6.1 Frequency Bands, DFS and Transmit Power Control

Independent OpenExamPrep teaching for CWNA-109 objective 2.6 is about where an 802.11 radio is allowed to transmit, not about how OFDM carries bits. A radio that can physically tune a frequency is still illegal to use if the regulatory domain has not authorized that channel, that indoor or outdoor class, or that equivalent isotropically radiated power (EIRP). Harborline Terminals operates a coastal cargo pier beside weather radar and a small airfield. Their WLAN fails this objective in the field when a 5 GHz radio goes quiet after a reboot, when an outdoor bridge carries the wrong country code, or when a 6 GHz indoor AP is bolted to a yard pole.

Regulatory domain versus PHY capability

IEEE 802.11 describes how a PHY sends symbols. National and regional regulators describe whether you may turn the transmitter on. Those two layers are easy to mash together on a test item. Current 802.11 PHYs used in WLANs occupy 2.4 GHz, 5 GHz, and 6 GHz. Not every PHY uses every band: classic 802.11b/g live on 2.4 GHz; 802.11a started the 5 GHz OFDM story; 802.11n (HT) can use 2.4 or 5 GHz; 802.11ac (VHT) is 5 GHz; 802.11ax (HE) can use 2.4 GHz, 5 GHz, and—with Wi-Fi 6E hardware—6 GHz.

OpenExamPrep does not publish an unofficial worldwide channel spreadsheet, and you should not memorize one. Available channels are regulatory-domain specific. Certified firmware, country codes, and local rules decide the legal set. A United States 2.4 GHz example that is valid to teach is channels 1 through 11. Other domains may allow a different 2.4 GHz set. If an item claims "the global 2.4 GHz plan," treat that claim as a trap.

Three bands 802.11 PHYs use

2.4 GHz is an industrial, scientific, and medical (ISM) neighborhood. Microwave ovens, Bluetooth, and analog cameras share it. There is no classic 802.11h DFS radar dance on 2.4 GHz. The scarcity is overlap: 2.4 GHz channel numbers are 5 MHz apart, so 20 MHz OFDM cells overlap unless you pick a non-overlapping set. In a US example, planners commonly treat 1, 6, and 11 as that set. Reuse math belongs with functional channel concepts; this section's job is to remember that the legal channel numbers themselves depend on the domain.

5 GHz is taught as Unlicensed National Information Infrastructure (UNII) slices, not as one flat "5 GHz Wi-Fi band." Conceptually:

• UNII-1 — the low block (about 5.15–5.25 GHz). Classic indoor WLAN channels live here. Outdoor use and power have been implementation- and rule-specific: some histories treated this block as indoor-only; some later rules allow outdoor operation with extra constraints. Do not assume every country copied one US revision.
• UNII-2 — the mid block (about 5.25–5.35 GHz), often nicknamed UNII-2A. Shared with radar. Dynamic Frequency Selection (DFS) is the usual coexistence tool.
• UNII-2e — UNII-2 extended (about 5.47–5.725 GHz), often nicknamed UNII-2C. Also radar-shared in typical teaching; DFS again.
• UNII-3 — the upper block (about 5.725–5.85 GHz in common US teaching). Often the outdoor-bridge favorite because classic DFS is not the headline requirement the way it is in UNII-2 and UNII-2e.

Many plans leave a gap between UNII-2 and UNII-2e. That gap is not a free WLAN playground. Do not invent channels there.

6 GHz is taught as UNII-5, UNII-6, UNII-7, and UNII-8 conceptually. The United States opened a wide 6 GHz range (about 5.925–7.125 GHz). Other regulators may authorize a smaller 6 GHz slice, or none yet. Do not treat UNII-5–8 as a guaranteed identical worldwide map. 6 GHz does not copy 5 GHz DFS; incumbent protection for higher-power operation often uses automated frequency coordination (AFC) and device-class rules instead of a radar listen.

Regulatory power constraints

Regulatory power is not "turn the AP to 11." Limits are written as conducted power at the connector, power spectral density, and EIRP after antenna gain. If Harborline hangs a high-gain dish on a UNII-3 bridge, the installer may have to reduce radio power so EIRP stays legal. A pretty link budget does not legalize an over-limit transmitter.

Rules vary by band and UNII slice, indoor versus outdoor class, access point versus client, and—in some outdoor 5 GHz rules—point-to-point versus point-to-multipoint. Antenna gain above a reference (a common US teaching example is 6 dBi) can force a 1-for-1 power reduction. Memorize the idea, not a fake international watt chart. On an item, match domain + indoor/outdoor + band to a cap, then apply EIRP = conducted power + antenna gain − cable loss.

