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Every Wi-Fi transmission happens on a channel inside one of three frequency bands: 2.4 GHz, 5 GHz or 6 GHz. Which band and channel an AP uses determines how far its signal travels, how much interference it meets, how wide its channels can be, which clients can join, and which regulatory rules apply. This post goes through each band in detail — its sub-bands, channel numbering, power limits and special rules — and finishes with practical channel-planning guidance.

What a Frequency Band Is

A radio band is a range of electromagnetic frequencies that regulators set aside for a particular use. Wi-Fi uses unlicensed bands: anyone may transmit without buying a spectrum licence, provided the equipment is certified and follows the rules — maximum power, required behaviours such as radar avoidance, and indoor/outdoor restrictions. The trade-off is that unlicensed bands are shared with everybody else, including other Wi-Fi networks and non-Wi-Fi devices.

In the US the FCC calls these bands the ISM band (Industrial, Scientific and Medical — 2.4 GHz) and the UNII bands (Unlicensed National Information Infrastructure — 5 and 6 GHz). Other regions use different names and slightly different boundaries, but the structure is similar everywhere.

A few physics basics explain most of the differences between bands:

• Wavelength is the speed of light divided by frequency: about 12.5 cm at 2.4 GHz, 5.5 cm at 5.5 GHz and 5 cm at 6 GHz.
• Path loss rises with frequency. With the same antenna type, a 5 GHz signal arrives roughly 7 dB weaker than a 2.4 GHz signal over the same distance, and 6 GHz about 1–2 dB weaker again. (The effect comes from the smaller capture area of an antenna at higher frequency.)
• Penetration through walls, glass, people and furniture generally worsens as frequency rises.
• Spectrum increases dramatically as frequency rises — and spectrum, not range, is what limits modern networks.

The 2.4 GHz Band

The 2.4 GHz channel numbering is a historical trap. Channel centres are only 5 MHz apart, but every transmission occupies about 20 MHz (22 MHz for the original DSSS signal). Adjacent channel numbers therefore overlap almost completely, and only channels five apart — 1, 6 and 11 — avoid each other. An AP on channel 3 does not find a quieter spot; it suffers adjacent-channel interference from both channel 1 and channel 6, which is worse than sharing channel 1 cleanly (co-channel stations at least coordinate through CSMA/CA, while adjacent-channel energy simply corrupts frames).

In regions that allow channels 12 and 13, a four-channel plan (1, 5, 9, 13) is sometimes used for OFDM-only networks, accepting slight overlap. The safe default everywhere is 1, 6 and 11.

2.4 GHz is crowded: Bluetooth, Zigbee, cordless phones, baby monitors, wireless video senders and microwave ovens (which leak energy around 2.45 GHz) all live here, along with every neighbour’s Wi-Fi and a long tail of legacy devices. It has the best range and penetration, which is exactly why it is so congested — every AP is heard far beyond its intended cell. With only three channels, 40 MHz channels should never be used in 2.4 GHz: a single 40 MHz AP consumes two of the three channels.

The modern role of 2.4 GHz is to serve devices that have no other option — IoT sensors, older scanners, medical devices — and to provide a coverage fallback. Dense enterprise designs often disable 2.4 GHz radios on a proportion of APs, because there are not enough channels to give every AP its own.

The 5 GHz Band

5 GHz is the workhorse of enterprise Wi-Fi: many more channels than 2.4 GHz, wider channel options, less non-Wi-Fi interference, and support in virtually every client sold in the last decade. Its complexity lies in the sub-bands, each with its own rules.

UNII Sub-Bands (US)

* Simplified. Exact limits depend on channel width, antenna gain, device class and point-to-point vs point-to-multipoint use. Enterprise APs normally transmit far below these maxima — typically 10–20 dBm — because cell size, not legality, is the design constraint.

Channel numbers in 5 GHz follow a simple formula: centre frequency = 5000 + 5 × channel MHz. Channel 36 is 5180 MHz, channel 149 is 5745 MHz. Channels are numbered every 5 MHz but used every 20 MHz, so valid 20 MHz channels step by four.

