cipher packets as they reach the UE (see Section 7.4). A gap in the sequence of acknowledged packets in RLC no longer stalls deciphering operations in PDCP. Second, the granularity of spectrum usage in NR goes below that of the carrier frequency of legacy systems: with the addition of Bandwidth Parts (BWPs), it becomes possible to constrain the UE to operate within a limited portion (i.e. a part) of the frequency bandwidth of a cell (hence, the name "bandwidth part" - as covered in Section 7.6.10). Furthermore, the BWP used by the UE can be ordered to change within the same cell, and all BWPs need not have the same numerology. Third, when connecting to 5GC, the QoS is not tied to the radio access bearer model of legacy systems. With 5GC, each packet is marked by the Core Network with a Quality of Service Flow ID (QFI), and the gNB can use that information to select an appropri- ate radio bearer. The one-to-one mapping between radio access bearer from the core network and the radio bearer from the radio access network (RAN) disappears, and the gNB is entrusted with configuring and selecting appropriate radio bearers to satisfy the QoS requirements (see Section 7.3). 7.3.1 Overview The SDAP layer handles the mapping of QoS flows from a PDU session onto Data Radio Bearers (DRBs). Unlike in previous generations, there is no one-to-one mapping 7 5G Radio Protocols between Radio Access Bearers (RABs) from the Core Network and Radio Bearers in the RAN. In 5G, the QoS model is not based on RABs but on QoS flows; it supports QoS flows that require a guaranteed flow bit rate (GBR QoS Flows) and QoS flows that do not require a guaranteed flow bit rate (non-GBR QoS Flows). The model gives a greater freedom to the RAN: 5GC only marks packets with a QFI, and the RAN is entrusted to use that information for configuring and selecting appropriate DRBs for the corresponding QoS flows. Not all QoS flows are expected to require a separate DRB, and several QoS flows can be multiplexed by the gNB onto the same DRB by the RAN. The QFI 