The 802.11ac PPDU format supports the legacy 802.11a/n PPDU formats and the 802.11ac Very High Throughput (VHT) format. As 802.11ac is a 5GHz only standard – it does not have to support 802.11b/g PPDU formats.
The PPDU format is shown below
Fig Courtesy: 802.11 Standard
The 802.11ac standard does not support a pure VHT format like the green field mode as in 802.11n. The Frame format supported is only the mixed mode PLCP frame.
Legacy fields in 802.11ac PPDU format
The L-STF and L-LTF is identical to the OFDM (Clause 18 in the 802.11-2012 standard) format for 20 and 40 MHz transmissions. For 80MHz and 160 MHz transmissions the 802.11ac standard defines the formula which is an extension for 20 and 40MHz transmission. Refer section 22.3.82.2 in the 802.11ac-2013 standard for more information on the 40 and 80 MHz L-STF and L-LTF formats.
The L-SIG is also identical to the OFDM (Clause 18 in the 802.11-2012 standard) and is shown below
The Rate field will be set to 6 Mb/s and the Length field is set according to the below formula
Wherein, the TXTIME field is the time in microseconds. The TXTIME parameter is calculated according to the formula
For short Guard Interval
For Long Guard Interval
The Reserved bit is set to zero and the Parity bit has the even parity of bits 0-16. The signal tail bits are set to zero.