Which of the following are 802.11 MAC-Layer technologies are now used by 802.11ax?

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The answer is C.

BSS Color (also referred to as BSS Coloring) is a method for addressing medium contention overhead due to overlapping basic service set (OBSS). BSS color aims to uniquely identify different BSSs even though they are transmitting on the same channel. 802.11ax radios can differentiate between BSSs by adding a number (color) to the PHY and MAC headers. The same color bit indicates an intra-BSS. Different color bits indicate inter-BSS. Inter-BSS detection means that a listening radio may not necessarily have to defer. Adaptive CCA implementation could raise the signal detect (SD) threshold for inter-BSS frames while maintaining a lower threshold for intra-BSS traffic. BSS Color potentially decreases the channel contention problem resulting from existing 4 dB signal detect (SD) thresholds.

BSS color, also known as BSS coloring, is a method for identifying overlapping basic service sets (OBSSs). BSS coloring was first defined in the 802.11ah-2016 amendment and is now also defined in the 802.11ax draft amendment. The BSS color is a numerical identifier of the BSS. 802.11ax radios are able to differentiate between BSSs using BSS color identifier when other radios transmit on the same channel. If the color is the same, this is considered to be an intra-BSS frame transmission. In other words, the transmitting radio belongs to the same BSS as the receiver. If the detected frame has a different BSS color from its own, then the STA considers that frame as an inter-BSS frame from an overlapping BSS.

BSS color information is communicated at both the PHY layer and the MAC sublayer. In the preamble of an 802.11ax PHY header, the SIG-A field contains a 6-bit BSS color field. This field can identify as many as 63 BSSs.

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If the question is multiple choice, then the correct answer is C & D according to the following diagram from the HCIA-WLAN Training material v3.0.

C.  BSS coloring

D. TWT