Pete guidesA field team faces an unresolved physical question: Where does the exact 54 Mbps number come from? They must answer it before changing guard interval on the real device. Predict the direction first.
See the relationship before changing it
The figure reads from left to right. The blue card is guard interval. The middle card applies this page's relationship. The green card is carrier spacing. 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 added model holds every other chapter fixture fixed, so the numeric fixture does not switch without explanation.
Derive the baseline in four named moves
- 1
Name the input. The chapter baseline for guard interval is 0.8.
- 2
Name the relationship. R=(6x3/4x48)/(3.2+0.8) us=54 Mbps
- 3
Substitute the chapter fixture. Set guard interval to 0.8. The page ledger gives carrier spacing as 312.5 kHz.
- 4
Read the result. Keep kHz beside the value. Use it only inside the technical boundary on this page.
Predict, then change guard interval
Try Predict the direction of carrier spacing. Move one control, calculate, then check your prediction.
Observe A longer guard tolerates more delay spread but spends more airtime without payload, so the same bits produce a lower raw rate. Reset the control to 0.8 and compare carrier spacing.
Explain Only guard interval moves here. The other chapter fixtures remain fixed.
Check yourself
What should you do before trusting a moved-control result?
What does this small model leave out?
1. OFDM trades one fast stream for many slow ones
Echoes smear a radio symbol in time. Parallel subcarriers make each useful symbol much longer, while a guard interval absorbs bounded delay spread before the next useful symbol begins.
2. Name every algebra move
Space the carriersΔf=20 MHz/64=312.5 kHz.
Invert the spacingTs=1/Δf=3.2 µs.
Count and divideR=(bits/carrier × code rate × 48)/(Ts+Tguard).
3. The 54 Mbps line is bookkeeping
QPSK 1/2 uses the same 48 data carriers and 4 µs total symbol, but carries 2×1/2×48=48 bits, so its raw rate is 12 Mbps.
4. Try one controlled change
TryChange only the guard interval. The useful symbol and payload bits stay fixed.
ObserveAt 0.8 µs, total symbol time is 4.0 µs, the echo-path budget is 240 m, and the raw rates are exactly 54 and 12 Mbps.
ExplainA longer guard tolerates more delay spread but spends more airtime without payload, so the same bits produce a lower raw rate.
This is legacy 20 MHz 802.11g PHY arithmetic, not an application-throughput prediction.
- MAC headers, contention, and acknowledgements
- Airtime overheads
- retries and aggregation
- Traffic-dependent effects
- channel width and spatial streams
- PHY configuration
- interference and implementation choices
- Deployment effects
Those terms determine actual throughput and need measured evidence.
5. Reproduce the chapter values
20 MHz/64=312.5 kHz and 1/312.5 kHz=3.2 µs. Add 0.8 µs to get 4.0 µs. Then 6×3/4×48=216 bits and 216/4 µs=54 Mbps; QPSK 1/2 gives 48/4 µs=12 Mbps.
6. Carry the evidence forward
Record channel width, guard interval, modulation/coding, spatial streams, RSSI/SNR, retries, airtime, channel occupancy, payload goodput, and latency distribution. Do not compare an advertised PHY rate with application bytes without the overhead ledger.
7. Check yourself
Why is useful symbol time 3.2 µs?
What does the 0.8 µs guard buy?
Is 54 Mbps application throughput?
These are the worked values and named assumptions for this bridge.
- 20 MHz
- Channel width
- 64
- Subcarrier count
- 312.5 kHz
- Carrier spacing
- 3.2 µs
- Useful symbol time
- 0.8 µs
- Guard interval
- 4.0 µs
- Total symbol time
- 216
- Coded bits per symbol
- 54 Mbps
- Raw 64-QAM rate
- 12 Mbps
- Raw QPSK rate
- 240 m
- Echo-path budget
This is legacy 20 MHz 802.11g PHY arithmetic. MAC headers, contention, acknowledgements, retries, aggregation, channel width, spatial streams, interference, and implementation choices determine actual throughput.