Today we are going to discuss about the 3 types of User Plane Bearers supported by EN-DC Base Station.
In case you are not aware of EN-DC Base Station. EN-DC refers to New Radio Dual Connectivity (EN-DC), where E-UTRA refers to 4G air-interface and New Radio refers to the 5G air-interface. EN-DC also means that E-UTRA is the Master Node followed by Secondary Node.
Now coming back to 3 types of user plane bearers.
- Master Cell Group (MCG) Bearer
- Secondary Cell Group (SCG) Bearer
- Split Bearer
Master Cell Group (MCG)
- An MCG Bearer users RLC, MAC and Physical layers within the Master Node. In case of EN-DC , 4G (E-UTRA) is the master node. MCG bearer uses 4G implementation of RLC, MAC and Physical layer and 4G air interface.
- PDCP layer can configured in both 4G and 5G mode. PDCP layer is considered to be located within Master Node if the S1 connection is between SGW and eNode.
PDCP Layer 4G Mode Services:
- 4G version of PDCP layer supports minimum sequence number (SN) length of 7 bits
- Speech services benefit from short sequence number as it helps minimize the overhead generated by PDCP layer. Therefore user plane bearers transferring speech content may be allocated an MCG bearer using 4G PDCP layer.
- Data services use larger packets so percentage overhead generated by PDCP layer is small. Larger significant number sizes can be used without having a significant impact
PDCP Layer 5G Mode Services:
- 5G version of the PDCP layer supports a mminimum Sequence Number (SN) length of 12 bits. The sequence number is included within PDCP header information.
SECONDARY CELL GROUP (SCG)
- As the name indicates, Secondary Cell group bearer uses the RLC, MAC and Physical layers within Secondary Node. In other words it uses the 5G implementation of these layers and 5G air-interface.
- SCG Bearers always use 5G version of the PDCP layer no matter it is Master or Secondary Node.
- Split Bearer uses the RLC, MAC and Physical layers within both the Master and Secondary Nodes, i.e., both 4G and 5G air-interfaces can be used.
- Split Bearer always use 5G version of the PDCP layer irrespective of the node providing the S1 connection towards the S-GW.
- UE cannot distinguish Split bearers as it always uses 5G version from the perspective of PDCP layer. Location of S1 interface is transparent to the UE and both Split bearers appear to be the same from UE perspective. This is known as Unified solution as shown in Figure 1 above.
- 3GPP TS 36.323
- 3GPP TS 38.323
- 5G NR in Bullets by Chris Johnson