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Content for  TS 23.502  Word version:  17.5.0

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4.11  System interworking procedures with EPCWord‑p. 222

4.11.0  GeneralWord‑p. 222

Clause 4.11 includes the following:
In clause 4.11, UEs are assumed to support both 5GC NAS and EPC NAS unless explicitly stated otherwise.
The procedures in clause 4.11 are not applicable for Disaster Roaming service (see clause 5.40 of TS 23.501).
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4.11.0a  Impacts to EPS ProceduresWord‑p. 222

4.11.0a.1  GeneralWord‑p. 222

This clause captures changes to procedures in TS 23.401 that are common to interworking based on N26 and interworking without N26.

4.11.0a.2  Interaction with PCCWord‑p. 222

When interworking with 5GS is supported and a "SMF+PGW-C" is selected for a PDN connection, policy interactions between PDN GW and PCRF specified in TS 23.401 are replaced by equivalent interactions between SMF+PGW-C and PCF as follows:
  • IP-CAN Session Establishment procedure defined in TS 23.203 is replaced by SM Policy Association Establishment Procedure as described in clause 4.16.4. The SMF+PGW-C includes the information elements received in Create Session Request message into the Npcf_SMPolicyControl_Create Service as follows: the SUPI contains the IMSI, the DNN contains the APN, the PEI contains either the IMEISV or IMEI, the Session AMBR contains the APN-AMBR and the default QoS information that contains the default EPS bearer QoS, note that QCI values are mapped into 5QI values. The SMF+PGW-C may receive PCC Rules and PDU Session Policy Information, 5G QoS information in the PCC Rule and in PDU Session Policy Information are mapped into EPS QoS information as defined in clause 4.11.1.1 and Annex C.
  • (PCEF-initiated) IP-CAN Session Modification procedure defined in TS 23.203 is replaced by SM Policy Association Modification procedure as described in clause 4.16.5.1.
  • The SMF+PGW-C includes the information elements received in Modify Bearer Request or Modify Bearer Command message into the Npcf_SMPolicyControl_Update Service with the following modifications, the subscribed Session AMBR includes the subscribed APN-AMBR and subscribed default QoS information includes the default EPS bearer QoS, note that QCI values are mapped into 5QI values. The SMF+PGW-C includes the stored SUPI. The SMF+PGW-C may receive PCC Rules and PDU Session Policy Information, 5G QoS information in the PCC Rule and in PDU Session Policy Information are mapped into EPS QoS information as defined in clause 4.11.1.1 and Annex C.
  • The SMF+PGW-C includes the information elements received in Delete Bearer Command message into the Npcf_SMPolicyControl_Update Service with the following modifications, The SMF+PGW-C includes the stored SUPI.
  • (PCRF-initiated) IP-CAN Session Modification procedure defined in TS 23.203 is replaced by SM Policy Association Modification procedure as described in clause 4.16.5.2. The SMF+PGW-C may receive PCC Rules and PDU Session Policy Information, 5G QoS information in the PCC Rule and in PDU Session Policy Information are mapped into EPS QoS information as defined in clause 4.11.1.1 and Annex C.
  • IP-CAN Session Termination procedure defined in TS 23.203 is replaced by SM Policy Association Termination procedure as described in clause 4.16.6. The SMF+PGW-C includes the information elements received in Delete Session Request message by the SMF+PGW-C into the Npcf_SMPolicyControl_Delete Service. The SMF+PGW-C includes the stored SUPI.
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4.11.0a.3  Mobility RestrictionsWord‑p. 223

The UE's subscription may include access restriction for NR in 5GS and restriction for Core Network Type (5GC). If so, the HSS provides these restrictions to the MME. The MME includes these restrictions in the Handover Restriction List to the E-UTRAN. The MME and E-UTRAN use these restrictions to determine if mobility of the UE to 5GS or NR connected to 5GS should be permitted.

