Information About BSS Coloring
The 802.11 Wi-Fi standard minimizes the chance of multiple devices interfering with one another by transmitting at the same time. This carrier-sense multiple access with collision avoidance (CSMA/CA) technology is based on static thresholds that allow Wi-Fi devices to avoid interfering with each other on air. However, with an increase in density and the number of Wi-Fi devices, these static thresholds often lead to CSMA/CA causing devices to defer transmissions unnecessarily.
For example, if two devices that are associated with different BSS, can hear every transmission from each other at relatively low signal strengths, each device should defer its transmission when it receives a transmission from the other. But if both the devices were to transmit at the same time, it is likely that neither would cause enough interference at the other BSS' receiver to cause reception failure for either transmission.
Devices today must demodulate packets to look at the MAC header in order to determine whether or not a received packet belongs to their own BSS. This process of demodulation consumes power, which can be saved if devices can quickly identify the BSS by looking at the PHY header alone, and subsequently drop packets that are from a different BSS. Prior to Wi-Fi 6, there was no provision for devices to do this.
The new 802.11ax (Wi-Fi 6) standard addresses both of the issues discussed above, through the new BSS Coloring and Spatial Reuse mechanism. BSS Coloring is a new provision that allows devices operating in the same frequency space to quickly distinguish between packets from their own BSS and packets from an Overlapping BSS (OBSS), by simply looking at the BSS color value contained in the HE PHY header. In some scenarios, Spatial Reuse allows devices, to transmit at the same time as the OBSS packets they receive, instead of deferring transmissions because of legacy interference thresholds. Since every Wi-Fi 6 device understands the BSS color, it can be leveraged to increase power savings by dropping packets earlier, and to identify spatial reuse opportunities.
BSS Coloring is a method used to differentiate between the BSS of access points and their clients on the same RF channel. Wi-Fi 6 enables each AP radio to assign a value (from 1 to 63), known as BSS color, to be included in the PHY header of all HE transmissions from devices in its BSS. With devices of each BSS transmitting a locally-unique color, a device can quickly and easily distinguish transmissions coming from its BSS from those of a neighboring BSS.
The following platforms support this feature:
Cisco Catalyst 9800 Series Wireless Controllers
Cisco Catalyst 9115 Access Points
Cisco Catalyst 9117 Access Point
Cisco Catalyst 9120AX Series Access Points
Cisco Catalyst 9124AX Series Access Points
Cisco Catalyst 9130AX Access Points
OBSS-PD and Spatial Reuse
Overlapping BSS Packet Detect (OBSS-PD) is a more aggressive Wi-Fi packet detect threshold for inter-BSS packets, which can be higher than the typical/legacy -82 dBm. Inter-BSS packets are easily identified by comparing the BSS color in the HE PHY header of the packets received with the BSS color of the device.
In OBSS-PD based Spatial Reuse, to improve throughput and network efficiency by increasing transmitting opportunities, a Wi-Fi 6 or 802.11ax device can transmit over an inter-BSS packet with an RSSI that is below the OBSS-PD threshold instead of deferring.
Cisco Catalyst 9120AX Series Access Points do not support OBSS-PD.