X2 Handover

X2-based handover:
After handover preparation is complete between the source and target eNodeBs, the target eNodeB instructs the MME to update the uplink bearer. The two eNodeBs directly exchange bearer information. During the handover, the MME remains unchanged; the original S-GW is used, or another S-GW is selected.

X2 HO Steps:

Before HO:
1/ UE Selects best Cell based on some conditions and measures some parameters continuously
2/ Start Control Plane Procedures with CN and RRC Connection with eNB
3/ Start User Plane ( DL/UL via SGW and Serving eNB )

Handover Preparation: ( neighboring Cell signal quality is better than serving cell now )
4/ UE send Measurement report to source eNB and inform it.
5/ source eNB checks some parameters with target eNB ( send HO request and receive HO Ack )
6/ downlink Data is coming from SGW to source eNB and it is forwarded to target eNB and then buffered.
7/ Uplink Data is still sent to source eNB.

Handover execution:
8/ source eNB sends HO command to UE and ask to connect to target eNB in RRC connection reconfiguration request message.
9/ UE uses the preamble assigned in the handover command to send a RACH to the target eNodeB.
10/ The target eNodeB accepts the request and responds back with a timing adjustment and an uplink resource grant.
11/ The UE uses the assigned resources to transmit the Handover Confirm message (RRC Connection Reconfiguration Complete).
12/ The UE is now connected to the target eNodeB. All the queued messages are now transmitted towards the UE.
( new Cell is selected and resources with old eNB is released In this step. )
13/ in this phase, all downlink is flowed from SGW to source eNB and forwarded to target eNB
14/ The uplink data is now being transmitted from the UE to the SGW via the target eNodeB.

Handover Completion:
15/ The target eNodeB requests the MME to switch the path from the source eNodeB to the target eNodeB in Path Switch request message.
16/ The MME sends a Modify Bearer Request message to the SGW for each PDN connection. The message contains the eNodeB Address and the TEIDs.
17/ The SGW uses the received address and TEID to send DL packets to the target eNodeB and then replies with a Modify Bearer Response message to the MME.
18/ The SGW sends “end marker” message on the old path immediately after switching the path.
( This marker will be used to sequence the data received from the source eNodeB and the new data received from the target eNodeB )
19/ The MME sends a Path Switch Request Acknowledge message to the target eNodeB. The message contains Serving GW address and uplink TEID(s) for the user plane
20/ The target eNodeB sends a Release Resource message to the source eNodeB to inform it of the handover success, triggering the release of resources

Above explanation is for X2 HO under Same SGW.
If target eNB is connected to same MME under different SGW, Create Session toward New SGW / modify bearer toward PGW and delete Session toward old SGW will be added to current steps.

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