8.7.4 SgNB Addition Preparation

36.4233GPPEvolved Universal Terrestrial Radio Access Network (E-UTRAN)Release 17TSX2 Application Protocol (X2AP)

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

8.7.4.1 General

The purpose of the SgNB Addition Preparation procedure is to request the en-gNB to allocate resources for EN-DC connectivity operation for a specific UE.

The procedure uses UE-associated signalling.

8.7.4.2 Successful Operation

Figure 8.7.4.2-1: SgNB Addition Preparation, successful operation

The MeNB initiates the procedure by sending the SGNB ADDITION REQUEST message to the en-gNB. When the MeNB sends the SGNB ADDITION REQUEST message, it shall start the timer T_DCprep.

The allocation of resources according to the values of the Allocation and Retention Priority IE included in the Full E-RAB Level QoS Parameters IE or in the Requested MCG E-RAB Level QoS Parameters IE or in the Requested SCG E-RAB Level QoS Parameters IE shall follow the principles described for the E-RAB Setup procedure in TS 36.413 [4].

If the SGNB ADDITION REQUEST message contains the Serving PLMN IE, the en-gNB may use it for RRM purposes.

If the SGNB ADDITION REQUEST message contains the Expected UE Behaviour IE, the en-gNB shall, if supported, store this information and may use it to optimize resource allocation.

If the SGNB ADDITION REQUEST message contains the Handover Restriction List IE, the en-gNB node shall, if supported, store this information and use it to select an appropriate NR cell.

If the SGNB ADDITION REQUEST message contains the MeNB Resource Coordination Information IE, the en-gNB should forward it to lower layers and it may use it for the purpose of resource coordination with the MeNB, or to coordinate with sidelink resources used in the MeNB. The en-gNB shall consider the received UL Coordination Information IE value valid until reception of a new update of the IE for the same UE. The en-gNB shall consider the received DL Coordination Information IE value valid until reception of a new update of the IE for the same UE. If the MeNB Coordination Assistance Information IE is contained in the MeNB Resource Coordination Information IE, the en-gNB shall, if supported, use the information to determine further coordination of resource utilisation between the en-gNB and the MeNB.

The en-gNB shall choose the ciphering algorithm based on the information in the NR UE Security Capabilities IE and locally configured priority list of AS encryption algorithms and apply the key indicated in the SgNB Security Key IE as specified in the TS 33.401 [18].

If the SGNB ADDITION REQUEST message contains the Subscriber Profile ID for RAT/Frequency Priority IE, the en-gNB may use it for RRM purposes.

If the SGNB ADDITION REQUEST message contains the Additional RRM Policy Index IE, the en-gNB may use it for RRM purposes.

The en-gNB shall search for the target NR cell among the NR neighbour cells of the E-UTRAN cell indicated in MeNB Cell ID IE, as specified in the TS 37.340 [32].

If the Masked IMEISV IE is contained in the SGNB ADDITION REQUEST message the en-gNB shall, if supported, use it to determine the characteristics of the UE for subsequent handling.

The en-gNB shall report to the MeNB, in the SGNB ADDITION REQUEST ACKNOWLEDGE message, the result for all the requested E-RABs in the following way:

– a list of E-RABs which are successfully established shall be included in the E-RABs Admitted To Be Added List IE;

– a list of E-RABs which failed to be established shall be included in the E-RABs Not Admitted List IE.

NOTE: The MeNB may trigger the SgNB Addition Preparation procedure in the course of the Inter-MeNB handover without SgNB change procedure as described in TS 37.340 [32]. The deleted E-RABs are not included in the E-RABs To Be Added List IE in the SGNB ADDITION REQUEST message, from MeNB point of view. If the en-gNB reports a certain E-RAB to be successfully established, respective SCG resources, from an en-gNB point of view, may be actually successfully established or modified or kept; if a certain E-RAB is reported to be failed to be established, respective SCG resources, from an en-gNB point of view, may be actually failed to be established or modified or kept.

For each E-RAB successfully established in the en-gNB, the en-gNB shall report to the MeNB, in the SGNB ADDITION REQUEST ACKNOWLEDGE message, the same value in the EN-DC Resource Configuration IE as received in the SGNB ADDITION REQUEST message.

