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CTS 161: 802.11ax BSS Coloring

Why is there a need for BSS Coloring? To help a receiving device identify the BSS from which a receiving PPDU originates from so that there’s a reduction in BSS collision reporting a busy medium. 802.11ax allows the medium to be reused more often between OBSSs by identifying those overlapping BSSs. The primary purpose is to improve the efficiency of Wi-Fi in a dense area. BSS Coloring will tackle the issue of frequency re-use.

An AP receives a neighbor report for the purpose of including the HE Operation element of neighboring High Efficienty (HE) APs to determine BSS Color information of those neighbors.

Which frames can you find the BSS Color field? HE Operation element will contain BSS color info which can be found in the Beacon frame, Association/reassociation, Probe response. It’s in the PHY Preamble.

BSS Color within the PHY

The HE Operation Element can be found in the following frames: Beacon, Probe Response and (Re)Association frames.

HE Operation Element – Notice the BSS Color Information Field

BSS color is an identifier of a BSS to assist a receiving device in an identifying BSS from which a PPDU originates for the purposes of channel access, reduce power consumption, or update NAV.

AP selects a value from 1 to 63 which is included in the BSS Color subfield of the HE Operation element or New BSS Color subfield of the BSS Color Change Announcement element.

The device will set the BSS Color subfield of HE Operation element to value indicated in the BSS Color subfield received from the AP. AP sets the parameter for BSS_COLOR of a HE PPDU.

BSS Color field is for the active BSS color. If a device roams to another BSS the value of the active BSS color will be entered in the New BSS Color field as received in the BSS Color Change Announcement element.

Image two BSSs on the same channel, 149. One BSS would use color yellow, and the other would use color blue. The BSS coloring changes channel access methods. Devices could transmit and receive at the same time. Won’t this cause a collision? Yes, if the BSS colors are the same.

Can a collision occur between colors?

An AP can determine if there is a BSS Color collision by receiving frames from an OBSS device or AP containing the same BSS color it has selected. If this occurs, the AP sets the BSS Color Disabled subfield. The subfield is set for a duration of a BSS Color Collision Period.

It is possible to have a BSS color collision with an OBSS. And when detected, AP will set value of BSS Color Disabled subfield within HE Operation element to 1 which informs others that BSS Color is disabled.

AP selects a BSS color and may change it under certain conditions such as detecting an OBSS using the same color. There is no method defined in how selecting a new BSS color should be performed. An AP may take colors used in its surroundings into account.

When AP is changing BSS color a BSS Color Change Announcement is sent in a Beacon, Probe Response and ReAssociation Response frame or using a HE BSS Color Change Announcement frame. What could cause a color change? Another BSS using the same color.

Ultimately, you’ll have SINR.

HE BSS Color Change Announcement

The HE BSS Color Change Announcement is an Action frame. Contains a BSS Color Change Announcement. The AP can change the BSS Color. And when it does so, it sends an announcement to associated devices.

BSS Color Change Announcement element – notice the last two bits.

The BSS Color Change Announcement Element can be found in the following frames: Beacon, Probe Response and (Re)Association response.

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