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Content for  TS 23.401  Word version:  17.6.0

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5.5.1.2  S1-based handoverp. 270

5.5.1.2.1  Generalp. 270
The S1-based handover procedure is used when the X2-based handover cannot be used. The source eNodeB initiates a handover by sending Handover Required message over the S1-MME reference point. This procedure may relocate the MME and/or the Serving-GW. The source MME selects the target MME. The MME should not be relocated during inter-eNodeB handover unless the UE leaves the MME Pool Area where the UE is served. The MME (target MME for MME relocation) determines if the Serving-GW needs to be relocated. If the Serving-GW needs to be relocated the MME selects the target Serving-GW, as specified in clause 4.3.8.2 on Serving-GW selection function.
The source eNodeB decides which of the EPS bearers are subject for forwarding of downlink and optionally also uplink data packets from the source eNodeB to the target eNodeB. The EPC does not change the decisions taken by the RAN node. Packet forwarding can take place either directly from the source eNodeB to the target eNodeB, or indirectly from the source eNodeB to the target eNodeB via the source and target Serving-GWs (or if the Serving-GW is not relocated, only the single Serving-GW).
The availability of a direct forwarding path is determined in the source eNodeB and indicated to the source MME. If X2 connectivity is available between the source and target eNodeBs, a direct forwarding path is available.
If a direct forwarding path is not available, indirect forwarding may be used. The source MME uses the indication from the source eNodeB to determine whether to apply indirect forwarding. The source MME indicates to the target MME whether indirect forwarding should apply. Based on this indication, the target MME determines whether it applies indirect forwarding.
If both source eNodeB and source MME support DAPS the source eNodeB may decide that some of the E-RABs are subject for DAPS handover as defined in TS 36.300; in this case, the source eNodeB provides the DAPS information indicating the request concerns a DAPS handover for that E-RAB as part of the Source to Target eNodeB Transparent Container. If the Target eNodeB accepts that the request concerns of DAPS handover and both Target eNodeB and Target MME support DAPS, the S1-based DAPS handover will be performed and the target eNodeB provides DAPS response information as part of the Target to Source eNodeB Transparent Container.
If the MME receives a rejection to an S1 interface procedure (e.g. dedicated bearer establishment/ modification/ release; location reporting control; NAS message transfer; etc.) from the eNodeB with an indication that an S1 handover is in progress (see TS 36.300), the MME shall reattempt the same S1 interface procedure when either the handover is complete or is deemed to have failed if the MME is still the serving MME, except in the case of Serving-GW relocation. If the S1 handover changes the serving MME, the source MME shall terminate any other ongoing S1 interface procedures except the handover procedure.
If the S1 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 S1 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 indication from the eNodeB with an indication that an S1 handover is in progress, the MME shall 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.
In order to minimise the number of procedures rejected by the eNodeB, the MME should pause non-handover related S1 interface procedures (e.g. downlink NAS message transfer, E-RAB Setup /Modify/ Release, etc.) while a handover is ongoing (i.e. from the time that a Handover Required has been received until either the Handover procedure has succeeded (Handover Notify) or failed (Handover Failure)) and continue them once the Handover procedure has completed if the MME is still the serving MME, except in the case of Serving-GW relocation.
If during the handover procedure the MME detects that the Serving-GW or/and the MME 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 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 emergency bearer services are ongoing for the UE, handover to the target CSG cell is performed independent of the UE's CSG subscription. If the handover is to a CSG cell that the UE is not subscribed, the target eNodeB only accepts the emergency bearers and the target MME releases the non-emergency PDN connections that were not accepted by the target eNodeB as specified in clause 5.10.3.
For inter-PLMN handover to a CSG cell, if the source MME has the CSG-ID list of the target PLMN, the source MME shall use it to validate the CSG membership of the UE in the target CSG cell. Otherwise, based on operator's configuration the source MME may allow the handover by validating the CSG membership of the UE in the target CSG cell using the CSG-ID list of the registered PLMN-ID. If neither the CSG-ID list of the target PLMN nor the operator's configuration permits the handover, the source MME shall reject the handover due to no CSG membership information of the target PLMN-ID.
