A Wi-Fi channel works like a meeting with one microphone: only one device can talk at a time, and everyone else waits. Add clients and watch each one's share shrink, then see how a second AP helps, but only on a different channel.
Listen before you talk
Wi-Fi devices share the air using CSMA/CA (carrier-sense multiple access with collision avoidance). Before sending, a device listens. If the channel is busy, it waits. When it goes quiet, each device waits a short fixed time plus a random backoff, and whoever's countdown ends first talks. If two finish at the very same moment, their frames collide and both try again later with a longer random wait.
The result: one transmission per channel at any moment. Every client on the channel takes turns, so each gets a slice of the airtime. And it's the channel that is shared, not the AP: two APs on the same channel share it just like one AP.
Setup
Clients
Who is talking right now
Everything is in one room: every client can hear both APs and every other client. Each phone is coloured by the channel it's on. The one with radio waves is transmitting; everyone else on that channel hears it and has to wait.
Speed
Channel 36Channel 40ACK from the APCollisionChannel idle: everyone counting down their backoff
More clients, smaller slices
Throughput per client as clients are added, for each setup (1 second of simulated traffic per point). Hover a point for the numbers.
1 AP2 APs, different channels2 APs, same channel (same line as 1 AP)
Notes and simplifications
Link: every client uses Wi-Fi 6 on a 20 MHz channel with 2 spatial streams at 286.8 Mbps (MCS 11), as if all were close to their AP. Real clients farther away use slower rates, take longer to send the same data, and slow everyone down further.
Each turn: the channel must stay idle for DIFS (34 µs), then each client counts down a random backoff of 0–15 slots of 9 µs, pausing while the channel is busy. The winner sends 1 ms of data (about 50 µs of it the 802.11ax preamble); after SIFS (16 µs) the AP answers with an ACK (28 µs, at the 24 Mbps basic rate). After a collision, the colliding clients double their contention window (up to 1,023 slots) before picking a new backoff.
Traffic: every client always has data to send to its AP, and only clients contend. In a real network the AP also contends to send downlink, as one more device on the channel; a busy AP gets roughly one turn in N+1.
Same channel, two APs: the model assumes every device can hear every other, as in one open room. That is the normal case for APs on the same channel close enough to share clients, and it's why they add no capacity. Walls and distance can hide devices from each other; see the Hidden Node tool for what happens then.