 session and QoS flow. For a color version of this figure, see www.iste.co.uk/launay/5g.zip 206 NG-RAN and 5G-NR At the NAS (Non Access Stratum) level, the QoS flow in a PDU session is identified by a QFI (QoS Flow Identifier). The QoS profile is managed at the NAS level (ARP + QCI (QoS Class Identifier)): - the maximum packet loss characteristics are configured only for multimedia streams at guaranteed GBR rate (voice, videoconferencing); - the Reflective QoS Attribute (RQA) only concerns non-GBR unsecured rate flows. The RQA attribute is optional. If the SMF transmits a request for a QoS parameter control notification relating to certain GBR flows to the base station, then the base station informs the SMF function when the actual throughput is lower than the minimum expected value or when the minimum throughput is reached again. At the AS (Access Stratum) level, the matching rules are based on the QFI parameter. QFI rules allow us to associate the stream with the corresponding DRB radio medium at both the uplink direction (mobile's rules) and downlink direction (base station's rules). Different DRB media can be configured for each stream with different packet routing specifications. Several streams belonging to the same PDU session can be multiplexed in the same DRB bearer. Only one SDAP entity is configured by the RRC layer per PDU session except for the dual connectivity mechanism or two SDAP entities are configured. Several SDAP entities can be simultaneously configured for a mobile. 8.1.1. Operations In the downlink direction, the NG-RAN radio node maps the QoS streams to the DRB radio bearers based on the QFI identifier and the QoS profile. The QoS rules at the mobile level are configured during the configuration of the radio link by RRC signaling messages. The mobile applies QoS or reflective QoS for the correspondence between the flow and the bearer. 5G-NR Radio Interface - Data Link Layer Reflective QoS allows the UE to create its QoS rules from the downlink traffic without additional signaling. The SDA