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Content for  TS 23.401  Word version:  17.5.0

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5.5  HandoverWord‑p. 263

5.5.1  Intra-E-UTRAN handoverWord‑p. 263

5.5.1.1  X2-based handoverWord‑p. 263

5.5.1.1.1  GeneralWord‑p. 263
These procedures are used to hand over a UE from a source eNodeB to a target eNodeB using the X2 reference point. In these procedures the MME is unchanged. Two procedures are defined depending on whether the Serving GW is unchanged or is relocated. In addition to the X2 reference point between the source and target eNodeB, the procedures rely on the presence of S1-MME reference point between the MME and the source eNodeB as well as between the MME and the target eNodeB.
The handover preparation and execution phases are performed as specified in TS 36.300. If emergency bearer services are ongoing for the UE handover to the target eNodeB is performed independent of the Handover Restriction List. The MME checks, as part of the Tracking Area Update in the execution phase, if the handover is to a restricted area and if so MME releases the non-emergency bearers as specified in clause 5.10.3.
If the serving PLMN changes during X2-based handover, the source eNodeB shall indicate to the target eNodeB (in the Handover Restriction List) the PLMN selected to be the new Serving PLMN.
When the UE receives the handover command it will remove any EPS bearers for which it did not receive the corresponding EPS radio bearers in the target cell. As part of handover execution, downlink and optionally also uplink packets are forwarded from the source eNodeB to the target eNodeB. When the UE has arrived to the target eNodeB, downlink data forwarded from the source eNodeB can be sent to it. Uplink data from the UE can be delivered via the (source) Serving GW to the PDN GW or optionally forwarded from the source eNodeB to the target eNodeB. Only the handover completion phase is affected by a potential change of the Serving GW, the handover preparation and execution phases are identical.
If the MME receives a rejection to a NAS procedure (e.g. dedicated bearer establishment/modification/release; location reporting control; NAS message transfer; etc.) from the eNodeB with an indication that an X2 handover is in progress (see TS 36.300), the MME shall reattempt the same NAS procedure either when the handover is complete or the handover is deemed to have failed, except in the case of Serving GW relocation. The failure is known by expiry of the timer guarding the NAS procedure.
If the X2 handover includes the Serving GW relocation, and if the MME receives a rejection to a NAS message transfer for a Downlink NAS Transport or Downlink Generic NAS Transport message from the eNodeB with an indication that an X2 handover is in progress, the MME should resend the corresponding message to the target eNodeB when either the handover is complete or to the source eNodeB when the handover is deemed to have failed if the MME is still the serving MME.
If the MME receives a rejection to a NAS message transfer for a CS Service Notification or to a UE Context Modification Request message with a CS Fallback indicator from the eNodeB with an indication that an X2 handover is in progress, the MME shall resend the corresponding message to the target eNodeB when the handover is complete or to the source eNodeB when the handover is deemed to have failed.
If during the handover procedure the MME detects that the Serving GW needs be relocated, the MME shall reject any PDN GW initiated EPS bearer(s) request received since handover procedure started and shall include an indication that the request has been temporarily rejected due to handover procedure in progress. The rejection is forwarded by the Serving GW to the PDN GW, with the indication that the request has been temporarily rejected.
Upon reception of a rejection for an EPS bearer(s) PDN GW initiated procedure with an indication that the request has been temporarily rejected due to handover procedure in progress, the PDN GW start a locally configured guard timer. The PDN GW shall re-attempt, up to a pre-configured number of times, when either it detects that the handover is completed or has failed using message reception or at expiry of the guard timer.
