ce to en-gNB. The master node eNB exchanges data in both directions, uplink and downlink, with the mobile. The secondary node en-gNB allows us to increase both uplink and downlink data rates. With a DC mechanism, data is transmitted to the mobile according to one of the following variations (Figure 1.3): - option 3: in plain option 3, all uplink and downlink data flows to and from the LTE part (MCG split bearer) of the LTE/NR base station, i.e. to and from the eNB. The eNB then decides which part of the data it wants to forward to the 5G gNB part of the base station over the Xx interface; - option 3a: both LTE eNG (MCG bearer) and 5G-NR en-gNB (SCG bearer) exchange traffic to the 4G core network directly. This means that a data bearer allocated to a node cannot share its load over the second node. This option does not suit the case of mobile use; - option 3x: user data traffic will directly flow to the 5G gNB part of the base station (SCG split bearer). The traffic is delivered over the 5G-NR interface to the device, and part of the data can be forwarded over the X2 interface to the 4G eNB. NG-RAN Network - Functional Architecture Option 3 Option 3a Option 3x S1-MME MCG Split Bearer S1-MME SCG Bearer S1-MME SCG Split Bearer Figure 1.3. Secondary node addition - option 3 Option 3 uses the MN terminated MCG bearer for user traffic. The eNB entity splits the S1 bearer into: - LTE radio support; - NR support. Option 3x uses the SN terminated SCG bearer for user traffic. The gNB entity splits the S1 bearer into: - LTE radio support; - NR support. 1.1.2. Option 4 Option 4 relies on the 5G core (5GC). The gNB acts as an MN; it supports signaling exchange (MCG signaling bearer) with the 5G core network's transport plane through the NG-C interface. The LTE user plane connections go via the 5G-NR through the NG-U interface. The ng-eNB base station acts as a secondary node. It is controlled by the gNB base station through the Xn-C interface. The ng-eNB is a new generation of the 4G base station. The gNB controls the ng-eNB 