Wi-Fi Airtime, SNR, and Modulation Margin

Wi-Fi Airtime, SNR, and Modulation Margin

Ada re-checks the Wi-Fi burst airtime, signal-to-noise headroom, and modulation margin from the chapter’s own link budget

foundations
math-foundations
wireless
wifi
intermediate
Ada ADA · CALCULATION AUDIT

Wi-Fi Airtime, SNR, and Modulation Margin

The chapter already gives enough numbers to check the design without inventing measurements. The useful question is not whether Wi-Fi can move the payload in theory; it is whether the selected modulation still has margin when interference appears — the chapter’s link budget lands a worst-case 29 dB SNR against a 25 dB requirement for 64-QAM, leaving a 4 dB cushion. This audit re-checks the airtime, SNR, and modulation margin from those figures.

Companion to the chapter Wireless Access: Wi-Fi — every number here comes from that chapter.

See the relationship before changing it

The figure reads from left to right. The blue card is interference allowance. The middle card applies the page rule. The green card is 64-qam margin. Walk the arrows once: set the input, apply the rule, then read the result with its unit.

Interference allowance changes 64-qam margin An input card leads through the rule margin = 39 dB clean SNR - interference - 25 dB requirement to the 64-qam margin result. INPUT PAGE INPUT APPLY THE RULE predict calculate check units OUTPUT RESULT
Walk the arrows. Interference spends modulation headroom even when the free-space link looks strong.

Derive the baseline in four named moves

  1. 1

    Name the input. The chapter baseline is 10 dB.

  2. 2

    Name the relationship. margin = 39 dB clean SNR - interference - 25 dB requirement

  3. 3

    Substitute with units. 39 - 10 - 25 = 4.00 dB

  4. 4

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

Predict, then change interference allowance

Try Predict the direction of margin = 39 dB clean SNR - interference - 25 dB requirement. Test another interference allowance, then compare 64-qam margin.

10 dB
Chapter baseline
64-QAM margin

Observe Interference spends modulation headroom even when the free-space link looks strong. Reset interference allowance to 10 and compare 64-qam margin.

Explain Interference spends modulation headroom even when the free-space link looks strong.

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 interference allowance moves here. Field effects named in the technical boundary stay fixed.

Honesty note. This teaching model changes only interference allowance. It is not a field prediction.

TryTreat the chapter's burst payload and 25 m, 2.4 GHz link with its 64-QAM requirement as the fixed audit case; press Calculate.
ObserveWorst-case SNR lands at 29 dB and the modulation-margin result shows only 4 dB above the 25 dB requirement.
ExplainPath loss and interference subtract from received signal headroom; a modulation remains viable only when the resulting SNR exceeds its decoding threshold.

Ready: use the stated baseline inputs, then compare each displayed result.

The airtime and margin checks

Check Arithmetic Result
Burst payload 10 KB x 1024 x 8 81,920 bits
Average report rate 81,920 bits / 30 s 2,730.7 bps = 2.73 kbps
Free-space path loss (25 m, 2.4 GHz) 32.45 + 20log10(25) + 20log10(2.4) 68.0 dB
Received power 20 dBm - 68 dB - 8 dB -56 dBm
Clean SNR -56 dBm - (-95 dBm) 39 dB
Worst-case SNR 39 dB - 10 dB 29 dB
64-QAM margin 29 dB - 25 dB 4 dB
1 Mbps airtime 81,920 / 1,000,000 81.92 ms
54 Mbps airtime 81,920 / 54,000,000 1.52 ms
72.2 Mbps airtime 81,920 / 72,200,000 1.13 ms

Audit conclusion: selecting 64-QAM 3/4 is the conservative answer because the worst-case SNR still keeps a 4 dB cushion above the 25 dB requirement. The faster 256-QAM option would need the whole 32 dB worst-case budget and falls 3 dB short of it, guaranteeing link failures for people, doors, multipath, or another busy neighbor, not just risking them.

CORRECTED: this audit previously computed the free-space path loss at 25 m as 60 dB, which silently drops the chapter’s own 20log10(f_GHz) frequency term (about 8 dB at 2.4 GHz) from its stated FSPL formula. Using that formula in full gives 68 dB, cascading to -56 dBm received power, 39 dB clean SNR, and (with the same 10 dB interference budget the chapter uses elsewhere) a 29 dB worst-case SNR and 4 dB 64-QAM margin, not 7 dB. The table and conclusion above now match the chapter’s own corrected derivation.

Every number above is taken from the chapter’s own Wi-Fi link-budget example and re-derived step by step.

Technical boundaries. CSMA contention, multipath evolution, rate adaptation, adjacent-channel traffic, packet retries, and antenna orientation are not simulated by this fixed Wi-Fi ledger.