transfer the user-plane PDUs. As a result, Xn-U interface can User traffic using different services or applications has different quality of service classes. A service data flow is an IP flow or an aggregation of IP flows of user traffic classified by the type of the service in use. Different service data flows have different quality of service class attributes, thereby a service data flow serves as a unit by which quality of service rules are applied in accordance with network policy and charging rules. GPRS tunneling protocol is a protocol that is used over various interfaces within the packet core in 3GPP networks to allow the user terminals to maintain a connection to a packet data network while on the move. The protocol uses tunnels to allow two GPRS support nodes to communicate over a GPRS tunneling protocol-based interface and separates traffic into different communication flows. GPRS tunneling protocol creates, modifies, and deletes tunnels for transporting IP payloads between the user equipment, the GPRS support node in the core network and the Internet. GPRS tunneling protocol comprises three types of traffic, namely control-plane (GTP-C), user-plane (GTP-U), and charging. GTP-C protocol supports exchange of control information for creation, modification, and termination of GPRS tunneling protocol tunnels. It creates data forwarding tunnels in case of handover. GTP-U protocol is used to forward user IP packets over certain network interfaces. When a GTP tunnel is established for data forwarding during LTE handover, an End Marker packet is transferred as the last packet over the GPRS tunneling protocol tunnel. 118 Chapter 1 User-plane PDUs Xn-AP GTP-U Data link layer Data link layer Physical layer Physical layer User-plane Control-plane Figure 1.54 Xn-U and Xn-C protocol stacks [16]. only provide non-guaranteed delivery of user-plane PDUs, but it supports data forwarding and flow control. The Xn interface specifications facilitate interconnection of NG-RAN nodes manufactured by different vendors as well 