LTE S1 Handover (S1 Handover) LTE Mobile eNodeB Network ...

1 LTE S1 Handover (S1 Handover )LTE MobileeNodeB NetworkCore NetworkUETargeteNodeBSourceeNodeBMMESGWP GWE ventStudio System Designer 617-Feb-14 21:15 (Page 1)This sequence diagram was generated with EventStudio Sytem Designer - UE is handed over using an S1 Handover if the X2 interface is not available between the source and target eNodeB . RRC-Connected The UE and Source eNodeB are in RRC gtp Downlink DataDownlink data is flowing from the SGW to the UE via theSource gtp Downlink Datarlc rlc Downlink DataDownlink data flow before Handover rlc rlc Uplink DataUplink data is flowing from the UE to the SGW via theSource gtp Uplink Datagtp gtp Uplink DataUplink data flow before Handover Before the Handover rrc rrc RRC Measurement ControlThe Network sets the measurement thresholds forsending measurement rrc RRC Measurement ReportSignal strength of serving cell,Signal strength of neighbors Neighboring cell signal quality is now better than theserving : Handover needs to beperformed.

2 A cell is selectedfor Handover . The RRC uses the latest measurement to decide if ahandover is needed to another cell. The target cell isselected. The eNodeB for the target cell is upon the type ofhandover: S1 The source eNodeB decides to initiate an S1-basedhandover to the target eNodeB as it does not have an X2interface with the target eNodeB . (The S1 Handover canalso be triggered if the X2 Handover had failed.)Derive KeNB* s1ap s1ap S1AP Handover RequiredDirect ForwardingPath Availability =Not Available,Source to Targettransparentcontainer,target eNodeBIdentity,target TAI,S1AP Cause,RAN assistancedata The source eNodeB sends a " Handover Required"message to the MME. The source eNodeB indicateswhich bearers are subject to data forwarding.

3 The X2interface is not available, direct forwarding is not anoption. The data will need to be tunneled via the target TAI is sent to MME to facilitate the selection ofa suitable target preparation LTE S1 Handover (S1 Handover )LTE MobileeNodeB NetworkCore NetworkUETargeteNodeBSourceeNodeBMMESGWP GWE ventStudio System Designer 617-Feb-14 21:15 (Page 2)s1ap s1ap S1AP Handover RequestEPS Bearers to Setup,AMBR,S1AP Cause,Source to Target transparentcontainer, Handover Restriction List,RAN assistance data The MME sends Handover Request message to thetarget eNodeB . This message creates the UE context inthe target eNodeB , including information about thebearers, and the security context. For each EPS Bearer,the "Bearers to Setup" includes Serving GW address anduplink TEID for user plane, and EPS Bearer QoS.

4 If thedirect forwarding flag indicates unavailability of directforwarding and the MME knows that there is no indirectdata forwarding connectivity between source and target,the Bearers to Setup shall include "Data forwarding notpossible" indication for each EPS bearer. HandoverRestriction List is sent if available in the AS keys fromKeNB (KeNB = KeNB*) Perform admission controlthe E-RABs in the message Check if resources are available at the target eNodeB toaccept this DRB ID (Uplink /Downlink) Assign Dedicated Radio Bearer ids for Uplink : Reserve downlink anduplink radio resources for thesession The Target eNodeB allocates radio resources for the UEthat will be handed RACH Preamble The Target eNodeB allocates a RACH preamble to theUE.

5 The UE will use this preamble to send a contentionfree C-RNTI A new C-RNTI is assigned to the : Prepare the HandoverCommand message (RRCC onnection ReconfigurationRequest) This message includes the RACH preamble that needs tobe sent to the terminal. This message includesinformation about the assigned radio s1ap S1AP Handover Request AcknowledgeEPS Bearer Setup list,EPS Bearers failed to setup list,Transparent container = HandoverCommand The Target eNodeB responds back to the MME with aHandover Request Acknowledge message. Thismessage carries the Handover Command message(RRC Connection Reconfiguration Request) in atransparent container. The "EPS Bearer Setup list"includes a list of addresses and TEIDs allocated at thetarget eNodeB for downlink traffic on S1 U referencepoint (one TEID per bearer) and addresses and TEIDs forreceiving forwarded data if s1ap S1AP Handover CommandBearers subjectto forwarding,Beare rs toRelease,Transpa rentcontainer =HandoverCommand The source MME sends a Handover Command messageto the source eNodeB .

6 The Bearers subject to forwardingincludes list of addresses and TEIDs allocated forforwarding. The Bearers to Release includes the list ofbearers to be downlink data At this point, the target eNodeB is ready to bufferdownlink data that will be received during the gtp GTP Create Indirect Data Forward Tunnel RequestIMSI,target eNodeBaddresses,Beare r Context {EPS Bearer Id, Target eNodeBF-TEID } No X2 path exists between the source and targeteNodeB, so a tunnel needs to be established betweenthe source and target eNodeBs via the Tunnel The SGW creates the indirect indirect tunnel to transportdownlink data from source eNodeB totarget eNodeB Handover execution LTE S1 Handover (S1 Handover )LTE MobileeNodeB NetworkCore NetworkUETargeteNodeBSourceeNodeBMMESGWP GWE ventStudio System Designer 617-Feb-14 21.

