Wi-Fi Professionals

Wi-Fi Pro Lab

2.4 GHz Wi-Fi: the original band

The original Wi-Fi band: long reach, good penetration through walls, only three clean channels, and a lot of company. Understanding it properly is the key to troubleshooting almost every home network.

83.5 MHzof spectrum (2400 to 2483.5 MHz)
3non-overlapping 20 MHz channels
about 12 cmwavelength
1997first 802.11 standard

1. DefinitionWhat is the 2.4 GHz band?

2.4 GHz is a slice of radio spectrum from 2400 to 2483.5 MHz. It belongs to the ISM (industrial, scientific and medical) bands, which regulators leave open to licence-exempt use as long as devices stay within power and other limits. Wi-Fi shares it with Bluetooth, Zigbee and Thread, cordless phones, baby monitors and microwave ovens.

Band, waveform, modulation, standard. Four different things get mixed up here. 2.4 GHz is where the signal sits (a frequency band). DSSS and OFDM are waveforms (how the signal is built). QPSK and 64-QAM are modulations (how bits map onto the signal). 802.11b, g, n, ax, be are standards that define all of the above. One band can host many technologies, and one technology can work in several bands.

2. PurposeWhy Wi-Fi started here

  • Licence-free. Anyone can build and use a compliant radio without applying for spectrum, which is what let Wi-Fi become a consumer product.
  • Good reach for the power. A wavelength of about 12 cm diffracts around obstacles and passes through walls well, so one access point covers a lot of floor area.
  • Cheap radios. Silicon for this band was cheap and mature early on.
  • The cost: very little spectrum. 83.5 MHz only fits a few wide channels, and everything else in the neighbourhood is using it too.

3. MechanismHow the channels work

A Wi-Fi channel number is shorthand for a centre frequency: centre = 2407 + 5 x channel MHz. Channel 1 is 2412 MHz and channel 6 is 2437 MHz. Channels are spaced only 5 MHz apart, but a standard OFDM channel is about 20 MHz wide, so neighbouring channel numbers overlap heavily.

That overlap is why the sensible plan is channels 1, 6 and 11. Two networks on the same channel can at least take turns, because each hears the other and waits (CSMA/CA). Two networks on partly overlapping channels cannot decode each other, so they simply add noise to one another and both slow down.

Try this. (1) Switch between UK and US: the UK allows channels 1 to 13, the US only 1 to 11. (2) Look at the lower peaks: a network on channel 3 sits between 1 and 6 and wrecks both. (3) Notice why three is the practical maximum for 20 MHz channels, and why 40 MHz channels almost never make sense here.

4. VisualWhy wavelength matters

Radio waves travel at the speed of light, so wavelength = speed of light / frequency. At 2.4 GHz that is about 12 cm, a little longer than your hand. A wave this size bends around furniture and door frames, which is part of why 2.4 GHz coverage reaches into so many rooms.

5. MathematicsPath loss and the PHY rate

Even in empty space a signal weakens with distance. Free-space path loss (FSPL) is 20 log10(d) + 20 log10(f) – 27.55 dB, with d in metres and f in MHz. At 10 m in 2.4 GHz (channel 6, 2437 MHz) that is 20 + 67.7 – 27.55 = 60.2 dB. Every doubling of distance adds 20 log10(2) = 6 dB, which means a quarter of the power: at 20 m the loss is 66.2 dB. Walls and furniture then come on top.

The second formula turns a modulation choice into a speed. The PHY data rate of one link is (data subcarriers x bits per subcarrier x coding rate x streams) / symbol time. Try different generations below: you will see that 802.11g tops out at 54 Mbps because it has only 48 data subcarriers and 64-QAM, and why a 20 MHz 802.11ax stream reaches 143.4 Mbps.

6. Generations over timeWhich Wi-Fi standards work in 2.4 GHz

Only the standards that define 2.4 GHz operation are shown: the original 802.11 (1997), 802.11b, 802.11g, 802.11n, 802.11ax and 802.11be. Click through to see the waveform, modulation, width and headline speed of each, with the date the standard was approved.

Go deeper. The timeline names the waveform each generation used: DSSS, CCK and OFDM. How those are built has its own track: Modulation and waveforms. A 2.4 GHz network that still allows the old 1, 2, 5.5 and 11 Mbit/s rates is forced to run the slow DSSS/CCK waveforms for its management traffic and to protect them from newer devices, which is very costly in airtime.

7. Rules and regulationsWhat the law allows in this band

Radio rules decide which channels you can use and how much power you may radiate, so they are part of the engineering. Each domain below starts with who sets the rules, then the limits. It opens on the UK (Ofcom); switch to the EU (CEPT and ETSI) or the US (FCC).

8. Real-world behaviourWhat goes wrong in 2.4 GHz

  • Neighbours. In flats and terraces, you may hear a dozen networks on three channels. Airtime, not signal strength, becomes the bottleneck.
  • Non-Wi-Fi interference. Bluetooth hops across 79 channels of 1 MHz; Zigbee and Thread use narrow 802.15.4 channels; microwave ovens leak noise around 2.45 GHz; cordless phones and video senders can occupy a lot of the band. A Wi-Fi radio cannot always tell these apart from bad luck.
  • Legacy overhead. Old 802.11b data rates (1, 2, 5.5 and 11 Mbps) are very slow. A beacon of about 300 bytes sent at 1 Mbps takes roughly 2.6 ms of airtime; at 6 Mbps about 0.4 ms. Beacons go out every 102.4 ms for each SSID, so 10 SSIDs at 1 Mbps can burn about a quarter of the channel before any user traffic. Disabling the lowest rates is one of the cheapest fixes there is.
  • Mixed mode. If an 802.11b client is present, newer devices use protection mechanisms (such as CTS-to-self) so the old device can hear them, which costs throughput for everyone.
  • IoT devices. Many smart-home devices support 2.4 GHz only, so the band cannot simply be switched off.

Practical validationProve it in a capture and on a spectrum analyser

Reading about a band is one thing; seeing it in your own evidence is the skill. These checks work with any monitor-mode capture:

  • Beacons only: wlan.fc.type_subtype == 0x0008
  • Everything on one channel: wlan_radio.channel == 6 (or by frequency: wlan_radio.frequency == 2437)
  • Retries, a sign of a congested or interfered channel: wlan.fc.retry == 1
  • Channel announced in the beacon (DS Parameter Set): wlan.ds.current_channel
  • Which waveform was used? Look at the PHY type and data rate in the radio information: wlan_radio.phy and wlan_radio.data_rate. Frames at 1, 2, 5.5 or 11 Mbit/s are DSSS or CCK; 6 to 54 Mbit/s frames are OFDM.
  • On a spectrum analyser, compare duty cycle with noise floor. High duty cycle at a steady level means busy Wi-Fi; a raised floor or a pulsing shape points to a non-Wi-Fi interferer.
  • Question to answer: with the same client and AP, how do retry rate and throughput change between channel 1, a clean channel, and channel 3, between two neighbours?

Check your understandingQuick quiz

ReferencesSources and further reading

IEEE standards are paid documents, so the link above goes to the publisher. Where a topic names a clause or table of IEEE Std 802.11-2020, its facts were checked against that clause; amendment dates come from the IEEE 802.11 Working Group’s public timelines. Regulations change, so check the regulator for the current text.