Content for  TS 23.401  Word version:  19.0.0

The source RNC decides to initiate an Inter-RAT handover to the E-UTRAN. At this point both uplink and downlink user data is transmitted via the following: Bearers between UE and source RNC, GTP tunnel(s) between source RNC, source SGSN (only if Direct Tunnel is not used), Serving-GW and PDN-GW.

The source RNC sends a Relocation Required (Cause, Target eNodeB Identifier, CSG ID, CSG access mode, Source RNC Identifier, Source RNC to Target RNC Transparent Container) message to the source SGSN to request the CN to establish resources in the target eNodeB, Target MME and the Serving-GW. The bearers that will be subject to data forwarding (if any) are identified by the target MME in a later step (see step 7 below). When the target cell is a CSG cell or a hybrid cell, the source RNC shall include the CSG ID of the target cell. If the target cell is a hybrid cell, the CSG access mode shall be indicated.

The source SGSN determines from the 'Target eNodeB Identifier' IE that the type of handover is IRAT Handover to E-UTRAN. The source SGSN selects the target MME as described in clause 4.3.8.3 on "MME Selection Function". The Source SGSN initiates the Handover resource allocation procedure by sending Forward Relocation Request (IMSI, Target Identification, CSG ID, CSG Membership Indication, MM Context, PDN Connections, SGSN Tunnel Endpoint Identifier for Control Plane, SGSN Address for Control plane, Source to Target Transparent Container, RAN Cause, MS Info Change Reporting Action (if available), CSG Information Reporting Action (if available), UE Time Zone, ISR Supported, Serving Network, Change to Report (if present)) message to the target MME. This message includes all EPS Bearer contexts corresponding to all the bearers established in the source system and the uplink Tunnel endpoint parameters of the Serving-GW. If the information ISR Supported is indicated, this indicates that the source SGSN and associated Serving-GW are capable to activate ISR for the UE. When ISR is activated the message should be sent to the MME that maintains ISR for the UE when this MME is serving the target identified by the Target Identification. RAN Cause indicates the Cause as received from source RNC. The Source to Target Transparent Container contains the value from the Source RNC to Target RNC Transparent Container received from the Source RNC. The old Serving Network is sent to target MME to support the target MME to resolve if Serving Network is changed. Change to Report flag is included by the source SGSN if reporting of change of UE Time Zone, or Serving Network, or both towards Serving-GW / PDN-GW was deferred by the source SGSN. The source SGSN shall perform access control by checking the UE's CSG subscription when CSG ID is provided by the source RNC. If there is no subscription data for this CSG ID or the CSG subscription is expired, and the target cell is a CSG cell, the source SGSN shall reject the handover with an appropriate cause unless the UE has emergency bearer services. The source SGSN includes the CSG ID in the Forward Relocation Request when the target cell is a CSG cell or hybrid cell. When the target cell is a hybrid cell, or if there are one or several emergency bearers and the target cell is a CSG cell, the CSG Membership Indication indicating whether the UE is a CSG member shall be included in the Forward Relocation Request message. This message includes all PDN Connections active in the source system and for each PDN Connection includes the associated APN, the address and the uplink tunnel endpoint parameters of the Serving-GW for control plane, and a list of EPS Bearer Contexts. Prioritization of EPS Bearer Contexts is performed by the target core network node. The MM context contains security related information, e.g. UE Network capabilities and used UMTS integrity and ciphering algorithm(s) as well as keys, as described in clause 5.7.2 (Information Storage for MME). The target MME selects the NAS ciphering and integrity algorithms to use. These algorithms will be sent transparently from the target eNodeB to the UE in the Target to Source Transparent Container (EPC part). The MME establishes the EPS bearer(s) in the prioritized order. The MME deactivates, as provided in step 8 of the execution phase, the EPS bearers which cannot be established. The target MME shall determine the Maximum APN restriction based on the APN Restriction of each bearer context received in the Forward Relocation Request, and shall subsequently store the new Maximum APN restriction value. If SIPTO at the Local Network is active for a PDN connection in the architecture with stand-alone GW the source SGSN shall include the Local Home Network ID of the source cell in the PDN Connections corresponding to the SIPTO at the Local Network PDN connection.

