ns of SDAP include mapping between a Access node UPF (PDN gateway) Radio interface Encapsulated IP IP flow 1 IP flow 2 LTE QoS Radio bearer S1 bearer + S5/S8 bearer IP flow N model EPS bearer IP flow IP flow 2 NR QoS Radio bearer User-plane tunnel IP flow N - model QoS flow(s) PDU session End-to-end service Figure 1.59 Comparison of QoS models in 4G LTE and 5G NR. 5G Network Architecture 127 QoS flow and a data radio bearer, marking QoS flow identifier (QFI) in the downlink and uplink packets. For each DRB, the UE monitors the QoS flow ID(s) of the downlink packets and applies the same mapping in the uplink, that is, for a DRB, the UE maps the uplink packets belonging to the QoS flows(s) corresponding to the QoS flow ID(s) and PDU ses- sion in the downlink packets for that DRB. In order to establish a PDU session, a PDU session establishment message is sent by 5GC to the gNB serving the UE, which includes the NAS message to be transferred to the UE containing the QoS-related information. The gNB sends a DRB setup request message to the UE and includes the DRB parameters and the NAS message that it received earlier. The UE establishes at least a default DRB associated with the new PDU session. It further cre- ates the QFI to DRB mapping and sends an RRC DRB setup complete message to the gNB. The gNB sends PDU session establishment acknowledgment message to 5GC, indicating successful establishment of the PDU session. Data is sent over the N3 tunnel to the gNB and then over the DRB to the UE. The data packets may optionally include a QoS marking (same as or corresponding to QFI) in their SDAP header. The UE sends uplink packets over the DRB to the gNB. The uplink data packets include a QoS marking (same as or corre- sponding to QFI) in the SDAP header [16]. Dual-connectivity was introduced as part of LTE Rel-12. In LTE DC, the radio protocol architecture that a particular bearer uses depends on how the bearer is setup. Three bearer types have been defined: master cell group (MCG), secondary cell group (SCG), and spl