If the source eNodeB and target eNodeB support RACS as defined in clause 5.11.3a, the source eNodeB provides the UE's UE Radio Capability ID to the target eNodeB. If the source eNodeB has knowledge that the target eNodeB might not have a local copy of the Radio Capability corresponding to the UE Radio Capability ID (i.e. because the source eNodeB had itself to retrieve the UE's Radio Capability from the MME) then the source eNodeB may also send some (or all) of the UE's Radio Capability to the target eNodeB (the size limit based on local configuration) in X2 signalling as defined in TS 36.423.. In the case of inter-PLMN handover, when the source and target eNodeB support RACS as defined in clause 5.11.3a and the source eNodeB determines based on local configuration that the target PLMN does not support the UE Radio Capability ID assigned by the source PLMN, then the source eNodeB shall provide the UE radio capabilities to the target eNodeB and shall not send the UE Radio Capability ID. If the target eNodeB does not have mapping between the specific UE Radio Capability ID and the UE radio capabilities and no UE radio capabilities are provided by source eNodeB, it shall use the procedure described in TS 36.413 to retrieve the mapping from the Core Network. If, as permitted in TS 36.423, the target eNodeB during the handover preparation received the UE radio capabilities but did not receive the UE Radio Capability ID the target eNodeB shall proceed with handover using the received UE radio capabilities. If the target eNodeB received both the UE radio capabilities and the UE Radio Capability ID, then the target eNodeB shall use any locally stored UE radio capability information corresponding to the UE Radio Capability ID. If none are stored locally, the target eNodeB may request the full UE radio capability information from the core network. If the full UE radio capability information is not promptly received from the core network, or the target eNodeB chooses not to request it, then the target eNodeB shall proceed with the UE radio capabilities sent by the source RAN node. The target eNodeB shall not use the UE radio capability information received from the source eNodeB for any other UE with the same UE Radio Capability ID.
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5.5.1.1.2  X2-based handover without Serving-GW relocationWord‑p. 264
This procedure is used to hand over a UE from a source eNodeB to a target eNodeB using X2 when the MME is unchanged and decides that the Serving GW is also unchanged. The presence of IP connectivity between the Serving GW and the source eNodeB, as well as between the Serving GW and the target eNodeB is assumed.
Reproduction of 3GPP TS 23.401, Fig. 5.5.1.1.2-1: X2-based handover without Serving GW relocation
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Step 1a.
If the PLMN has configured secondary RAT usage reporting, the source eNodeB during the handover execution phase may provide RAN usage data Report (Secondary RAT usage data, handover flag) to the MME. The source eNodeB shall provide this only when the Target eNodeB has confirmed handover over X2 interface (see TS 36.300 and the source eNodeB has sent a HO command to the UE). The handover flag indicates to the MME that it should buffer the usage data report before forwarding it to the Serving GW.
Step 1b.
The target eNodeB sends a Path Switch Request message to MME to inform that the UE has changed cell, including the TAI+ECGI of the target cell and the list of EPS bearers to be switched.
If Dual Connectivity is activated for the UE, the PSCell ID shall be included in the Path Switch Request message.
If the target cell is a CSG cell , the target eNodeB includes the CSG ID of the target cell in Path Switch Request message. If the target cell is in hybrid mode, it includes the CSG ID of the target cell and CSG Access Mode set to "hybrid" in the Path Switch Request message. Moreover, the Path Switch Request message contains the CSG Membership Status IE if the hybrid cell accessed by the UE has a different CSG from the source cell or the source cell does not have a CSG ID. The MME determines the CSG membership based on the CSG ID and the target PLMN id received from the target eNodeB.The MME updates the User CSG information based on the CSG ID and CSG Access Mode received from the target eNodeB and CSG membership if one of the parameters has changed.
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 Path Switch Request message.
The MME determines that the Serving GW can continue to serve the UE.
Step 2.
The MME sends a Modify Bearer Request (eNodeB address(es) and TEIDs for downlink user plane for the accepted EPS bearers, ISR Activated, Secondary RAT usage data, User Location Information, PSCell ID) message per PDN connection to the Serving GW for each PDN connection where the default bearer has been accepted by the target eNodeB. If the PDN GW requested location information change reporting, the MME also includes the User Location Information IE in this message if it is different compared to the previously sent information. If the UE Time Zone has changed, the MME includes the UE Time Zone IE in this message. If the Serving Network has changed, the MME includes the new Serving Network IE in this message. If ISR was activated before this procedure, MME should maintain ISR. The UE is informed about the ISR status in the Tracking Area Update procedure. If the Serving GW supports Modify Access Bearers Request procedure and if there is no need for the SGW to send the signalling to the PDN GW, the MME may send Modify Access Bearers Request (eNodeB address(es) and TEIDs for downlink user plane for the accepted EPS bearers, ISR Activated) per UE to the Serving GW to optimise the signalling. The MME includes the Secondary RAT usage data if the MME received it from the source eNodeB in step 1a. The MME includes the PSCell ID if the MME received it from the target eNodeB in step 1b.