The target MME determines if the Serving-GW is to be relocated, e.g., due to PLMN change. If the Serving-GW is to be relocated, the target MME selects the target Serving-GW as described under clause 4.3.8.2 on "Serving-GW selection function". The target MME sends a Create Session Request message (IMSI, MME Address and TEID, MME Tunnel Endpoint Identifier for Control Plane, MME Address for Control plane, PDN-GW address(es) for user plane, PDN-GW UL TEID(s) for user plane, PDN-GW address for control plane, and PDN-GW TEID(s) for control plane, the Protocol Type over S5/S8, Serving Network) per PDN connection to the target Serving-GW. The Protocol Type over S5/S8 is provided to Serving-GW which protocol should be used over S5/S8 interface.

The target Serving-GW allocates its local resources and returns them in a Create Session Response (Serving-GW address(es) for user plane, Serving-GW UL TEID(s) for user plane, Serving-GW Address for control plane, Serving-GW TEID for control plane) message to the target MME.

The target MME requests the target eNodeB to establish the bearer(s) by sending the message Handover Request (UE Identifier, S1AP Cause, KeNB, allowed AS Integrity Protection and Ciphering algorithm(s), NAS Security Parameters to E-UTRAN, EPS Bearers to be setup list, CSG ID, CSG Membership Indication, Source to Target Transparent Container). The NAS Security Parameters to E-UTRAN includes the NAS Integrity Protection and Ciphering algorithm(s), eKSI and NONCEMME are targeted for the UE. S1AP Cause indicates the RAN Cause as received from source SGSN. The Source to Target Transparent Container contains the value from the RAN Transparent Container received from the source SGSN. For each EPS bearer requested to be established, 'EPS Bearers To Be Setup' IE shall contain information such as ID, bearer parameters, Transport Layer Address, "Data forwarding not possible" indication, and S1 Transport Association. The target MME ignores any Activity Status Indicator within an EPS Bearer Context and requests the target eNodeB to allocate resources for all EPS Bearer Contexts received from the source side. The Transport Layer Address is the Serving-GW Address for user data, and the S1 Transport Association corresponds to the uplink Tunnel Endpoint Identifier Data. "Data forwarding not possible" indication shall be included if the target MME decides the corresponding bearer will not be subject to data forwarding. The target MME shall include the CSG ID and CSG Membership Indication when provided by the source SGSN in the Handover Request message. The information about the selected NAS ciphering and integrity protection algorithm(s), KSI and NONCEMME will be sent transparently from the target eNodeB to the UE in the Target to Source Transparent Container, and in the message UTRAN HO Command from source RNC to the UE. This will then allow data transfer to continue in the new RAT/mode target cell without requiring a new AKA (Authentication and Key Agreement) procedure. More details are described in TS 33.401. If the target cell is a CSG cell, the target eNodeB shall verify the CSG ID provided by the target MME, and reject the handover with an appropriate cause if it does not match the CSG ID for the target cell. If the target eNodeB is in hybrid mode, it may use the CSG Membership Status to perform differentiated treatment for CSG and non-CSG members. If the target cell is a CSG cell, and if the CSG Membership Indication is "non member", the target eNodeB only accepts the emergency bearers.

The target eNodeB allocates the requested resources and returns the applicable parameters to the target MME in the message Handover Request Acknowledge (Target to Source Transparent Container, EPS Bearers setup list, EPS Bearers failed to setup list). The target eNodeB shall ignore it if the number of radio bearers in the Source to Target Transparent container does not comply with the number of bearers requested by the MME and allocate bearers as requested by the MME. Upon sending the Handover Request Acknowledge message the target eNodeB shall be prepared to receive downlink GTP PDUs from the Serving-GW for the accepted EPS bearers. The target eNodeB selects AS integrity and ciphering algorithm(s). In addition to the information provided by the MME (eKSI, NAS Integrity Protection and Ciphering algorithm(s) and NONCEMME), the target eNodeB inserts AS integrity and ciphering algorithm(s) into the UTRAN RRC message, which is contained in the Target to Source Transparent Container.

If 'Indirect Forwarding' and relocation of Serving-GW apply the target MME sends a Create Indirect Data Forwarding Tunnel Request message (Target eNodeB Address, TEID(s) for DL data forwarding) to the Serving-GW. Indirect forwarding may be performed via a Serving-GW which is different from the Serving-GW used as the anchor point for the UE.

