Section: 4.8.2.3.1 Interworking between NG-RAN and E-UTRAN
Content: At inter-system change from N1 mode to S1 mode in EMM-IDLE mode when: a) the UE supports non-IP PDN type and at least one PDU session of Unstructured PDU session type is active; b) the UE supports IPv4 PDN type and at least one PDU session of IPv4 PDU session type is active; c) the UE supports IPv6 PDN type and at least one PDU session of IPv6 PDU session type is active; d) the UE supports IPv4v6 PDN type and at least one PDU session of IPv4v6 PDU session type is active; or e) at least one PDU session of Ethernet PDU session type is active and: 1) the UE supports non-IP PDN type; or 2) the UE and the network support Ethernet PDN type in S1 mode; the UE shall proceed as follows: a) if the UE supports 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 and the UE has received an "interworking without N26 interface supported" indication from the network, the UE shall: 1) enter substates EMM-DEREGISTERED.NORMAL-SERVICE and 5GMM- DEREGISTERED.NO-CELL-AVAILABLE for 3GPP access; NOTE: Since MM context transfer is not possible between MME and AMF in a network that indicates "Interworking without N26 supported", it is up to the UE implementation as to how to keep the 5GMM and EMM states in the UE in sync. 2) map the PDU session(s) which the UE intends to transfer to EPS to the default EPS bearer context of the corresponding PDN connection(s) as specified in subclause 6.1.4.2; and 3) initiate an EPS attach procedure and include in the ATTACH REQUEST message a PDN CONNECTIVITY REQUEST message with: - the request type set to "handover of emergency bearer services" to activate a default EPS bearer context for an active emergency PDU session, if the session to be transferred is an emergency PDU session; or - the request type set to "handover" message to activate a default EPS bearer context for an active non-emergency PDU session, if the session to be transferred is a non-emergency PDU session. If the selected PDU session is an MA PDU session established over 3GPP access, the UE shall include the ATSSS request parameter in the Protocol configuration options IE or the Extended protocol configuration options IE of the ESM INFORMATION RESPONSE message. After successful completion of the EPS attach procedure, the UE shall reset the registration attempt counter for 3GPP access and the attach attempt counter (see 3GPP TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [15]), enter substates EMM-REGISTERED.NORMAL-SERVICE and 5GMM-REGISTERED.NO-CELL-AVAILABLE for 3GPP access and attempt to activate each of the other default EPS bearer contexts, if any, by initiating a stand-alone 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. If the EPS attach procedure is unsuccessful the UE shall enter substates EMM-DEREGISTERED.NORMAL-SERVICE and 5GMM-DEREGISTERED.NO-CELL-AVAILABLE for 3GPP access; and b) otherwise, enter substates EMM-REGISTERED.NORMAL-SERVICE and 5GMM-REGISTERED.NO-CELL-AVAILABLE for 3GPP access and initiate a tracking area update procedure (see 3GPP TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [15]). At inter-system change from N1 mode to S1 mode in EMM-IDLE mode when: a) the UE does not support non-IP PDN type or no PDU session of Unstructured PDU session type is active; b) the UE does not support IPv4 PDN type or no PDU session of IPv4 PDU session type is active; c) the UE does not support IPv6 PDN type or no PDU session of IPv6 PDU session type is active; d) the UE does not support IPv4v6 PDN type or no PDU session of IPv4v6 PDU session type is active; and e) no PDU session of Ethernet PDU session type is active or: 1) the UE does not support non-IP PDN type; and 2) the UE, the network or both do not support Ethernet PDN type in S1 mode; the UE shall enter substates EMM-DEREGISTERED.NORMAL-SERVICE and 5GMM-DEREGISTERED.NO-CELL-AVAILABLE for 3GPP access, and initiate an attach procedure. At inter-system change from S1 mode to N1 mode in 5GMM-IDLE mode, the UE shall: a) enter substate 5GMM-REGISTERED.NORMAL-SERVICE for 3GPP access and substate EMM-REGISTERED.NO-CELL-AVAILABLE; b) map the default EPS bearer context(s) of the PDN connection(s) which the UE intends to transfer to 5GS, if any, to the corresponding PDU session(s) as specified in subclause 6.1.4.2; and c) initiate the registration procedure for mobility and periodic registration update over 3GPP access indicating "mobility registration updating" in the 5GS registration type IE of the REGISTRATION REQUEST message (see subclause 5.5.1.3). After having successfully registered in N1 mode over 3GPP access, the UE shall reset the registration attempt counter for 3GPP access, and the attach attempt counter or tracking area updating attempt counter (see 3GPP TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [15]) and: a) if the UE supports the PDU session establishment procedure with request type set to "existing PDU session" or "existing emergency PDU session" to transfer a PDN connection from S1 mode to N1 mode and the UE has received an "interworking without N26 interface supported" indication from the network, attempt to transfer the PDN connection(s) which the UE intends to transfer to 5GS, if any, from S1 mode to N1 mode by: - if the PDN connection which the UE intends to transfer is a PDN connection for emergency bearer services, initiating the PDU session establishment procedure with request type 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, initiating the PDU session establishment procedure with request type set to: 1) "MA PDU request", if the PDN connection to be transferred is a user-plane resource of an MA PDU session; or 2) "existing PDU session" to transfer the non-emergency PDN connection; and b) otherwise, establish PDU session(s) corresponding to the PDN connection(s) which the UE intends to transfer to 5GS, if any, by initiating the PDU session establishment procedure with request type set to "initial request". See subclause 5.1.4.3 for coordination between 5GMM and EMM and subclause 6.1.4.2 for coordination between 5GSM and ESM.