For each E-RAB for which allocation of the PDCP entity is requested at the en-gNB:

– the MeNB may propose to apply forwarding of downlink data by including the DL Forwarding IE within the E-RABs To be Added Item IE of the SGNB ADDITION REQUEST message. For each E-RAB that it has decided to admit, the en-gNB may include the DL Forwarding GTP Tunnel Endpoint IE within the E-RABs Admitted To Be Added Item IE of the SGNB ADDITION REQUEST ACKNOWLEDGE message to indicate that it accepts the proposed forwarding of downlink data for this bearer. This GTP tunnel endpoint may be different from the corresponding GTP tunnel endpoint, i.e the information contained in the Transport Layer Address IE and the DL GTP TEID IE in the E-RAB To Be Modified List IE of the E-RAB MODIFICATION INDICATION message (see TS 36.413 [4]) depending on implementation choice;

– the en-gNB may include for each bearer in the E-RABs Admitted To Be Added List IE the UL Forwarding GTP Tunnel Endpoint IE to indicate that it requests data forwarding of uplink packets to be performed for that bearer.

– the en-gNB shall use the S1 UL GTP Tunnel Endpoint IE of the SGNB ADDITION REQUEST message as the UL S1-U address.

– the MeNB shall use the SgNB UL GTP Tunnel Endpoint at PDCP IE of the SGNB ADDITION REQUEST ACKNOWLEDGE message as the UL X2-U address.

– if the SGNB ADDITION REQUEST message contains for an E-RAB to be added which is requested to be configured with MCG resources the MeNB DL GTP Tunnel Endpoint at MCG IE the en-gNB shall use it as the DL X2-U address for delivery of DL PDCP PDUs.

– the en-gNB shall include in the SGNB ADDITION REQUEST ACKNOWLEDGE message the S1 DL GTP Tunnel Endpoint at the SgNB IE.

– the en-gNB shall include in the SGNB ADDITION REQUEST ACKNOWLEDGE message the RLC Mode IE.

– the en-gNB may include for each bearer in the E-RABs Admitted To Be Added List IE in the SGNB ADDITION REQUEST ACKNOWLEDGE the PDCP SN Length IE to indicate the PDCP SN length for that bearer.

– If the RLC Mode IE is included for an E-RAB within the E-RABs To be Added List IE in the SGNB ADDITION REQUEST message, it indicates the mode that the MeNB used for the E-RAB when it was hosted at the MeNB.

– If the Bearer Type IE for the concerned E-RAB is received by the en-gNB and is set to "non IP", the en-gNB shall, if supported, not perform IP header compression for the concerned E-RAB.

– If the Ethernet Type IE for the concerned E-RAB is received by the en-gNB and is set to "True", the en-gNB shall, if supported, take this into account to perform header compression appropriately for the concerned E-RAB.

Upon reception of the SGNB ADDITION REQUEST ACKNOWLEDGE message the MeNB shall stop the timer T_DCprep.

If the SGNB ADDITION ACKNOWLEDGE message contains the SgNB Resource Coordination Information IE, the MeNB may use it for the purpose of resource coordination with the en-gNB. The MeNB shall consider the received UL Coordination Information IE value valid until reception of a new update of the IE for the same UE. The MeNB shall consider the received DL Coordination Information IE value valid until reception of a new update of the IE for the same UE. If the SgNB Coordination Assistance Information IE is contained in the SgNB Resource Coordination Information IE, the MeNB shall, if supported, use the information to determine further coordination of resource utilisation between the en-gNB and the MeNB.

If the SgNB UE X2AP ID IE is contained in the SGNB ADDITION REQUEST message, the en-gNB shall, if supported, store this information and use it as defined in TS 37.340 [32].

If the SGNB ADDITION REQUEST message contains the SGNB Addition Trigger Indication, the en-gNB shall include the RRC config indication IE in the SGNB ADDITION REQUEST ACKNOWLEDGE message to inform the MeNB if the en-gNB applied full or delta configuration, as specified in TS 37.340 [32].

If the en-gNB receives for an E-RAB for which the PDCP entiy is allocated at the MeNB the Secondary MeNB UL GTP Tunnel Endpoint at PDCP IE and the Duplication Activation IE in the SGNB ADDITION REQUEST message, it may provide the Secondary SgNB DL GTP Tunnel Endpoint at SCG IE and the LCID IE to the MeNB in the SGNB ADDITION REQUEST ACKNOWLEDGE message if PDCP duplication is configured at the en-gNB.

