How it Works

To interwork with EPS, a UE that supports both 5GC and EPS NAS works in one of the following modes:

  • Single-registration Mode—In this mode, the UE has only one active MM state, which is either the RM state in 5GC or EMM state in EPS. In addition, this state is either in 5GC NAS mode or in EPS NAS mode when connected to 5GC or EPS, respectively.

  • Dual-registration Mode—In this mode, the UE handles independent registrations for 5GC and EPS using separate RRC connections. The UE may be registered to 5GC only, EPS only, or to both 5GC and EPS.

Alternate RAT Tunnel Creation for 4G, 5G, and Wi-Fi

SMF creates an alternate RAT tunnel when the inter-PLMN handover CLI is enabled under the DNN profile. In addition, SMF creates this tunnel based on meeting the specific conditions. The following table lists the conditions for creation and non-creation of this tunnel.

Conditions for Alternate RAT Tunnel Creation and Non-Creation

Create Session Request

N11 SMContext Create

Subscription Data

Alternate RAT Tunnel Creation

epsInterworkingInd’: ‘WITH_N26’

dnnInfo->IwkEpsInd=true

Yes

epsInterworkingInd’: ‘WITH_N26’

dnnInfo->IwkEpsInd=false

No

epsInterworkingInd’: ‘WITHOUT_N26’

dnnInfo->IwkEpsInd=false

No

epsInterworkingInd’: ‘WITHOUT_N26’

dnnInfo->IwkEpsInd=true

No

IWK_5GS indication flag=true, pduSessionId received from UE

Yes

dnnInfo->IwkEpsInd=true

Yes

IWK_5GS indication flag=true, pduSessionId received from UE

No

dnnInfo->IwkEpsInd=false

No

IWK_5GS indication flag=false, pduSessionId not received from UE

Yes

dnnInfo->IwkEpsInd=false

No

IWK_5GS indication flag=false, pduSessionId not received from UE

No

dnnInfo->IwkEpsInd=true

No