SDAP
Service Data Adaptation Protocol: a NR-specific protocol layer that maps QoS flows to data radio bearers and handles QoS flow marking between the 5GC and the RAN.
SDAP is the newest Layer 2 sublayer and exists only in NR, because 5G changed how QoS works. The 5G Core deals in QoS flows, but the radio side carries data on data radio bearers, and those don't map one-to-one. SDAP sits at the top of the user-plane stack and does the mapping — routing each QoS flow into the right DRB in both directions.
It also handles QoS flow marking. SDAP can stamp packets with a QoS Flow Identifier so the receiving end knows which flow a packet belongs to, which keeps the end-to-end QoS handling consistent from core to air interface. There's exactly one SDAP entity per PDU session. The whole point is flexibility: one DRB can carry several QoS flows with similar treatment, and the network can remap flows to bearers dynamically as conditions change, without renegotiating the PDU session. It's a thin layer, but it's the hinge that connects 5G's flow-based QoS model to the bearer-based radio.
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