If the SGNB ADDITION REQUEST message contains the UL PDCP SN Length IE and the DL PDCP SN Length IE, the en-gNB shall, if supported, store this information and use it for lower layer configuration of the concerned MN terminated bearer.

The SgNB may include the Location Information at SgNB IE in the SGNB ADDITION REQUEST ACKNOWLEDGE message, if respective information is available at the SgNB.

If the Location Information at SgNB Reporting IE set to "pscell" is included in the SGNB ADDITION REQUEST, the SgNB shall start providing information about the current location of the UE. If the Location Information at SgNB IE is included in the SGNB ADDITION REQUEST ACKNOWLEDGE, the MeNB shall store the included information so that it may be transferred towards the MME.

If Trace Activation IE has previously been received for this UE, it shall be included in the SGNB ADDITION REQUEST message. If the Trace Activation IE is included in the SGNB ADDITION REQUEST message, the en-gNB shall, if supported, initiate the requested trace function as described in TS 32.422 [6]. If the Trace Activation IE includes the MDT Configuration NR IE, the en-gNB shall take it into account for MDT function as described in TS 37.320 [31].

If the Management Based MDT Allowed IE only or the Management Based MDT Allowed IE and the Management Based MDT PLMN List IE is contained in the SGNB ADDITION REQUEST message, the en-gNB shall, if supported, store the received information in the UE context, and use this information to allow subsequent selection of the UE for management based MDT defined in TS 32.422 [6].

The MeNB shall, if supported and available in the UE context, include the Management Based MDT Allowed IE and the Management Based MDT PLMN List IE in the SGNB ADDITION REQUEST message.

If the UE Context Reference at Source NG-RAN IE is contained in the SGNB ADDITION REQUEST message, the en-gNB shall, if supported, store this information and use it for UE context retrieval and allocate data forwarding resources as specified in TS 37.340 [32].

If the Requested Fast MCG recovery via SRB3 IE set to "true" is included in the SGNB ADDITION REQUEST message and the en-gNB decides to configure fast MCG link recovery via SRB3 as specified in TS 37.340 [32], the en-gNB shall, if supported, include the Available fast MCG recovery via SRB3 IE set to "true" in the SGNB ADDITION REQUEST ACKNOWLEDGE message.

If the UE Radio Capability ID IE is contained in the SGNB ADDITION REQUEST message, the en-gNB shall, if supported, store this information and use it as specified in TS 23.401 [12].

If the SGNB ADDITION REQUEST message contains the IAB Node Indication IE, the en-gNB shall, if supported, consider that the request is for an IAB node.

For each requested E-RAB configured as MN-terminated split bearer/SCG bearer, if the QoS Mapping Information IE is contained in the GTP Tunnel Endpoint IE in the SGNB ADDITION REQUEST ACKNOWLEDGE message, the MeNB shall, if supported, use it to set DSCP and/or flow label fields for the downlink IP packets which are transmitted from MeNB to en-gNB through the GTP tunnels indicated by the GTP Tunnel Endpoint IE.

If the Source NG-RAN Node ID IE is included in the SGNB ADDITION REQUEST message, the en-gNB shall, if supported, use it to decide the direct data forwarding path availability with the indicated source NG-RAN node, and if the direct data forwarding path is available, include the Direct Forwarding Path Availability IE in the SGNB ADDITION REQUEST ACKNOWLEDGE message.

For each E-RAB for which the Security Indication IE is included in the E-RABs To Be Added Item IE of the SGNB ADDITION REQUEST message:

– if the Integrity Protection Indication IE is set to "required", the en-gNB shall, if supported, perform user plane integrity protection for the concerned E-RAB as specified in TS 33.401 [18], otherwise it shall reject the addition of the concerned E-RAB with an appropriate cause value.

– if the Integrity Protection Indication IE is set to "preferred", the en-gNB should perform user plane integrity protection for the concerned E-RAB as specified in TS 33.401 [18], and it shall notify the MeNB whether it performed the user plane integrity protection by including the Integrity Protection result IE in the Security Result IE of the SGNB ADDITION REQUEST ACKNOWLEDGE message.

