QoS Flow
The finest granularity of QoS differentiation in 5G, identified by a QoS Flow Identifier (QFI) within a PDU session. Replaces the LTE bearer concept.
In 5G the QoS flow is the unit the network actually differentiates on, and it lives entirely inside a single PDU session. Every flow carries a QoS Flow Identifier (QFI), and packets get marked with that QFI so each box along the path knows how to treat them. This is the clean break from LTE: there, QoS was tied to bearers, and the radio bearer and the core bearer were the same object end to end.
5G splits that apart. Inside the core and on the N3 tunnel, traffic is organised by QoS flow; over the air, the gNB maps one or more QoS flows onto Data Radio Bearers (DRBs) however it sees fit. That decoupling is the point — the core decides QoS treatment, the RAN decides how to realise it on the radio, and the two can evolve independently. A single PDU session can hold several flows: say one GBR flow for a voice call alongside a non-GBR flow for everything else.
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