The Serving-GW returns a Create Indirect Data Forwarding Tunnel Response (Cause, Serving-GW Address(es) and Serving-GW DL TEID(s) for data forwarding) message to the target MME.

The target MME sends the message Forward Relocation Response (Cause, List of Set Up RABs, EPS Bearers setup list, MME Tunnel Endpoint Identifier for Control Plane, RAN Cause, MME Address for control plane, Target to Source Transparent Container, Address(es) and TEID(s) for Data Forwarding, Serving-GW change indication) to the source SGSN. Serving-GW change indication indicates whether a new Serving-GW has been selected. The Target to Source Transparent Container includes the value from the Target to Source Transparent Container received from the target eNodeB. The IE 'Address(es) and TEID(s) for User Traffic Data Forwarding' defines the destination tunnelling endpoint for data forwarding in target system, and it is set as follows. If 'Direct Forwarding' or if 'Indirect Forwarding' but no relocation of Serving-GW applies, then the IEs 'Address(es) and TEID(s) for Data Forwarding' contains the forwarding DL GTP-U tunnel endpoint parameters to the eNodeB received in step 5a. If 'Indirect Forwarding' and relocation of Serving-GW apply the IEs 'Address(es) and TEID(s) for Data Forwarding' contains the DL GTP-U tunnel endpoint parameters to the Target eNodeB or to the forwarding Serving-GW received in step 6a.

If "Indirect Forwarding" applies, the source SGSN shall send the message Create Indirect Data Forwarding Tunnel Request (Address(es) and TEID(s) for Data Forwarding (received in step 7)) to the Serving-GW used for indirect forwarding. Indirect forwarding may be performed via a Serving-GW which is different from the Serving-GW used as the anchor point for the UE.

The Serving-GW returns the forwarding user plane parameters by sending the message Create Indirect Data Forwarding Tunnel Response (Cause, Serving-GW Address(es) and TEID(s) for data forwarding). If the Serving-GW doesn't support data forwarding, an appropriate cause value shall be returned and the Serving-GW Address(es) and TEID(s) will not be included in the message.

The source SGSN completes the preparation phase towards source RNC by sending the message Relocation Command (Target RNC to Source RNC Transparent Container, RABs to be Released List, RABs Subject to Data Forwarding List). The "RABs to be Released list" IE will be the list of all NSAPIs (RAB Ids) for which a Bearer was not established in Target eNodeB. The "RABs Subject to Data forwarding list" IE may be included in the message and it shall be a list of 'Address(es) and TEID(s) for user traffic data forwarding' received from target side in step 7 of the preparation phase when 'Direct Forwarding' applies. If 'Indirect Forwarding' is applicable and Direct Tunnel is used the "RABs Subject to Data Forwarding List" IE includes the parameters received in Step 8a of the preparation phase. If 'Indirect Forwarding' is applicable and Direct Tunnel is not used the "RABs Subject to Data Forwarding List" IE includes the source SGSN address(es) and TEID(s) allocated for indirect data forwarding by Source SGSN. The Target RNC to Source RNC Transparent Container contains the value from the Target to Source Transparent Container received from the target MME.

The source RNC will command to the UE to handover to the target eNodeB via the message HO from UTRAN Command. The access network specific message to UE includes a transparent container including radio aspect parameters that the target eNodeB has set-up in the preparation phase. The source RNC may initiate data forwarding for the indicated RABs/EPS Bearer contexts specified in the "RABs Subject to Data Forwarding List". The data forwarding may go directly to target eNodeB, or alternatively go via the Serving-GW if so decided by source SGSN and/or target MME in the preparation phase. Upon the reception of the HO from UTRAN Command message containing the Relocation Command message, the UE shall associate its RAB IDs to the respective bearers ID based on the relation with the NSAPI and shall suspend the uplink transmission of the user plane data.

Void.

The UE moves to the E-UTRAN and performs access procedures toward target eNodeB.

When the UE has got access to target eNodeB it sends the message HO to E-UTRAN Complete. The UE shall implicitly derive the EPS bearers for which an E-RAB was not established from the HO from UTRAN Command and deactivate them locally without an explicit NAS message at this step.