As specified in clause 4.3.8.3, with regard to CIoT EPS Optimisations, the source MME attempts to perform handover to a target MME that can support the UE's Preferred Network Behaviour. For a UE that is using a Non-IP connection to a PDN Gateway, or a PDN connection to a SCEF, if these bearers cannot be supported by the target MME, the source MME does not attempt to handover those bearers, but instead releases them upon successful completion of the handover. If the MME does not have any bearer for the UE that can be transferred, then the MME sends an S1-AP Handover Preparation Failure message to the source eNodeB.
For PDN connection of Ethernet Type, if the target MME does not support Ethernet PDN Type, the source MME does not attempt to handover those bearers, but instead releases them upon successful completion of the handover.
If the source eNodeB and target eNodeB support RACS as defined in clause 5.11.3a, the Source to Target transparent container need not carry the UE radio capabilities (instead the UE Radio Capability ID is supplied from the CN to the target eNodeB). However, 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 send some (or all) of the UE's Radio Capability to the target eNodeB (the size limit based on local configuration. 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 include the UE radio capabilities in the Source to Target transparent container. At such an inter-PLMN handover, the source CN node shall not provide the UE Radio Capability ID to the target CN node (or in intra-CN node case not to the target eNodeB). If the target eNodeB does not have a mapping between the UE Radio Capability ID received from the MME and the UE radio capabilities and no UE radio capability is provided in the Source to Target transparent container, it shall use the procedure described in TS 36.413 to retrieve the mapping from the Core Network. If the target eNodeB received both the UE radio capabilities (in the Source to Target transparent container) and the UE Radio Capability ID (from the MME), 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 RAN node for any other UE with the same UE Radio Capability ID.
The Inter eNodeB S1 based handover procedure specified in clause 5.5.1.2.2 may also be used for intra-eNodeB handover.
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5.5.1.2.2  S1-based handover, normalp. 272
This procedure describes the S1-based handover in the normal case, clause 5.5.1.2.3 describes it when the procedure is rejected by the target eNodeB or the target MME and clause 5.5.1.2.4 describes when the procedure is canceled by the source eNodeB.
Reproduction of 3GPP TS 23.401, Fig. 5.5.1.2.2-1: S1-based handover
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Step 1.
The source eNodeB decides to initiate an S1-based handover to the target eNodeB. This can be triggered e.g. by no X2 connectivity to the target eNodeB, or by an error indication from the target eNodeB after an unsuccessful X2-based handover, or by dynamic information learnt by the source eNodeB.
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Step 2.
The source eNodeB sends Handover Required (Direct Forwarding Path Availability, Source to Target transparent container, target eNodeB Identity, CSG ID, CSG access mode, target TAI, S1AP Cause) to the source MME. The source eNodeB indicates which bearers are subject to data forwarding. Direct Forwarding Path Availability indicates whether direct forwarding is available from the source eNodeB to the target eNodeB. This indication from source eNodeB can be based on e.g. the presence of X2. The target TAI is sent to MME to facilitate the selection of a suitable target MME. When the target cell is a CSG cell or a hybrid cell, the source eNodeB shall include the CSG ID of the target cell. If the target cell is a hybrid cell, the CSG access mode shall be indicated.
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Step 3.
The source MME selects the target MME as described in clause 4.3.8.3 on "MME Selection Function" and if it has determined to relocate the MME, it sends a Forward Relocation Request (MME UE context, Source to Target transparent container, RAN Cause, target eNodeB Identity, CSG ID, CSG Membership Indication, target TAI, MS Info Change Reporting Action (if available), CSG Information Reporting Action (if available), UE Time Zone, Direct Forwarding Flag, Serving Network, Local Home Network ID, LTE-M UE Indication) message to the target MME. The target TAI is sent to the target MME to help it to determine whether S-GW relocation is needed (and, if needed, aid SGW selection). The old Serving Network is sent to target MME to support the target MME to resolve if Serving Network is changed. In network sharing scenarios Serving Network denotes the serving core network.
The source MME shall perform access control by checking the UE's CSG subscription when CSG ID is provided by the source eNodeB. 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 MME shall reject the handover with an appropriate cause unless the UE has emergency bearer services.
The MME UE context includes IMSI, ME Identity, UE security context, UE Network Capability, AMBR, Selected CN operator ID, APN restriction, Serving-GW address and TEID for control signalling, and EPS Bearer context(s), UE Radio Capability ID.
An EPS Bearer context includes the 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, APN, Serving-GW addresses and TEIDs for uplink traffic, and TI.
