11 Wi-Fi Bands: Channels, DFS, and 6 GHz

11.5 Three Bands, Many Channels, Several Widths

This section: Three Bands, Many Channels, Several Widths — Wi-Fi operates across three license-exempt bands where local rules permit, and each behaves differently for IoT.

Wi-Fi operates across three license-exempt bands where local rules permit, and each behaves differently for IoT. 2.4 GHz generally reaches farthest and penetrates walls best but is crowded; channels 1, 6, and 11 are the usual non-overlapping 20 MHz planning set. 5 GHz has more channels and generally shorter reach. 6 GHz (Wi-Fi 6E/7) offers up to 1200 MHz where the full band is allowed, with wide-channel room for compatible clients and generally shorter reach than lower bands.

Before choosing a channel width, inspect Figure 11.1 to ask how a wider peak rate changes reuse and exposure to other radio use.

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Read Figure 11.1 as evidence that a band is not a speed rank. In the worked strip, 80 MHz / 20 MHz = 4, so one 80 MHz channel consumes the space of four 20 MHz channels and leaves about one quarter as many independent choices. A 2.4 GHz plan may fit fixed sensors because the radios are common and the link reaches farther. A 5 GHz plan offers more reuse, but its mounts and DFS repair need proof. A 6 GHz plan gives clean space only to clients and sites that support it.

Channel width is the second half of the decision. A narrow 20 MHz channel often gives a dense IoT site more predictable reuse than a wide channel that looks faster in a single-link test. Wide channels belong in the approval only when the traffic needs the throughput, the AP layout can still reuse spectrum, and the support team knows which device classes are excluded. The approval record should name the band, width, client group, AP profile, and retest trigger together.

11.6 The 5 GHz UNII Sub-Bands and Channel Count

This section: The 5 GHz UNII Sub-Bands and Channel Count — The 5 GHz band is organised into UNII sub-bands, each with its own rules.

The 5 GHz band is organised into UNII sub-bands, each with its own rules. Knowing them explains why an AP offers the channels it does:

Channel width consumes these channels fast. A 20 MHz plan yields many non-overlapping channels for reuse; an 80 MHz plan bonds four 20 MHz channels into one, so the same spectrum yields only a quarter as many independent channels. In a dense site that means far less reuse and more co-channel contention.

Worked example. In a plan allowing only UNII-1 and UNII-3 non-DFS channels, an installer sets every AP to 80 MHz and finds two non-overlapping 80 MHz blocks: 36–48 and 149–161. Channel 165 does not form another 80 MHz block. Dropping to 40 MHz gives four blocks in the same spectrum: 2 × 2 = 4, doubling the independent choices. Adjacent APs still need a surveyed reuse plan; a channel change alone does not guarantee interference-free service. Choose width for channel reuse in dense areas, and reserve 80/160 MHz for sparse, high-throughput links.

For IoT reviews, turn that trade-off into a channel evidence record. Start with the actual traffic class: low-rate sensors, cameras, support phones, gateways, and moving tools should not automatically share one width. Then record the channel list available under local rules, which channels the AP profile is allowed to use, which channels neighbouring APs already occupy, and whether a DFS event would interrupt service. The best width is the narrowest one that meets the service objective while leaving enough clean reuse for the installed cell pattern.

A useful field test has two views. The spectrum view shows neighbouring Wi-Fi, partial overlap, non-Wi-Fi interference, and channel changes over time. The service view shows association, retries, missing telemetry, video stalls, command latency, and update success for the client class. If the spectrum view looks clean but the service view fails, the record should keep investigating AP load, roaming, firmware, and backhaul instead of declaring the band solved.

11.7 DFS Mechanics and the 6 GHz Power Classes

This section: DFS Mechanics and the 6 GHz Power Classes — The 6 GHz band manages incumbent coexistence without 5 GHz DFS.

The DFS-governed UNII-2 channels (52–144) are shared with radar, so an AP that wants them must obey a strict protocol: perform a channel-availability check (about 60 seconds of listening, longer on some weather-radar channels) before transmitting, then keep monitoring; on detecting a radar pattern it must stop transmitting on that channel within seconds and move. This unlocks many extra channels but risks a mid-service channel change.

The 6 GHz band manages incumbent coexistence without 5 GHz DFS. In the US, Low-Power Indoor (LPI) access points may operate indoors at reduced power without AFC; Standard-Power access points, indoors or outdoors, use an Automated Frequency Coordination (AFC) system to obtain permitted frequencies and power for their location. Other markets can authorize different portions of the band and power classes, so check the local rules.

Worked example. A campus wants dependable 5 GHz channels for outdoor cameras. UNII-2C offers many channels but they are DFS, so a passing weather radar could force a camera’s AP to vacate mid-stream. The planner reserves non-DFS UNII-1/UNII-3 channels for the cameras’ critical links and uses DFS channels for tolerant bulk traffic — a design driven directly by the band’s regulatory mechanics.

The hidden implementation detail is that a channel move is not just a radio event. Clients must hear the change, reassociate or follow the AP, refresh security state if needed, and rebuild the application path before the service timeout expires. That is why a DFS review should include recovery evidence, not only a list of allowed channels. For a moving tool or camera stream, record whether the application buffered through the move, whether telemetry was delayed or lost, and whether the support team can see the event in logs.

For 6 GHz, the equivalent review is compatibility and operating scope. LPI can be attractive indoors, but old clients, setup phones, gateways, and battery sensors may not support the band. Standard-power operation adds a location-aware coordination dependency. A bounded approval therefore names the client hardware, AP profile, indoor or outdoor scope, fallback SSID, and retest trigger for region, antenna, firmware, or mounting changes. Without those limits, a clean spectrum label can turn into an operations problem.