Section: 4.11.1 N26 based Interworking Procedures 4.11.1.1 General
Content: N26 interface is used to provide seamless session continuity for single registration mode UE. Interworking between EPS and 5GS is supported with IP address preservation by assuming SSC mode 1. When the UE is served by the 5GC, during PDU Session establishment and GBR QoS Flow establishment, SMF+PGW-C performs EPS QoS mappings, from the 5G QoS parameters obtained from the PCF and allocates TFT with the PCC rules obtained from the PCF if PCC is deployed. Otherwise, EPS QoS mappings and TFT allocation are mapped by the SMF+PGW-C locally. The PGW+SMF ignores 5G QoS parameters that are not applicable to EPC (e.g. QoS Notification control). If a TFT is to be allocated for a downlink unidirectional EPS bearer mapped from a downlink only QoS Flow, the SMF+PGW-C shall allocate a TFT packet filter that effectively disallows any useful uplink packet as specified in TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13]. EPS Bearer IDs are allocated by the serving AMF requested by the SMF if the SMF determines that EPS Bearer IDs need to be assigned to the QoS Flows. For each PDU Session, EPS bearer IDs are allocated to the default EPS bearer and dedicated bearers. The SMF shall be able to determine the QoS flows that require EPS Bearer IDs, based on the QoS profile and operator policies. NOTE 1: Based on operator policies, an SMF can map all non-GBR QoS flows to default EPS bearer in which case it requests only one EBI for all the non-GBR QoS flows. Alternatively, an SMF can also map one non-GBR QoS flow to one dedicated EPS bearer in which case it requests a dedicated EBI for non-GBR QoS flow that should be mapped to dedicated EPS bearer. In between these two extreme cases, the SMF can also map more than one (but not all) non-GBR QoS Flow to the same EPS bearer (either default EPS bearer or dedicated EPS bearer). NOTE 2: To reduce the probability of AMF revoking the EBI corresponding to the QoS Flow associated with the default QoS rule, ARP priority level of dedicated QoS Flows can be set to higher value than that of the QoS Flow associated with the default QoS rule within the same PDU Session. When a new QoS Flow needs to be mapped to an EPS Bearer ID that has already been assigned for an existing QoS Flow, the SMF includes the already assigned EPS Bearer ID in the QoS Flow description sent to the UE. If there is a possibility to run into a restriction regarding the number of TFT packet filters that can be allocated for the PDU Session, - in the case that PCC is deployed, the PCF may include in the PCC rules the Precedence for TFT packet filter allocation parameter, which determines the order of the PCC rules in allocation of TFT packet filter(s) by the SMF+PGW-C. - in the case that PCC is deployed but the Precedence for TFT packet filter allocation parameter is not received, or in the case that PCC is not deployed, the SMF+PGW-C makes the decision how to allocate TFT packet filter(s) based on operator policy or implementation. NOTE 3: An EPS bearer can have up to 16 TFT packet filters. For Ethernet and Unstructured PDU Session Types, only EPS Bearer ID for the default EPS Bearer is allocated. The EPS Bearer IDs for these EPS bearers are provided to the SMF+PGW-C by the AMF and are provided to the UE and NG-RAN by the SMF+PGW-C using N1 SM NAS message and N2 SM message. The UE is also provided with the mapped QoS parameters. The UE and the SMF+PGW-C store the association between the QoS Flow and the corresponding EBI and the EPS QoS parameters. When the QoS Flow is deleted e.g. due to PDU Session status synchronization or PDU Session Modification, the UE and the SMF+PGW-C delete any possibly existing EPS QoS parameters associated with the deleted QoS Flow. In this release, for a PDU Session for a LADN or for Multi-homed IPv6 PDU Session, the SMF doesn't allocate any EBI or mapped QoS parameters. For PDU Sessions with UP integrity protection of UP Security Enforcement Information set to Required, the SMF does not allocate any EBI or mapped QoS parameters unless the UE support User Plane Integrity Protection with EPS and the AMF supports the associated functionality. If the UE supports User Plane Integrity Protection with EPS, as indicated in the S1 UE network capability and the AMF supports the related functionality, the AMF indicates the UE support for EPS User Plane Integrity Protection to SMF. Then, for PDU Sessions with UP integrity protection of UP Security Enforcement Information set to Required, the SMF may perform the EPS bearer ID allocation procedure as described in clause 4.11.1.4. If the MME indicates support for EPS User Plane Integrity Protection to SMF+PGW-C, the SMF+PGW-C provides User Plane Security Enforcement Information for the EPS bearer contexts to the MME (via the SGW). When the UE is served by the EPC, during PDN connection establishment, the UE allocates the PDU Session ID and sends it to the SMF+PGW-C via PCO. During PDN Connection establishment and dedicated bearer establishment, SMF+PGW-C performs EPS QoS mappings, from the 5G QoS parameters obtained from the PCF and allocates TFT with the PCC rules obtained from the PCF if PCC is deployed. Otherwise, EPS QoS mappings