If the PDN GW requested UE's User CSG information (determined from the UE context), the MME includes the User CSG Information IE in this message if the User CSG Information has changed.
The MME uses the list of EPS bearers to be switched, received in step 1, to determine whether any dedicated EPS bearers in the UE context have not been accepted by the target eNodeB. 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 multiple PDN connections active, the MME shall consider all bearers of that PDN connection as failed and release that PDN connection by triggering the MME requested PDN disconnection procedure specified in clause 5.10.3.
If none of the default EPS bearers have been accepted by the target eNodeB or there is a LIPA PDN connection that has not been released, the MME shall act as specified in step 6.
Step 3.
If the Serving GW has received the User Location Information IE and/or the UE Time Zone IE and/or the Serving Network IE and/or User CSG Information IE from the MME in step 2 the Serving GW informs the PDN GW(s) about this information that e.g. can be used for charging, by sending the message Modify Bearer Request (Serving GW Address and TEID, User Location Information IE and/or UE Time Zone IE and/or Serving Network IE and/or User CSG Information IE, Secondary RAT usage data) per PDN connection to the PDN GW(s) concerned. The Serving GW shall return a Modify Bearer Response (Serving GW address and TEID for uplink traffic) message to the MME as a response to a Modify Bearer Request message, or a Modify Access Bearers Response (Serving GW address and TEID for uplink traffic) as a response to a Modify Access Bearers Request message. If the Serving GW cannot serve the MME Request in the Modify Access Bearers Request message without S5/S8 signalling or without corresponding Gxc signalling when PMIP is used over the S5/S8 interface, it shall respond to the MME with indicating that the modifications are not limited to S1-U bearers, and the MME shall repeat its request using Modify Bearer Request message per PDN connection. The Serving GW forwards the Secondary RAT usage data to the PDN GW, if the Serving GW received it in step 2 and if PGW secondary RAT usage data reporting is active.
Step 4.
The Serving GW starts sending downlink packets to the target eNodeB using the newly received address and TEIDs. A Modify Bearer Response message is sent back to the MME.
Step 5.
In order to assist the reordering function in the target eNodeB, the Serving GW shall send one or more "end marker" packets on the old path immediately after switching the path as defined in TS 36.300, clause 10.1.2.2.
Step 6.
The MME confirms the Path Switch Request message with the Path Switch Request Ack message. If the UE-AMBR is changed, e.g. all the EPS bearers which are associated to the same APN are rejected in the target eNodeB, the MME shall provide the updated value of UE-AMBR to the target eNodeB in the Path Switch Request Ack message.
If the CSG membership status was included in the Path Switch Request message, the MME shall include its verified CSG membership status in the Path Switch Request Ack message.
If some EPS bearers have not been switched successfully in the core network, the MME shall indicate in the Path Switch Request Ack message which bearers failed to be established (see more detail in TS 36.413) and for dedicated bearers initiate the bearer release procedure as specified in clause 5.4.4.2 to release the core network resources of the failed dedicated EPS bearers. The target eNodeB shall delete the corresponding bearer contexts when it is informed that bearers have not been established in the core network.
If none of the default EPS bearers have been switched successfully in the core network or if they have not been accepted by the target eNodeB or the LIPA PDN connection has not been released, the MME shall send a Path Switch Request Failure message (see more detail in TS 36.413) to the target eNodeB. The MME performs explicit detach of the UE as described in the MME initiated detach procedure of clause 5.3.8.3.
