A while back we got called in for "the slow WiFi in the meeting room". The phone showed full bars standing right next to the AP, and the video call still froze every couple of minutes. The culprit turned up at the end of the hallway: an ancient IP camera hanging off the same AP, negotiating 6 Mbps from the storage room. Signal was never the problem — the microphone was. WiFi works like karaoke night: one singer at a time, and that camera never let go of the mic.
One microphone: WiFi is a shared medium
A modern UTP cable is full-duplex: every device gets its own dedicated copper pairs and can talk and listen at the same time. Radio can't. On a WiFi channel, one station transmits at a time: the AP and all of its clients share the same air, half-duplex. The protocol that manages the queue is CSMA/CA, and it behaves like a polite conversation: listen before talking, wait a random turn, transmit, and hope for the ACK. If two stations talk at once, the frame is lost and both retry — slower.
Almost everything else follows from that one fact: channels define how many karaoke rooms exist, channel width is the size of each room, the MCS is how fast each singer can go, and airtime is how long each client hogs the mic. Once that clicks, you stop chasing signal bars and start designing cells.
Channels: the map of 2.4, 5 and 6 GHz
The 2.4 GHz band is an old neighborhood with narrow streets. There are 13 numbered channels (in most of the world), with center frequencies every 5 MHz from 2412 to 2472 MHz. The cruel detail: each channel is 20 MHz wide, so neighboring channels overlap each other. With centers 25 MHz apart, only one combination coexists cleanly: 1, 6 and 11.
On paper, 1/5/9/13 would also work (exactly 20 MHz apart), but only if the entire neighborhood agrees — and one router on channel 6 ruins the party. Add microwave ovens, Bluetooth and wireless doorbells all living in 2.4 GHz, and you see why it's the band of inherited problems.
The 5 GHz band is a different world: roughly 25 channels of 20 MHz (depending on local regulation) and no microwave ovens butting into the conversation. The toll is the DFS channels (52 through 144), where WiFi is a guest of weather and military radar. And 6 GHz (WiFi 6E) is the wide-open prairie: up to 1200 MHz of fresh spectrum — 59 channels of 20 MHz, 14 of 80 or 7 of 160 — and Chile was among the first countries to open the full band. Before pinning channels by hand, check the full map in the WiFi channels calculator.
Channel width: more lanes, more friction
Doubling the channel width roughly doubles the rate: going from 20 to 40 MHz multiplies the data subcarriers by 2.08, and from 40 to 80 MHz by 2.17 (guard bands get used more efficiently). Sounds like free money, but a wide channel occupies your neighbor's spectrum: a single 40 MHz channel in 2.4 GHz stomps on two of the three clean channels all by itself, and in 5 GHz four APs on 80 MHz can exhaust the non-DFS channels in the area.
The field rule: in dense deployments (offices, buildings, classrooms) use 20 or 40 MHz in 5 GHz and save 80 MHz for where spectrum is plentiful — a detached house, or 6 GHz. A narrow, clean channel outperforms a wide, contested one: co-channel interference never shows up in the signal bars, but it eats your airtime in retries.
MCS: the signal decides how fast you go
Within the channel, speed is set by the modulation. Think of dictating a phone number: on a crystal-clear line you rattle it off in one go; on a noisy line you end up spelling it digit by digit. The MCS index is exactly that ladder: with poor signal, the client drops to BPSK (1 bit per symbol, slow but sturdy); with excellent signal it climbs to 256-QAM (8 bits per symbol) or, on WiFi 6, to 1024-QAM.
