GFBR
Guaranteed Flow Bit Rate: the minimum bit rate that the network guarantees to deliver for a GBR QoS flow under normal conditions.
GFBR is the floor the network commits to for a GBR flow — the rate it will keep reserving radio and transport resources to sustain under normal conditions. It's defined per direction, so a flow has an uplink GFBR and a downlink GFBR, and they don't have to match. For a voice call those numbers are small and steady; for video they're larger.
The practical role is admission control. Before the gNB accepts a new GBR flow it checks whether it can honour the GFBR on top of everything already running; if not, the flow is rejected rather than admitted and starved. Once admitted, the guarantee holds as long as the source actually offers traffic up to that rate. Pair it with MFBR — GFBR is what you're promised, MFBR is the ceiling you're allowed to burst to — and you have the GBR flow's full rate envelope.
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