A complete non-HT transmission: the preamble the receiver uses to find and synchronise to the signal, a SIGNAL field describing how to demodulate what follows, and the DATA field carrying the frame itself.
Reading a capture
The preamble and SIGNAL field are fixed at 20 microseconds regardless of how much data follows. That overhead is the reason a short frame is far more expensive per octet than a long one, and it is the argument aggregation exists to answer.
Subfields
Legacy Short Training Field — Ten repetitions of a short symbol. The receiver uses them to detect that a signal is present, set its gain, and make a coarse estimate of frequency offset and symbol timing.
Legacy Long Training Field — A guard interval followed by two identical long symbols, used for fine frequency correction and to estimate the channel on every subcarrier.
SIGNAL field — One OFDM symbol, always sent at 6 Mb/s with BPSK and rate-1/2 coding so that any receiver can read it. Tells the receiver the rate and length of what follows.
DATA field — Everything after the SIGNAL field: a service field, the PSDU handed down by the MAC, and the tail and padding needed to fill whole OFDM symbols.
Where it sits
non-HT PPDU
Authored in non-HT OFDM PPDU . Drawn in 20 frames.