An 802.11n HT station supports three modes of transmissions.
- Legacy 802.11 transmission Mode
- HT-Mixed Mode Transmission Mode
- HT-Green Field Mode Transmission Mode
Legacy 802.11 transmission Mode
The Frame Transmission is understandable by all 802.11 stations including legacy 802.11a/b/g stations.
- L-STF – Legacy Short Training Field
- L-LTF – Legacy Long Training Field
- L-SIG – Legacy Signal Field
HT-Mixed Mode Transmission Mode
The HT Mixed Mode Packet Transmission contains a Frame Preamble which is understandable by both Legacy WLAN Stations and 802.11 Stations. The HT Mixed mode Frame format is provided below
- L-STF – Legacy Short Training Field
- L-LTF – Legacy Long Training Field
- L-SIG – Legacy Signal Field
- HT-SIG – HT Signal Field
- HT-LTF – HT Long Training Field
- HT-STF – HT Short Training Field
Since, the initial frame preamble is the legacy preamble, the legacy 802.11 stations can back off for the duration of the HT frame transmission.
HT-Green Field Mode Transmission Mode
The Green-Field Mode Frame format does not contain any legacy frame preamble.
- HT-GF-STF – HT Green Field Short Training Field
- HT-LTF – HT Long Training Field
- L-STF – Legacy Short Training Field
- L-LTF – Legacy Long Training Field
- L-SIG – Legacy Signal Field
- HT-SIG – HT Signal Field
- HT-LTF – HT Long Training Field
- HT-STF – HT Short Training Field
As is seen from the Figure above, the Green Field Format does not have the Legacy frame preamble and hence legacy 802.11 Stations will not be capable of deciphering them. Green Field mode transmissions need to be protected with Legacy RTS-CTS frame transmissions which indicate the duration of the frame transmission or the TXOP.