cell tower shared across multiple nonvirtual operators); Internet offload models; and 52 Open Radio Access Network (O-RAN) Systems Architecture and Design core network deployment models can force traffic differentiation (again, user plane and control plane separation being an example). As a small but important last point, note that IP fragmentation is required to be sup- ported on all the F1, NG, and other links. Quality of Service and related concepts The purpose of QoS is to guarantee sufficiently good customer experience on a network that has limited capacity. QoS is measured though metrics such as packet loss, throughput, latency, jitter availability, etc. Some applications (say, voice traffic) are intuitively understood to be sensitive to packet loss and latency, while others (large downloads) can sustain latency but need to have high (average) throughput, and SO on. QoS in 3GPP (both 4G and 5G but we will focus on 5G first and cover changes from 4G after) is interesting because it carries aspects from wireline QoS and wireless QoS (the latter being the scheduler algorithm that we discuss extensively later). The wireline piece of QoS is defined by 3GPP to be based on DiffServ concepts. A short introduction to DiffServ is added to the end of this chapter to explain its basic concepts. QoS in 5G centers around the concept of QoS flows. What is a QoS flow? This is defined by 3GPP as the finest granularity of QoS, defined within a PDU session, and identified by a QoS flow Id (QFI) (Fig. 2-8). In the downlink direction (from UPF to UE), classification of traffic [mapping from DiffServ code point (DSCP) and/or application detection] is done by NG-RAN PDU Session Radio Bearer NG-U Tunnel QoS Flow QoS Flow Radio Bearer QoS Flow Radio FIGURE 2-8 QoS flows in 5GC and NG-RAN (reproduced from 3GPP 38.300). NG-RAN, Next-Generation Radio Access Network; QoS, Quality of Service. Chapter 2 System components, requirements, and interfaces 53 the UPF, and it is the UPF that assigns the QFI. In the uplink direction, this job is