DFS Channels

UNII-2A and UNII-2C share spectrum with radar systems — military, airport and weather radar. To use these channels, an AP must perform Dynamic Frequency Selection (DFS):

• Channel Availability Check (CAC) — before transmitting on a DFS channel, the AP listens for radar for 60 seconds (10 minutes on weather-radar channels in some regions, notably Europe).
• In-service monitoring — while operating, the AP keeps watching for radar.
• Channel move — if radar is detected, the AP must stop transmitting within 10 seconds (sending a Channel Switch Announcement so clients can follow) and move to another channel.
• Non-occupancy period — the abandoned channel may not be used for 30 minutes.

DFS channels make up 16 of the 25 classic 5 GHz channels in the US, so avoiding them leaves only nine (36–48 and 149–165) — barely enough for a small building with 20 MHz channels. Practical guidance:

• Use DFS channels by default; they more than double usable spectrum.
• Check where radar events actually occur (APs log them). Sites near airports, harbours, military bases and weather radar may see frequent hits on some channels; exclude those channels locally rather than everywhere.
• Channels 120–128 overlap Terminal Doppler Weather Radar; many vendors and regions treat them specially or exclude them.
• Some clients — older devices and certain IoT, medical or industrial equipment — do not support DFS channels. Know the client population before relying on them for critical SSIDs.
• DFS is covered further in Spectrum Management, QoS and Security, which explains the 802.11h mechanisms behind it.

Regional Differences in 5 GHz

Rules change regularly. Every AP must be configured with the correct country code (regulatory domain), and the vendor’s current channel tables are the authority for a given model and region.

The 6 GHz Band

In 2020 the FCC opened 1,200 MHz of spectrum from 5.925 to 7.125 GHz for unlicensed use — more than all previous Wi-Fi spectrum combined. Devices that use it are certified as Wi-Fi 6E (802.11ax) or Wi-Fi 7 (802.11be).

Centre frequency = 5950 + 5 × channel MHz, with 20 MHz channels numbered 1, 5, 9 … 233. The US band holds 59 × 20 MHz, 29 × 40 MHz, 14 × 80 MHz, 7 × 160 MHz and 3 × 320 MHz channels.

Power Classes and AFC

• Low Power Indoor (LPI) — any 6 GHz channel, indoor APs only (no weatherproofing, no battery power, permanently connected). Power is defined as power spectral density: 5 dBm per MHz for APs, which allows about 18 dBm EIRP on a 20 MHz channel, 24 dBm on 80 MHz and 30 dBm on 320 MHz. Clients must operate 6 dB below their AP. LPI is how most enterprise 6 GHz networks operate today.
• Standard Power (SP) — UNII-5 and UNII-7 only, indoor or outdoor, up to 36 dBm EIRP. Before transmitting, the AP must query an Automated Frequency Coordination (AFC) system with its location; the AFC database replies with the channels and power levels that will not interfere with licensed incumbents such as fixed microwave links. Standard power gives 6 GHz the range of 5 GHz and makes outdoor 6 GHz possible.
• Very Low Power (VLP) — added by the FCC in 2023 for portable and wearable devices (for example phone-to-headset links), at about 14 dBm EIRP, without AFC.

Europe and many other regions have opened only the lower 480 MHz (5.945–6.425 GHz), giving 24 × 20 MHz, 6 × 80 MHz or 3 × 160 MHz channels, with LPI (23 dBm EIRP) and VLP classes. Other countries fall between the two models.

Rules That Make 6 GHz Different

• No legacy devices. Only Wi-Fi 6E and Wi-Fi 7 devices can use the band, so there is no protection overhead and no slow clients.
• Security is mandatory. Only WPA3 (Personal with SAE, or Enterprise) and Enhanced Open (OWE) are allowed, with Protected Management Frames. A WPA2 SSID is simply not offered on 6 GHz.
• Discovery is different. Clients may not send broadcast probe requests freely across 59 channels. Instead they learn about 6 GHz networks from the same AP’s 2.4/5 GHz beacons (Reduced Neighbor Report), or scan only the 15 Preferred Scanning Channels (5, 21, 37 … 229), where APs place their primary channel. APs can also send short FILS discovery frames or unsolicited probe responses every 20 TU to speed up discovery.
• Wide channels are realistic. With 14 × 80 MHz channels, every AP on a floor can have its own 80 MHz channel; 160 MHz is practical in many buildings; 320 MHz (Wi-Fi 7) suits very high-throughput, low-density areas.

