Why Channels 15, 20, 25, and 26 Land in the Gaps

Why Channels 15, 20, 25, and 26 Land in the Gaps

Ada re-derives this chapter’s own numbers step by step, at full precision

ADA · CALCULATION AUDIT

Why Channels 15, 20, 25, and 26 Land in the Gaps

A Zigbee network on channel 20 runs fine until an access point is retuned near Wi-Fi channel 11, retries climb, and the fix is to move the PAN to channel 25 rather than turn up power. That works because an 802.15.4 channel sits at 2405 + 5 x (k - 11) MHz, so channels 15, 20, 25, and 26 each land in a ~3 MHz seam between the Wi-Fi blocks, while a 1 mW radio faces a 20-to-30 dB deficit against a 100 mW-to-1 W access point. This audit computes every centre frequency and asks whether shifting a few megahertz into a gap really beats shouting, or is only a shortlist entry until on-site packet loss confirms it.

Companion to the chapter 802.15.4 Coexistence — every number here comes from that chapter.

See the relationship before changing it

The figure reads from left to right. The blue card is 802.15.4 channel. The middle card applies this page's rule. The green card is centre frequency. Walk the arrows once: set the input, apply the rule, then read the result with its unit.

The retained audit below checks several chapter fixtures. This model keeps those stated values fixed and changes only 802.15.4 channel, so the numeric fixture does not switch without explanation.

Walk the arrows. Channel number moves the signal in five-megahertz steps; measured interference still decides whether a gap is usable.

Derive the baseline in four named moves

1. 1

   Name the input. The chapter baseline is 25 channel.

2. 2

   Name the relationship. centre frequency = 2,405 + 5 x (channel - 11)

3. 3

   Substitute with units. 2,405 + 5 x (25 - 11) = 2,475 MHz

4. 4

   Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.

Predict, then change 802.15.4 channel

Try Predict the direction of centre frequency = 2,405 + 5 x (channel - 11). Test another 802.15.4 channel, then compare centre frequency.

802.15.4 channel

25 channel

Chapter baseline

Centre frequency

Observe Channel number moves the signal in five-megahertz steps; measured interference still decides whether a gap is usable. Reset 802.15.4 channel to 25 and compare centre frequency.

Explain Channel number moves the signal in five-megahertz steps; measured interference still decides whether a gap is usable.

Check yourself

What should you do before trusting a moved-control result?

Answer: Predict its direction, apply the shown relationship, keep the units, and reset to the worked baseline.

What does this small model leave out?

Answer: Only 802.15.4 channel moves here. Field effects named in the technical boundary stay fixed.

Honesty note. This teaching model changes only 802.15.4 channel. It is not a field prediction.

Ada: The chapter names channels 15, 20, 25, and 26 as the classic candidate set and puts Wi-Fi 1/6/11 at 2412, 2437, and 2462 MHz. Those are not arbitrary picks - let me compute the centre frequencies and show each candidate lands in a Wi-Fi gap.

• An 802.15.4 2.4 GHz channel k sits at 2405 + 5 x (k - 11) MHz: channel 15 is 2405 + 5 x 4 = 2425 MHz, channel 20 is 2405 + 5 x 9 = 2450 MHz, channel 25 is 2405 + 5 x 14 = 2475 MHz, and channel 26 is 2480 MHz.
• A 20 MHz Wi-Fi channel spans 20 / 5 = 4 of the 5-MHz-spaced 802.15.4 channels, so Wi-Fi 1 (2401-2423) blankets 802.15.4 channels 11-14, Wi-Fi 6 (2426-2448) covers 16-19, and Wi-Fi 11 (2451-2473) covers 21-24 - matching the chapter’s table.
• Channel 15 at 2425 MHz falls in the 2423 to 2426 gap, channel 20 at 2450 MHz in the 2448 to 2451 gap, and channels 25 and 26 sit above 2473 MHz. Every candidate lands in a ~3 MHz seam between the Wi-Fi blocks.

The power arithmetic explains why dodging beats shouting: 10 x log10(100 / 1) = 20 dB and 10 x log10(1000 / 1) = 30 dB, so a 1 mW radio next to a 100 mW-to-1 W access point runs a 20-to-30 dB deficit. When you cannot add 100-to-1000x of power, shifting a few megahertz into a spectral gap is the only lever left - but the seams are only ~3 MHz wide, so the candidate is a shortlist entry until on-site packet loss confirms it.

Every number above is taken from the chapter’s own material and re-derived step by step.

TryCalculate the centre frequencies for channels 15, 20, 25, and 26.

ObservePlace those centres beside the stated 20 MHz Wi-Fi blocks and compare the 20-to-30 dB power deficit.

ExplainMoving into a spectral seam can help more than transmit power, but only a site survey proves it.

Technical boundaries. The arithmetic treats Wi-Fi blocks and 802.15.4 centres as fixed ideal spans. It does not model spectral masks, adjacent-channel rejection, access-point bandwidth, antenna placement, fading, traffic duty, channel 26 restrictions, or measured packet loss.

Audit result

The candidates land at 2425, 2450, 2475, and 2480 MHz; the narrow gaps are candidates, not guaranteed interference-free channels.