Content for  TS 23.401  Word version:  19.0.0

The source RNC decides to initiate a handover to E-UTRAN. Step 2. Step 3.

The MME selects a Serving-GW and sends a Create Session Request (bearer context(s) with PDN-GW addresses and TEIDs for uplink traffic, APN-AMBR, Serving Network, UE Time Zone) message per PDN connection to the target Serving-GW. For relocation from Gn/Gp SGSN, the target MME provides the APN-AMBR if not received explicitly from the Gn/Gp SGSN based on the mapping from MBR (as specified in Annex E) to the Serving-GW.

The Serving-GW allocates the S-GW addresses and TEIDs for the uplink traffic on S1_U reference point (one TEID per bearer). The target Serving-GW sends a Create Session Response (Serving-GW addresses and uplink TEID(s) for user plane) message back to the target MME.

The new MME requests the target eNodeB to establish the bearer(s) by sending the message Handover Request (UE Identifier, S1AP Cause, CN Domain Indicator, KeNB, NAS Security Parameters to E-UTRAN, EPS Bearers to be setup list, Source to Target Transparent Container, Serving-GW Address(es) and TEID(s) for User Traffic Data, Handover Restriction List). S1AP Cause indicates the RANAP Cause as received from SGSN. Source to Target Transparent Container contains the RAN Transparent Container as received from SGSN. The NAS Security Parameters to E-UTRAN includes the NAS Integrity Protection and Ciphering algorithm(s), eKSI and NONCEMME information elements. Handover Restriction List is sent if it is available in the Target MME; it is described in clause 4.3.5.7. If the MME did not receive the UE Network Capability information from the old SGSN, then the MME will not have received information on the E-UTRAN Integrity Protection and Encryption algorithms that the UE supports. In this case, the MME can assume that the UE supports both EIA1/EEA1 and EIA2/EEA2. The MME shall not request the target eNodeB to establish EPS GBR bearers with maximum bitrate set to 0 and those EPS bearers should not be included in the EPS Bearers to be setup list and should be deactivated by the MME. For the remaining EPS Bearer Contexts the MME ignores any Activity Status Indicator within an EPS Bearer Context and requests the target eNodeB to allocate resources for all the remaining EPS Bearer Contexts. The MME shall compute the UE-AMBR, according to clause 4.7.3, based on explicit APN-AMBR values received from the Gn/Gp SGSN. If explicit APN-AMBR values are not received by the MME, a local UE-AMBR shall be included in the 'EPS Bearers be setup list' IE. The local UE-AMBR is described in Annex E. "Data forwarding not possible" indication per bearer shall be included in the 'EPS Bearers to be setup list' if the target MME decides the corresponding bearer will not be subject to data forwarding.

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, Cause). 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. The target eNodeB inserts the information provided by the MME (KSI, selected NAS Integrity Protection and Ciphering algorithm(s), NONCEMME) and selected 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 (IMSI, MME Tunnel Endpoint Identifier for Control Plane, MME Address for Control plane, Target eNodeB Address and TEID(s) for DL user plane) to the Serving-GW. The allocation of a new Serving-GW by steps 4 and 5 the MME shall consider as a Serving-GW change.

The Serving-GW returns a Create Indirect Data Forwarding Tunnel Response (Cause, Serving-GW DL TEID(s)) message to the source MME. If the Serving-GW doesn't support data forwarding, an appropriate cause value shall be returned. Step 10. For each RAB, if the MME has determined no Data forwarding, i.e. the data forwarding from the source RNC to the target eNodeB is not required, the MME indicates the reserved TEID and IP address parameters to the old SGSN in the Target RNC Information. The packets received on that reserved TEID and IP address are discarded. Step 11. Step 12. Step 13.

There is no RAN context transfer during inter RAT handovers with E-UTRAN. If the source RNC originates any SRNC contexts the MME acknowledges the receipt towards the SGSN and ignores the message content.

When the UE has successfully accessed the target eNodeB, the target eNodeB informs the target MME by sending the message Handover Notify (TAI+ECGI). 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. If Dual Connectivity is activated for the UE, the PSCell ID shall be included in the Handover Notify message.

Upon receipt of Handover Notify message, if the SRNS Relocation is an inter SGSN SRNS relocation, the new SGSN signals to the old SGSN the completion of the SRNS relocation procedure by sending a Forward Relocation Complete message. Upon receipt of the Relocation Complete message the new MME starts a timer.

The target MME will now complete the handover procedure by informing the 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, Tunnel Endpoint Identifier Control Plane, MME Address for Control Plane, eNodeB Address(es) and TEID(s) for User Traffic, RAT type, APN-AMBR, User Location Information, PSCell ID) per PDN connection. If the PDN-GW requested UE's location information and/or User CSG information (determined from the UE context), the MME also includes the User Location Information IE and/or User CSG Information IE in this message. If the UE Time Zone has changed, the MME includes the UE Time Zone IE in this message. If the MME has received PSCell ID in step 15, the MME shall include it in this message. The MME releases the non-accepted 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.

The Serving-GW informs the PDN-GW the APN-AMBR and the change of for example the RAT type that e.g. can be used for charging, by sending the message Modify Bearer Request (APN-AMBR, Serving Network, PDN Charging Pause Support Indication) 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 it is present in step 17. The Serving-GW allocates DL TEIDs on S5/S8 even for non-accepted bearers. The PDN-GW must acknowledge the request with the message Modify Bearer Response (Default bearer id, APN Restriction, PDN Charging Pause Enabled Indication (if PDN-GW has chosen to enable the function)). When the UE moves from Gn/Gp SGSN to the MME, the PDN-GW shall send the APN restriction of each bearer context to the Serving-GW.

The Serving-GW acknowledges the user plane switch to the target MME via the message Modify Bearer Response (Cause, Tunnel Endpoint Identifier Control Plane, Serving-GW Address for Control Plane, Default bearer id, APN restriction). The Serving-GW shall forward the received APN Restriction to the MME. At this stage the user plane path is established for all bearers between the UE, target eNodeB, Serving-GW and PDN-GW. Step 20.

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. The target MME gets the subscribed UE-AMBR value and the subscribed APN-AMBR value from the HSS during the TA update procedure. If the Subscription Data received from the HSS (during the TAU) 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, Unlicensed Spectrum or a combination of them, and the MME has not provided this information to the target eNodeB during step 6 (the Handover Request), then the MME sends an updated Handover Restriction List in the Downlink NAS Transport message that it sends to E-UTRAN.

The target MME calculates UE-AMBR as defined in clause 4.7.3. If this calculated value is different from the UE-AMBR computed during step 6, or the APN-AMBR mapped from the subscribed MBR is different from the subscribed APN-AMBR, or the mapped subscribed QoS profile (i.e. the subscribed QoS profile mapped according to Annex E) of the default bearer is different from the EPS Subscribed QoS profile received from the HSS, the new MME shall initiate Subscribed QoS Modification procedure as described in clause 5.4.2.2, Figure 5.4.2.2-1.

When the timer started in step 16 expires the new MME releases the resources that have been allocated for indirect forwarding.