Wi-Fi is radio before it is networking. Every problem you will debug — slow rooms, sticky clients, an AP that reboots under load — reduces to spectrum, power, or airtime. This article covers the first two.
Three bands, three personalities
| Band | Frequencies | Non-overlapping channels | Behavior |
|---|---|---|---|
| 2.4 GHz | 2.400–2.4835 | 3 (1, 6, 11) | Long reach, low speed, crowded |
| 5 GHz | 5.150–5.850 | ~25 (with DFS) | Shorter reach, high speed, the workhorse |
| 6 GHz | 5.925–7.125 | up to 59 (varies) | Shortest reach, cleanest, newest clients only |
2.4 GHz travels through walls and microwaves alike, which is why it is both the most reliable coverage band and the slowest one. There are exactly three channels that do not overlap, and in a district of 197 APs, most of them will hear each other.
5 GHz is where capacity lives. More channels, wider channels, less interference — and shorter range, which is fine because you have an AP in every other classroom.
6 GHz is pristine spectrum with a catch: only Wi-Fi 6E and 7 clients can use it, so it is capacity for the newest devices, not coverage for the oldest.
Channel numbers are not sequential in frequency
2.4 GHz channel 1 is 2.412 GHz, channel 6 is 2.437, channel 11 is 2.462. Each channel is 20 MHz wide but they are spaced 5 MHz apart, which is why 1/6/11 are the only three that do not overlap.
Width is a trade, not a setting to maximize
A channel is a slice of spectrum. Wider slices carry more data per transmission and are more sensitive to interference.
| Width | Effect | When to use |
|---|---|---|
| 20 MHz | Most channels, lowest noise floor | 2.4 GHz always; dense 5 GHz |
| 40 MHz | Double throughput, half the channels | 5 GHz in classroom wings |
| 80 MHz | High peak throughput, very few channels | 5 GHz in open areas, few APs |
| 160 MHz | One or two channels in the band | 6 GHz, low density only |
Every doubling of width raises the noise floor by 3 dB, because you are listening to twice as much spectrum. In a dense building that is the difference between a clean channel and a marginal one.
The default on most access points is 80 MHz on 5 GHz and 160 MHz on 6 GHz. In a classroom wing with an AP every two rooms, that default guarantees co-channel interference: there are only six non-DFS 80 MHz channels in the whole 5 GHz band.
Power is a coverage decision
Transmit power is measured in dBm. Enterprise APs usually run 14–20 dBm on 5 GHz and 7–14 dBm on 2.4 GHz, and more power is not better:
- Higher power extends the cell, which extends the interference footprint.
- Clients transmit at 12–15 dBm, so a 20 dBm AP can be heard by a client that cannot be heard back — the "sticky client" problem.
- Asymmetric power between bands pushes devices onto 2.4 GHz, where they stay.
The goal is cells sized to the client's ability to talk back, not to the AP's ability to shout.
Worked example: a 197-AP district
The plan that a district this size can actually maintain:
| Band | Width | Channels | Power |
|---|---|---|---|
| 2.4 GHz | 20 MHz | 1, 6, 11 only | 7–10 dBm |
| 5 GHz | 40 MHz (20 in dense wings) | UNII-1 and UNII-3, staggered | 14 dBm |
| 6 GHz | 80 MHz | staggered, PSC channels | 14 dBm |
Then check the distribution. If 97 APs are on 2.4 GHz channel 6 and 85 are on 5 GHz channel 48, the plan exists on paper and not in the controller — which is what automated channel optimization produces when it is left running.
What breaks in production
- 80 MHz everywhere. Fewer channels than APs, so every cell overlaps. Throughput per client looks fine in a speed test and terrible under load.
- 40 MHz on 2.4 GHz. Two channels' worth of interference for a band that cannot use it.
- DFS channels without planning. A radar event forces a channel change, and every client on that AP reconnects. In some buildings this happens weekly.
- Maximum power on 2.4 GHz, which makes every AP hear every other AP and drags clients onto the slow band.
- 6 GHz treated as coverage. It is not; it is capacity for devices that support it.
Checklist
- 2.4 GHz is 20 MHz on 1/6/11, with power low enough that cells do not overlap.
- 5 GHz width is chosen per building, not globally.
- 6 GHz is planned separately and only where clients support it.
- Channel distribution is checked after every change, not assumed.
- Power is fixed, not auto, so cells stay the size you designed.
Next
Channels decide where the signal goes. Continue with Why Your Wi-Fi Is Slow: Airtime, Not Bandwidth for the part that decides how fast it feels.