4.11.0a.4  PGW SelectionWord‑p. 223

When the UE requests to establish a non-emergency PDN connection to an APN, the MME may use the UE's support for 5GC NAS indication included in the UE Network Capability and/or UE's subscription from HSS that includes UE's mobility restriction parameters related to 5GS and/or indication of support for interworking with 5GS for this APN to determine if SMF+PGW-C or a standalone PGW-C should be selected. If both PGW-C and SMF+PGW-C is available, then MME may select SMF+PGW-C when UE's subscription from HSS indicate support for interworking with 5GS for the APN.
When the UE performs emergency attach or requests to establish an emergency PDN connection, the MME may use the UE's support for 5GC NAS indication included in the UE Network Capability and/or local configuration to determine if an emergency SMF+PGW-C or a standalone emergency PGW-C should be selected. An emergency SMF+PGW-C needs to be configured in Emergency Configuration Data in the MME if it is to be selected.
To enable a differentiation of the emergency gateway based on UE's support of 5GC NAS, the Emergency Configuration Data in the MME defined in TS 23.401 is extended with the IE listed in Table 4.11.0a.4-1.
Field Description
Emergency PDN GW-C/SMF identityThe statically configured identity of a combined PDN GW-C/SMF used for emergency APN. The PDN GW-C/SMF identity may be either an FQDN or an IP address. (NOTE)
NOTE:
The FQDN always resolves to one PDN GW.
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4.11.0a.5  PDN Connection EstablishmentWord‑p. 224