Based on the CIoT EPS Optimisation capabilities of the target MME (determined according to the target MME selection procedure of clause 4.3.8.3) the source MME only includes the EPS Bearer Context(s) that the target MME can support. If none of the UE's EPS Bearers can be supported by the selected target MME, the source MME rejects the S1 handover attempt by sending a Handover Preparation Failure (Cause) message to the Source eNodeB. If the target MME supports CIoT EPS Optimisation and the use of header compression has been negotiated between the UE and the source MME, the source MME also includes in the Forward Relocation Request the previously negotiated Header Compression Configuration that includes the information necessary for the ROHC channel setup but not the RoHC context itself.
If the source MME includes EPS Bearer Context, in addition to the Serving-GW IP address and TEID for S1-U use plane, the source MME also includes Serving-GW IP address and TEID for S11-U, if available.
If SIPTO at the Local Network is active for a PDN connection in the architecture with stand-alone GW the source MME shall include the Local Home Network ID of the source cell in the EPS Bearer context corresponding to the SIPTO at the Local Network PDN connection.
RAN Cause indicates the S1AP Cause as received from source eNodeB.
The source MME includes the CSG ID in the Forward Relocation Request when the target cell is a CSG 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.
The Direct Forwarding Flag indicates if direct forwarding is applied, or if indirect forwarding is going to be set up by the source side.
The target MME shall determine the Maximum APN restriction based on the APN Restriction of each bearer context in the Forward Relocation Request, and shall subsequently store the new Maximum APN restriction value.
If the UE receives only emergency services and the UE is UICCless, IMSI can not be included in the MME UE context in Forward Relocation Request message. For emergency attached UEs, if the IMSI cannot be authenticated, then the IMSI shall be marked as unauthenticated. Also, in this case, security parameters are included only if available.
If a UE is RLOS attached, the old MME includes an RLOS indication to the new MME. If the RLOS attached UE in the old MME does not have a USIM, IMSI can not be included in the Forward Relocation Request message. If the RLOS attached UE has USIM but the IMSI cannot be successfully authenticated, then the IMSI shall be marked as unauthenticated. Also, in this case, security parameters are included only if available.
If the Old MME is aware the UE is a LTE-M UE, it provides the LTE-M UE Indication to the new MME.
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Step 4.
If the MME has been relocated, the target MME verifies whether the source Serving-GW can continue to serve the UE. If not, it selects a new Serving-GW as described in clause 4.3.8.2 on "Serving-GW Selection Function". If the MME has not been relocated, the source MME decides on this Serving-GW re-selection.
If the source Serving-GW continues to serve the UE, no message is sent in this step. In this case, the target Serving-GW is identical to the source Serving-GW.
If a new Serving-GW is selected, the target 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, Serving Network, UE Time Zone) message per PDN connection to the target Serving-GW. 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 target Serving-GW sends a Create Session Response (Serving-GW addresses and uplink TEID(s) for user plane) message back to the target MME.
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Step 5.
The Target MME sends Handover Request (EPS Bearers to Setup, AMBR, S1AP Cause, Source to Target transparent container, CSG ID, CSG Membership Indication, Handover Restriction List, UE Radio Capability ID) message to the target eNodeB. This message creates the UE context in the target eNodeB, including information about the bearers, and the security context. For each EPS Bearer, the Bearers to Setup includes Serving-GW address and uplink TEID for user plane, and EPS Bearer QoS. If the direct forwarding flag indicates unavailability of direct forwarding and the target MME knows that there is no indirect data forwarding connectivity between source and target, the Bearers to Setup shall include "Data forwarding not possible" indication for each EPS bearer. Handover Restriction List is sent if available in the Target MME; it is described in clause 4.3.5.7 "Mobility Restrictions".
S1AP Cause indicates the RAN Cause as received from source MME.
The Target MME shall include the CSG ID and CSG Membership Indication when provided by the source MME in the Forward Relocation Request message.
The target eNodeB sends a Handover Request Acknowledge (EPS Bearer Setup list, EPS Bearers failed to setup list Target to Source transparent container) message to the target MME. The EPS Bearer Setup list includes a list of addresses and TEIDs allocated at the target eNodeB for downlink traffic on S1-U reference point (one TEID per bearer) and addresses and TEIDs for receiving forwarded data if necessary. 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 recalculate the new UE-AMBR and signal the modified UE-AMBR value to the target eNodeB.
If none of the default EPS bearers have been accepted by the target eNodeB, the target MME shall reject the handover as specified in clause 5.5.1.2.3.
