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Dual connectivity: split bearer related

+1 vote

I guess the EN-DC Dual connectivity concept comes LTE Advanced. But since I am not aware of radio side of LTE, I have a basic question regarding the uplink path.

Lets consider the MN terminated split bearer, I understand that the downlink PDCP data gets split into MeNB and SeNB. But whatabout the uplink from UE, will it come in both ways, from MeNB and SeNB and gets combined at PDCP in MeNB ? Not gone in detail in spec. Could some LTE or 5G RAN people help me with this information..

posted Nov 15, 2018 by Pdk

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From 3gpp spec 36423 v 15.3.0, In the message, defined in section "SENB ADDITION REQUEST", under the
IE "E-RABs To Be Added List", and again under "Split Bearer", we can see the IE, "MeNB GTP Tunnel Endpoint".
In the description of this IE it says, "MeNB endpoint of the X2 transport bearer. For delivery of UL PDUs."
So from this message it looks like only "MeNB terminated split bearer" can be configured.
Is this correct ?
Also, this endpoint is for the SeNB to send the UL PDUs of the "MeNB split bearer" back to
MeNB ?
@Harshita or Jagan, could you give your thoughts on this please ?

1 Answer

+1 vote


yes , UL is possible in Dual connectivity . But it is least bothered as most data flows in DL. Those tunneling pint is used to transfer status of the DL data as well as UL data from UE.

answer Nov 19, 2018 by Jaganathan
Thanks a lot Jagan for the reply.

I am trying to  map the GTP tunnel ids in the X2AP specs (36423 v 15.3.0) to the E1AP specs. If you see in the message "SENB ADDITION REQUEST", the E-RABs To Be Added List
>E-RABs To Be Added Item
>>CHOICE Bearer Option
>>>SCG Bearer
>>>>E-RAB ID
>>>>E-RAB Level QoS Parameters
>>>>DL Forwarding
>>>>S1 UL GTP Tunnel Endpoint

The "S1 UL GTP Tunnel Endpoint" is for the SCG bearers and is transparently passed in the "    BEARER CONTEXT SETUP REQUEST" as part of the E1 message.
>>DRB To Setup List
>>>DRB To Setup Item
>>>>DRB ID
>>>>PDCP Configuration
>>>>S1 UL UP Transport Layer Information

But whatabout the IE,
>>>Split Bearer
>>>>E-RAB ID
>>>>E-RAB Level QoS Parameters
>>>>MeNB GTP Tunnel Endpoint
which is there in the X2AP message (SENB ADDITION REQUEST", the E-RABs To Be Added List) , But how is that mapped to E1AP ?
Just to let you know, the message I was looking is in Section "" which is used for LTE dual connectivity.

But for EUTRAN-NR dual connectivity, the section needs to be looked.

Another Q is :
SENB ADDITION REQUEST has a field ">>>SCG Bearer" and ">>>Split Bearer" under the "E-RABs To Be Added Item".

But there is no such option under "SGNB ADDITION REQUEST" message, so how to know which E-RAB is configured as SCG and which one as split ?

Hope to find the answer this new year :)
Happy new year to everybody in the great forum

 i.e "SGNB ADDITION REQUEST". Sorry for the confusion. But my doubt is still not resolved.
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+2 votes

While digging into 5G NR dual connectivity, ( and as you are aware that it is based on LTE dual connectivity). I want to learn a bit about how LTE put the foundation for this feature to come towards 5g.

I want to read the LTE architecture specs related to dual connectivity.
I donot have much RAN background in LTE. Could somebody from this background give me some quick hints, to look into which 3gpp TS or TR to understand some basics.

Btw there is 3GPP TS 36.401 doesnot have much info it looks like.

0 votes

In Release14/15 for to support Dual Connectivity Xn user plane interface was introduced.
which is similar to S5(PGW <=> SGW)/S1(eNB <=> SGW) interface means GTP-u over UDP.

In 36.425 Xn has status report which lost packet range with start and last packet. What is the significance of having those in status report?

Does MeNB Xn has to re-transmit lost packets over Xu?
Or Also in Xn status report we have "Highest successful delivered PDCP seqence number", how SeNB Xn will get the PDCP sequence number while sending the status report?

If SeNB Xn has to send feedback report at interval it has to send?

Could anyone please clarify the above queries.

0 votes

Bearer split works if same SGW serving MeNB and SeNB. But if two different SGW is serving then is it possible to have bearer split?