AMBR
Aggregate Maximum Bit Rate: the maximum total bit rate across all non-GBR QoS flows/bearers for a UE, enforced per session (Session-AMBR) or per subscriber (UE-AMBR).
AMBR is the cap that keeps best-effort traffic from running wild. It applies only to non-GBR flows — GBR flows have their own per-flow guarantees and are accounted separately — and it limits the total rate those non-GBR flows can collectively reach. There are two scopes you need to keep distinct. Session-AMBR (the 5G term; APN-AMBR in 4G) bounds the aggregate across all non-GBR flows within one PDU session. UE-AMBR bounds the aggregate across the subscriber's non-GBR traffic and is enforced by the RAN.
The split in where they're enforced trips people up. Session-AMBR is typically policed at the UPF in the downlink for that session; UE-AMBR is the gNB's job, since only the RAN sees all of a given UE's flows together. Both come from the subscription data, so they're effectively the subscriber's "speed tier" expressed as a guarantee the network won't let them exceed. When a user on an unlimited-but-throttled plan hits a rate ceiling, the AMBR is usually the parameter doing the throttling.
Frequently asked questions
- Does AMBR limit GBR flows too?
- No. AMBR only governs the aggregate of non-GBR flows. GBR flows are handled by their own per-flow parameters — the guaranteed bit rate (GFBR) and the maximum flow bit rate (MFBR) — and their rates sit outside the AMBR budget. So a voice call on a GBR flow isn't squeezed by the same cap that limits a user's background data; the two are accounted independently.
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