An HT Station can perform a pure HT transmission (termed green-field mode) which is not understandable to legacy 802.11 stations (802.11b/g). An HT station that intends to perform HT transmissions which will not be decipherable by a non-HT station needs to perform protection of such transmissions.
The HT Operation element describes the HT protection field which describes what kind of protection each WLAN station in the BSS needs to conform to.
Fig Courtesy: 802.11-2016 Standard
As per the 802.11 standard
- The HT Protection field may be set to no protection mode only if the following are true:
- All STAs detected (by any means) in the primary or the secondary channel are HT STAs, and
- All STAs that are known by the transmitting STA to be a member of this BSS are either
- 20/40 MHz HT STAs in a 20/40 MHz BSS, or
- 20 MHz HT STAs in a 20 MHz BSS.
- The HT Protection field may be set to nonmember protection mode only if the following are true:
- A non-HT STA is detected (by any means) in either the primary or the secondary channel or in both the primary and secondary channels, that is not known by the transmitting STA to be a member of this BSS, and
- All STAs that are known by the transmitting STA to be a member of this BSS are HT STAs.
- The HT Protection field may be set to 20 MHz protection mode only if the following are true:
- All STAs detected (by any means) in the primary channel and all STAs detected (by any means) in the secondary channel are HT STAs and all STAs that are members of this BSS are HT STAs, and
- This BSS is a 20/40 MHz BSS, and
- There is at least one 20 MHz HT STA associated with this BSS.
If a BSS contains HT and Non-HT stations, it needs to protect the transmission via an RTS-CTS frame or send the transmission as mixed mode HT transmission.
Also, an adjoining BSS may contain non-HT stations present. The presence of such stations is provided by the HT Operation element – OBSS Non-HT STAs Present field.
Fig Courtesy: 802.11-2016 standard