e PDU sessions, which are associations between the UE and a data network. When a UE registers with the network, it requests establishment of one or more PDU sessions. For every PDU session of a UE, there is one tunnel on the NG-U interface. However, each NG-U tunnel may have multiple QoS flows configured. This is one of the key differences compared with the S1-U interfaces used in LTE, where the EPC (and therefore S1) supported the E-UTRAN Radio Access Bearers (E-RAB bearers) and for every bearer there was only one tunnel on the S1-U interface. There was no further QoS differentiation within an E-RAB bearer. In the downlink, a UPF maps a PDU to an NG-U tunnel and marks a PDU in accordance with its QoS flow. An NG-RAN node maps PDUs from QoS flows to radio access-specific 3 5G System Overview User Plane Figure 3.3.6 NG user-plane protocol stack. (Source: Reproduced by permission of © 3GPP). Radio Network Layer GTP-U Transport Network Layer Data link layer Physical layer resources based on the QoS flow information, together with the information on NG-U tunnel. A similar process happens in the uplink. An NG-RAN node selects an NG-U tunnel for an uplink PDU, and sends it to the UPF, together with the QoS flow information. The QoS-related information is piggybacked on every GTP-U PDU in a GTP-U container, dedicated for the NG-U, and defined in 3GPP TS 29.281, whereas the content of the con- tainer is defined in 3GPP TS 38.415. The formats used in uplink (UL PDU SESSION INFOR- MATION) and downlink (UL PDU SESSION INFORMATION) are different. Both carry the QoS Flow Identifier (QFI), which indicates the QoS flow to which the transferred packet belongs. Additionally, the downlink packet may carry a Paging Policy Indicator (PPI) used for paging policy differentiation and a Reflective QoS Indicator (RQI), used for activation of the reflective QoS toward the UE. 3.3.4 Xn Interface The Xn interface connects a gNB to another gNB or ng-eNB. It is primarily used for: 1. UE mobility control, i.e. handovers and UE mobility in inac