5G/NR - Network Architecture - N26  

 

 

 

Interworking with N26

N26 interface is an inter-CN(Core Network) interface between the MME and 5GS AMF in order to enable interworking between EPC and the NG core. Support of N26 interface in the network is optional for interworking.

Features of N26 Interface

Imporant features of N26 can be listed as below.

  • The N26 interface is an inter-Core Network interface between the 4G LTE's MME and the 5G core's 5GS AMF.
  • It facilitates interworking between 4G's EPC and 5G's NG core, especially for mobility between LTE-EPC and 5G-NR-5GS.
  • The interface ensures seamless continuity of voice services during inter-RAT handovers between LTE and 5G.
  • While optional, the presence of the N26 interface can reduce latency during Inter-RAT procedures.
  • The UE can operate in either Single Registration mode (one active MM state) or Dual Registration mode (independent registrations for 5GC and EPC).

How to figure out N26 is supported ?

As mentioned above, the support of N26 is optional (not mandatory). So there might be some mechanism for Network to inform UE on whether it support N26 or not. In addition, N26 is related both to LTE core (MME) and NR Core (AMF) meaning that there should be such a notification mechanism on both LTE and NR signaling procedure.

In NR, this notification is done by Registration Accept message and in LTE it is done by Attach Accept message as indicated by following IE (Information Element)

NR : Registration Accept -> 5GS network feature support

    Interworking without N26 interface indicator (IWK N26) (octet 3, bit 7)

    This bit indicates whether interworking without N26 interface is supported

    Bit 7

      0 ---> Interworking without N26 interface not supported : N26 is supported

      1 ---> Interworking without N26 interface supported  : N26 is not supported

    < Example >

    Protocol discriminator = 0x7e (5GS Mobility Management)

    Security header = 0x2 (Integrity protected and ciphered)

    Auth code = 0xc93c7d52

    Sequence number = 0x02

    Protocol discriminator = 0x7e (5GS Mobility Management)

    Security header = 0x0 (Plain 5GS NAS message, not security protected)

    Message type = 0x42 (Registration accept)

    5GS registration result = 0x09 (Emergency registered=0, NSSAA to be performed=0, SMS allowed=1, 3GPP access)

    ...

    5GS network feature support:

      0x03 (MPSI=0, IWK N26=0, EMF=not supported, EMC=not supported, IMS-VoPS-N3GPP=1, IMS-VoPS-3GPP=1)

      0x00 (5G-UP CIoT=0, 5G-IPHC-CP CIoT=0, N3 data=0, 5G-CP CIoT=0,

              RestrictEC=both CE mode A and CE mode B are not restricted, MCSI=0, EMCN3=0)

    ...

LTE : Attach Accept -> EPS network feature support

    Interworking without N26 interface indicator (IWK N26) (octet 4, bit 7)

    This bit indicates whether the network supports interworking procedure without N26 interface.

    Bit 7

      0 ---> Interworking without N26 not supported : N26 is supported

      1 --->  Interworking without N26 supported  : N26 is not supported

     

     

    < Example >

    Protocol discriminator = 0x7 (EPS Mobility Management)

    Security header = 0x2 (Integrity protected and ciphered)

    Auth code = 0xa585b8bd

    Sequence number = 0x01

    Protocol discriminator = 0x7 (EPS Mobility Management)

    Security header = 0x0 (Plain NAS message, not security protected)

    Message type = 0x42 (Attach accept)

    EPS attach result = 1 (EPS only)

    ....

    EPS network feature support:

      0x01 (CP CIoT=0, ERw/oPDN=0, ESRPS=0, CS-LCS=0, EPC-LCS=0, EMC BS=0, IMS VoPS=1)

      0x08 (15 bearers=0, IWK N26=0, RestrictDCNR=0, RestrictEC=0, ePCO=1, HC-CP CIoT=0, S1-U data=0,

              UP CIoT=0)

Single Registration Mode vs. Dual Registration Mode

Registration Mode refers to how a User Equipment (UE) manages its registration with the network. The registration mode determines the UE's ability to register with one or both of the 4G and 5G core networks simultaneously.

There are two primary registration modes:

Single Registration Mode:

  • In this mode, the UE maintains only one active Mobility Management (MM) state at a time
  • The UE can be registered to either the 5G Core (5GC) or the 4G Evolved Packet Core (EPC), but not both simultaneously(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
  • This means that if the UE is registered in the 5GC, it must deregister from the EPC, and vice versa.
  • It simplifies the registration process but may involve more steps when transitioning between 4G and 5G coverage areas.

Dual Registration Mode:

  • Here, the UE can handle independent registrations for both the 5GC and the EPC.
  • This allows the UE to be simultaneously registered to both the 5GC and the EPC (UE keeps independent registration for 5GC and EPC)
  • UE maintains 5G-GUTI and EPC-GUTI independently.
  • UE can perform 5GC or EPC re-registration/TAU using corresponding GUTIs.
  • The UE manages these registrations using separate Radio Resource Control (RRC) connections.
  • This mode facilitates smoother transitions between 4G and 5G networks, especially in areas where both network types are available.

N26 Architectures

 

< 23.501 - Figure 4.3.1-1: Non-roaming architecture for interworking between 5GS and EPC/E-UTRAN >

 

< 23.501 - Figure 4.3.2-1: Local breakout roaming architecture for interworking between 5GS and EPC/E-UTRAN >

 

< 23.501 - Figure 4.3.2-2: Home-routed roaming architecture for interworking between 5GS and EPC/E-UTRAN >

 

Reference