5G Architecture

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In option 3 there are 3 deployment scenarios for NSA NR they consist of at least two nodes, one for LTE and one for NR.
LTE-NR Dual Connectivity feature 
-The LTE-NR Dual Connectivity feature introduces the support for EN-DC in the NR Node used in an NSA deployment.
-The feature covers the fundamental interaction between LTE and NR in the EN-DC context.
-The feature makes it possible to configure split DRBs with one leg in LTE and one leg in NR.
 
The benefits of this setup are the following:
  Higher peak rate of network data traffic.
  Sustainable capacity and performance growth.

-CONTROL PLANE
  Master Cell Group (MCG) SRB (SRB1, SRB2): SRB (Signal Radio Bearer)
  Direct SRB between the master node and the mobile device that can be used for conveying master node RRC messages which can also embed secondary node RRC configurations.
  Split SRB (SRB1+SRB1S, SRB2+SRB2S): 
  SRB that is split between the master node and the secondary node (at a higher layer, PDCP) towards the mobile device, allowing a master node RRC message to be sent via the lower layers (RLC, MAC and PHY) of either the   master node or secondary node; or to be sent via the lower layers of both the master and secondary nodes. Here, the master node RRC message can also embed secondary node RRC configurations.
  Secondary Cell Group (SCG) SRB (SRB3): 
  Direct SRB between the secondary node and the mobile device by which secondary node RRC messages are sent.
-USER PLANE
  MCG DRBs: DRB (Data Radio Bearer)
  Bearers terminated at the master node and using only the master node lower layers.
  MCG split DRBs:
  Bearers terminated at the master node but that can use the lower layers of either the master node or secondary node; or can use the lower  layers of both the master and secondary nodes.
  SCG DRBs:
  Bearers terminated at the SN and using only the secondary node lower layers.
An additional data radio bearer has been introduced in EN-DC:
  SCG split DRBs:
  Bearers terminated at the secondary node but that can use the lower layers of either the master node or secondary node, or can use the lower   layers of both the master and secondary nodes.
Adopting current LTE DC procedures for the introduction of the SCG split DRB entails that every bearer type change will require a re-establishment in the PDCP layer, as well as in some cases signaling towards the core network to switch the path from the core network to the radio access network. To minimize the impact of such bearer type changes, and minimize implementation and testing efforts on mobile devices, 3GPP has agreed to harmonize the bearer definitions. With the harmonized bearer concept, there will be only two kinds of bearers from the mobile device perspective:
  Direct DRBs: 
  DRBs with only one lower layer configuration, either corresponding to LTE or NR lower layers. If the LTE lower layers are configured, either the   NR or the LTE version of the higher layer can be used by the bearer. If the NR lower layers are configured, the NR version of the higher layer will   be used.
  Split DRBs: 
  DRBs with two lower layer configurations, corresponding to LTE and NR lower layers. It will always use the NR version of the higher layer. In case   of split DRBs, packet duplication, when it is finalized as a new functionality in Release 15, could be used for additional reliability.
Limitations.
It is not recommended for EN-DC UEs to use VoLTE due to the lack of support for important VoLTE related features.
The VoLTE performance is expected to be poor due to the following:
  TTI (Transmission Time Interval) Bundling cannot be used.
  Handover support is limited.
  RRC re-establishment for EN-DC UEs is not supported.

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