– if the Integrity Protection Indication IE is set to "not needed", the en-gNB shall not perform user plane integrity protection for the concerned E-RAB.

If the SCG Activation Request IE is included in the SGNB ADDITION REQUEST message, the en-gNB may use it to configure SCG resources as specified in TS 37.340 [32], and shall, if supported, include the SCG Activation Status IE in the SGNB ADDITION REQUEST ACKNOWLEDGE message. If the SCG Activation Request IE in the SGNB ADDITION REQUEST message is set to "Activate SCG", the en-gNB shall, if supported, activate the SCG resources and set the SCG Activation Status IE in the SGNB ADDITION REQUEST ACKNOWLEDGE message to "SCG activated".

If the Source DL Forwarding IP Address IE or both Source DL Forwarding IP Address IE and Source Node DL Forwarding IP Address IE is included in the SGNB ADDITION REQUEST message, the en-gNB shall, if supported, store this information and use it as part of its ACL functionality configuration actions, if such ACL functionality is deployed.

If the Source DL Forwarding IP Address IE is included in the SGNB ADDITION REQUEST ACKNOWLEDGE message, the MeNB shall, if supported, store this information and use it as part of its ACL functionality configuration actions to identify source TNL address for data forwarding in case of subsequent handover preparation, if such ACL functionality is deployed.

Upon reception of the SGNB ADDITION REQUEST message, the en-gNB shall, if supported, start collecting the SCG information and continue for as long as the UE stays in one of its cells.

If the UE History Information from the UE IE is included in the SGNB ADDITION REQUEST message, the en-gNB shall, if supported, store this information.

If the PScell Change History IE set to “reporting full history” is included in the SGNB ADDITION REQUEST message, the en-gNB shall, if supported, signal the latest SCG UE History Information upon each PSCell change, to the MeNB, using the SgNB initiated SgNB Modification procedure.

If the CHO Information SN Addition IE is included in the SGNB ADDITION REQUEST message, the en-gNB shall, if supported, consider that the SgNB Addition Preparation procedure has been triggered as part of a conditional handover. It may use the Source eNB ID IE and the Source eNB UE X2AP ID IE to identify other active SgNB Addition Preparations related to this UE. If the Estimated Arrival Probability IE is contained in the CHO Information SN Addition IE included in the SGNB ADDITION REQUEST message, then the en-gNB may use the information to allocate necessary resources for the UE.

If the Conditional PSCell Addition Information Request IE is included in the SGNB ADDITION REQUEST message, the en-gNB shall, if supported, consider that the request concerns CPAC, as described in TS 37.340 [32]. Accordingly, the en-gNB shall, if supported, include the Conditional PSCell Addition Information Acknowledge IE in the SGNB ADDITION REQUEST ACKNOWLEDGE message.

If the CG-CandidateList is included in the SgNB to MeNB Container IE in the SGNB ADDITION REQUEST ACKNOWLEDGE message, the MeNB shall, if supported, use it for the purpose of CPAC.

If the Estimated Arrival Probability IE is contained in the Conditional PSCell Addition Information Request IE included in the SGNB ADDITION REQUEST message, then the candidate target en-gNB may use the information to allocate necessary resources for the incoming CPAC procedure.

Interactions with the MeNB initiated SgNB Modification procedure:

If the en-gNB provides for an E-RAB for which the PDCP entiy is allocated at the MeNB the Secondary SgNB DL GTP Tunnel Endpoint at SCG IE to the MeNB in the SGNB ADDITION REQUEST ACKNOWLEDGE message and the MeNB has not provided the Secondary MeNB UL GTP Tunnel Endpoint at PDCP IE in the SGNB ADDITION REQUEST message, the MeNB shall trigger the MeNB initiated SgNB Modification procedure to provide the Secondary MeNB UL GTP Tunnel Endpoint at PDCP IE to the SgNB.

Interactions with the SgNB Reconfiguration Completion procedure:

If the en-gNB admits at least one E-RAB, the en-gNB shall start the timer T_DCoverall when sending the SGNB ADDITION REQUEST ACKNOWLEDGE message to the MeNB except for a request for conditional configuration. The reception of the SGNB RECONFIGURATION COMPLETE message shall stop the timer T_DCoverall if T_DCoverall is running.

