What is ENDC in 5G Technology
ENDC stands for Evolved-Universal Terrestrial Radio Access New Radio Dual Connectivity. You can also say EPS core New Radio Dual connectivity. In EN-DC, the UE has a single RRC state, based on the MN RRC and a single C-plane connection towards the EPC Core Network. ENDC is also known as E-UTRA NR Dual Connectivity.
C-Plane
If you see the above picture clearly, there is no direct control plane connectivity between gNB and EPC core. LTE eNB acts as the master node and 5G gNB acts as the secondary node. There is a direction interface x2 between eNB and gNB. 5G gNB has a user plane interface with the SGW.
Each radio node has its own RRC entity. eNB has the E-UTRA version of RRC entity. gNB has the NR version of RRC entity. Both the RRC entity can generate RRC PDUs to be sent to the UE. RRC PDUs generated by the SN (Secondary Node) can be transported via the MN (Master Node) to the UE. The MN always sends the initial SN RRC configuration via MCG SRB (SRB1), but subsequent re configurations may be transported via MN or SN can directly send to the UE over SRB3. When transporting RRC PDU from the SN, the MN does not modify the UE configuration provided by the SN.
U-Plane
In EN-DC, from a UE perspective, three bearer types exist: MCG bearer, SCG bearer and split bearer. These three bearer types are depicted in the Figure for EN-DC. For EN-DC, the network can configure either E-UTRA PDCP or NR PDCP for MCG bearers while NR PDCP is always used for SCG and split bearers.
Basically ENDC is 5g non standalone approach. This is going to be the initial launch of 5G. So, in this dual connectivity LTE would become MCG (Master Cell Group) and 5g NR would become SGC (Secondary Cell Group). So MCG act as a anchor and UE will have to perform initial registration with master cell group. After successful initial registration, this MCG would add one or more 5g secondary Cells.
ENDC is a concept of dual connectivity. A UE can connect to LTE and then add 5g cells. So UE will be connected to LTE eNB as well as 5g gNB. In fact there is no 5g core in ENDC, so 5g gNB will interact with LTE eNB for any core network functionality. The idea of ENDC is to utilize the power of 5G NR while core network is till the EPC. This the good step towards the 5g technology. As we all know 5G core network is still not deployed anywhere in the world. Dual mode looks the only solution for initial 5G deployment.
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