Content for  TS 23.502  Word version:  18.5.0

This clause captures changes to procedures in TS 23.401 due to interworking with 5GS based on N26. The handover procedures between EPS and 5GS captured in clause 4.11.1.2 capture impacts to clause 5.5.1.2.2 of TS 23.401 (S1-based handover, normal).

The E-UTRAN Initial Attach Procedure specified in clause 5.3.2.1 of TS 23.401 is impacted as shown in Figure 4.11.1.5.2-1 when interworking with 5GS using N26 interface is supported.

The following changes are applied to clause 5.3.3.1 (Tracking area update procedure with Serving GW change) in TS 23.401:

• Step 2: The UE shall in Access Stratum signalling include GUMMEI that is mapped from 5G-GUTI following the mapping rules specified in TS 23.501 and the UE indicates it as a native GUMMEI and should in addition indicate it as "Mapped from 5G-GUTI". The UE shall, in the TAU request message, include EPS GUTI that is mapped from 5G-GUTI following the mapping rules specified in TS 23.501. The UE indicates that it is moving from 5GC. The UE integrity protects the TAU request message using the 5G security context. If the UE supports any of the CIoT 5GS Optimisations included in 5GC Preferred Network Behaviour, then the UE shall include its 5GC Preferred Network Behaviour if it included its EPC Preferred Network Behaviour in the TAU request.
  MME may steer the UE from EPC by rejecting the TAU request with an appropriate cause value. The MME should take into account availability of 5GC to the UE and the Preferred and Supported Network Behaviour (see clause 5.31.2 of TS 23.501) before steering the UE from EPC.
• Step 5 and message Context Response may include new information Return preferred.
  Return preferred is an indication by the AMF of a preferred return of the UE to the last used 5GS PLMN at a later access change to a 5GS shared network.
  RFSP Index in Use Validity Time is provided by the AMF to the MME if the AMF selects the RFSP Index in use identical to the authorized RFSP Index as specified in clause 5.3.4.3 of TS 23.501 and validity time is received from PCF as specified in clause 4.16.2.2 and in clause 6.1.2.1 of TS 23.503. The MME handles RFSP Index as specified in clause 4.11.1.5.8.
  The MME may store the last used 5GS PLMN ID in UE's MM Context.
  The MME may provide E-UTRAN with a Handover Restriction List taking into account the last used 5GS PLMN ID and the Return Preferred indication. The Handover Restriction List contains a list of PLMN IDs as specified by TS 23.251.
• Step 9a IP-CAN Session Modification procedure:
  It is replaced by SM Policy Association Modification as specified in clause 4.16.5.
• Step 13 and HSS use of Cancel Location
  The HSS/UDM de-registers any old AMF node by sending an Nudm_UECM_DeregistrationNotification service operation to the registered AMF for 3GPP access. The registered AMF for 3GPP access initiates AM Policy Association Termination procedure as defined in clause 4.16.3.2 and UE Policy Association Termination procedure as defined in clause 4.16.13.1.
• Step 17: If the DNN and SMF+PGW-C FQDN for S5/S8 interface association exist, the HSS/UDM sends APN mapped form DNN and SMF+PGW-C FQDN for S5/S8 to UE.
• Step 20 and MME processing of the partial Tracking Area Update (TAU) procedure.
  The MME may use an indication Return preferred from Context Response at step 6 when deciding the PLMN list content.
  The MME may provide the eNodeB with a PLMN list. The Handover Restriction List contains a list of PLMN IDs as specified by TS 23.501.

In step 3 of clause 4.11.1.2.1, the Forward Relocation Request may include new information Return Preferred. Return Preferred is an indication by the AMF of a preferred return of the UE to the last used 5GS PLMN at a later access change to a 5GS shared network. RFSP Index in Use Validity Time is provided by the AMF to the MME if the AMF selects the RFSP Index in use identical to the authorized RFSP Index as specified in clause 5.3.4.3 of TS 23.501 and validity time is received from PCF as specified in clause 4.16.2.2 and in clause 6.1.2.1 of TS 23.503. The MME handles RFSP Index as specified in clause 4.11.1.5.8. The MME may store the last used 5GS PLMN ID in UE's MM Context. The MME may provide E-UTRAN with a Handover Restriction List taking into account the last used 5GS PLMN ID and the Return Preferred indication. The Handover Restriction List contains a list of PLMN IDs as specified by TS 23.251.

The following changes are applied to clause 5.3.4.1 (UE triggered Service Request) in TS 23.401:

• Step 4: MME may steer the UE from EPC by rejecting the service request with an appropriate cause value. The MME should take into account availability of 5GC to the UE and the Preferred and Supported Network Behaviour (see clause 5.31.2 of TS 23.501) before steering the UE from EPC.

The following changes are applied to clause 5.3.4B.4 (Establishment of S1-U bearer during Data Transport in Control Plane CIoT EPS Optimisation) in TS 23.401:

• Step 3: MME may steer the UE from EPC by rejecting the Control Plane Service Request with an appropriate cause value. The MME should take into account availability of 5GC to the UE and the Preferred and Supported Network Behaviour (see clause 5.31.2 of TS 23.501) before steering the UE from EPC.

The following changes are applied to clause 4.3.6 (Radio Resource Management functions) in TS 23.401:

• At 5GS to EPS mobility or during inter-MME mobility, if RFSP Index in Use Validity Time is received from the AMF or the old MME, the new MME uses the received RFSP Index in use by the time indicated in RFSP Index in Use Validity Time. Only when RFSP Index in Use Validity Time expires, the MME re-evaluates the RFSP Index in use as in clause 4.3.6 of TS 23.401.

The following new parameter is to be added in Table 5.7.2-1 of TS 23.401: