802.11 PROTOCOL INTELLIGENCE

PPDU Formats - PHY Preamble Reference

A PPDU (Physical Protocol Data Unit) is the complete unit delivered to the RF medium. Every PPDU begins with a preamble - fields that grow more complex with each Wi-Fi generation to carry MIMO parameters, OFDMA allocations, and multi-user information. The legacy preamble (L-STF + L-LTF + L-SIG) is present in every format from 802.11a through Wi-Fi 7.

IEEE 802.11-2020 802.11be EHT PHY Layer CWNA / CWAP Domain

The preamble is the part of every Wi-Fi frame you never see in Wireshark's data columns. It is also what every device on the channel reads to decide whether to defer - which makes HT-Greenfield's removal of the legacy preamble the most consequential omission in 802.11n deployment history.

PSDU → PPDU - MAC to PHY

MSDU
MAC Service Data Unit
IP packet from upper layer. The actual user data.
MPDU
MAC Protocol Data Unit
MSDU + MAC header + FCS. What the 802.11 MAC manages.
A-MPDU / PSDU
Aggregated MPDU / PHY SDU
Multiple MPDUs packed. Handed from MAC to PHY as PSDU.
PPDU
PHY Protocol Data Unit
PSDU + PHY preamble + PHY header. What goes on air.
KEY FUNCTION OF THE PREAMBLE

The preamble exists entirely to help the receiver. It provides: (1) AGC calibration - the STF lets the receiver set its gain before the useful signal arrives, (2) timing and frequency synchronisation via the LTF, (3) channel estimation for equalisation, (4) signalling the modulation and coding scheme, bandwidth, and MIMO configuration in the SIG fields. Without the preamble, the PSDU payload cannot be demodulated.

802.11a/g

Non-HT (Legacy)

Backward compatibility anchor - present in every PPDU format since 802.11a
Preamble overhead
20 µs
L-STF 8 µs
L-LTF 8 µs
L-SIG 4 µs
DATA variable
L-STF 8 µs Legacy Short Training Field - AGC calibration, timing sync, frequency offset estimation
L-LTF 8 µs Legacy Long Training Field - channel estimation, fine frequency/time sync
L-SIG 4 µs Rate (4 bits), Reserved, Length (12 bits), Parity, Tail. BPSK, R=1/2. All STAs read this to set NAV.
DATA variable PSDU payload + SERVICE field (16b) + tail (6b) + pad bits
KEY NOTE

All STAs (n/ac/ax/be) must decode L-SIG to set NAV. L-SIG length field encodes TXOP duration. Non-HT preamble = 20 µs.

802.11n

HT-Mixed (HT-MF)

Mandatory backward compat - non-HT STAs coexist via L-SIG NAV
Preamble overhead
32 µs + 4N
L-STF 8 µs
L-LTF 8 µs
L-SIG 4 µs
HT-SIG 8 µs
HT-STF 4 µs
HT-LTF×N 4N µs
DATA variable
L-STF 8 µs Legacy compat - AGC, timing sync
L-LTF 8 µs Legacy compat - channel estimate for non-HT STAs
L-SIG 4 µs NAV protection for non-HT STAs. Length set so L-SIG duration covers full PPDU.
HT-SIG 8 µs 2 OFDM symbols. MCS (7b), BW, HT-Length, Smoothing, Not Sounding, Aggregation, STBC, FEC, Short-GI, Num-ESS, Pre-FEC-Pad, Tx-BF-LTF, ASEL, CRC, Tail.
HT-STF 4 µs HT Short Training Field - AGC restart for MIMO receiver
HT-LTF×N 4N µs One LTF per spatial stream (N = Nsts). MIMO channel matrix estimation. 40 MHz: 4 µs each.
DATA variable PSDU + SERVICE + tail + pad bits
KEY NOTE

Most widely deployed format. Legacy STAs decode L-SIG and wait. HT STAs decode HT-SIG for MIMO parameters. Short GI reduces symbol period from 800ns to 400ns (+11% throughput).

