G RAN, whereas in option 4a this is directly sent from the 5GC to the LTE RAN. Similarly, Options 7, 7a, and 7x (Fig. 2-15) are also NSA with a 5G NGC, but the differ- ence between Option 4 and Options 7 is that the primary connection (and C-Plane) connec- tion between core and RAN is to the LTE eNB. Options 5 and 6 (Fig. 2-16) are reverse options 1 and 2 where an EPC is connected to a 5G RAN (Option 5) and a 5GC is connected to a 4G RAN (Option 6): Keep in mind that the core network choice defines if the C-Plane signaling is carried over the 4G or 5G RAN. If the core network is EPC, C-Plane is carried over the LTE RAN and if the core network is 5G, C-Plane is carried over the 5G/NR RAN. The concept of dual connectivity (Fig. 2-17) adds a level of complexity to the definition of the PDCP implementation to take care of buffering and packet distribution from MeNB to Chapter 2 System components, requirements, and interfaces 59 Secondary eNB (SeNB). The concept is that an MeNB "feeds" enough packets to the SeNB to keep the SeNB - > UE link occupied while not overloading the SeNB with packets because (assuming the SeNB is a smaller cell size), in the case of mobility, the SeNB would no longer be able to maintain a connection toward the UE. Flow control between the MeNB and SeNB is a topic of differentiation between vendors. When implemented in different physical units (i.e., CU and DU), this communication link is defined in the 3GPP 38.425 standard. Initial definition of DC in Release 12 for LTE assumed a single MeNB and a single SeNB and was designed for downlink traffic only. As standardization evolved, features were included to support uplink (Release 13), and in Release 14, the concept of multiconnectivity where the UE/client can connect to multiple nodes as the same time. This concept is carried forward in 5G. ENDC stands for E-UTRAN New Radio Dual Connectivity where the client device is connected to LTE and 5G networks at the same time, reducing handovers between the two networks and increasing (cumulative) avai