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 Indication 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.
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 Handover Request message, if target eNodeB supports RACS.
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Step 6.
If indirect forwarding applies and the Serving-GW is relocated, the target MME sets up forwarding parameters by sending Create Indirect Data Forwarding Tunnel Request (target eNodeB addresses and TEIDs for forwarding) to the Serving-GW. The Serving-GW sends a Create Indirect Data Forwarding Tunnel Response (target Serving-GW addresses and TEIDs for forwarding) to the target MME. If the Serving-GW is not relocated, indirect forwarding may be set up in step 8 below.
Indirect forwarding may be performed via a Serving-GW which is different from the Serving-GW used as the anchor point for the UE.
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Step 7.
If the MME has been relocated, the target MME sends a Forward Relocation Response (Cause, Target to Source transparent container, Serving-GW change indication, EPS Bearer Setup List, Addresses and TEIDs) message to the source MME. For indirect forwarding, this message includes Serving-GW Address and TEIDs for indirect forwarding (source or target). Serving-GW change indication indicates a new Serving-GW has been selected.
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Step 8.
If indirect forwarding applies, the source MME sends Create Indirect Data Forwarding Tunnel Request (addresses and TEIDs for forwarding) to the Serving-GW. If the Serving-GW is relocated it includes the tunnel identifier to the target serving GW.
The Serving-GW responds with a Create Indirect Data Forwarding Tunnel Response (Serving-GW addresses and TEIDs for forwarding) message to the source MME.
Indirect forwarding may be performed via a Serving-GW which is different from the Serving-GW used as the anchor point for the UE.
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Step 9.
The source MME sends a Handover Command (Target to Source transparent container, Bearers subject to forwarding, Bearers to Release) message to the source eNodeB. The Bearers subject to forwarding includes list of addresses and TEIDs allocated for forwarding. The Bearers to Release includes the list of bearers to be released.
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Step 9a.
The Handover Command is constructed using the Target to Source transparent container and is sent to the UE. Upon reception of this message the UE will remove any EPS bearers for which it did not receive the corresponding EPS radio bearers in the target cell.
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Step 9b.
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 source MME. The handover flag indicates to the MME that it should buffer the report before forwarding the Secondary RAT usage data.
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Step 10.
The source eNodeB sends the eNodeB Status Transfer message to the target eNodeB via the MME(s) to convey the PDCP and HFN status of the E-RABs for which PDCP status preservation applies, as specified in TS 36.300. The source eNodeB may omit sending this message if none of the E-RABs of the UE shall be treated with PDCP status preservation.
If there is an MME relocation the source MME sends this information to the target MME via the Forward Access Context Notification message which the target MME acknowledges. The source MME or, if the MME is relocated, the target MME, sends the information to the target eNodeB via the MME Status Transfer message.
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Step 11.
The source eNodeB should start forwarding of downlink data from the source eNodeB towards the target eNodeB for bearers subject to data forwarding. This may be either direct (step 11a) or indirect forwarding (step 11b).
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Step 12.
After the UE has successfully synchronized to the target cell, it sends a Handover Confirm message to the target eNodeB. Downlink packets forwarded from the source eNodeB can be sent to the UE. Also, uplink packets can be sent from the UE, which are forwarded to the target Serving-GW and on to the PDN-GW.
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Step 13.
The target eNodeB sends a Handover Notify (TAI+ECGI, Local Home Network ID) message to the target MME. If Dual Connectivity is activated for the UE, the PSCell ID shall be included in the Handover Notify message.
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.
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Step 14.
If the MME has been relocated, the target MME sends a Forward Relocation Complete Notification message to the source MME. The source MME in response sends a Forward Relocation Complete Acknowledge (Secondary RAT usage data) message to the target MME. The source MME includes Secondary RAT usage data in this message if it received this in step 9b. Regardless if MME has been relocated or not, a timer in source MME is started to supervise when resources in Source eNodeB and if the Serving-GW is relocated, also resources in Source Serving-GW shall be released.
Upon receipt of the Forward Relocation Complete Acknowledge message the target MME starts a timer if the target MME allocated S-GW resources for indirect forwarding.
For all bearers that were not included in the Forward Relocation Request message sent in step 3, the MME now releases them by sending a Delete Bearer Command to the SGW, or, the appropriate message to the SCEF.
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Step 15.