802.11n

HT-Greenfield (HT-GF)

Prohibited in mixed b/g/n networks - causes collisions with non-HT STAs
Preamble overhead
24 µs + 4N
HT-GF-STF 8 µs
HT-LTF1 8 µs
HT-SIG 8 µs
HT-LTF×(N-1) 4(N-1) µs
DATA variable
HT-GF-STF 8 µs HT Greenfield STF - AGC + timing. No legacy preamble - non-HT STAs cannot decode.
HT-LTF1 8 µs First HT-LTF - extended for channel estimation. 8 µs (twice normal) for first LTF only.
HT-SIG 8 µs Same as HT-MF HT-SIG
HT-LTF×(N-1) 4(N-1) µs Remaining spatial stream LTFs
DATA variable PSDU + SERVICE + tail + pad
KEY NOTE

Greenfield drops legacy preamble entirely - slightly lower overhead. BUT: non-HT STAs cannot detect it at all, causing hidden-node collisions. Prohibited in mixed networks by IEEE 802.11-2020. Rarely used in production.

802.11ac

VHT

Mandatory A-MPDU - all VHT frames wrapped in A-MPDU even if single MPDU
Preamble overhead
36 µs + 4N
L-STF 8 µs
L-LTF 8 µs
L-SIG 4 µs
VHT-SIG-A 8 µs
VHT-STF 4 µs
VHT-LTF×N 4N µs
VHT-SIG-B 4 µs
DATA variable
L-STF 8 µs Legacy compat
L-LTF 8 µs Legacy compat
L-SIG 4 µs NAV for legacy STAs. L-SIG length encodes TXOP duration.
VHT-SIG-A 8 µs 2 symbols. BW (2b), STBC, Group ID (6b), Nsts (3b per user), Partial AID, TX-BF, RSSI, MCS, Coding, Short-GI, LDPC extra symbol, CRC, Tail.
VHT-STF 4 µs AGC for MIMO streams in wider channels (80/160 MHz)
VHT-LTF×N 4N µs N = Nsts (1–8 streams). MIMO channel matrix.
VHT-SIG-B 4 µs Per-user allocation for MU-MIMO DL. SU: encodes LENGTH. MU: per-user MCS + Nss + CB (coding+BF).
DATA variable PSDU
KEY NOTE

VHT-SIG-B enables MU-MIMO downlink to 4 users simultaneously. Group ID field (6 bits) identifies which users are in this MU-MIMO transmission. SU transmissions use Group ID 0 or 63.

802.11ax

HE-SU

Spatial Reuse via BSS Color - first standard to enable true OBSS coexistence
Preamble overhead
44 µs + variable
L-STF 8 µs
L-LTF 8 µs
L-SIG 4 µs
RL-SIG 4 µs
HE-SIG-A 8 µs
HE-STF 4 µs
HE-LTF×N 4-32 µs
DATA variable
L-STF 8 µs Legacy compat
L-LTF 8 µs Legacy compat
L-SIG 4 µs NAV for legacy. L-SIG length mod 3 ≠ 0 distinguishes HE from EHT.
RL-SIG 4 µs Repeated L-SIG (inverted). Identifies HE PPDU to receivers. Repeated = HE or EHT.
HE-SIG-A 8 µs 2 symbols. Format (2b), BSS Color (6b), Spatial Reuse, TXOP, MCS, DCM, Coding, STBC, LDPC, Doppler, Nsts, PE Disambiguity.
HE-STF 4 µs HE AGC for OFDMA resource units
HE-LTF×N 4-32 µs 1×/2×/4× LTF modes. Nsts streams. 4× LTF = 16 µs each for better MIMO channel tracking.
DATA variable PSDU + PE (Packet Extension) field for OFDMA timing
KEY NOTE

BSS Color (6-bit) in HE-SIG-A enables Spatial Reuse - STAs from different BSSs can transmit simultaneously when OBSS_PD threshold allows. RL-SIG inverts L-SIG bits to confirm HE PPDU type.

802.11be (Wi-Fi 7)

