Section: 5.5.1.2.2 S1-based handover, normal
Content: 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. Figure 5.5.1.2.2-1: S1-based handover NOTE 1: For a PMIP-based S5/S8, procedure steps (A) and (B) are defined in TS 23.402[ Architecture enhancements for non-3GPP accesses ]  [2]. Steps 16 and 16a concern GTP based S5/S8. NOTE 2: If the Serving GW is not relocated, the box "Source Serving GW" in figure 5.5.1.2.2-1 is acting as the target Serving GW. 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. 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. 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, MSISDN, 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. The MME UE context received from the source MME contains the MSISDN if the MSISDN was available at the source MME. 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. NOTE 3: If the handover is successful, the source MME will signal to the SGW and/or SCEF to release any non-included EPS Bearers after step 14. The non-included bearers are locally released by the UE following the Bearer Context Status synchronisation that occurs during the Tracking Area Update at step 18. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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[ Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 ]  [5]. 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. 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). 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. 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. 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. 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. 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. 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. 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. 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. 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.