7 15 (Page 3)gtp gtp GTP Create Indirect Data Forward Tunnel ResponseCause = RequestAccepted SGW replies back to the gtp Downlink Datagtp gtp Downlink DataThe downlink data is transported from the SourceeNodeB to the Target eNodeB via the just establishedindirect data received fromthe Source eNodeB The data cannot be sent to the target until the RRCreconfiguration is rrc RRC Connection Reconfiguration RequestRACH Preamble Assignment,Target C-RNTI,Target DRB ID (UL/DL),Target eNB AS Security Algorithm The Source eNodeB sends a Handover command to theUE. The message contains a new C-RNTI and new DRBIDs. A RACH preamble is also included for contentionfree RACH access. Upon reception of this message theUE will remove any EPS bearers for which it did notreceive the corresponding EPS radio bearers in the s1ap eNB Status TransferThe source eNodeB sends the eNodeB Status Transfermessage to the target eNodeB via the MME(s) to conveythe PDCP and HFN status of the E-RABs for whichPDCP status preservation toTarget eNodeB Meanwhile, the UE has received the Handover commandand it is switching to the new target fromSource eNodeB Generates KeNB(KeNB = KeNB*)and AS keys RRC-Idle At this point, the UE has detached from the sourceeNodeB but is still not communicating with the targeteNodeB.

8 The UE is in the RRC-Idle rrc RACH PreambleUE uses the preamble assigned in the handovercommand to send a RACH to the target rrc Random Access ResponseTiming Advance,Target C-RNTI,Uplink grant The target eNodeB accepts the request and respondsback with a timing adjustment and an uplink with target cell rrc rrc RRC Connection Reconfiguration CompleteTarget C-RNTI The UE uses the assigned resources to transmit theHandover Confirm message (RRC ConnectionReconfiguration Complete).endendSwitching toTarget eNodeB AS layer security procedure RRC-Connected The UE is not connected to the Target eNodeB . Thus ittransitions to the RRC-Connected reconfiguration Copying data from source eNodeB to target eNodeB LTE S1 Handover (S1 Handover )LTE MobileeNodeB NetworkCore NetworkUETargeteNodeBSourceeNodeBMMESGWP GWE ventStudio System Designer 617-Feb-14 21:15 (Page 4)rrc rrc RRC Handover ConfirmAfter the UE has successfully synchronized to the targetcell, it sends a Handover Confirm message to the targeteNodeB.

9 Downlink packets forwarded from the sourceeNodeB can be sent to the UE. Also, uplink packets canbe sent from the UE, which are forwarded to the targetServing GW and on to the PDN rlc Downlink DataThe buffered downlink data is sent to the eNodeB Forwardingbuffered downlink data to theUE rlc rlc Uplink DataUplink data is now being received from the gtp Uplink DataThe uplink data is now flowing directly from the TargeteNodeB to the data is now being received at the target eNodeB s1ap s1ap S1AP Handover NotifyTAI+ECGI,Local Home Network ID The target eNodeB sends a Handover Notify message tothe target is successful. Start a timer for a delayedresource gtp GTP Modify Bearer RequestHandover has been successful. So the Downlink path canbe switched from the Source eNodeB to the gtp GTP Modify Bearer ResponseSGW starts forwarding downlink datadirectly to the target eNodeB gtp gtp Downlink dataSGW has switched the path to the Target eNodeB , so thedownlink data is directly delivered to the target rlc Downlink dataDownlink data flows directly to the target eNodeB Tracking Area Update (Click to Tracking Area Update Sequence Diagram) The UE may perform a tracing area update due to release timer has expired.

10 Now resources canbe cleaned up at the source s1ap S1AP UE Context Release CommandInitiate resource release on the Source context for the UEthat has been successfullyhanded over s1ap s1ap S1AP UE Context Release CommandRelease resources on Source eNodeB LTE S1 Handover (S1 Handover )LTE MobileeNodeB NetworkCore NetworkUETargeteNodeBSourceeNodeBMMESGWP GWE ventStudio System Designer 617-Feb-14 21:15 (Page 5)gtp gtp GTP Delete Indirect Data Forward Tunnel RequestIMSI,target eNodeBaddresses,Beare r Context {EPS Bearer Id,Target eNodeBF-TEID } MME requests removal of the indirect tunnel betweensource and target Tunnel The SGW frees the source eNodeB to target eNodeBindirect gtp GTP Delete Indirect Data Forward Tunnel ResponseCause = RequestAccepted Delete indirect tunnel to transportdownlink data from source eNodeB totarget eNodeB This sequence diagram was generated with EventStudio Sytem Designer - Learn more about LTE at.