5GC, EPC interworking without N26 Interface

5GC, EPC interworking without N26 Interface

Inter-working Concepts

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.

prasanna

I am Prasanna Sahu. I live in Dublin Ireland. I work in 3gpp wireless technology like UMTS and LTE and 5G. I love Photography, painting. Know more about me: http://in.linkedin.com/pub/prasanna-sahu/29/257/91a
See my photographs: http://www.flickr.com/photos/24986299@N05/

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prasanna

I am Prasanna Sahu. I live in Dublin Ireland. I work in 3gpp wireless technology like UMTS and LTE and 5G. I love Photography, painting. Know more about me: http://in.linkedin.com/pub/prasanna-sahu/29/257/91a See my photographs: http://www.flickr.com/photos/24986299@N05/