Wifi channels and interference: fixing the evening slowdown

Wifi slows down in the evening because radio space is shared, and at seven o'clock everyone in range starts using it. A channel is a slice of spectrum, and every network within earshot of that slice takes turns inside it. Your router is not degrading: the airtime you had to yourself at eleven in the morning is now split a dozen ways. The fixes that help are picking a less crowded slice, choosing the right width, and moving what you can onto bands that still have room.

How a channel works, and why taking turns matters

Wifi radios are polite by design. Before transmitting, a device listens, and if the channel is busy it waits and retries a fraction of a second later. Ten networks on one channel is not chaos: everybody gets a share of the talking time, and everybody is slower.

Partial overlap is what actually hurts. If a neighbor sits halfway across your channel, the two radios cannot decode each other well enough to know when to wait. They talk over each other, frames arrive corrupted, and every corrupted frame is sent again, costing the airtime twice. So a strong neighbor sitting exactly on your channel is usually less damaging than one three channels away.

The currency here is airtime, not megabits. A slow device holds the channel open far longer to move the same data, so one elderly smart plug on 2.4 GHz can consume more of the channel than a laptop downloading on 5 GHz. The differences in range, speed and crowding between the bands are laid out in what 2.4, 5 and 6 GHz are actually for.

Why 2.4 GHz only has three real channels

The 2.4 GHz band is numbered in channels spaced 5 MHz apart: 1 to 11 in the United States, up to 13 in most of Europe and Asia. But a standard channel is 20 MHz wide, four of those steps. So channel 1 does not occupy only channel 1. It spreads across the space where 2, 3, 4 and 5 are numbered.

That is why only 1, 6 and 11 are non-overlapping. Everything else is a compromise between two of them. Regions that allow channel 13 can also run 1, 5, 9 and 13, which is why some European routers behave differently out of the box.

Hence the classic mistake: you see no names on channel 3, move there, and collide with the networks on both 1 and 6 instead of sharing politely with one.

Channel width and the radar shared channels

Routers can bond channels into wider ones: 20, 40, 80 or 160 MHz. Doubling the width roughly doubles the peak link rate, which is why wide channels look good on the box. It also doubles the clear spectrum you must find and lets more noise into the receiver, so the range at which you hold that rate shrinks.

In a detached house, 80 MHz on 5 GHz is usually right. In a flat where you can see fifteen networks, 40 MHz often measures faster than 80, because fewer frames get destroyed and resent. On 2.4 GHz a 40 MHz channel swallows more than half the band.

DFS: the empty channels your router may be avoiding

A large middle section of the 5 GHz band is shared with radar: weather, aviation and military systems. To use it, a router must run Dynamic Frequency Selection. It has to listen on the channel before transmitting at all, typically about a minute and longer on the sub-band used by weather radar, and it must vacate within seconds if it detects a radar pulse.

The consequences are visible. After a reboot the 5 GHz network can take a minute to appear, and near an airport, a coastline or a weather station you may get occasional forced moves with a short drop. Some devices, especially cheap smart home gadgets and older phones, cannot use these channels and will not see the network there.

The payoff is that these channels are often far emptier than those at either end of the band, because many consumer routers avoid them by default. If your devices cope, this is usually the biggest free win available. Note too that the top group of 5 GHz channels is allowed in the United States but not generally across much of Europe, so channel lists differ by country.

When the interference is not wifi at all

Some of the worst 2.4 GHz problems come from things that are not networks, and they never appear in an analyzer app because they broadcast no name.

The USB 3 one traps people regularly: if a wireless dongle sits in the port beside an external drive, move it to a short extension cable. The 5 and 6 GHz bands are immune to nearly all of this, which is the strongest argument for leaving 2.4 GHz behind.

Diagnosing it with a phone in ten minutes

Analyzer apps on Android list every network in range with its channel, width and signal strength in dBm. iPhones cannot do this, because Apple does not expose the scanning interface to apps. Instead use a Mac's wireless diagnostics scan, netsh wlan show networks mode=bssid on Windows, or your router's own site survey page.

Read three things: how many networks sit on each channel, how strong they are (a neighbor at -80 dBm is background hiss, one at -55 dBm is a real competitor), and how wide each one is, because a single 80 MHz neighbor occupies four channels worth of space.

Then check your own signal in the room that suffers. Around -67 dBm is the usual target for smooth video, and below roughly -75 dBm things fall apart. A weak signal is a coverage problem no channel change will rescue: that is a job for where the router sits and what is blocking it, and when placement is not enough, for choosing between mesh, an extender and a cable.

Survey at the bad hour: a scan at eleven in the morning tells you nothing about eight in the evening. And before changing anything, plug a laptop into the router and test, so you know whether the ceiling is the wifi or the line. Reading a speed test properly separates the two, and the full order of diagnosis is in why your internet is slow.

The settings worth changing, and the myths

Worth doing, in rough order of payoff:

1. Set 2.4 GHz to 20 MHz, fixed to whichever of 1, 6 or 11 has the fewest strong neighbors.
2. Set 5 GHz width by density: 80 MHz in a house, 40 MHz in a crowded block.
3. Allow radar shared channels if your devices support them and you see no repeated drops.
4. Give both bands one network name and let the router steer, unless a stubborn gadget needs its own 2.4 GHz name.
5. If offered, disable the oldest legacy data rates so slow distant devices cannot hog airtime.

Now the myths. Turning transmit power to maximum does not help: your router shouting louder does not make your phone's reply louder, and the link needs both directions. Hiding the network name does nothing for interference. Automatic channel selection is not always best either, because many routers only re-evaluate at boot, so yours may have chosen at three in the morning when the band was empty. Reboot it during a busy evening, or set the channel by hand. And if your speed already matches what you pay for when you stand next to the router, channels are not the problem at all: check how much speed you actually need before spending anything.

What to do this evening

Change one thing at a time and test at the same hour each night, or you will never know which change worked. Start with a wired speed test to establish the ceiling. Scan during the slow period, not the quiet one. Set 2.4 GHz to 20 MHz on 1, 6 or 11, match the 5 GHz width to how many neighbors you can see, and move everything capable onto 5 or 6 GHz. Most evening slowdowns come down to two or three of those settings. The ones that survive all of it are coverage problems wearing an interference costume.