The MME sends a Modify Bearer Request (eNodeB address and TEID allocated at the target eNodeB for downlink traffic on S1 U for the accepted EPS bearers, ISR Activated, Secondary RAT usage data if PGW secondary RAT usage data reporting is active, User Location Information, PSCell ID) message to the target Serving-GW for each PDN connection, including the PDN connections that need to be released. If the PDN-GW requested location information change reporting and/or User CSG information (determined from the UE context), the MME also includes the User Location Information IE (if it is different compared to the previously sent information) 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 Serving-GW is not relocated but the Serving Network has changed or if the MME has not received any old Serving Network information from the old MME, the MME includes the Serving Network IE in this message. For the case that neither MME nor S-GW changed, 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 and TEID allocated at the target eNodeB for downlink traffic on S1 U for the accepted EPS bearers, ISR Activated) per UE to the Serving-GW to optimise the signalling. If Serving-GW is not relocated and if Secondary RAT usage data was received in step 9a, the MME includes the Secondary RAT usage data in the message. If the Serving-GW has been relocated and if PGW Secondary RAT reporting is active, the MME includes the Secondary RAT usage data and also includes a flag stating that the Serving-GW should not process the information and only forward it to the PDN-GW. If PSCell ID is received in step 13, the MME includes it in Modify Bearer Request message.
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.
When the Modify Bearer Request does not indicate ISR Activated the Serving-GW deletes any ISR resources by sending a Delete Bearer Request to the other CN node that has bearer resources on the Serving-GW reserved.
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Step 16.
If the Serving-GW is relocated, the target Serving-GW assigns addresses and TEIDs (one per bearer) for downlink traffic from the PDN-GW. 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 they are present in step 15. The Serving-GW also includes Serving Network IE if it is present in step 4 or step 15. The Serving-GW allocates DL TEIDs on S5/S8 even for non-accepted bearers. 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 target 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 Secondary RAT usage data is included if it was received in step 15 and if PGW secondary RAT usage data reporting is active.
If the Serving-GW is not relocated, but has received the User Location Information IE and/or UE Time Zone IE and/or User CSG Information IE and/or Serving Network IE from the MME in step 15, the Serving-GW shall inform the PDN-GW(s) about these information that e.g. can be used for charging, by sending the message Modify Bearer Request (User Location Information IE, UE Time Zone IE, User CSG Information IE, Serving Network IE) to the PDN-GW(s) concerned. A Modify Bearer Response message is sent back to the Serving-GW.
If the Serving-GW is not relocated and it has not received User Location Information IE nor UE Time Zone IE nor User CSG Information IE nor Serving Network IE from the MME in step 15, no message is sent in this step and downlink packets from the Serving-GW are immediately sent on to the target eNodeB.
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. If data forwarding -direct or indirect) occurs, the source eNodeB shall forward the "end marker" packets to the target eNodeB via the forwarding tunnel.
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Step 17.
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 other than to unpause charging in the PDN-GW 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.
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. If data forwarding -direct or indirect) occurs, the source eNodeB shall forward the "end marker" packets to the target eNodeB via the forwarding tunnel.18. The UE initiates a Tracking Area Update procedure when one of the conditions listed in clause "Triggers for tracking area update" applies.
For a UE supporting CIoT EPS Optimisations, the EPS bearer status information shall be included in the TAU Request. The MME shall then indicate the EPS bearer status to the UE in the TAU Accept and the UE shall locally release any non-transferred bearer.
The target MME knows that it is a Handover procedure that 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 MME and target MME. In this case, the target MME shall set the Header Compression Context Status for each EPS Bearer in the TAU Accept message based on information obtained in step 3.
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Step 19.
When the timer started in step 14 expires the source MME sends a UE Context Release Command () message to the source eNodeB. The source eNodeB releases its resources related to the UE and responds with a UE Context Release Complete () message. When the timer started in step 14 expires and if the source MME received the Serving-GW change indication in the Forward Relocation Response message, it deletes the EPS bearer resources by sending Delete Session Request (Cause, LBI, Operation Indication, Secondary RAT usage data, User Location Information, PSCell ID) 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. Secondary RAT usage data is included if it was received in step 9b. PSCell ID is included if it was received in step 9b. 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.
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Step 20.
If indirect forwarding was used then the expiry of the timer at source MME started at step 14 triggers the source MME to send a Delete Indirect Data Forwarding Tunnel Request message to the S-GW to release the temporary resources used for indirect forwarding that were allocated at step 8.
