(III) ) ) ((H)) ng-eNB ng-eNB Figure 1.53 Overall access network architecture [16]. 5G Network Architecture 117 idle-mode UE reachability which includes paging control, registration area management; support of intra-system and inter-system mobility; access authentication and authorization; mobility management control; support of network slicing; and SMF selection. The role of SMF in the new architecture is similar to that of EPC PGW entity hosting user- plane functions such as session management; allocating and managing UE IP address; selecting and controlling of UPFs; configuring traffic steering at UPF in order to route traf- fic toward proper destination; controlling part of policy enforcement; and the QoS. The new architecture further features a UPF entity which is similar to the EPC SGW user- plane entity. This entity hosts the UPFs such as anchor point for intra- or inter-RAT mobil- ity, external PDU session point of interconnect to data network, packet routing and forward- ing, packet inspection and user plane part of policy rule enforcement, traffic usage reporting, uplink classifier to support routing traffic flows to a data network, QoS handling for user plane which includes packet filtering, gating, UL/DL rate enforcement, uplink traf- fic verification, that is, service data flow (SDF)55 to QoS flow mapping, and downlink packet buffering and downlink data notification triggering [16]. 1.2.1.1.1 Xn Control-Plane/User-Plane Functions and Procedures Xn is a reference point connecting the NG-RAN access nodes, supporting the transfer of signaling information and forwarding of user traffic between those nodes. Xn is a logical point-to-point interface between two NG-RAN access nodes which establishes a logical con- nection between the two nodes even when a direct physical connection does not exist between the two nodes (which is often the case). It comprises both user-plane and control-- plane protocols. The user-plane protocol stack of Xn interface is shown in Fig. 1.54. The TNL uses GTP-U 56 over UDP/IP to 