4.8.2.3.2 Interworking between TNGF or N3IWF connected to 5GCN and E-UTRAN

If:

• the UE has registered in neither N1 mode over 3GPP access nor S1 mode yet; and
• the UE has at least one active PDU session associated with non-3GPP access which the UE intends to transfer to EPS,

the UE shall initiate an EPS attach procedure and include a PDN CONNECTIVITY REQUEST message with a request type in the ATTACH REQUEST message to activate a default EPS bearer context for one of the active PDU sessions which the UE intends to transfer to EPS (see 3GPP TS 24.301 [15]). The request type is set as follows:

• if the PDU session which the UE intends to transfer is a non-emergency PDU session, the request type is set to "handover"; and
• if the PDU session which the UE intends to transfer is an emergency PDU session, the request type is set to "handover of emergency bearer services" and the default bearer to be activated is the default EPS bearer context for the emergency PDU session.

NOTE 1: It is necessary for the UE to support sending an ATTACH REQUEST message containing a PDN CONNECTIVITY REQUEST message with request type set to "handover" or "handover of emergency bearer services" to transfer a PDU session from N1 mode to S1 mode for interworking between TNGF or N3IWF connected to 5GCN and E-UTRAN.

NOTE 2: The order of PDU sessions to be tranferred to EPS is up to UE implementation.

After successful completion of the EPS attach procedure where the activated default EPS bearer context is not for emergency service, the UE shall initiate a UE requested PDN connectivity procedure with request type set to "handover" for non-emergency PDU session or "handover of emergency bearer services" for emergency PDU session in the PDN CONNECTIVITY REQUEST message to transfer each of the other PDU sessions which the UE intends to transfer to EPS, if any.

If:

• the UE has not registered in N1 mode over non-3GPP access yet; and
• the UE has at least one active PDN connection which the UE intends to transfer to TNGF or N3IWF connected to 5GCN,

the UE shall initiate an initial registration procedure over non-3GPP access (see clause 5.5.1.2).

After successful completion of the 5GS initial registration in N1 mode over non-3GPP access, the UE shall initiate a UE-requested PDU session establishment procedure with a request type to transfer each of the PDN connections which the UE intends to transfer to TNGF or N3IWF connected to 5GCN, if any. The request type is set as follows:

• if the PDN connection which the UE intends to transfer is a PDN connection for emergency bearer services, the request type is set to "existing emergency PDU session" to transfer the PDN connection for emergency bearer services; and
• if the PDN connection which the UE intends to transfer is a non-emergency PDN connection, the request type is set to "existing PDU session" to transfer the non-emergency PDN connection.

NOTE 3: If the UE has no active PDU session associated with non-3GPP access which the UE in N1 mode intends to transfer to EPS or no active PDN connection associated with 3GPP access which the UE in S1 mode intends to transfer to TNGF or N3IWF connected to 5GCN, the interworking between TNGF or N3IWF connected to 5GCN and E-UTRAN is not supported.

See clause 6.1.4.2 for coordination between 5GSM and ESM.