When the UE has successfully accessed the target eNodeB, the target eNodeB informs the target MME by sending the message Handover Notify (TAI+ECGI, Local Home Network ID). For SIPTO at the Local Network with stand-alone GW architecture, the target eNodeB shall include the Local Home Network ID of the target cell in the Handover Notify message.

Then the target MME knows that the UE has arrived to the target side and target MME informs the source SGSN by sending the Forward Relocation Complete Notification (ISR Activated, Serving-GW change) message. If ISR Activated is indicated, this indicates to the source SGSN that it shall maintain the UE's contexts and activate ISR, which is only possible when the S-GW is not changed. The source SGSN shall also acknowledge that information. A timer in source SGSN is started to supervise when resources in the in Source RNC and Source Serving-GW (for Serving-GW relocation) shall be released Upon receipt of the Forward Relocation Complete Acknowledge message the target MME starts a timer if the target MME applies indirect forwarding.

The target MME will now complete the Inter-RAT Handover procedure by informing the Serving-GW (for Serving-GW relocation this will be the Target Serving-GW) that the target MME is now responsible for all the bearers the UE have established. This is performed in the message Modify Bearer Request (Cause, MME Tunnel Endpoint Identifier for Control Plane, EPS Bearer ID, MME Address for Control Plane, eNodeB Address(es) and TEID(s) for User Traffic for the accepted EPS bearers and RAT type, ISR Activated) per PDN connection. As it is a mobility from UTRAN, if the target MME supports location information change reporting, the target MME shall include the User Location Information (according to the supported granularity) in the Modify Bearer Request, regardless of whether location information change reporting had been requested in the previous RAT by the PDN-GW. If the PDN-GW requested User CSG information (determined from the UE context), the MME also includes the User CSG Information IE in this message. If either the UE Time Zone has changed or Forward Relocation Request message from source SGSN indicated pending UE Time Zone change reporting (via Change to Report flag), the MME includes the UE Time Zone IE in this message. If either Serving-GW is not relocated but the Serving Network has changed or Forward Relocation Request message from source SGSN indicated pending Serving Network change reporting (via Change to Report flag), the MME includes the new Serving Network IE in this message. If indicated, the information ISR Activated indicates that ISR is activated, which is only possible when the S-GW was not changed. When the Modify Bearer Request does not indicate ISR Activated and S-GW is not changed, the S-GW deletes any ISR resources by sending a Delete Bearer Request to the other CN node that has bearer resources on the S-GW reserved. The MME releases the non-accepted dedicated bearers by triggering the bearer release procedure as specified in clause 5.4.4.2. If the Serving-GW receives a DL packet for a non-accepted bearer, the Serving-GW drops the DL packet and does not send a Downlink Data Notification to the MME. If the default bearer of a PDN connection has not been accepted by the target eNodeB and there are other PDN connections active, the MME shall handle it in the same way as if all bearers of a PDN connection have not been accepted. The MME releases these PDN connections by triggering the MME requested PDN disconnection procedure specified in clause 5.10.3.

The Serving-GW (for Serving-GW relocation this will be the Target Serving-GW) may inform the PDN-GW the change of for example for Serving-GW relocation or the RAT type that e.g. can be used for charging, by sending the message Modify Bearer Request per PDN connection. The S-GW also includes User Location Information IE and/or UE Time Zone IE and/or User CSG Information IE if they are present in step 8. Serving Network should be included if it is received in step 8 or in step 4 in clause 5.5.2.2.2. For Serving-GW relocation, the Serving-GW allocates DL TEIDs on S5/S8 even for non-accepted bearers and may include the PDN Charging Pause Support Indication. The PDN-GW must acknowledge the request with the message Modify Bearer Response. In the case of Serving-GW relocation, the PDN-GW updates its context field and returns a Modify Bearer Response (Charging Id, MSISDN, PDN Charging Pause Enabled Indication (if PDN-GW has chosen to enable the function), etc.) message to the Serving-GW. The MSISDN is included if the PDN-GW has it stored in its UE context. If location information change reporting is required and supported in the target MME, the PDN-GW shall provide MS Info Change Reporting Action in the Modify Bearer Response. If PCC infrastructure is used, the PDN-GW informs the PCRF about the change of, for example, the RAT type. If the Serving-GW is relocated, the PDN-GW shall send one or more "end marker" packets on the old path immediately after switching the path in order to assist the reordering function in the target eNodeB. The source Serving-GW shall forward the "end marker" packets to the source SGSN or RNC.

