6.1.4.2 Coordination between 5GSM and ESM without N26 interface

When the network does not support N26 interface, the SMF does not provide the UE with the mapped EPS bearer context for a PDU session.

NOTE 1: Since the SMF does not provide the UE with the mapped EPS bearer context for a PDU session, the UE does not know whether interworking with EPS is supported for a PDU session before attempting to transfer the PDU session from N1 mode to S1 mode.

NOTE 2: It is up to UE implementation to decide which PDU session(s) to be attempted to transfer from N1 mode to S1 mode, e.g. based on UE policy or UE local configuration.

Upon inter-system change from N1 mode to S1 mode in EMM-IDLE mode, the UE shall not transfer a PDU session for LADN to EPS.

Upon inter-system change from N1 mode to S1 mode in EMM-IDLE mode, the UE shall not transfer a multi-homed IPv6 PDU session to EPS.

Upon inter-system change from N1 mode to S1 mode in EMM-IDLE mode, the UE shall not transfer a PDU session with the selected SSC mode set to "SSC mode 2" or "SSC mode 3" to EPS.

Upon inter-system change from N1 mode to S1 mode in EMM-IDLE mode, the UE shall use the parameters from each PDU session which the UE intends to transfer to EPS to create the contents of a PDN CONNECTIVITY REQUEST message as follows:

• if the PDU session is an emergency PDU session, the request type shall be set to "handover of emergency bearer services". Otherwise the request type shall be set to "handover";
• the PDU session type of the PDU session shall be mapped to the PDN type of the default EPS bearer context as follows:
• the PDN type shall be set to "non-IP" if the PDU session type is "Unstructured";
• the PDN type shall be set to "IPv4" if the PDU session type is "IPv4";
• the PDN type shall be set to "IPv6" if the PDU session type is "IPv6";
• the PDN type shall be set to "IPv4v6" if the PDU session type is "IPv4v6";
• the PDN type shall be set to "non-IP" if the PDU session type is "Ethernet" and the UE, the network or both of them do not support Ethernet PDN type in S1 mode; and
• the PDN type shall be set to "Ethernet" if the PDU session type is "Ethernet" and the UE and the network support Ethernet PDN type in S1 mode;
• the DNN of the PDU session shall be mapped to the APN of the default EPS bearer context, unless the PDU session is an emergency PDU session;
• the PDU session ID parameter in the Protocol configuration options IE or Extended protocol configuration options IE shall be set to the PDU session identity of the PDU session; and
• if the PDU session is an MA PDU session established over 3GPP access, the ATSSS request parameter shall be included in the Protocol configuration options IE or Extended protocol configuration options IE.

After inter-system change from N1 mode to S1 mode, the UE shall associate the PDU session identity with the default EPS bearer context. If the PDU session being transferred is a non-emergency PDU session, the UE shall in addition associate the S-NSSAI and the PLMN ID of the current PLMN with the default EPS bearer context.

Upon successful completion of an EPS attach procedure after inter-system change from N1 mode to S1 mode (see 3GPP TS 24.301 [15]), the UE shall delete any UE derived QoS rules except when the PDU session is an MA PDU session established over 3GPP access and non-3GPP access both connected to the 5GCN.

The UE shall perform a local release of the PDU session(s) and QoS flow(s) associated with the 3GPP access which have not been transferred to EPS. The UE shall also perform a local release of any QoS flow description not associated with any QoS rule.

For PDU session(s) associated with non-3GPP access in 5GS, if present, the UE may:

• keep some or all of these PDU sessions still associated with non-3GPP access in 5GS, if supported;
• release some or all of these PDU sessions explicitly by initiating the UE requested PDU session release procedure(s); or
• attempt to transfer some or all of these PDU sessions from N1 mode to S1 mode by initiating the UE requested PDN connectivity procedure(s) with the PDN CONNECTIVITY REQUEST message created as above.