During establishment of non-emergency PDN connection in the EPC, the UE and the SMF+PGW-C exchange information via PCO as described in clause 5.15.7 of TS 23.501. For UE with 5GC NAS capability disabled (i.e. N1 mode is disabled), the UE may also allocate a PDU Session ID and send it to the SMF+PGW-C via PCO. If the SMF+PGW-C supports more than one S-NSSAI and the APN is valid for more than one S-NSSAI, before the SMF+PGW-C provides an S-NSSAI to the UE, the SMF+PGW-C should check such that the selected S-NSSAI is among the UE's subscribed S-NSSAIs and that the S-NSSAI is not subject to Network Slice-Specific Authentication and Authorization, by retrieving the Subscribed S-NSSAI from UDM using the Nudm_SDM_Get service operation (the SMF+PGW-C discovers and selects a UDM as described in clause 6.3.8 of TS 23.501). If the SMF+PGW-C is in a VPLMN, the SMF+PGW-C uses the Nnssf_NSSelection_Get service operation to retrieve a mapping of the Subscribed S-NSSAIs to Serving PLMN S-NSSAI values. If the S-NSSAIs supported by the SMF+PGW-C are all subject to NSSAA, then the SMF+PGW-C should reject the PDN connection establishment. If the selected S-NSSAI is subject to NSAC and EPS counting is required for the S-NSSAI, the SMF+PGW-C uses the Nnsacf_NSAC_NumberOfUEsUpdate services operation and/or the Nnsacf_NSAC_NumberOfPDUsUpdate services operation to check if the selected S-NSSAI is available as described in clause 4.11.5.9. The SMF+PGW-C uses the Nudm_SDM_Subscribe service operation to subscribe the change of the Session Management Subscription data. If the SMF+PGW-C is notified from UDM with subscription data change, the SMF+ PGW-C takes actions for the PDN connection as described in clause 5.17.2.1 of TS 23.501.
As described in TS 23.548, during establishment of a PDN connection, a UE that hosts EEC(s) may indicate to the SMF+PGW-C, in the PCO, that it supports the ability to receive ECS address(es) via NAS and to transfer the ECS Address(es) to the EEC(s). If the UE indicated in the PCO that it supports the ability to receive ECS address(es) via NAS, the SMF+PGW-C may provide the ECS Address Configuration Information (as described in clause 6.5.2 of TS 23.548) to the UE in the PCO. The SMF+PGW-C may derive the Edge Configuration Server Information based on local configuration, the UE's location, and/or UE subscription information.
The SMF+PGW-C may use the bearer modification procedure without bearer QoS update to send the UE a PCO with updated ECS Address Configuration Information as defined in clause 6.5.2 of TS 23.548 to the UE.
During establishment of non-emergency PDN connection in the EPC, if PGW-C+SMF is selected for a UE that has 5GS subscription, the SMF may be configured to obtain the subscribed IP index from UDM as part of subscription data using the Nudm_SDM_Get service operation (the PGW-C+SMF discovers and selects a UDM as described in clause 6.3.8 of TS 23.501).
During establishment of non-emergency PDN connection in the EPC, if SMF+PGW-C is selected for a UE that has 5GS subscription but does not support 5GC NAS and is accessing via EPC/E-UTRAN and if the SMF+PGW-C supports more than one S-NSSAI and the APN is valid for more than one S-NSSAI, the SMF+PGW-C+PGW-C may proceed as specified in first paragraph of this clause or select any S-NSSAI associated with the APN of the PDN connection. The SMF+PGW-C shall not provide any 5GS related parameters to the UE.
During establishment of emergency PDN connection:
  • The SMF+PGW-C is to be derived from the emergency APN or to be statically configured in the Emergency Configuration Data in MME.
  • 5GC interworking support with N26 or without N26 is determined based on UE's 5G NAS capability and local configuration (in the Emergency Configuration Data in MME).
  • The S-NSSAI configured for the emergency APN in SMF+PGW-C is not sent to the UE by the SMF+PGW-C. One S-NSSAI is configured for the emergency APN.
During establishment of non-emergency PDN connection and emergency PDN connection, if SMF+PGW-C is selected for a UE that does not support 5GC NAS, the SMF+PGW-C creates unique PDU Session ID for each PDN connection of the UE.
  • The unique PDU Session ID can be created based on the EPS Bearer IDs assigned by the MME for the PDN Connections associated with the UE and not be in the range of PDU Session ID values that can be created by a 5GC NAS capable UE.
  • If handover between EPS and EPC/ePDG (as specified in clause 8.6 of TS 23.402) is required, the SMF+PGW-C, based on operator configuration, may perform the following to ensure the uniqueness of PDU Session ID:
  • the SMF+PGW-C queries the UDM for any PDU Session ID(s) that are already registered in the UDM for the UE by invoking Nudm_UECM_Get service operation.
  • the SMF+PGW-C creates a PDU session ID that does not collide with the received PDU session ID(s).
  • When the SMF+PGW-C establishes the PDN connection successfully, the SMF+PGW-C performs UDM registration using the Nudm_UECM_Registration service operation.
When the SMF+PGW-C establishes the PDN connection successfully, the SMF+PGW-C provides the ID of the PCF ID selected for the PDN connection in the UDM using the Nudm_UECM_Registration service operation.
A SMF+PGW-C may support L2TP as described in clause 4.3.2.4. In this case step 1 and step 7 of Figure 4.3.2.4-1 correspond to a PDN Connection establishment and a SMF+PGW-C replaces the SMF in that Figure.
To support User Plane Integrity Protection with EPS and policies that Require User Plane integrity protection to be used, at PDN connection establishment, the MME shall indicate to the SMF+PGW-C (via the Serving GW) whether the UE, the current eNB and the MME support User Plane Integrity Protection in EPS. If the MME and the UE support User Plane Integrity Protection, then the SMF+PGW-C informs the MME of the User Plane integrity protection policy (Required, Preferred, Not Needed) applicable to the PDN connection on a per-EPS bearer basis. In turn, the MME informs the eNB.
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4.11.0a.6  Network Configuration |R16|Word‑p. 225

To avoid the need for identifier coordination between MMEs, AMFs and SGSNs, the MME provides to the eNB its served GUMMEIs by separating the values between native MME GUMMEI values, values mapped from AMF and values mapped from SGSN.

4.11.0a.7  Interactions with DN-AAA Server |R16|Word‑p. 225

In this version of the specification, secondary authentication with a DN-AAA Server, as specified in clause 4.3.2.3, is not supported when the UE is in EPS. Secondary authorization (without authentication of the UE) with a DN-AAA Server, as specified in clause 4.3.2.3, is supported when the UE is in EPS.
If secondary authentication and authorization has been performed for the PDU Session while the UE was in 5GS and the UE has moved to EPS, the following applies:
  • DN-AAA re-authorization (without re-authentication signalling) can be performed even when the UE is in EPS, e.g. to provide new parameters from the DN-AAA Server to SMF+PGW-C.
  • Re-authentication cannot be performed while the UE is in EPS because there is no support of the related signalling in EPS. In case the SMF+PGW-C receives a re-authentication request from the DN-AAA, the SMF+PGW-C informs the DN-AAA Server that the UE is not available for re-authentication at the moment. The SMF+PGW-C should not initiate PDN connection release at this point: the DN-AAA decides the actions to take, based on the reply from SMF+PGW-C and local policy which may also trigger a DN-AAA Server request to release the PDU Session/PDN connection of the UE.
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4.11.0a.8  5GC NAS capability (re-)enabled and disabled |R17|Word‑p. 226

