Six microseconds of nothing after the data, so that receivers in the same band have time to finish.
Airtime that carries no information and cannot be omitted. It is here because 802.11b receivers need longer to decode than OFDM ones, and without it an 802.11g station could start transmitting while one was still working. The cost is paid on every ERP-OFDM frame ever sent, including on networks where no 802.11b device has existed for twenty years — which is what backward compatibility actually looks like once it is in silicon.
Signal Extension
Authored in ERP-OFDM PPDU . Drawn in 20 frames.