The following figure shows an example message flow when the gNB receives a request to release a QoS flow from CN that involves explicit NAS signalling. NAS procedures details between gNB and 5GC can be found in TS 23.501
, TS 23.502
and TS 38.413
PDU session and DRB(s) have been already established.
gNB receives a PDU SESSION RESOURCE MODIFY REQUEST message from AMF to release a QoS flow.
The gNB decides to release corresponding the QFI to DRB mapping rule. Since the DRB also carries other QoS flows, the DRB is not released.
gNB sends an RRCReconfiguration message to UE to release the QFI to DRB mapping rule.
UE updates the AS QFI to DRB mapping rules to release this QFI to DRB mapping rule.
UE sends an RRCReconfigurationComplete message to gNB.
gNB sends a PDU SESSION RESOURCE MODIFY RESPONSE message to AMF.
The following figure shows an example message flow when the UE AS receives an UL packet for a new QoS flow for which a QFI to DRB mapping rule does not exist.
PDU session and DRBs (including a default DRB) have been already established.
UE AS receives a packet with a new QFI from UE NAS.
UE uses the QFI of the packet to map it to a DRB. If there is no mapping of the QFI to a DRB in the AS mapping rules for this PDU session, then the packet is assigned to the default DRB.
UE sends the UL packet on the default DRB. The UE includes the QFI in the SDAP header.
gNB sends UL packets to UPF and includes the corresponding QFI.
If gNB wants to use a new DRB for this QoS flow, it sets up one. It can also choose to move the QoS flow to an existing DRB using RQoS or RRC signalling (see clauses A.2 and A.3).
User Plane Data for the new QoS flow can then be exchanged between UE and gNB over the DRB according to the updated mapping rules and between UPF and gNB over the tunnel for the PDU session.