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LTE : Data forwarding over X2ap during handover ?

+1 vote

During X2-handover, In PDCP how to forward prepared packets which are not yet got ACK from UE and packets getting from SGW ?

Because why I getting this doubt is, Packets which are not got ACK all are prepared one means assigned PDCP SN but packets getting from SGW are SDU's means not assigned PDCP SN for those (we sent SN status info to Target eNB when handover triggered).how to differentiate these packets at target eNB which one is PDU and which one is SDU.

posted Sep 23, 2014 by Bheemappa G

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1 Answer

+1 vote
Best answer

PDCP buffers SDUs also. Until discard timer expires or it gets acknowledged.

In lossless handover pending downlink packets (i.e. PDCP SDUs for which no Acks are received from UE, PDCP SDU which were not transmitted and new GTP packets that are coming from S1 interface in source eNB )are sent to the target eNB.

Control plane in source eNB sends the 'Next Transmit Seq num' to target eNB.This information is passed to the the target eNB via SN-STATUS-TRANSFER..
The Status Transfer message provides the sequence number (SN) and the Hyper Frame Number (HFN) that the target eNodeB should assign to the first packet with no SN yet assigned that it must deliver. This first packet can
either be one received over the target S1 path or one received over X2, if data forwarding over X2
is used.

I hope it will help you...
Let me know if you have any doubt.

answer Sep 24, 2014 by Aarti Jain
thank you for reply.
Still i have one more doubt ,in your explanation you have mentioned,  PDCP SDUs for which no Acks are received from UE and new GTP packets that are coming from S1-intf in source enb all packets will be forwarded to target enb.        But here PDCP SDUs for which no Acks are received from UE are PDCP SN assigned  SDU's and new GTP packets that are coming from S1 interface in source eNB are not PDCP SN assigned SDU's so how do differentiates at target which one is PDCP SN assigned SDU and which one is not assigned SDU's?
1)should i remove SN assigned to SDU's and forward to target eNB?
   if yes then what about if that SDU is ciphered?
once again thanks for your king input.
Source eNB PDCP will assign SN to all the packets which it has received from S1 interface and at the time of sending it to target eNB will send end marker. So because of end marker target eNB knows this much packet has received from eNB have SN value. And whatever target eNB will receive from S1 interface, target eNB PDCP assign SN to those packets based on SN status(received from source eNB).
thanks aarti jain,
what if that packets are ciphered ? Because once handover happened security keys will be changed at both (UE and target eNB side) so UE con't deciphered it (PDCP SDUs for which no Acks are received from UE) when we forward  from target eNB.
And one more doubt i have ,should need to send SN status transfer after i got END MARKER only to target eNB ?
Sender PDCP(source eNB) always have one re transmission buffer in which it buffers IP packets which are not ciphered nor SN assigned(but we can assign SN while queuing the packets, like its all upto our implementation).
1.)So if they are not SN assigned and not ciphered then based on SN status report target eNB will take care.
2.)If they are SN assigned and not ciphered then also target eNB can handle.(as I replied in my previous comment)
And like if in some implementation we are buffering after ciphering. Then packets should be deciphered before sending to target eNB. Because yeah ciphering algo. will be reset and UE cant decipher those packets later.  

And SN status report is very important for target eNB. For both uplink and downlink. Control plane in source eNB sends the 'Next Transmit Seq num' and 'Next expected receive sequence number' to the target eNB for each RAB via SN status transfer.

Let me know if you have any doubt and in case if I am wrong .
thanks  Aarti Jain,you have clear many of my doubts.
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