Interaction with the Activity Notification procedure

Upon receiving an SGNB ADDITION REQUEST message containing the Desired Activity Notification Level IE, the en-gNB shall, if supported, use this information to decide whether to trigger subsequent SgNB Activitity Notification procedures according to the requested notification level.

8.7.4.3 Unsuccessful Operation

Figure 8.7.4.3-1: SgNB Addition Preparation, unsuccessful operation

If the en-gNB is not able to accept any of the bearers or a failure occurs during the SgNB Addition Preparation, the en-gNB sends the SGNB ADDITION REQUEST REJECT message with an appropriate cause value to the MeNB.

8.7.4.4 Abnormal Conditions

If the en-gNB receives a SGNB ADDITION REQUEST message containing multiple E-RAB ID IEs (in the E-RABs To Be Added List IE) set to the same value, the en-gNB shall consider the establishment of the corresponding E-RAB as failed.

If the en-gNB receives a SGNB ADDITION REQUEST message containing a E-RAB Level QoS Parameters IE which contains a QCI IE indicating a GBR bearer (as defined in TS 23.203 [13]), and which does not contain the GBR QoS Information IE, the en-gNB shall consider the establishment of the corresponding E-RAB as failed.

If the supported algorithms for encryption defined in the NR Encryption Algorithms IE in the NR UE Security Capabilities IE, plus the mandated support of NEA0 in all UEs (TS 33.401 [18]), do not match any algorithms defined in the configured list of allowed encryption algorithms in the en-gNB (TS 33.401 [18]), the en-gNB shall reject the procedure using the SGNB ADDITION REQUEST REJECT message.

If the supported algorithms for integrity defined in the NR Integrity Protection Algorithms IE in the NR UE Security Capabilities IE do not match any algorithms defined in the configured list of allowed integrity protection algorithms in the en-gNB (TS 33.401 [18]), the en-gNB shall reject the procedure using the SGNB ADDITION REQUEST REJECT message.

If the en-gNB receives a SGNB ADDITION REQUEST message containing a SgNB UE X2AP ID IE that does not match any existing UE Context that has such ID, the en-gNB shall reject the procedure using the SGNB ADDITION REQUEST REJECT message.

If the MeNB has provided the en-gNB for an E-RAB for which the PDCP entiy is allocated at the MeNB the Secondary MeNB UL GTP Tunnel Endpoint at PDCP IE and the Duplication Activation IE in the SGNB ADDITION REQUEST message, and the en-gNB does not provide the Secondary SgNB DL GTP Tunnel Endpoint at SCG IE and the LCID IE to the MeNB in the SGNB ADDITION REQUEST ACKNOWLEDGE message, the MeNB shall assume that PDCP duplication was not configured at the en-gNB and releases duplication resources.

If the en-gNB provides for an E-RAB for which the PDCP entiy is allocated at the MeNB the Secondary SgNB DL GTP Tunnel Endpoint at SCG IE and the LCID IE to the MeNB in the SGNB ADDITION REQUEST ACKNOWLEDGE message and the MeNB has not provided the Secondary MeNB UL GTP Tunnel Endpoint at PDCP IE and the Duplication Activation IE in the SGNB ADDITION REQUEST message, and the MeNB does not trigger the MeNB initiated SgNB Modification procedure to provide the Secondary MeNB UL GTP Tunnel Endpoint at PDCP IE and the Duplication Activation IE to the SgNB the en-gNB before the SgNB Reconfigurationi Completion procedure was triggered, the en-gNB shall trigger the release of the concerned E-RAB.

Interactions with the SgNB Reconfiguration Completion and SgNB initiated SgNB Release procedure:

If the timer T_DCoverall expires before the en-gNB has received the SGNB RECONFIGURATION COMPLETE or the SGNB RELEASE REQUEST message, the en-gNB shall regard the requested RRC connection reconfiguration as being not applied by the UE and shall trigger the SgNB initiated SgNB Release procedure.

Interactions with the MeNB initiated SgNB Release procedure:

If the timer T_DCprep expires before the MeNB has received the SGNB ADDITION REQUEST ACKNOWLEDGE message, the MeNB shall regard the SgNB Addition Preparation procedure as being failed and shall trigger the MeNB initiated SgNB Release procedure.