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Step 21.
If indirect forwarding was used and the Serving-GW is relocated, then the expiry of the timer at target MME started at step 14 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 that were allocated at step 6.
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5.5.1.2.2a  S1-based DAPS handover, normal |R16|p. 279
This procedure describes the S1-based DAPS handover in the normal case.
Reproduction of 3GPP TS 23.401, Fig. 5.5.1.2.2a-1: S1-based DAPS handover
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Step 1.
Step 1 to step 9b are performed as described in clause 5.5.1.2.2 with the following changes:
The Source to Target Transparent Container which is included in Handover Required message in step 2 in clause 5.5.1.2.2 will contain the DAPS Information if DAPS HO is supported by source eNodeB and source MME and DAPS HO is requested for one or more DRBs as described in TS 36.300.
If DAPS HO is supported by the target eNodeB and target MME and the DAPS Information is received for one or more DRBs in the Source to Target transparent container, the target eNodeB provides the DAPS Response Information in the Target to Source transparent container in Handover Request Acknowledge message in step 5a in clause 5.5.1.2.2.
Step 2.
The source eNodeB sends the eNodeB Early Status Transfer message to the target eNodeB via the MME(s) to convey the PDCP and HFN status of the E-RABs for which PDCP status preservation applies, as specified in TS 36.300. For the DRBs not subject to DAPS, step 10 to step 10c in clause 5.5.1.2.2 may be performed.
If there is an MME relocation the source MME sends this information to the target MME via the Forward Access Context Notification message which the target MME acknowledges. The source MME or, if the MME is relocated, the target MME, sends the information to the target eNodeB via the MME Early Status Transfer message.
Step 3.
This step is the same as step 11 in clause 5.5.1.2.2.
Step 4.
This step is the same as step 12 in clause 5.5.1.2.2.
Step 5.
This step is the same as step 13 in clause 5.5.1.2.2 with the difference that the Notify Source eNodeB IE is included in the Handover Notify message.
Step 6.
This step is the same as step 14 in clause 5.5.1.2.2 with following changes:
If there is an MME relocation and the target MME receives this Notify Source eNodeB information from the target eNodeB, the target MME provides the Notify Source eNodeB information via the Forward Relocation Complete Notification message.
Step 7.
If the Notify Source eNodeB IE is provided in either step 5 or step 6, the source MME sends the Handover Success message to source eNodeB to inform the source eNodeB that the UE has successfully accessed the target eNodeB.
Step 8.
The Source eNodeB initiates the eNodeB Status Transfer message for the DRB(s) subject to DAPS. This step is the same as step 10 in clause 5.5.1.2.2.
Step 9.
Steps 15 to 21b in clause 5.5.1.2.2 are performed.
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5.5.1.2.3  S1-based handover, Rejectp. 280
The Target eNodeB rejects the use of the Handover procedure if none of the requested bearers in the Handover Request message could be established. In this case no UE context is established in the target MME/eNodeB and no resources are allocated. Further, the Target MME rejects the handover request and clears all resource in Target eNodeB and Target MME if the Target eNodeB accepts the handover request but none of the default EPS bearers gets resources allocated. In both cases, the UE remains in the Source eNodeB/MME.
Reproduction of 3GPP TS 23.401, Fig. 5.5.1.2.3-1: S1-based handover, Reject
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Step 1-5.
Steps 1 to 5 in the flow are identical to steps 1-5 in clause 5.5.1.2.2.
Step 6a.
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. The Target MME clears any reserved resources for this UE in the target MME.
Step 6b.
If the Target MME receives a Handover Request Acknowledge message from the Target eNodeB but none of the default EPS bearers are in the EPS Bearer Setup list IE, the Target MME clears any reserved resources for this UE in both the Target MME and the Target eNodeB.
Step 7.
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.
Step 8.
The Target MME sends the Forward Relocation Response (Cause) message to the Source MME.
Step 9.
When the Source MME receives the Forward Relocation Response message, it sends a Handover Preparation Failure (Cause) message to the Source eNodeB.
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5.5.1.2.4  S1-based handover, Cancelp. 281
Instead of completing the handover procedure, the source eNodeB may at any time during the handover procedure, up to the time when a handover command message is sent to the UE cancel the handover.
The MME shall cancel the handover resources as defined in clause 5.5.2.5.1 for case the source RAN is eNodeB.

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