MCG ,SCG, SPLIT Bearer and Use Case's


Talk about MCG /SCG and Split Bearers

For EN-DC three bearer types exist those are

1.      MCG Bearer
2.      SCG Bearer
3.      Split Bearer

In ENDC , Network can configure either E-utran PDCP or NR PDCP for MCG bearer whereas NR PDCP is must for SCG bearer and Split bearer


Data Flow :


MCG Bearer Using NR PDCP
User plane data flow from the NR PDCP to LTE RlC Via MenB
MCG Bearer using E-utran PDCP
User plane data flow from the E-utran PDCP to LTE RlC Via MenB
SCG bearer
SCG bearer always Uses NR PDCP , and the data flow is from NR -PDCP to NR-RLC and NR-MAC via Senb
Split Bearer
Split bearer also Uses NR PDCP and the data flow is from NR-PDCP to LTE-RLC & NR-RLC
For ex:  DRB -5 -LCID-4 (LTE-RLC ) and LCID-5(NR-RLC)

NR PDCP can be created using R15 IE's i.e NR radio bearer config information
-NRRadioBearerConfig will allow to configure DRB's through DRB_toAddModList and each DRB will have Identity number and other Security configuration 

There is one important IE i.e MorethanOneRLC  -> IF DRB data has been splited in DL which mean data splited in to two paths via two different logical channels one would be NR-RLC logical channel and another one would be LTE-RLC logical channel .
Let's suppose  incase If UE would like to Use defined path for UL data , UE can choose either NR logical channel or LTE logical channel by defining i.e primary path (with information cell group , LCID)

Let's consider some of Use cases with short description

1.      UE in RRC_CONNECTED state with EN-DC, and, MCG (E-UTRA PDCP)
·         In short -  while attach procedure in reconfiguration E-Utran PDCP will configure to that specific DRB and user data will terminate at MN (LTE-PDCP-> LTE-RLC)
2.      UE in RRC_CONNECTED state with EN-DC, and, MCG (NR PDCP)
·         In short -  while attach procedure in reconfiguration NR PDCP will configure to that specific DRB and user data will terminate at SN .(NR-PDCP- LTE-RLC)
3.      UE in RRC_CONNECTED state with EN-DC, and, SCG (NR PDCP)
·         In short -  while attach procedure in reconfiguration Message,  configure NR_secondary cellgroup config as well Nr Radio bearer configuration which will configure NR PDCP , user data will terminate at SN (NR-PDCP- NR-RLC)
4.      UE in RRC_CONNECTED state with EN-DC, and, SPLIT (NR PDCP)
·         In short -  while attach procedure in reconfiguration Message,  configure NR_secondary cellgroup config as well Nr Radio bearer configuration which will configure NR PDCP , user data will terminate at SN (NR-PDCP- NR-RLC & LTE-RLC)

Like that there could be many use cases with different bearer combinations .






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