EHT-MU

U-SIG = universal header for all future 802.11 versions. MLO operates A-MPDU per link.
Preamble overhead
52 µs + variable
L-STF 8 µs
L-LTF 8 µs
L-SIG 4 µs
RL-SIG 4 µs
U-SIG 8 µs
EHT-SIG variable
EHT-STF 4 µs
EHT-LTF×N 4N µs
DATA variable
L-STF 8 µs Legacy compat
L-LTF 8 µs Legacy compat
L-SIG 4 µs NAV for legacy STAs. L-SIG length mod 3 = 0 distinguishes EHT from HE.
RL-SIG 4 µs Repeated L-SIG. Together with L-SIG identifies EHT PPDU.
U-SIG 8 µs Universal Signal (new in 802.11be). 2 symbols. PHY version (3b), BW, UL/DL, BSS Color, TXOP, PPDU type (SU/MU/TB/EXT-SU), Puncture pattern, EHT-SIG-MCS.
EHT-SIG variable EHT-specific user allocations. RU/MRU allocations, per-user MCS, Nss, Coding. Only in EHT-MU, not EHT-TB.
EHT-STF 4 µs AGC for EHT MIMO/OFDMA streams
EHT-LTF×N 4N µs N = Nsts (1–16 streams in Wi-Fi 7). Supports MLO per-link channel estimation.
DATA variable PSDU + PE field
KEY NOTE

U-SIG replaces the per-generation SIG-A field with a universal header that supports version negotiation. Preamble puncturing pattern encoded in U-SIG - enables 320 MHz operation with selective subchannel usage. EHT-TB drops EHT-SIG; EHT-STF is doubled (8 µs) for UL timing.

Quick Comparison

FormatStandardPreamble OHFieldsKey SIG fieldBackward compat
Non-HT 802.11a/g 20 µs 4 L-SIG (Rate+Length) All STAs
HT-Mixed 802.11n 32 µs + 4N 7 HT-SIG (MCS, BW, STBC) Legacy STAs read L-SIG
HT-Greenfield 802.11n 24 µs + 4N 5 HT-SIG NONE - HT-only networks only
VHT 802.11ac 36 µs + 4N 8 VHT-SIG-A + VHT-SIG-B Legacy STAs read L-SIG
HE-SU 802.11ax 44 µs + var 8 HE-SIG-A (BSS Color, SR) RL-SIG marks HE; legacy NAV via L-SIG
EHT-MU 802.11be 52 µs + var 9 U-SIG + EHT-SIG L-SIG mod3=0 marks EHT

L-SIG NAV - The Backward Compatibility Mechanism

Every PPDU format from HT-Mixed onwards starts with the legacy L-STF + L-LTF + L-SIG preamble. This is deliberate - a legacy 802.11a/g device that cannot decode HT-SIG or VHT-SIG-A still decodes L-SIG, reads the Duration/Length field, and sets its NAV (Network Allocation Vector) timer accordingly. This prevents the legacy device from transmitting and causing a collision during the HT/VHT/HE/EHT exchange it cannot understand.

L-SIG Length Field Trick

The LENGTH field in L-SIG encodes the number of OFDM symbols in the PPDU at the legacy 6 Mbps rate. This makes the L-SIG-computed duration match the actual PPDU duration - even for the HT/VHT/HE payload the legacy STA cannot decode.

EHT Differentiation from HE

L-SIG LENGTH field value: if mod(length, 3) = 0 → EHT PPDU. If mod(length, 3) ≠ 0 → HE PPDU. The RL-SIG field (repeated inverted L-SIG) appears in both HE and EHT - it is the mod-3 check that differentiates them.

HT-Greenfield Has No L-SIG

HT-GF drops the legacy preamble entirely (no L-STF/L-LTF/L-SIG). Legacy STAs cannot detect the channel as busy → they transmit → collision at the AP. This is why HT-GF is prohibited by IEEE 802.11-2020 in mixed networks containing non-HT STAs.

PCAP Identification - How to Spot PPDU Types

Wireshark + a monitor mode adapter shows PPDU type in the radiotap header. Look for the following fields:

Radiotap: MCS field present, HT flag set
→ HT-Mixed or HT-GF
Radiotap: VHT field present
→ VHT (802.11ac)
Radiotap: HE-MU or HE-SU data rate format
→ HE (802.11ax)
Radiotap: EHT data rate format
→ EHT (802.11be / Wi-Fi 7)
No MCS/VHT/HE fields - OFDM rate shown
→ Non-HT Legacy (802.11a/g)
Radiotap guard interval = 400 ns
→ Short GI (802.11n/ac) - +11% throughput
WIRESHARK FILTER
radiotap.he.data_1.ppdu_format == 0 // HE-SU
radiotap.vht.present.mcs == 1 // VHT frame
radiotap.mcs.index // HT MCS index
// related reference
MCS Rate Calculator →Preamble Puncturing →Channel Reference →Frame Aggregation →Interframe Spacing →