5GC, EPC interworking without N26 Interface

Technical Product Manager at Viavi Solutions

Telecom Professional

There can be single or dual registration mode UE supports to register with 5GC and EPC

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

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.

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.