When 5G NAS (i.e. N1 mode) capability is (re-)enabled, the UE triggers Tracking Area Update procedure as specified in clause 5.3.3.0 of TS 23.401.
If the MME determines that interworking with 5GS is changed to Supported and if the MME has selected a standalone PGW, the MME may initiate PDN disconnection with reactivation required as specified in clause 5.10.3 of TS 23.401 at the end of the tracking area update procedure.
If a UE disables its support of N1 mode and moves from 5GS to EPS, the UE and the SMF+PGW-C maintain the mapped 5GS parameters for PDU session(s) that are transferred to EPS.
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4.11.0a.9  PDN Connection Release |R17|Word‑p. 226

If the SMF+PGW-C has registered with the UDM, the SMF+PGW-C shall deregister with UDM (using Nudm_UECM_Deregistration) during the PDN Connection Release procedure.

4.11.1  N26 based Interworking ProceduresWord‑p. 226

4.11.1.1  GeneralWord‑p. 226

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. 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.
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.
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.
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.
  • 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.
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4.11.1.2  Handover proceduresWord‑p. 228

4.11.1.2.1  5GS to EPS handover using N26 interfaceWord‑p. 228
Figure 4.11.1.2.1-1 describes the handover procedure from 5GS to EPS when N26 is supported.
In the case of handover to a shared EPS network, the source NG-RAN determines a PLMN to be used in the target network as specified by TS 23.501. The source NG-RAN shall indicate the selected PLMN ID to be used in the target network to the AMF as part of the TAI sent in the HO Required message.
In the case of handover from a shared NG-RAN, the AMF may provide the MME with an indication that the 5GS PLMN is a preferred PLMN at later change of the UE to a 5GS shared networks.
During the handover procedure, as specified in clause 4.9.1.3.1, the source AMF shall reject any SMF+PGW-C initiated N2 request received since handover procedure started and shall include an indication that the request has been temporarily rejected due to handover procedure in progress.
Upon reception of a rejection for an SMF+PGW-C initiated N2 request(s) with an indication that the request has been temporarily rejected due to handover procedure in progress, the SMF+PGW-C behaves as specified in TS 23.401.
Reproduction of 3GPP TS 23.502, Fig. 4.11.1.2.1-1: 5GS to EPS handover for single-registration mode with N26 interface
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The procedure involves a handover to EPC and setup of default EPS bearer and dedicated bearers for QoS Flows that have EBI assigned, in EPC in steps 1-16 and re-activation, if required, of dedicated EPS bearers for non-GBR QoS Flows that have no EBI assigned, in step 19. This procedure can be triggered, for example, due to new radio conditions, load balancing or in the presence of QoS Flow for normal voice or IMS emergency voice, the source NG-RAN node may trigger handover to EPC.
For Ethernet and Unstructured PDU Session Types, the PDN Type Ethernet and non-IP respectively are used, when supported, in EPS.
When EPS supports PDN Type non-IP but not PDN type Ethernet, PDN type non-IP is used also for Ethernet PDU sessions. The SMF shall also set the PDN Type of the EPS Bearer Context to non-IP in this case. After the handover to EPS, the PDN Connection will have PDN Type non-IP, but it shall be locally associated in UE and SMF to PDU Session Type Ethernet or Unstructured respectively.
In the roaming home routed case, the SMF+PGW-C always provides the EPS Bearer ID and the mapped QoS parameters to UE. The V-SMF caches the EPS Bearer ID and the mapped QoS parameters obtained from H-SMF for this PDU session. This also applies in the case that the HPLMN operates the interworking procedure without N26.
Step 1.
NG-RAN decides that the UE should be handed over to the E-UTRAN. If NG-RAN is configured to perform Inter RAT mobility due to IMS voice fallback triggered by QoS flow setup and request to setup QoS flow for IMS voice was received, NG-RAN responds indicating rejection of the QoS flow establishment because of mobility due to fallback for IMS voice via N2 SM information and triggers handover to E-UTRAN. The NG-RAN sends a Handover Required (Target eNB ID, Direct Forwarding Path Availability, Source to Target Transparent Container, inter system handover indication) message to the AMF. NG-RAN indicates bearers corresponding to the 5G QoS Flows for data forwarding in Source to Target Transparent Container.
