5 GHz Wi-Fi: wide channels and DFS

Many channels, wide channels, and a set of rules (DFS) that sets it apart. This is the band where most modern Wi-Fi performance is won or lost.

1. DefinitionWhat is the 5 GHz band?

5 GHz Wi-Fi uses several separate blocks of spectrum between roughly 5150 and 5850 MHz. It is not one tidy band: each block has its own rules. The UK calls them Bands A, B and C; the US calls them U-NII-1, 2A, 2C and 3. Both end up with the same channel numbers, but the rules around those channels differ.

2. PurposeWhy move to 5 GHz?

• Room. Hundreds of MHz of spectrum, so many non-overlapping channels and room for 40, 80 and 160 MHz widths.
• Less crowding. Few non-Wi-Fi devices use it, so interference is mostly from other Wi-Fi.
• Smaller cells. At about 5.5 cm the waves are absorbed and blocked by walls, which limits how far a signal travels. That limits range, but it also limits how far your neighbours’ signals reach you, and small cells are a feature in dense deployments.

2b. Rules firstWhy 5 GHz has radar rules

The 5 GHz blocks are shared with other services, including weather and military radar and some satellite links. To protect them, regulators require Dynamic Frequency Selection (DFS): the access point must listen before using a channel, and leave if it detects radar. Transmit Power Control (TPC) keeps power no higher than needed. The standards support came from IEEE 802.11h (approved in 2003). In the UK, DFS applies in 5250 to 5350 MHz and 5470 to 5730 MHz, and channels 52 to 64 (5250 to 5350 MHz) are indoor-only. Channels 36 to 48 (5150 to 5250 MHz) were liberalised in 2023: they may be used outdoors, but not as part of a fixed outdoor installation.

3. MechanismHow the channels work

Channel number to frequency is centre = 5000 + 5 x channel MHz. The 20 MHz channels are four numbers apart (36, 40, 44, 48) so they do not overlap. Wider channels bond adjacent 20 MHz channels: 40 MHz takes two, 80 MHz takes four and 160 MHz takes eight. The primary 20 MHz channel is where beacons live and where devices contend for the medium.

4. VisualWavelength and reach

5. MathematicsPath loss and PHY rate

At 10 m the free-space loss at 5.5 GHz (channel 100) is 20 + 74.8 – 27.55 = 67.3 dB, and every doubling of distance adds another 6 dB. In practice walls add loss on top, so coverage per access point is limited. What 5 GHz pays back is rate: wider channels have more data subcarriers, so rate scales roughly with width.

Try 802.11ac at 80 MHz, MCS 9, one stream, short guard interval: 234 x 8 x 5/6 / 3.6 = 433.3 Mbps. Now take it to 160 MHz and four streams.

6. Techniques over timeFrom 802.11a to Wi-Fi 7

• OFDM (1999). The first Wi-Fi OFDM: 48 data subcarriers, 64-QAM, 54 Mbps in 20 MHz.
• DFS and TPC (2003). Regulatory plumbing added by 802.11h.
• MIMO and bonding (2009). 40 MHz channels and up to four streams.
• VHT (2013). 80 and 160 MHz channels, 256-QAM, downlink multi-user MIMO.
• HE and EHT (2021, 2024). OFDMA, 1024-QAM then 4096-QAM, and multi-link operation across bands.

7. Rules and regulationsUK Ofcom first, then the EU and the US

This is where 5 GHz is most confusing, because the UK and the US use the same channel numbers with different rules. Start with the UK, then switch to the EU and the US.

8. Real-world behaviourWhat goes wrong in 5 GHz

• DFS events. When radar is detected, the AP must leave the channel and stay off it for a period (at least 30 minutes under the FCC rule). Clients see a sudden disconnect, which can look like a random fault. Before using a DFS channel the AP must also listen first (the channel availability check; the FCC rule is 60 seconds). That is why an AP can take a minute or more to start up on some channels. The FCC rule also gives an AP 10 seconds to stop transmitting after radar is detected.
• Client support. Some older or low-cost client radios do not support DFS channels, so a network on channel 100 can look invisible to them.
• Wide-channel fragility. A 160 MHz channel uses up the whole main plan and needs DFS channels, so it fails (or falls back to 80 MHz) whenever radar is seen or neighbours overlap.
• Range. Walls absorb 5 GHz signals strongly, so coverage per access point is limited, and a client far from its AP drops to a low MCS quickly.

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:

• Which channel and width is the AP really using? Look at the beacon’s HT/VHT/HE Operation elements, not just the settings page.
• Channel by frequency: wlan_radio.frequency == 5500 or by number: wlan_radio.channel == 100
• Retries: wlan.fc.retry == 1, compared at 20, 40 and 80 MHz on the same link.
• Reduced Neighbor Report elements, with which an AP advertises its other bands: wlan.tag.number == 201
• DFS evidence: look for a channel switch announcement (wlan.tag.number == 37) in beacons after a radar event.
• Question to answer: at what distance and through how many walls does a 5 GHz client drop from its top MCS to its lowest usable one?

Check your understandingQuick quiz

ReferencesSources and further reading

• IEEE Std 802.11-2020 (IEEE Xplore)
• IEEE 802.11 Working Group: standards timelines (approval and publication dates)
• IEEE 802.11 Working Group
• Ofcom: UK Interface Requirement 2030 (IR 2030), current edition
• ETSI EN 301 893 (5 GHz RLAN harmonised standard)
• eCFR: 47 CFR 15.407 (US 5 GHz U-NII rules)
• Wi-Fi Alliance: history

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.