SDAP – Service Data Adaption Protocol
(SDAP) layer is the topmost L2 sublayer at 5G NR protocol stack Standalone Architecture (SA) where gNB connects to 5G Core network. It interfaces to 5GC UPF using NG-U interface via QoS flows and to the PDCP lower layer via Data Radio Bearers ( DRBs).
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 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. SDAP 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 primary services and functions of SDAP include: Mapping between a QoS flow and a data radio bearer + Marking QoS flow ID (QFI) in both DL and UL packets.
