Channel planning: co-channel contention is the real enemy

Wi-Fi is half-duplex and polite. Every device on a channel — yours and your neighbour's — takes turns. Understanding that one fact explains most of what looks like inexplicable slowness.

CSMA/CA in one paragraph

Before transmitting, a station listens. If the channel is busy it waits, then waits a further random backoff. If two stations transmit at once, both fail and both back off further. Airtime is a single shared resource per channel, and it is consumed by whoever is transmitting, regardless of which AP they are associated with.

Co-channel interference (CCI)

Two APs on the same channel that can hear each other share airtime. They do not interfere in the destructive sense; they queue. Each AP gets roughly half the airtime, so each gets roughly half the throughput. Add a third and it is a third each.

This is why the classic response to "the Wi-Fi is slow" — add more access points — so often makes things worse. More APs on the same channels means more devices sharing the same airtime with more overhead.

Adjacent-channel interference (ACI)

Two APs on overlapping channels cannot decode each other, so they do not defer politely. They transmit over each other and both signals are corrupted. ACI is genuinely destructive and far worse than CCI. This is the reason for channels 1, 6 and 11 in 2.4 GHz, and the reason a badly configured 80 MHz plan is worse than a careful 40 MHz one.

Given a choice, always take co-channel over adjacent-channel.

Building a channel plan

1. Count your channels honestly. Decide on DFS first — it determines whether you have 9 or 25 channels at 5 GHz. Decide on channel width second.
2. Divide, do not multiply. Channel budget at 40 MHz is half what it is at 20 MHz. At 80 MHz it is a quarter. Wide channels are a capacity trade, not a free upgrade.
3. Reuse in three dimensions. The AP two floors up is a co-channel neighbour. Floor-to-floor leakage through a suspended ceiling and a concrete slab is often only 15 to 25 dB. Plan buildings as volumes, not as stacked plans.
4. Stagger, do not stripe. Avoid repeating patterns that line up across a floor. Offset each floor's plan.
5. Let automatic RRM do the tuning, not the strategy. Modern controllers do a decent job of channel and power optimisation within constraints you set. Set the constraints — allowed channels, width, min and max power — deliberately, and then let it work. Leaving it fully automatic with all channels and all widths enabled is how you end up with a 160 MHz channel in a call centre.

The 2.4 GHz special case

There are three channels. Not four, not "1, 5, 9 works fine". In any real deployment:

• Use 20 MHz only.
• Disable the 2.4 GHz radio on 50 to 70 percent of APs in a dense design.
• Reduce power on the radios you keep, so the cells do not overlap into each other.
• Accept that 2.4 GHz is an IoT band and stop trying to make it perform.

How to tell if CCI is your problem

Look at channel utilisation, not client count or signal strength. Above roughly 50 percent utilisation, latency and jitter climb noticeably. Above 70 percent, users describe the network as broken even though every client shows full bars and a strong RSSI.

If utilisation is high and your own APs account for most of it, you have a channel plan or channel width problem. If it is high and the energy is not from your APs, you have neighbours or non-Wi-Fi interference — and you need a spectrum analyser, not another access point.