One can refer the first part of this blog here — <802.11g PLCP Frame Formats – Part 1>

DSSS-OFDM (Long Preamble Format)

The DSSS-OFDM Preamble provides co-existence with legacy 802.11b devices as the Preamble and the PLCP header are sent at a rate that the legacy 802.11b devices can hear and understand. The DSS-OFDM PPDU format is shown below

Fig Courtesy: 802.11 Standard

Short Preamble Format

As can be seen from the two Figures – the preamble in the short preamble format is of length 72 bits as compared to 144 bits in the Long preamble format.

The PLCP Signal field for both DSSS-OFDM formats will be set to 0x1E (3 Mbps)

The PLCP header is the same as the 802.11 Long/short Preamble formats discussed earlier.

The Signal extension field is a quiet time of 6 microseconds. This is to make the TXTIME Calculation for ERP-OFDM to contain 6 microseconds longer. The need for this additional time is due to the fact that the OFDM PHY (clause 18 in 802.11-2012 standard – OFDM PHY) had a SIFS time duration of 16 microseconds to accommodate the convolutional decode to finish. However, the High rate PHY (802.11b Clause 17 in 802.11-2012 standard) has a 10 microseconds SIFS time defined. Hence – to make sure that the duration is computed properly at the 802.11g transmitter and the NAV value in Legacy 802.11b High rate PHY devices is set properly – additional 6 microseconds is added.

802.11g PLCP Frame Formats – Part 3