Robust versus fast

A rate for each frame

Wi-Fi has no single speed. Each frame is sent at a rate the sender picks for it. For 802.11n with one stream on a 20 MHz channel there are eight, MCS 0 to 7: BPSK with 1/2 coding at the bottom, 64-QAM with 5/6 coding at the top, 6.5 to 65 Mbit/s, or 7.2 to 72.2 Mbit/s with the short 400 ns guard interval. Denser modulation carries more bits in each symbol and leaves less room for noise to push one symbol into another. Lighter coding spends fewer bits on error correction. Wi-Fi 5 goes up to 256-QAM and Wi-Fi 6 to 1024-QAM; the ESP32-C6 supports 802.11ax MCS 0 to 9, so it stops at 256-QAM (Wi-Fi 6 on the ESP32).

The signal sets the ceiling

The faster the rate, the stronger the signal it needs. The classic ESP32's datasheet gives about -98 dBm for 802.11b at 1 Mbit/s and about -73 dBm for MCS 7, typical figures on the same chip. A link that weakens, with distance or a wall in the way, drops to slower rates.

How a radio chooses is not in the standard. Rate adaptation is each maker's algorithm, and it reacts to retries and errors as well as to signal level: the radio steps down when frames keep failing. Lower rates with distance is a tendency, not a law.

One slow station

Contention gives every station roughly equal chances to send, frame for frame (sharing airtime). A 1500-byte payload takes about 0.17 ms at 72.2 Mbit/s and about 1.67 ms at 7.2. When a fast station and a slow one take turns, the fast one waits through every slow frame, and both get fewer frames a second. Martin Heusse and colleagues in Grenoble described this in 2003 as the performance anomaly: one slow station can slow everyone on the channel. Newer access points can schedule airtime differently.

A link rate is not a download

Headers, gaps, acknowledgements and retries all take airtime, so what an application gets is less than the link rate. The "up to 150 Mbps" in the ESP32-C6 datasheet is the physical rate, 40 MHz with the short guard interval.

Espressif's Long Range mode is its own proprietary mode, not standard Wi-Fi; only ESP devices understand it.

Common mistakes

• Reading the link rate as throughput. It is the speed of the radio link while a frame is on the air. Everything around the frame comes out of it.
• Blaming only the fast devices for a slow network. One distant station sending at MCS 0 holds the air ten times as long per frame as one at MCS 7.
• Expecting a fixed rate at a fixed distance. The rate depends on the signal, the errors and the maker's algorithm, and changes from moment to moment.