If the source NG RAN and target E-UTRAN support RACS as defined in TS 23.501, the Source to Target transparent container need not carry the UE radio access capabilities (instead the UE Radio Capability ID is supplied from the CN to the target E-UTRAN). However, if the source NG-RAN has knowledge that the target E-UTRAN might not have a local copy of the Radio Capability corresponding to the UE Radio Capability ID (i.e. because the source NG-RAN had itself to retrieve the UE's Radio Capability from the AMF) then the source NG-RAN may also send some (or all) of the UE's Radio Capability to the target E-UTRAN (the size limit based on configuration). In the case of inter-PLMN handover, when the source NG-RAN and target E-UTRAN support RACS as defined in TS 23.501 and TS 23.401 and the source NG-RAN determines that the target PLMN does not support the UE Radio Capability ID assigned by the source PLMN based on local configuration, then the source NG-RAN includes the UE radio access capabilities in the Source to Target transparent container.
Direct Forwarding Path Availability indicates whether direct forwarding is available from the NG-RAN to the E-UTRAN. This indication from NG-RAN can be based on e.g. the presence of IP connectivity and security association(s) between the NG-RAN and the E-UTRAN.
If the handover is triggered due to Emergency fallback, the NG-RAN may forward the Emergency indication to the target eNB in the Source to Target Transparent Container and the target eNB allocates radio bearer resources taking received indication into account.
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Step 2a-2c.
TThe AMF determines from the 'Target eNB Identifier' IE that the type of handover is Handover to E-UTRAN. The AMF selects an MME as described in clause 4.3.8.3 of TS 23.401.
The AMF determines for a PDU Session whether to retrieve context including mapped UE EPS PDN Connection from the V-SMF (in the case of HR roaming) or the SMF+PGW-C (in the case of non roaming or LBO roaming) as follows:
  • If the AMF determines that one or more of the EBI(s) can be transferred, the AMF sends Nsmf_PDUSession_ContextRequest to the V-SMF or SMF+PGW-C and includes in the message EBI value(s) if any that cannot be transferred.
  • The EBI values(s) that cannot be transferred is determined by the AMF if the target MME does not support 15 EPS bearers, i.e. the AMF determines the EBI values in range 1-4 as not to be transferred to EPS and if there are still more than 8 EBI values associated with PDU Sessions, the AMF then determines EBI value(s) not to be transferred to EPS based on S-NSSAI and ARP as specified in clause 5.17.2.2.1 of TS 23.501.
  • The AMF does not retrieve the context for a PDU Session that cannot be transferred to EPS due to no EBI allocated, or allocated EBIs not transferrable, or combination of the two.
When the AMF sends Nsmf_PDUSession_ContextRequest the AMF provides also the target MME capability to the V-SMF or the SMF+PGW-C to allow it to determine whether to include EPS Bearer context for Ethernet PDN Type or non-IP PDN Type or not.
When the AMF sends Nsmf_PDUSession_ContextRequest to the V-SMF or the SMF+PGW-C, the AMF indicates whether the target MME supports User Plane Integrity Protection with EPS.
When Nsmf_PDUSession_Context Request is received in the V-SMF or the SMF+PGW-C, the V-SMF or the SMF+PGW-C provides context that includes the mapped EPS PDN Connection as follows:
  • If there is EBI list not to be transferred and the EBI value of the QoS Flow associated with the default QoS Rule is included in that list, the V-SMF or the SMF+PGW-C shall not return the PDN Connection context (which implies the whole PDU Session is not transferred to EPS), otherwise if the EBI value of the QoS Flow associated with the default QoS Rule is not included in EBI list not to be transferred, the V-SMF or PGW C+SMF shall not provide the EPS bearer context(s) mapped from QoS Flow(s) associated with the EBI list not to be transferred.
