A direct-sequence transmission with the shortened synchronisation field and a header sent at twice the rate.
Reading a capture
Same rates, same payload, half the overhead — and optional, because a station that does not implement it cannot decode the frame at all. That is the trade at the centre of every backward-compatibility decision in this protocol: the saving is available only when nothing old is listening.
Subfields
SYNC — Fifty-six scrambled zero bits at 1 Mb/s, for the receiver to lock on to.
SFD — The start frame delimiter, sent in reverse bit order from the long preamble's.
PLCP Header — The same forty-eight header bits as the long preamble, sent at 2 Mb/s rather than 1.
PSDU — The frame itself, at the rate named in the SIGNAL field.
Where it sits
HR/DSSS PPDU (short preamble)
Authored in HR/DSSS short preamble PPDU . Drawn in 20 frames.