WiFi Airtime & Capacity
Define the clients (PHY rate and demand) and I compute the airtime each one consumes, the cell total and whether it saturates. See why a slow client hogs the medium (airtime fairness).
Cell clients
Each row is a WiFi client. Set its PHY rate (Mbps, the one
negotiated with the AP) and its actual traffic demand (Mbps).
The airtime it consumes is demand / (PHY × efficiency).
Total cell airtime
—
Per-client breakdown
| Client | PHY | Demand | Airtime | Real throughput |
|---|
How it is computed · airtime, saturation and the slow client
1. For each client, the airtime (fraction of air time
it occupies) is airtime = demand(Mbps) / (PHY(Mbps) × efficiency).
Efficiency (MACeff) accounts for 802.11 overhead: preambles, ACKs, contention and
inter-frame spaces. Typically 0.65 on a/g/n, up to 0.75 on ax
thanks to OFDMA; in crowded 2.4 GHz it drops to ~0.5.
2. The cell's total airtime is the sum of all of them:
total = Σ airtime_i. The air is a shared resource, so only 100%
is available.
3. If total ≤ 100% the cell does not
saturate: every client gets its demand and some airtime is left free. If
total > 100% the cell saturates: there is not enough air and each
client's real throughput is scaled by 1/total (proportional to demand).
4. Slow-client effect: a client at a low PHY rate
consumes a lot of airtime even with modest demand. E.g. 5 Mbps at 6 Mbps PHY
→ 5/(6×0.65) = 128%, it alone saturates the cell. That client hogs the medium
and slows everyone down (which is why raising its rate or moving it off the channel helps).
5. Simplified model: it does not include retransmissions, real collisions, MU-MIMO or aggregation (A-MPDU). It is meant to show the airtime-fairness dynamics, not as an exact capacity figure.
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