and TFT allocation are mapped by the SMF+PGW-C locally. Other 5G QoS parameters corresponding to the PDN connection, e.g. Session AMBR and QoS rules and QoS Flow level QoS parameters if needed for the QoS Flow(s) associated with the QoS rule(s), are sent to UE in PCO. The UE and the SMF+PGW-C store the association between the EPS Context and the PDU Session Context to use it in the case of handover from EPS to 5GS. During the EPS bearer establishment/modification procedure, QoS rules corresponding to the related EPS bearers are allocated and sent to UE in PCO. The 5G QoS parameters are stored in the UE and are to be used when the UE is handed over from EPS to the 5GS. The 5G QoS parameters may be provided to SMF+PGW-C by the PCF, if PCC is deployed. On mobility from EPS to 5GS, the UE sets the SSC mode of the mapped PDU Session to SSC mode 1. The UE and the SMF+PGW-C store the association between the EPS bearer and the corresponding 5G QoS Rules and QoS Flow level QoS parameters if needed for the QoS Flow(s) associated with the QoS rule(s). When the EPS bearer is deleted e.g. due to EPS bearer status synchronization or bearer deactivation, the UE and the SMF+PGW-C delete any possibly existing 5G QoS Rule(s) and QoS Flow level QoS parameters if any for the QoS Flow(s) associated with the QoS rule(s) associated with the deleted EPS bearer. In the roaming case, if the VPLMN supports interworking with N26, the UE shall operate in Single Registration mode. During the 5GS-EPS handover, indirect forwarding may apply for the downlink data forwarding performed as part of the handover. From its configuration data the AMF knows whether indirect forwarding applies and it requests to allocate downlink data forwarding paths on UPFs for indirect forwarding. From its configuration data the MME knows whether indirect forwarding applies and it requests to allocate downlink data forwarding paths on Serving GWs for indirect forwarding. It is configured on AMF and MME whether indirect downlink data forwarding does not apply, applies always or applies only for inter PLMN inter RAT handovers. During the 5GS-EPS handover, direct forwarding may apply for the downlink data forwarding performed as part of the handover. From its configuration data the source RAN node knows whether direct forwarding applies and indicates to source CN the direct data forwarding is available. During 5GS-EPS handover, on the target side, the CN informs the target RAN node whether data forwarding is possible or not. During interworking from EPS to 5GS, as the SMF+PGW-C may have different IP addresses when being accessed over S5/S8 and N11/N16 respectively, the AMF shall discover the SMF instance by an NF/NF service discovery procedure using the FQDN for the S5/S8 interface received from the MME as a query parameter. This is required for both non-roaming and roaming with local breakout, as well as for home routed roaming. NOTE 4: As the AMF is not aware of the S-NSSAI assigned for the PDN Connection, the NF/NF service discovery used to find the SMF instance can use PLMN level NRF. During interworking from 5GS to EPS, for QoS Flows without EPS bearer ID(s) assigned or for QoS Flows with PCC rules that do not have allocated TFT packet filters, the SMF+PGW-C deletes the PCC rules associated with those QoS Flows and informs the PCF about the removed PCC rule(s). During interworking from 5GS to EPS, as a PDU Session may be released while the UE is served by EPS, if Small Data Rate Control is used the SMF+PGW-C obtains the Small Data Rate Control Status from the PGW-U+UPF in the N4 Session Modification procedure or from the SCEF+NEF and passes the Small Data Rate Control Status in the PDU Session Context Response to the AMF, for the AMF to store. The time to store the Small Data Rate Control Statuses is implementation specific. If the UE and PGW-U+UPF / SCEF+NEF have stored APN Rate Control parameters and optionally APN Rate Control Status they are only applied when the UE is served by EPS. During interworking from 5GS to EPS, for PDU Sessions with UP integrity protection of UP Security Enforcement Information set to Required, the SMF+PGW-C does not provide the EPS bearer context unless both the UE, the target eNB and the target MME support User Plane Integrity Protection with EPS. During interworking from EPS to 5GS the UE and PGW-U+UPF / SCEF+NEF store the APN Rate Control parameters and APN Rate Control Status while the UE is served by 5GS, so they can be used if the UE moves back to EPS. At EPS to 5GS mobility: - The UE considers the PDN connections released if those PDN connections were established over EPS and for which the UE has not received mapped 5GS QoS parameters from the network. NOTE 5: UE not receiving mapped 5GS QoS parameters from the network covers the case that a UE did not provide a PDU Session ID to the network when establishing a PDN connection while UE's N1 mode is disabled and the case that a UE provided PDU Session ID where the network (SMF+PGW-C) does not provide mapped 5GS parameters. - The MME does not transfer to 5GS a PDN connection that does not support 5GS interworking, e.g. PDN connection was established on a stand-alone PGW, or 5GS interworking is restricted by subscription data.