Watch out for the number the driver shows: that's the PHY rate, the instantaneous speed while a frame is on the air. Between preambles, ACKs, contention and inter-frame spacing, real throughput on a healthy cell lands around 65 % of PHY. A client at MCS9 on 80 MHz with 1 spatial stream reports 433 Mbps, but iperf will show close to 280. Nothing is broken — that's just how the 802.11 MAC works.
| MCS | Modulation | PHY 80 MHz, 1 stream | ≈ Real (×0.65) | Where you get it |
|---|---|---|---|---|
| MCS0 | BPSK 1/2 | 32.5 Mbps | ≈ 21 Mbps | far cell edge |
| MCS2 | QPSK 3/4 | 97.5 Mbps | ≈ 63 Mbps | weak signal |
| MCS4 | 16-QAM 3/4 | 195 Mbps | ≈ 127 Mbps | medium signal |
| MCS7 | 64-QAM 5/6 | 325 Mbps | ≈ 211 Mbps | good signal |
| MCS8 | 256-QAM 3/4 | 390 Mbps | ≈ 254 Mbps | very good signal, high SNR |
| MCS9 | 256-QAM 5/6 | 433.3 Mbps | ≈ 282 Mbps | close to the AP, clean cell |
To see which MCS a Linux client negotiated, just ask the driver:
$ iw dev wlan0 station dump | grep -E 'signal|tx bitrate'
signal: -58 dBm
tx bitrate: 433.3 MBit/s VHT-MCS 9 80MHz short GI VHT-NSS 1 The full ladder by standard, width, guard interval and spatial streams lives in the PHY rate (MCS) table. With WiFi 6 and 1024-QAM, the same 80 MHz channel with 1 stream reaches 600 Mbps PHY — if the signal can sustain the modulation, which is a big ask far from the AP.
Airtime: the slow singer hogs the mic
This is where it all comes together. Since there's only one air, what matters isn't how many megabits each client wants, but how much airtime it needs to get them. And time depends on rate: shipping 1 MB at MCS9 takes milliseconds; the same MB at 6 Mbps takes nearly a full second, with everyone else checking their watches. The math is simple: airtime = requested throughput ÷ (PHY × 0.65). Let's run it for the camera from the story:
- The old camera sits far away and negotiates PHY 6 Mbps (the slowest 802.11g base rate). Its real ceiling is 6 × 0.65 = 3.9 Mbps.
- Someone configured a 5 Mbps video stream on it. Airtime needed: 5 ÷ 3.9 = 128 %. It doesn't fit: the camera burns 100 % of the cell's air all by itself, delivers barely 3.9 Mbps, and the video still stutters.
- The meeting-room laptop negotiates MCS9 on 80 MHz: 433 Mbps PHY, about 282 Mbps real (433 × 0.65).
- Its video call wants 50 Mbps: 50 ÷ 282 = 17.8 % of the air. Five laptops like that could coexist and there'd still be mic time to spare.
- The moral, with a number: every megabit from the camera costs 72 times more air than one from the laptop (282 ÷ 3.9 ≈ 72). A single slow client degrades every fast one — not because it moves a lot of data, but because it hogs the turn.
You can play with these scenarios — mix clients, rates and demands, and watch the cell hit the wall — in the airtime and capacity calculator.
Quick design: the cell that performs
With all of the above, decent cell design fits in three rules:
- Cell edge between -65 and -67 dBm, measured with your weakest real client (the cheap phone, not your big-antenna laptop). At that signal, clients hold high MCS and nobody sings at 6 Mbps.
- Alternate channels between neighboring APs: 1/6/11 in 2.4 GHz, and in 5 GHz never repeat a channel between cells that can hear each other. Two APs on the same channel don't add capacity — they share one microphone.
- Moderate power. The link is a round trip: an AP at full power reaches far, but the phone — transmitting a lot less — can't answer back. The result is that lying cell with full bars and packet loss. Aim for AP and client at similar power; you can work out EIRP in the antenna and RF calculator.
And if the problem is getting signal from one building to another, that's not a cell anymore — it's a radio link. Size it with the wireless link calculator instead of stretching an AP past its dignity.
Now, into the field: walk the area watching signal and negotiated rate on your phone, pin channels with the channels calculator, and run your slowest clients through the airtime tool. If the cell is still slow after that, you know exactly whose microphone to take away.