When the network does not support N26 interface, the MME does not provide the UE with the mapped PDU session for a PDN connection but provides the UE with an S-NSSAI if the PDN connection is not for emergency bearer services. When establishing a new PDN connection in S1 mode, to enable the UE to attempt to transfer the PDN connection from S1 mode to N1 mode in case of inter-system change, the UE shall allocate a PDU session identity, indicate the allocated PDU session identity in the PDU session ID parameter in the Protocol configuration options IE of the PDN CONNECTIVITY REQUEST message and associate the allocated PDU session identity with the default EPS bearer context of the PDN connection. If an N5CW device supporting 3GPP access establishes a new PDN connection in S1 mode, the N5CW device supporting 3GPP access shall refrain from allocating "PDU session identity value 15". The network provides the UE with an S-NSSAI and the related PLMN ID in the Protocol configuration options IE or Extended protocol configuration options IE of the ACTIVATE DEFAULT EPS BEARER REQUEST message, the UE shall delete the stored S-NSSAI and the related PLMN ID, if any, and shall store the S-NSSAI and the related PLMN ID provided in the ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST message.

NOTE 3: Since the MME does not provide the UE with the mapped PDU session for a PDN connection, the UE does not know whether interworking to 5GS is supported for a PDN connection for which the UE assigned a PDU Session identity before attempting to transfer the PDN connection from S1 mode to N1 mode.

NOTE 4: It is up to UE implementation to decide which PDN connection(s) to be attempted to transfer from S1 mode to N1 mode, e.g. based on UE policy or UE local configuration.

NOTE 5: If the PDN connection has been transferred from a PDN connection established via non-3GPP access to EPC, it is possible that the network provided the S-NSSAI already during the establishment via non-3GPP access (see 3GPP TS 24.302 [16]).

Upon inter-system change from S1 mode to N1 mode in 5GMM-IDLE mode, the UE uses the parameters from the default EPS bearer context of each PDN connection which the UE intends to transfer to 5GS and for which the UE has allocated a PDU session identity to create a PDU SESSION ESTABLISHMENT REQUEST message as follows:

• if the PDN connection is for emergency bearer services, the request type shall be set to "existing emergency PDU session". Otherwise the request type shall be set to:
• "MA PDU request", if the PDN connection to be transferred is a user-plane resource of an MA PDU session; or
• "existing PDU session";
• the PDN type of the default EPS bearer context shall be mapped to the PDU session type of the PDU session as follows:
• if the PDN type is "non-IP":
• the PDU session type is set to the locally available information associated with the PDN connection (either "Ethernet" or "Unstructured"), if available; or
• otherwise, the PDU session type is set to "Unstructured";
• if the PDN type is "IPv4" the PDU session type is set to "IPv4";
• if the PDN type is "IPv6", the PDU session type is set to "IPv6";
• if the PDN type is "IPv4v6", the PDU session type is set to "IPv4v6"; and
• if the PDN type is "Ethernet", the PDU session type is set to "Ethernet"; and
• the APN of the default EPS bearer context shall be mapped to the DNN of the PDU session, unless the PDN connection is an emergency PDN connection;
• the PDU session ID shall be set to the PDU session identity included by the UE in the Protocol configuration options IE or Extended protocol configuration options IE in the PDN CONNECTIVITY REQUEST message, or to the PDU session ID associated with the default EPS bearer context;
• if the PDU session is not an emergency PDU session, the S-NSSAI of the PDU session shall be set to the S-NSSAI included by the network in the Protocol configuration options IE or Extended protocol configuration options IE in the ACTIVATE DEFAULT EPS BEARER REQUEST message, if provided by the network, or the S-NSSAI associated with the default EPS bearer context, if available; and
• the SSC mode IE shall be set to "SSC mode 1".

NOTE 6: If T3584 is running or deactivated for the S-NSSAI and optionally the DNN combination, the UE is allowed to initiate ESM procedures in EPS with or without APN corresponding to that DNN, and if the APN is congested in EPS, the MME can send a back-off timer for the APN to the UE as specified in 3GPP TS 24.301 [15].

The UE shall locally release the PDN connection(s) and EPS bearer(s) associated with the 3GPP access which have not been transferred to 5GS.

Upon inter-system change from N1 mode to S1 mode, if the UE has any PDU sessions associated with one or more MBS multicast sessions, the UE shall locally leave the associated multicast MBS sessions and the network shall consider the UE as removed from the associated multicast MBS sessions.

Upon inter-system change from S1 mode to N1 mode, for the PDN connection(s) in the EPS transferred to the 5GS, if the UE supports reporting of URSP rule enforcement and is indicated to send URSP rule enforcement report to the network based on the matching URSP rule newly enforced while the UE was in S1 mode, which contains the URSP rule enforcement report indication set to "URSP rule enforcement report is required", the UE shall include the URSP rule enforcement reports IE in the PDU SESSION ESTABLISHMENT REQUEST message.