LTE: How PDCP layer at eNodeB make sure seamless and lossless handover ?

Question

I came through these terms like seamless and lossless handover, what is the significance's in these type of handovers and when these thing happens?

Answer 1

Once the handover execution is complete.
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. 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 eNodeB Status Transfer message.
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 or indirect forwarding.

This way the PDCP status is transferred and preserved between the SenB and TenB.
Hope this answers.

Answer 2

Some more things need to verify:

During Handout case:
Check the src ENB last received ACK or NACK, that should be the least SN of the Forwarded packet(among all forward packets).

During Hand-in case:

Verify the PDCP status report from UE for which SN onwards expecting in UL from the target.

In all forwarded packets, the large SN from all SNs of the forwarded packets +1 should be present in the SN status transfer message and the same should use target ENB(verify in the UE logs).