  • For PDU Sessions with PDU Session Type Ethernet, if the UE and target MME supports Ethernet PDN type, the V-SMF or the PGW C+SMF provides Context for Ethernet PDN Type, otherwise if the target MME does not support Ethernet Type but support non-IP Type, the V-SMF or the PGW C+SMF provides Context for non-IP PDN Type. For PDU Sessions with PDU Session Type Unstructured, the V-SMF or the SMF+PGW-C provides Context for non-IP PDN Type.
  • If the UP integrity protection policy for the EPS bearer context is set to "Required", the V-SMF or the PGW C+SMF shall not provide the EPS bearer context unless the MME capability indicates support for User Plane Integrity Protection with EPS and the UE supports User Plane Integrity Protection with EPS.
In the case of non roaming or LBO roaming, when Nsmf_PDUSession_ContextRequest is received in PGW C+SMF, if the SMF+PGW-C determines that EPS Bearer Context can be transferred to EPS and the CN Tunnel Info for EPS bearer(s) have not been allocated before, the SMF+PGW-C sends N4 Session modification to the PGW-U+UPF to establish the CN tunnel for each EPS bearer and provides EPS Bearer Contexts to AMF, as described in step 8 of clause 4.11.1.4.1. The PGW-U+UPF is ready to receive the uplink packet from E-UTRAN.
This step is performed with all the SMF+PGW-Cs corresponding to PDU Sessions of the UE which are associated with 3GPP access and have at leaset one EBI(s) determined to be transferred to EPS.
In home routed roaming scenario, the UE's EPS PDN Contexts are obtained from the V-SMF. If Small Data Rate Control applies on PDU Session, the V-SMF retrieves the SM Context, including Small Rate Control Status information from the H-SMF using Nsmf_PDUSession_Context Request.
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Step 3.
The AMF sends a Forward Relocation Request as in step 3 in clause 5.5.1.2.2 (S1-based handover, normal) in TS 23.401, with the following modifications and clarifications:
  • Parameter "Return preferred" may be included. Return preferred is an optional indication by the MME of a preferred return of the UE to the 5GS PLMN at a later access change to a 5GS shared network. An MME may use this information as specified by TS 23.501.
  • The SGW address and TEID for both the control-plane or EPS bearers in the message are such that target MME selects a new SGW.
  • The AMF determines, based on configuration and the Direct Forwarding Path Availability, the Direct Forwarding Flag to inform the target MME whether direct data forwarding is applicable.
  • The AMF includes the mapped SM EPS UE Contexts for PDU Sessions with and without active UP connections.
  • Subject to operator policy if the secondary RAT access restriction condition is the same for EPS and 5GS, the AMF may set EPS secondary RAT access restriction condition based on the UE's subscription data.
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Step 4-5.
Step 4 and 4a respectively in clause 5.5.1.2.2 (S1-based handover, normal) in TS 23.401.
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Step 6.
Step 5 (Handover Request) in clause 5.5.1.2.2 (S1-based handover, normal) in TS 23.401 with the following modification:
  • Handover Request may contain information Handover Restriction List with information about PLMN IDs as specified by clause 5.2a of TS 23.251 for eNodeB functions.
  • The target eNB should establish E-RABs indicated by the list of EPS bearer to be setup provided by the MME, even if they are not included in the source to target container.
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Step 7-9.
Step 5a through 7 in clause 5.5.1.2.2 (S1-based handover, normal) in TS 23.401.
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Step 10a.
If data forwarding applies, the AMF sends the Nsmf_PDUSession_UpdateSMContext Request (data forwarding information) to the SMF+PGW-C. If multiple SMF+PGW-Cs serves the UE, the AMF maps the EPS bearers for Data forwarding to the SMF+PGW-C address(es) based on the association between the EPS bearer ID(s) and PDU Session ID(s). In home-routed roaming case, the AMF requests the V-SMF to create indirect forwarding tunnel if indirect forwarding applies.
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Step 10b.
If indirect data forwarding applies, the SMF+PGW-C may select an intermediate PGW-U+UPF for data forwarding. The SMF+PGW-C maps the EPS bearers for Data forwarding to the 5G QoS flows based on the association between the EPS bearer ID(s) and QFI(s) for the QoS flow(s) in the SMF+PGW-C and then sends the QFIs, Serving GW Address(es) and TEID(s) for data forwarding to the PGW-U+UPF. The CN Tunnel Info is provided by the PGW-U+UPF to SMF+PGW-C in this response. In home-routed roaming case, the V-SMF selects the V-UPF for data forwarding.
