if allocated earlier) with attach request.
- In PDN connectivity request, UE sends Request type as “Handover” to indicate that it is moving from 5GS to EPS.
- If Request Type is Handover and previous node is 5G AMF then MME is configured to send a location update request to HSS/UDM to tell them not to cancel Old AMF registration, as UE may move back to 5GS again.
- After successful Attach and PDN connection PGW-C/SMF allocates same IP address as the 5GS.
- And PGW-C/SMF initiates UE De-Registration and PDU Session release from 5GS.
EPS to 5G mobility without N26 interface flow
Message flow Steps
- Initially UE is Attached to EPC and establishes the PDN Connection.
- In order to move to 5G system, UE triggers Registration and PDU session procedure in 5GS.
- In single registration mode, UE provides Registration Type as “mobility Registration update” and provides 5G-GUTI derived from 4G-GUTI.
- UE in Dual Registration mode Provides Registration type as “initial registration” along with a native 5G-GUTI or SUPI.
- UE will include, NSSAI from current serving PLMN.
- AMF supports interworking with EPC, so it treats this as initial registration and skips PDU session status synchronization with SMF.
- If UE is moving in step 1, then AMF updates UE location in HSS/UDM and tells them not to cancel the MME registration for the UE if any by setting ULR-Flags.
- During PDU session establishment with 5GS, UE indicates “Existing PDU session” as Request Type to preserve the IP address.
- After Successful Registration and PDU session establishment, PGW-C triggers the PDN connection release and detach of the UE from EPS.