Transmit Power Control

Transmit Power Control (TPC) arrived with the same spectrum-management work as DFS (IEEE 802.11h, later folded into the consolidated 802.11 standard). TPC lets an AP advertise a local maximum transmit power so associated stations reduce their output when full blast is unnecessary. The goals are coexistence (including satellite and other incumbents in some 5 GHz stories) and staying inside the regulatory envelope. Many UNII-2 and UNII-2e certifications expect DFS and TPC together.

TPC is not the same exam bullet as a vendor's automatic radio-resource power algorithm. Those proprietary "auto-RF" features show up later. TPC is the 802.11 mechanism: country information, a power constraint, and TPC request/report exchanges that tell a station "do not exceed this local max." Harborline's indoor warehouse APs should not run at outdoor-bridge EIRP. Clients should not scream at maximum conducted power when a lower setting still closes the link. Smaller cells reuse channels; oversized cells fight each other and can violate the cap once antenna gain is counted.

Indoor versus outdoor deployments and implementation variants

"Indoor" and "outdoor" are regulatory classes and hardware SKUs, not just whether it is raining.

Indoor variants typically assume a building envelope, integrated or limited antennas, and lower EIRP. Outdoor variants add weatherization, wider temperature ratings, connectorized antennas, and sometimes higher allowed EIRP—or stricter elevation-angle limits so energy does not punch into satellite receivers. Copying an indoor AP into a plastic enclosure does not change its certified class.

6 GHz makes the split louder. Low-power indoor (LPI) operation is the usual indoor 6 GHz class: the AP is expected to live inside, often across a wide UNII-5 through UNII-8 range under indoor rules. Standard-power 6 GHz devices that want more range, including many outdoor or high-ceiling designs, typically must consult AFC so they do not land on licensed microwave paths. Very low power (VLP) is another class used for short-range portable use in some rules. Harborline should not assume a 6 GHz indoor AP on a yard pole is legal just because 5 GHz outdoor APs exist.

Implementation variants you will see in the field:

1. Country code / regulatory domain locked in firmware
2. Indoor AP, outdoor AP, and industrial AP SKUs
3. Dedicated bridge radios versus dual-band access APs
4. Internal versus external antennas (gain and pattern change EIRP)
5. 6 GHz LPI versus standard-power plus AFC

If Harborline ships a US-certified AP to a crane cabin in another country without a matching SKU and domain, channels and power will be wrong even if the SSID looks the same. Clients also carry a certified domain. A visiting laptop does not magically inherit Harborline's US channel list when it roams onto a foreign pier.

Dynamic Frequency Selection

Dynamic Frequency Selection (DFS) is how 5 GHz WLANs share UNII-2 and UNII-2e with radar (weather, military, aviation). Wi-Fi is the secondary user. Radar wins.

Classic WLAN teaching for DFS behavior:

Before the AP may transmit (beacons, probes, data) on a DFS channel, it listens for radar. During CAC the SSID is absent on that radio. Weather-radar occupancy (Terminal Doppler Weather Radar and similar use around the 5.60–5.65 GHz neighborhood) can require the longer 10-minute CAC in rules that protect those radars. Harborline's pier is exactly the geography where that longer listen shows up.

If a radar pulse pattern is recognized while the BSS is live, the AP must stop using that channel, complete a channel move, and treat the tainted channel as non-occupancy for often 30 minutes. If the replacement channel is also DFS, expect another CAC before beacons return.

The exam trap: a DFS outage looks like the WLAN died for a minute. After a power cycle, a firmware reload, or a controller moving an AP onto UNII-2e, Harborline's 5 GHz SSID is silent while CAC runs. Helpdesk tickets say "Wi-Fi is down." The 2.4 GHz radio may still be up. A spectrum analyzer shows no AP energy on the new DFS channel because silence is the compliance behavior, not a PoE failure.

Design implications:

• Voice and location tags in the warehouse: prefer non-DFS 5 GHz (UNII-1 / UNII-3 where legal) if radar hits are common.
• Outdoor capacity: DFS channels are useful spectrum, not forbidden spectrum—use them with eyes open.
• Do not disable DFS in firmware to "make it stable." That is a regulatory violation, not a tuning trick.
• After a radar event, expect 30 minutes before that exact channel is a candidate again.

TPC and DFS together are why 802.11h belongs in the same memory palace: change frequency when radar is there; change power so you do not cook the neighbors.

Harborline walk-through

Harborline's indoor packing floor uses UNII-1 20 MHz channels for voice-capable handhelds. The yard backhaul and the slip point-to-point bridge sit on UNII-3 where classic DFS is not the first failure mode. A visiting engineer enables UNII-2e for more 80 MHz width, reboots, and watches 5 GHz vanish for a minute. That is CAC, not a dead switch. Near the airfield, a later radar hit dumps clients and starts a 30-minute non-occupancy. Weather-radar channels with a 10-minute CAC never make the production template. Indoor 6 GHz stays inside the building on LPI-class APs. Outdoor 6 GHz, if used at all, is a different implementation variant with AFC—not a copy-paste of the indoor channel plan.