MACLTE MeNB (LTE) SgNB (NR) MeNB (NR) SgNB (LTE) Figure 1.62 Illustration of split bearer via SCG [13]. the S1-U/NG-U interface terminates in the master node, whereas for SCG bearer, the S1- U/NG-U interface terminates at the secondary node [13]. 3GPP NR supports DC in which a UE in RRC_CONNECTED is configured to utilize radio resources provided by two distinct schedulers located in two gNBs connected via a non- ideal backhaul. The gNBs involved in DC operation for a certain UE may assume two dif- ferent roles, that is, a gNB may either act as an MgNB or as SgNB. In DC operation, a UE connected to one MgNB and one SgNB. There are four bearer types in NR DC, namely MCG bearer, MCG split bearer, SCG bearer, and SCG split bearer. The dual-connectivity between LTE and NR supports similar bearer types. Split bearer via MCG, SCG bearer (a bearer whose radio protocols are split at the SgNB and belongs to both SCG and MCG), and MCG split bearer (a bearer whose radio protocols are split at the MgNB and belongs to both MCG and SCG). The MCG bearer and one SCG bearer are used for two different QoS flows [16]. 5G Network Architecture 129 In the downlink, the incoming data packets are classified by the UPF based on SDF6 tem- plates according to their precedence (without initiating additional N4 signaling). The UPF conveys the classification of user-plane traffic associated with a QoS flow through an N3 (and N9) user-plane marking using a QFI. The AN binds QoS flows to AN resources (i.e., data radio bearers). There is no one-to-one relationship between QoS flows and AN resources and it is the responsibility of the AN to establish the necessary resources for map- ping to the QoS flows [13]. In 3GPP NR, the DRB defines the packet treatment on the radio interface. A DRB serves packets with the same packet forwarding treatment. Separate DRBs may be established for QoS flows requiring different packet handling. In the downlink, the RAN maps QoS flows to DRBs based on QoS flow ID and the associated QoS profiles. In the uplink, the U