The Serving-GW (for Serving-GW relocation this will be the Target Serving-GW) acknowledges the user plane switch to the target MME via the message Modify Bearer Response (Cause, Serving-GW Tunnel Endpoint Identifier for Control Plane, Serving-GW Address for Control Plane, Protocol Configuration Options, MS Info Change Reporting Action). At this stage the user plane path is established for all bearers between the UE, target eNodeB, Serving-GW (for Serving-GW relocation this will be the Target Serving-GW) and PDN-GW. If the Serving-GW does not change, the Serving-GW shall send one or more "end marker" packets on the old path immediately after switching the path in order to assist the reordering function in the target eNodeB.

The UE initiates a Tracking Area Update procedure when one of the conditions listed in clause "Triggers for tracking area update" applies. The target MME knows that an IRAT Handover has been performed for this UE as it received the bearer context(s) by handover messages and therefore the target MME performs only a subset of the TA update procedure, specifically it excludes the context transfer procedures between source SGSN and target MME. If the Subscription Data received from the HSS (during the TAU in step 11) contains information that is necessary for the E-UTRAN to be aware of (e.g. a restriction in the UE's permission to use NR as a secondary RAT, Unlicensed Spectrum in the form of LAA/LWA/LWIP/NR-U (as specified in clause 4.3.30) or a combination of them), or an existing UE context in the MME indicates that the UE is not permitted to use NR as a secondary RAT or Unlicensed Spectrum or a combination of them and the MME has not provided this information to the target eNodeB during step 5 of the handover preparation phase, then the MME sends an updated Handover Restriction List in the Downlink NAS Transport message that it sends to RAN. If the UE is not allowed to use NR as Secondary RAT, the MME indicates that to the UE in TAU Accept message.

When the timer started in step 7 expires the source SGSN will clean-up all its resources towards source RNC by sending the Iu Release Command to the RNC. When there is no longer any need for the RNC to forward data, the source RNC responds with an Iu Release Complete message. When the timer started in step 7 expires and if the source SGSN received the Serving-GW change indication in the Forward Relocation Response message, it deletes the EPS bearer resources by sending Delete Session Request (Cause, Operation Indication) messages to the Source Serving-GW. The operation Indication flag is not set, that indicates to the Source Serving-GW that the Source Serving-GW shall not initiate a delete procedure towards the PDN-GW. The Source Serving-GW acknowledges with Delete Session Response (Cause) messages. If ISR has been activated before this procedure, the cause indicates to the Source S-GW that the Source S-GW shall delete the bearer resources on the other old CN node by sending Delete Bearer Request message(s) to that CN node.

If indirect forwarding was used then the expiry of the timer at source SGSN started at step 7 triggers the source SGSN to send a Delete Indirect Data Forwarding Tunnel Request message to the S-GW to release the temporary resources used for indirect forwarding.

If indirect forwarding was used and the Serving-GW is relocated, then the expiry of the timer at target MME started at step 7 triggers the target MME to send a Delete Indirect Data Forwarding Tunnel Request message to the target S-GW to release temporary resources used for indirect forwarding.

Steps 1 to 5 in the flow are identical to the ones in clause 5.5.2.2.2.

If the Target eNodeB fails to allocate any resources for any of the requested EPS Bearers it sends a Handover Failure (Cause) message to the Target MME. When the Target MME receives the Handover Failure message from Target eNodeB the Target MME clears any reserved resources for this UE.

This step is only performed for Serving-GW relocation, i.e. if Steps 4/4a have been performed. The Target MME deletes the EPS bearer resources by sending Delete Session Request (Cause) messages to the Target Serving-GW. The Target Serving-GW acknowledges with Delete Session Response (Cause) messages.

The Target MME sends the Forward Relocation Response (Cause) message to the Source SGSN.

When the Source SGSN receives the Forward Relocation Response message it send a Relocation Preparation Failure (Cause) message to the Source RNC.