If the MME supports RACS as defined in clause 5.11.3a and has UE Radio Capability ID stored in the UE's context it includes it in the Path Switch Request Ack message, if the target eNodeB supports RACS.
Step 7.
By sending Release Resource the target eNodeB informs success of the handover to source eNodeB and triggers the release of resources. This step is specified in TS 36.300.
Step 8.
The UE initiates a Tracking Area Update procedure when one of the conditions listed in clause "Triggers for tracking area update" applies. If ISR is activated for the UE when the MME receives the Tracking Area Update Request, the MME should maintain ISR by indicating ISR Activated in the Tracking Area Update Accept message.
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5.5.1.1.3  X2-based handover with Serving-GW relocationWord‑p. 266
This procedure is used to hand over a UE from a source eNodeB to a target eNodeB using X2 when the MME is unchanged and the MME decides that the Serving GW is to be relocated. The presence of IP connectivity between the source Serving GW and the source eNodeB, between the source Serving GW and the target eNodeB, and between the target Serving GW and target eNodeB is assumed. (If there is no IP connectivity between target eNodeB and source Serving GW, it is assumed that the S1-based handover procedure in clause 5.5.1.2 shall be used instead.)
Reproduction of 3GPP TS 23.401, Fig. 5.5.1.1.3-1: X2-based handover with Serving GW relocation
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Step 1a.
If the PLMN has configured Secondary RAT usage data reporting and the source eNodeB has Secondary RAT usage data to report, the eNodeB sends a RAN usage data Report (Secondary RAT usage data, handover flag) message to the MME. The eNodeB shall provide this only when it is to perform a Path Switch (i.e. the Target eNodeB has confirmed it is ready over X2 interface (see TS 36.300 and the source eNodeB has sent a HO command to the UE). The handover flag indicates to the MME that it should buffer the report before forwarding the Secondary RAT usage charging data.
Step 1b.
The target eNodeB sends a Path Switch Request message to MME to inform that the UE has changed cell, including the ECGI of the target cell and the list of EPS bearers to be switched.
If Dual Connectivity is activated for the UE, the PSCell ID shall be included in the Path Switch Request message.
If the target cell is a CSG cell, the target eNodeB includes the CSG ID of the target cell in Path Switch Request message. If the target cell is in hybrid mode, it includes the CSG ID of the target cell and CSG Access Mode set to "hybrid" in the Path Switch Request message. Moreover, the Path Switch Request message contains the CSG Membership Status IE if the hybrid cell accessed by the UE has a different CSG from the source cell or the source cell does not have a CSG ID. The MME determines the CSG membership based on the CSG ID and the target PLMN id received from the target eNodeB. The MME updates the User CSG information based on the CSG ID and CSG Access Mode received from the target eNodeB and CSG membership if one of the parameters has changed.
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 Path Switch Request message.
The MME determines that the Serving GW is relocated and selects a new Serving GW according to clause 4.3.8.2 on "Serving GW Selection Function".
Step 2.
The MME sends a Create Session Request (bearer context(s) with PDN GW addresses and TEIDs (for GTP-based S5/S8) or GRE keys (for PMIP-based S5/S8) at the PDN GW(s) for uplink traffic, eNodeB address(es) and TEIDs for downlink user plane for the accepted EPS bearers, the Protocol Type over S5/S8, Serving Network, UE Time Zone, Secondary RAT usage data, User Location Information, PSCell ID) message per PDN connection to the target Serving GW for each PDN connection where the default bearer has been accepted by the target eNodeB. The target Serving GW allocates the S GW addresses and TEIDs for the uplink traffic on S1_U reference point (one TEID per bearer). The Protocol Type over S5/S8 is provided to Serving GW which protocol should be used over S5/S8 interface. If the PDN GW requested location information change reporting, the MME also includes the User Location Information IE in this message if it is different compared to the previously sent information. If the PDN GW requested UE's User CSG information (determined from the UE context), the MME includes the User CSG Information IE in this message if the User CSG Information has changed.