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Step 10c.
The SMF+PGW-C returns an Nsmf_PDUSession_UpdateSMContext Response (Cause, Data Forwarding tunnel Info, QoS flows for Data Forwarding). Based on the correlation between QFI(s) and Serving GW Address(es) and TEID(s) for data forwarding, the PGW-U+UPF maps the QoS flow(s) into the data forwarding tunnel(s) in EPC.
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Step 11.
The AMF sends the Handover Command to the source NG-RAN (Transparent container (radio aspect parameters that the target eNB has set-up in the preparation phase), Data forwarding tunnel info, QoS flows for Data Forwarding). The source NG-RAN commands the UE to handover to the target Access Network by sending the HO Command. The UE correlates the ongoing QoS Flows with the indicated EPS Bearer IDs to be setup in the HO command. The UE locally deletes the PDU Session if the QoS Flow associated with the default QoS rule in the PDU Session does not have an EPS Bearer ID assigned. If the QoS Flow associated with the default QoS rule has an EPS Bearer ID assigned, the UE keeps the PDU Session (PDN connection) and for the remaining QoS Flow(s) that do not have EPS bearer ID(s) assigned, the UE locally deletes the QoS rule(s) and the QoS Flow level QoS parameters if any associated with those QoS Flow(s) and notifies the impacted applications that the dedicated QoS resource has been released. The UE deletes any UE derived QoS rules. The EPS Bearer ID that was assigned for the QoS flow of the default QoS rule in the PDU Session becomes the EPS Bearer ID of the default bearer in the corresponding PDN connection.
If indirect data forwarding is applied, Data forwarding tunnel info includes CN tunnel info for data forwarding per PDU session. For the QoS Flows indicated in the "QoS Flows for Data Forwarding", NG-RAN initiate data forwarding via to the PGW-U+UPF based on the CN Tunnel Info for Data Forwarding per PDU Session. Then the PGW-U+UPF maps data received from the data forwarding tunnel(s) in the 5GS to the data forwarding tunnel(s) in EPS and sends the data to the target eNodeB via the Serving GW.
If direct data forwarding is applied, Data forwarding tunnel info includes E-UTRAN tunnel info for data forwarding per EPS bearer. NG-RAN initiate data forwarding to the target E-UTRAN based on the Data Forwarding Tunnel Info for Data Forwarding per EPS bearer.
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Step 12-12c.
Step 13 to step 14 from clause 5.5.1.2.2 (S1-based handover, normal) in TS 23.401 with the following clarification:
  • The AMF requests the release of the PDU Session which is associated with 3GPP access and not expected to be transferred to EPC, i.e. the AMF requests the release of:
  • PDU Session(s) whose corresponding SMF+PGW-C(s) are not contacted by AMF for SM context because the AMF determines that none of EBI(s) for the PDU Session can be transferred to EPS at step 2a; and
  • PDU Session(s) for which the SM context retrieval failed at step 2c.
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Step 12d.
The AMF acknowledges MME with Relocation Complete Ack message. A timer in AMF is started to supervise when resource in NG-RAN shall be released.
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Step 12e.
In the case of home routed roaming, the AMF invokes Nsmf_PDUSession_ReleaseSMContext Request (V-SMF only indication) to the V-SMF. This service operation request the V-SMF to remove only the SM context in V-SMF, i.e. not release PDU Session context in the SMF+PGW-C.
If indirect forwarding tunnel(s) were previously established, the V-SMF starts a timer and releases the SM context on expiry of the timer. If no indirect forwarding tunnel has been established, the V-SMF immediately releases the SM context and its UP resources for this PDU Session in V-UPF locally.
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Step 13.
Step 15 from clause 5.5.1.2.2 (S1-based handover, normal) in TS 23.401.
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Step 14a.
Step 16 (Modify Bearer Request) from clause 5.5.1.2.2 (S1-based handover, normal) in TS 23.401 with the following clarification:
