ervices provided by different types of networks which include fixed, mobile, and Internet. 126 Chapter 1 when service is needed and independent of the attachment procedure of UE, that is, attach- ment without any PDU session is possible. 5GS also supports the UE to establish multiple PDU sessions to the same data network or to different data networks over a single or multi- ple access networks including 3GPP and non-3GPP. The number of UPFs for a PDU session is not specified. The deployment with at least one UPF is essential to serve a given PDU session. For a UE with multiple PDU sessions, there is no need for a convergence point such as SGW in the EPC. In other words, the user-plane paths of different PDU sessions are completely disjoint. This implies that there is a distinct buffering node per PDU session for the UE in the RRC_IDLE state. 5G QoS framework has been designed to allow detection and differentiation of sub-service flows in order to provide improved quality of experience relative to the previous generations of 3GPP radio interface standards. Since LTE bearer framework was insufficient to address certain 5G service requirements, a refined QoS model based on the concept of QoS flow was introduced in 5G. The QoS flow is the finest granularity for QoS enforcement in 3GPP 5G systems. All traffic mapped to the same 5G QoS flow receives the same forwarding treatment. Providing different QoS forwarding treatment requires the use of different 5G QoS flows. Fig. 1.59 illustrates the comparison between 4G and 5G QoS models. It is shown that the 5G concept allows flexible mapping of the 5G QoS flows to radio bearers, for example, the first 5G QoS flow is transported over the first 5G radio bearer while the second and third 5G QoS flows are transported together in the second 5G radio bearer. In order to support 5G QoS flows, either the existing protocols (e.g., PDCP) had to be enhanced or a new (layer-2) sublayer known as service data adaptation protocol (SDAP) had to be introduced. The main services and functio