5G NR SDAP: QoS Flow to DRB mapping

  • The Session Management Function (SMF) is responsible for distributing 5G Quality of Service (QoS) parameters among the entities (UE, gNB and UPFs) in the 5G System (5GS). The gNB using the received QoS parameters(e.g. QoS profiles) and the internal configuration determines how the incoming QoS flows from the N3 tunnel between the gNB and the UPF is mapped to the Data Radio Bearers(DRBs) between the gNB and the UE. The gNB also configures the UE how to map QoS Flows to DRBs in the uplink direction with explicit signalling (RRC Reconfiguration message) or by using Reflective QoS (RQoS). In the case of RQoS, gNB adds the SDAP header to the user data packet and sets the RDI (Reflective QoS flow to DRB Identifier) flag to 1 inside the header and also includes QoS Flow Identifier (QFI).

  • The Service Data Adaptation Protocol (SDAP) layer at the gNB and UE is responsible for mapping QoS flows to DRBs.

    • The SDAP sublayer is configured by RRC.
    • One or more QoS flows may be mapped onto one DRB.
    • Depending on the number of PDU sessions , one or multiple SDAP entities can be configured within the SDAP layer.
  • Here I’ll try to depict how the QoS flows in the uplink and downlink direction are mapped to the Data Radio Bearers (DRBs) by using SDAP entity.

  • RQoS (Reflective QoS) allows the UE connected to a gNB or ng-eNB to deduce the Uplink mapping rules from the Downlink mappings, i.e. the Uplink rules are copied from the Downlink.
    The mapping rules are related to the Access Stratum (QoS Flow to DRB mapping) or related to the Non-Access Stratum (IP flows to the QoS FLow).
    Two parameters in SDAP header (RDI and RQI) are used to inform the UE to create the rule.

  • When a QoS flow to DRB mapping rule is updated, the UE sends an End- Marker control PDU on the old bearer.

  • There is a single SDAP entity for each PDU Session (the logical association between the UE and the anchor User Plane Function (UPF)).
    In the case of Dual connectivity like NR-DC , each Cell Group, i.e. Master Cell Group(MCG) on the Master gNB(MgNB) and Secondary Cell Group (SCG) on the Secondary gNB(SgNB) will have a separate SDAP configuration on the UE.

  • Example: the gNB can use explicit RRC signalling to inform the UE to map a new QoS flow, e.g. to an existing Data Radio Bearer (DRB):

    • In this case, the gNB receives from the UPF a first downlink packet associated with a QFI, for which the QoS parameters are already known from the PDU session establishment, but for which there is no association to any DRB yet in Access Stratum (AS).

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