Wi-Fi PHY Frames: From 802.11b to Wi-Fi 7

Before any data, every Wi-Fi transmission sends a preamble: fields that let the receiver find the signal, tune in, and learn how the data is sent. Explore each generation's frame, see the part they all still share, and what an older device makes of a newer one.

What's in front of the data

A Wi-Fi transmission at the physical layer is a PPDU (PHY protocol data unit): a preamble followed by the data (the MAC frame). The preamble has three kinds of fields. Training fields (STF, LTF) let the receiver detect the signal, set its gain and measure the channel. Signal fields (SIG) say how the data is sent: its rate, length, bandwidth and more. And every OFDM format since 802.11a starts with the same legacy fields, so that even old devices can tell how long the channel will be busy.

Each new generation added fields for its new features: more streams need more training, and multi-user transmissions need more signalling.

1 · PPDU explorer

Format

Data (PSDU) size

View

DSSS preamble and header Legacy fields (L-STF, L-LTF, L-SIG) Signal fields: how the data is sent Training fields: gain and channel measurement Data Extension / packet extension

2 · Every preamble side by side

Time from the start of the transmission until the data begins, all on one scale (1 stream, 20 MHz, 0.8 µs guard interval; multi-user formats with 4 users). Every OFDM format starts with the same 20 µs of legacy fields. Zoom in to compare the OFDM preambles field by field.

3 · What an older device hears

Pick a receiver. For the format selected above, see which fields it can read, and how it still knows how long to stay quiet.

Receiver

decoded (in the field's own colour)not understood: heard only as energy

4 · PHY parameters by generation

	802.11b	802.11a/g	Wi-Fi 4 (802.11n)	Wi-Fi 5 (802.11ac)	Wi-Fi 6/6E (802.11ax)	Wi-Fi 7 (802.11be)
Published	1999	1999 (a), 2003 (g)	2009	2013	2021	2024
Bands	2.4 GHz	5 GHz (a), 2.4 GHz (g)	2.4 and 5 GHz	5 GHz	2.4, 5 and 6 GHz (6E)	2.4, 5 and 6 GHz
Channel widths	22 MHz	20 MHz	20, 40 MHz	20, 40, 80, 160 MHz	20, 40, 80, 160 MHz	20 to 320 MHz
How the signal is built	Single carrier: DSSS / CCK	OFDM, 64 subcarriers per 20 MHz	OFDM, 64 per 20 MHz	OFDM, 64 per 20 MHz	OFDMA, 256 per 20 MHz	OFDMA, 256 per 20 MHz
Subcarrier spacing	—	312.5 kHz	312.5 kHz	312.5 kHz	78.125 kHz	78.125 kHz
Symbol (+ guard interval)	1 µs (DSSS), 0.727 µs (CCK)	3.2 µs + 0.8	3.2 µs + 0.8 or 0.4	3.2 µs + 0.8 or 0.4	12.8 µs + 0.8, 1.6 or 3.2	12.8 µs + 0.8, 1.6 or 3.2
Highest modulation	DQPSK / CCK	64-QAM	64-QAM	256-QAM	1024-QAM	4096-QAM
Spatial streams	1	1	up to 4	up to 8	up to 8	up to 8
Top PHY rate	11 Mbps	54 Mbps	600 Mbps	6.9 Gbps	9.6 Gbps	23 Gbps
Multi-user	—	—	—	Downlink MU-MIMO	OFDMA and MU-MIMO, down- and uplink	OFDMA (multiple RUs per user) and MU-MIMO
New preamble fields	SYNC, SFD, PLCP header	L-STF, L-LTF, L-SIG	HT-SIG, HT-STF, HT-LTF	VHT-SIG-A, VHT-STF, VHT-LTF, VHT-SIG-B	RL-SIG, HE-SIG-A, HE-SIG-B, HE-STF, HE-LTF, PE	U-SIG, EHT-SIG, EHT-STF, EHT-LTF

Notes and simplifications

Timing: field durations follow the 802.11 standards for 20 MHz-based channels. Data length is SERVICE (16 bits) + data + tail (6 bits), rounded up to whole symbols. Multi-user signal fields (HE-SIG-B, EHT-SIG) are estimated at about one symbol plus one per two users; their real length depends on the resource-unit layout and signalling MCS.
Training fields: the number of long training fields grows with the number of spatial streams (1, 2, 4, 4, 6, 6, 8, 8 for 1–8 streams). Wi-Fi 6 and 7 LTFs here use the 2× size with a 0.8 or 1.6 µs guard interval, or 4× with 3.2 µs.
Packet extension: Wi-Fi 6 can add 0–16 µs after the data (IEEE 802.11-2024, Table 27-13) and Wi-Fi 7 0–20 µs (802.11be-2024, 36.3.14), to give receivers time to process wide, high-order signals. The actual value depends on what the receiver asked for; shown here: 20 µs for Wi-Fi 7 at 320 MHz or with 4096-QAM, 16 µs for 80 MHz and wider at MCS 10 and above, 8 µs otherwise.
Wi-Fi 7 has no separate single-user format: a single-user transmission uses the EHT MU format with one user. Rates for some Wi-Fi 5 combinations are not allowed by the standard (for example MCS 9 at 20 MHz with 1 stream); the tool doesn't block them.