Range Versus Capacity

It is tempting to see 2.4 GHz as the “long-range” band and the others as compromises. Modern design inverts that view. A WLAN serving hundreds of clients is limited by airtime, and airtime per client increases only by adding APs on separate channels. Shorter range is a feature: it lets cells be smaller, so more APs can reuse channels without hearing each other. That is why 5 and 6 GHz carry almost all enterprise traffic, and why 2.4 GHz power is usually set several dB below 5 GHz power — to keep its cells similar in size rather than letting it shout across the building.

Channel Planning Guidance

• Let the WLAN’s radio resource management assign channels and power, but restrict its choices (allowed channels and widths) to what suits the site.
• Never use 40 MHz in 2.4 GHz, and never use channels other than 1, 6 and 11 in a three-channel region.
• Prefer more channels over wider channels when the network is dense: four APs on separate 40 MHz channels beat four APs sharing one 160 MHz channel.
• Include DFS channels unless radar or client support proves otherwise.
• Set the correct country code on every AP; a wrong code can be illegal and can strand clients that do not support the resulting channels.
• Plan 6 GHz APs at roughly the same density as 5 GHz, since LPI power and the extra path loss make 6 GHz cells slightly smaller.

Common Mistakes

• Using channels 2–5, 7–10 in 2.4 GHz “to avoid the neighbours” — it creates adjacent-channel interference with two clean channels.
• Turning off DFS globally because of one radar event, leaving too few channels for a clean plan.
• 80 MHz everywhere in 5 GHz on a dense floor, producing six channels for dozens of APs.
• WPA2-only SSIDs on 6 GHz radios, which then broadcast nothing on 6 GHz.
• Assuming every client supports UNII-4, DFS or 6 GHz without checking.
• Maximum transmit power to “improve coverage” — creating oversized cells, co-channel contention and sticky clients.

Exercises

1. Calculate the centre frequencies of 2.4 GHz channel 6, 5 GHz channel 100 and 6 GHz channel 37.

2. A neighbour’s AP is on 2.4 GHz channel 6. An engineer puts a new AP on channel 8 “to stay away from it”. Is that a good choice?

3. A US office wants to avoid DFS. How many 20, 40 and 80 MHz channels remain in 5 GHz (ignoring UNII-4)?

4. An AP on channel 112 logs a radar detection. What happens to the AP and its clients, and when can channel 112 be used again?

5. What maximum EIRP may a US LPI access point use on a 160 MHz channel?

6. Why can a 6 GHz client not simply send probe requests on every channel, as 5 GHz clients do, and how does it find networks instead?

Summary

Wi-Fi uses three unlicensed bands. 2.4 GHz has the best range but only 83.5 MHz and three non-overlapping channels (1, 6, 11), heavy interference and many legacy devices. 5 GHz offers about 25 channels across the UNII-1, 2A, 2C, 3 and (newly) 4 sub-bands; DFS channels require radar detection — a 60-second check, a 10-second move on detection and a 30-minute non-occupancy period — but more than double the usable spectrum. 6 GHz adds 1,200 MHz in the US (480 MHz in Europe) for Wi-Fi 6E and 7 only, with mandatory WPA3/OWE, discovery via RNR and PSCs, and power classes defined by spectral density: LPI indoors, Standard Power with AFC, and VLP for portables. Higher frequencies give smaller cells but far more channels, which is exactly what dense networks need.

Next: RF Power and OFDM covers the maths behind signal strength — dB and dBm, EIRP, antennas, path loss and link budgets — and how OFDM carries data on these channels.