Survey the spectrum you are sitting in

short · 35 min · Objective 2.3

Task

Scan the wireless networks around you, record channel, band, signal strength and width for each, and work out where the congestion is. Everything in objective 2.3 — channels, bands, RSSI, overlap — becomes concrete in one scan.

Steps

  1. Run the scan and capture the output to a file so you can work on it: sudo iw dev wlan0 scan > /tmp/scan.txt.
  2. Extract, for each network, the SSID, channel, signal in dBm, and channel width. Group them by band — channels 1 to 14 are 2.4 GHz, 36 and above are 5 GHz, and anything above 1 in the 6 GHz list is Wi-Fi 6E.
  3. Count how many networks sit on each 2.4 GHz channel. Then check how many are on 1, 6 or 11 versus something else. Any network not on 1, 6 or 11 overlaps two of them.
  4. Rank the networks by signal. Note where yours falls, and convert the dBm figures to the practical bands: better than −65 dBm is good enough for voice, −70 is adequate for data, below −80 is unusable.
  5. Decide which 2.4 GHz channel you would move to and justify it from your own counts, not from the usual advice.

Verify

sudo iw dev wlan0 scan | grep -E "SSID|freq|signal|DS Parameter" | head -40
sudo iw dev wlan0 scan | grep -c "^BSS"
python3 -c "
for ch in (1,6,11):
    print('2.4 GHz channel', ch, 'centre', 2407 + ch*5, 'MHz')
print('non-overlapping 2.4 GHz channels: 1, 6, 11 only -- 22 MHz wide, 5 MHz apart')
print('good -65 dBm  |  adequate -70 dBm  |  unusable below -80 dBm')
"

The scan must list multiple BSSIDs with signal values in dBm. Your channel count per 2.4 GHz channel is the finding — and if the busiest channel is not 1, 6 or 11, you have found a network making the problem worse for everyone.

Notes

The 2.4 GHz arithmetic is worth understanding rather than memorising. Channels are 5 MHz apart and each carrier is about 22 MHz wide, so a channel overlaps its four neighbours on each side. Only 1, 6 and 11 are far enough apart to avoid each other — and channel 3 overlaps both 1 and 6, which is why "I picked an empty channel" so often makes things worse.

5 GHz has many non-overlapping channels, which is most of why it performs better in dense areas. The trade is range: higher frequency attenuates faster, so a 5 GHz cell is smaller than a 2.4 GHz one at the same power.

Channel width is the other lever. 40, 80 and 160 MHz channels are faster and consume more of the band, so in a crowded environment a narrower channel often delivers more real throughput than a wider one.