  • If the PDU Session (PDN connection) has QoS Flows that do not have EPS bearer ID(s) assigned, or QoS Flow(s) for which the mapped EPS bearers are not included in Modify Bearer Request, the SMF+PGW-C deletes the PCC rule(s) associated with those QoS Flows and informs the PCF about the removed PCC rule(s). If there are QoS Flow(s) with PCC rule(s) that do not have allocated TFT packet filters, the SMF+PGW-C deletes those PCC rule(s) and informs the PCF about the removed PCC rule(s).
The SMF+PGW-C may need to report some subscribed event to the PCF by performing an SMF initiated SM Policy Association Modification procedure as defined in clause 4.16.5.
If the MME does not indicate support of User Plane integrity protection, or the new eNB does not support User Plane integrity protection, or the UE does not support User Plane Integrity Protection with EPS and the UP integrity protection policy is set to "Required" then the SMF+PGW-C releases the bearers associated with the PDN CONNECTION.
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Step 15.
The SMF+PGW-C initiates a N4 Session Modification procedure towards the UPF+PGW-U to update the User Plane path, i.e. the downlink User Plane for the indicated PDU Session is switched to E-UTRAN. The SMF+PGW-C releases the resource of the CN tunnel for PDU Session in UPF+PGW-U.
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Step 16.
Step 16a (Modify Bearer Response) from clause 5.5.1.2.2 (S1-based handover, normal) in TS 23.401. At this stage the User Plane path is established for the default bearer and the dedicated EPS bearers between the UE, target eNodeB, Serving GW and the PGW-U+UPF. The SMF+PGW-C uses the EPS QoS parameters as assigned for the dedicated EPS bearers during the QoS Flow establishment. SMF+PGW-C maps all the other IP flows to the default EPS bearer (see NOTE 4).
If indirect forwarding tunnel(s) were previously established, the SMF+PGW-C starts a timer, to be used to release the resource used for indirect data forwarding.
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Step 17.
Step 17 from clause 5.5.1.2.2 (S1-based handover, normal) in TS 23.401.
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Step 18.
The UE initiates a Tracking Area Update procedure as specified in step 18 of clause 5.5.1.2.2 (S1-based handover, normal) in TS 23.401.
This includes the deregistration of the old AMF for 3GPP access from the HSS+UDM as specified in clause 4.11.1.5.3. Any registration associated with the non-3GPP access in the old AMF is not removed (i.e. an AMF that was serving the UE over both 3GPP and non-3GPP accesses does not consider the UE as deregistered over non 3GPP access and will remain registered and subscribed to subscription data updates in UDM).
When the UE decides to deregister over non-3GPP access or the old AMF decides not to maintain a UE registration for non-3GPP access anymore, the old AMF then deregisters from UDM by sending a Nudm_UECM_Deregistration service operation, unsubscribes from Subscription Data updates by sending an Nudm_SDM_Unsubscribe service operation to UDM and releases all the AMF and AN resources related to the UE.
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Step 19.
If PCC is deployed, the PCF may decide to provide the previously removed PCC rules to the SMF+PGW-C again thus triggering the SMF+PGW-C to initiate dedicated bearer activation procedure. This procedure is specified in clause 5.4.1 of TS 23.401 with modification captured in clause 4.11.1.5.4. This step is applicable for PDN Type IP or Ethernet, but not for non-IP PDN Type.
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Step 20.
Step 21 from clause 5.5.1.2.2 (S1-based handover, normal) in TS 23.401.
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Step 21.
In the case of home routed roaming, at the expiry of the timer at V-SMF started at step 12e, the V-SMF locally releases the SM context and the UP resource for the PDU Session including the resources used for indirect forwarding tunnel(s) that were allocated at step 10.
In non-roaming or local breakout roaming, if SMF+PGW-C has started a timer in step 16, at the expiry of the timer, the SMF+PGW-C sends N4 Session Modification Request to PGW-U+UPF to release the resources used for the indirect forwarding tunnel(s) that were allocated at step 10.
When the timer set in step 12d expires, AMF also sends a UE Context Release Command message to the source NG RAN. The source NG RAN releases its resources related to the UE and responds with a UE Context Release Complete message.
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