The MME uses the list of EPS bearers to be switched, received in step 1, to determine whether any dedicated EPS bearers in the UE context have not been accepted by the target eNodeB. The MME releases the non-accepted dedicated bearers by triggering the bearer release procedure as specified in clause 5.4.4.2 via target Serving GW. 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 multiple PDN connections active, the MME shall consider all bearers of that PDN connection as failed and release that PDN connection by triggering the MME requested PDN disconnection procedure specified in clause 5.10.3 via source Serving GW.
If none of the default EPS bearers have been accepted by the target eNodeB or there is a LIPA PDN connection that has not been released, the MME shall act as specified in step 5.
If the MME received it from the source eNodeB in step 1a and PDN GW Secondary RAT reporting is active, the MME includes the Secondary RAT usage data with a flag stating that the target SGW shall not process the information and only forward it to the PDN GW. If MME received PSCell ID in step 1b, the MME shall include it in Create Session Request.
Step 3.
The target Serving GW assigns addresses and TEIDs (one per bearer) for downlink traffic from the PDN GW. The Serving GW allocates DL TEIDs on S5/S8 even for non-accepted bearers. It sends a Modify Bearer Request (Serving GW addresses for user plane and TEID(s), Serving Network, PDN Charging Pause Support Indication, Secondary RAT usage data) message per PDN connection to the PDN GW(s). 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 2. 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. The PDN GW starts sending downlink packets to the target GW using the newly received address and TEIDs. These downlink packets will use the new downlink path via the target Serving GW to the target eNodeB. The Serving GW shall allocate TEIDs for the failed bearers and inform to the MME. The Serving GW forwards the Secondary RAT usage data to the PDN GW, if the Serving GW received it in step 2 and if PGW secondary RAT usage data reporting is active.
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 eNodeB.
Step 4.
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 MME starts a timer, to be used in step 7.
Step 5.
The MME confirms the Path Switch Request message with the Path Switch Request Ack (Serving GW addresses and uplink TEID(s) for user plane) message. If the UE-AMBR is changed, e.g. all the EPS bearers which are associated to the same APN are rejected in the target eNodeB, the MME shall provide the updated value of UE-AMBR to the target eNodeB in the Path Switch Request Ack message. The target eNodeB starts using the new Serving GW address(es) and TEID(s) for forwarding subsequent uplink packets.
If the CSG membership status was included in the Path Switch Request message, the MME shall include its verified CSG membership status in the Path Switch Request Ack message.
If some EPS bearers have not been switched successfully in the core network, the MME shall indicate in the Path Switch Request Ack message which bearers failed to be established (see more detail in TS 36.413) and for dedicated bearers initiate the bearer release procedure as specified in clause 5.4.4.2 to release the core network resources of the failed dedicated EPS bearers. The target eNodeB shall delete the corresponding bearer contexts when it is informed that bearers have not been established in the core network.
If none of the default EPS bearers have been switched successfully in the core network or if they have not been accepted by the target eNodeB or the LIPA PDN connection has not been released, the MME shall send a Path Switch Request Failure message (see more detail in TS 36.413) to the target eNodeB. The MME performs explicit detach of the UE as described in the MME initiated detach procedure of clause 5.3.8.3.
If the MME supports RACS as defined in clause 5.11.3a and has UE Radio Capability ID stored in the UE's context it includes it in the Path Switch Request Ack message.
Step 6.
By sending Release Resource the target eNodeB informs success of the handover to source eNodeB and triggers the release of resources. This step is specified in TS 36.300.
Step 7.
When the timer has expired after step 4, the source MME releases the bearer(s) in the source Serving GW by sending a Delete Session Request message (Cause, Operation Indication, Secondary RAT usage data, User Location Information, PSCell ID). 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. If PSCell ID was received in step 1a, the MME includes it in Delete Session Request message. The Source Serving GW acknowledges with Delete Session Response 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. The MME includes the Secondary RAT usage data in this message if it received it in step 1a.
Step 8.
The UE initiates a Tracking Area Update procedure when one of the conditions listed in clause "Triggers for tracking area update" applies.
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